Staff Profiles

Books

• Tzanakis I, Eskin D, Ed., Ultrasonic Cavitation Treatment of Metallic Alloys, MDPI (2020)
  ISBN: 9783039281961 eISBN: 9783039281978
  Abstract

  The current trend in solidification research is to develop a generic, energy-efficient, economical,
  sustainable, and pollution-free technology that can be applied to different alloy systems.
  Ultrasonic-cavitation melt treatment (UST) is a rather universal technology that can be applied
  to conventional and advanced metallic materials, regardless of their composition, while being
  environmentally friendly, cost effective, and ready to be implemented in conventional casting
  technologies such as direct-chill, continuous, or shape casting, as well as in emerging technologies
  of additive manufacturing and nanocomposite materials.
  

  The beneficial effects of UST—such as in assisted nucleation, activation of substrates (wetting),
  deagglomeration and fragmentation of solid phases, degassing of the melt, and grain refinement of
  the as-cast product—stem from the growth, collapse, and implosion of cavitation bubbles as a result of
  alternate fluctuations in ultrasonic pressure. Although successfully demonstrated on the laboratory
  and pilot scale, UST has not yet found widespread industrial implementation. This is mostly due
  to the lack of in-depth understanding of the fundamental mechanisms behind the improved metal
  quality and structure refinement.
  

  Thus, fundamental research is needed to answer the following practical questions: What is the
  optimum melt flow rate that maximizes treatment efficiency whilst minimizing input power, cost, and
  plant complexity? What is the optimum operating frequency and acoustic power that accelerates the
  treatment effects? What is the optimum location of an ultrasonic power source in the melt transfer
  system in relation to the melt pool geometry? Answering these questions will pave the way for
  widespread industrial use of ultrasonic melt processing with the benefit of improving the properties
  of lightweight structural alloys, simultaneously alleviating the present use of polluting (Cl, F) for
  degassing or expensive (Zr, Ti, B, Ar) grain refinement additives.
  

  Website 

Journal articles

• Morton JA, Khavari M, Qin L, Maciejewska BM, Tyurnina AV, Grobert N, Eskin DG, Mi J, Porfyrakis K, Prentice P, Tzanakis I, 'New insights into sono-exfoliation mechanisms of graphite: in situ high-speed imaging studies and acoustic measurements'
  Materials Today x (x) (2021)
  ISSN: 1369-7021
  Abstract

  The application of ultrasound and acoustic cavitation in liquid exfoliation of bulk layered materials is a widely used method. However, despite extensive research, the fundamental mechanisms remain far from being fully understood. A number of theories have been proposed to interpret the interactions between cavitation and bulk layered materials and hence to explain the mechanisms of ultrasound assisted exfoliation. Unfortunately, most of the research reported to date is ambiguous or inconclusive due to lack of direct real-time experimental evidence. In this paper, we report systematic work characterising cavitation emissions and observing the exfoliation of graphite in situ, in deionised water under the dynamic interaction with laser and ultrasound induced cavitation bubbles. Using ultra-high-speed optical imaging, we were able to determine the dynamic sequence of graphite exfoliation events on a time scale never reported before. Real-time observations also revealed that shock waves with a pressure magnitude up to 5 MPa and liquid-jets in the range of 80 ms−1, from transient cavitation bubble implosions, were essential for the initiation and propagation of the exfoliation process. On the other hand, bubble oscillations associated with stable cavitation were beneficial for promoting a gentler delamination of graphite layers.
  

  Website 
• Khavari M, Priyadarshi A, Subroto T, Beckwith C, Pericleous K, Eskin DG, Tzanakis I, 'Scale Up Design Study on Process Vessel Dimensions for Ultrasonic Processing of Water and Liquid Aluminium'
  Ultrasonics Sonochemistry 76 (2021)
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract

  Scaling up ultrasonic cavitation melt treatment (UST) requires effective flow management with minimised energy requirements. To this end, container dimensions leading to the resonance play a crucial role in amplifying pressure amplitude for cavitation. To quantify the importance of resonance length during the treatment of liquid aluminium, we used calibrated high-temperature cavitometers (in the range of 8–400 kHz), to measure and record the acoustic pressure profiles inside the cavitation-induced environment of liquid Al and deionized water (used as an analogue to Al) excited at 19.5 kHz. To achieve a comprehensive map of the acoustic pressure field, measurements were conducted at three different cavitometer positions relative to the vibrating sonotrode probe and for a number of resonant and non-resonant container lengths based on the speed of sound in the treated medium. The results showed that the resonance length affected the pressure magnitude in liquid Al in all cavitometer positions, while water showed no sensitivity to resonance length. An important practical application of UST in aluminium processing concerns grain refinement. For this reason, grain size analysis of UST-treated Al-Cu-Zr-Ti alloy was used as an indicator of the melt treatment efficiency. The result showed that the treatment in a resonance tank of  (the wavelength of sound in Al) gave the best structure refinement as compared to other tested lengths. The data given here contribute to the optimisation of the ultrasonic process in continuous casting, by providing an optimum value for the critical compartment (e.g. in a launder of direct-chill casting) dimension.
  

  Website 
• Beckwith C, Djambazov G, Pericleous K, Subroto T, Eskin DG, Roberts D, Skalicky I, Tzanakis I, 'Multiphysics Modelling of Ultrasonic Melt Treatment in the Hot-Top and Launder during Direct-Chill Casting: Path to Indirect Microstructure Simulation'
  Metals 11 (5) (2021)
  eISSN: 2075-4701
  Abstract

  This study concerns the numerical simulation of two competing ultrasonic treatment (UST) strategies for microstructure refinement in the direct-chill (DC) casting of aluminium alloys. In the first, more conventional, case, the sonotrode vibrating at 17.3 kHz is immersed in the hop-top to treat the sump melt pool, in the second case, the sonotrode is inserted between baffles in the launder. It is known that microstructure refinement depends on the intensity of acoustic cavitation and the residence time of the treated fluid in the cavitation zone. The geometry, acoustic field intensity, induced flow velocities, and local temperature are factors which affect this treatment. The mathematical model developed in this work couples flow velocity, acoustics modified by cavitation, heat transfer, and solidification at the macroscale, with Lagrangian refiner particles, used to determine: (a) their residence time in the active zones, and (b) their eventual distribution in the sump as a function of the velocity field. This is the first attempt at using particle models as an efficient, though indirect, alternative to microstructure simulation, and the results indicate that UST in the launder, assisted with baffle separators, yields a more uniform distribution of refining particles, avoiding the strong acoustic streaming jet that, otherwise, accompanies hot-top treatment, and may lead to the strong segregation of refining particles. Experiments conducted in parallel to the numerical studies in this work appeared to support the results obtained in the simulation.
  

  Website 
• Subroto T, Lebon GSB, Eskin DG, Skalicky I, Roberts D, Tzanakis I, Pericleous K, 'Numerical modelling and experimental validation of the effect of ultrasonic melt treatment in a direct-chill cast AA6008 alloy billet'
  Journal of Materials Research and Technology 12 (2021) pp.1582-1596
  ISSN: 2238-7854
  Abstract

  In this work, we study how ultrasonic cavitation melt treatment (UST) affects the temperature distribution, sump profile, and resulting microstructure in the direct-chill (DC) casting of an AA6008 aluminum alloy. Two 152 mm diameter billets were cast; one was treated with UST (UST-DC casting) in the hot top while the other was not (conventional DC casting). To investigate the temperature distribution, temperature was measured at multiple points in both billets. The sump profile was visualized by pouring Zn into the sump during casting. The microstructure was analyzed by measuring the grain size of as-cast billets. A numerical model of DC casting and UST-DC casting has been validated with the temperature measurements across the billets, and the experimental results agrees well with the numerical model. It is found that the sump profile quantification with thermocouple measurements is more accurate and less prone to interpretation than with Zn tracing. Numerical simulation results show that UST application in the hot top with sonotrode position at 20 mm above the graphite ring level depresses the liquidus isotherm but does not affect the solidus isotherm, resulting in a thinner transition region compared with conventional DC casting. Grain structure analysis verifies that structure refinement with UST has been achieved at the given sonotrode position. The strongest grain refinement was at the center of the billet with the average grain size 50% smaller than that without UST. The results are discussed in terms of the known mechanisms of UST, i.e. dendrite fragmentation and deagglomeration of nucleating substrates.
  

  Website 
• Khavari M, Priyadarshi A, Hurrell A, Pericleous K, Eskin D, Tzanakis I, 'Characterization of shock waves in power ultrasound'
  Journal of Fluid Mechanics 915 (2021)
  ISSN: 0022-1120 eISSN: 1469-7645
  Abstract

  The application of cavitation-induced shock waves generated at low driving frequencies, known as power ultrasound, is essential for a wide range of fields, such as sonochemistry, lithotripsy, nanomaterials, emulsions and casting, to name but a few. In this paper, we present measurements of the shock wave pressures emitted by cavitating bubbles in water, under ultrasonic excitation produced by an immersed probe oscillating at 24 kHz. A broad-spectrum fibre-optic hydrophone calibrated in the range of 1–30 MHz was used for this purpose. Spectral analysis of the data reveals a consistent resonance peak at a very narrow range of frequencies (3.27–3.43 MHz). Results were confirmed using real-time analysis of high-speed recordings. By eliminating other possible sources, we propose that this new peak might be associated with shock wave emissions from collapsing bubbles. Spatial maps obtained by collating individual shock wave pressures highlight the effect of pressure shielding with increasing input power, attributed to a cloud of bubbles surrounding the probe. This work contributes towards the elucidation of the key properties of cavitation-driven shock waves and the underlying mechanisms, essential in controlling the effectiveness of the external processing conditions on various physical, chemical and biological systems.
  

  Website 
• Hansen JT, Mahak M, Tzanakis I, 'Numerical modelling and optimization of vertical axis wind turbine pairs: A scale up approach'
  Renewable Energy 171 (2021) pp.1371-1381
  ISSN: 0960-1481 eISSN: 1879-0682
  Abstract

  The performance augmentation of pairs of vertical axis wind turbines (VAWTs) is known to be dependent on incident wind direction, turbine spacing and direction of rotation. Yet, there is a lack of robust numerical models investigating the impact of these parameters. In this study two-dimensional CFD simulations of an isolated VAWT and of co- and counter-rotating pairs of VAWTs were performed with the aim to determine turbine layouts that can increase the power output of VAWT farms. More than 11,500 h of simulations were conducted at a turbine diameter Reynolds number of 1.35 · 107. A mesh convergence study was conducted, investigating the influence of mesh size, domain size, azimuth increment, number of iterations per time step, and domain cell density. Results showed that mesh size, domain size, and azimuth increment proved to have the biggest impact on the converged results. For the configurations analysed, pairs of VAWTs exhibited a 15% increase in power output compared to operating in isolation, when the second rotor was spaced three turbine diameters downstream and at an angle of 60° to the wind direction. Furthermore, when three turbines were positioned in series, the power output was greater than a pair by an additional 3%.
  

  Website 
• Wu WH, Eskin DG, Priyadarshi A, Subroto T, Tzanakis I, Zhai W, 'New insights into the mechanisms of ultrasonic emulsification in the oil–water system and the role of gas bubbles'
  Ultrasonics Sonochemistry 73 (2021)
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract

  Ultrasonic emulsification (USE) assisted by cavitation is an effective method to produce emulsion droplets. However, the role of gas bubbles in the USE process still remains unclear. Hence, in the present paper, high-speed camera observations of bubble evolution and emulsion droplets formation in oil and water were used to capture in real-time the emulsification process, while experiments with different gas concentrations were carried out to investigate the effect of gas bubbles on droplet size. The results show that at the interface of oil and water, gas bubbles with a radius larger than the resonance radius collapse and sink into the water phase, inducing (oil–water) blended liquid jets across bubbles to generate oil-in-water-in-oil (O/W/O) and water-in-oil (W/O) droplets in the oil phase and oil-in-water (O/W) droplets in the water phase, respectively. Gas bubbles with a radius smaller than the resonance radius at the interface always move towards the oil phase, accompanied with the generation of water droplets in the oil phase. In the oil phase, gas bubbles, which can attract bubbles nearby the interface, migrate to the interface of oil and water due to acoustic streaming, and generate numerous droplets. As for the gas bubbles in the water phase, those can break neighboring droplets into numerous finer ones during bubble oscillation. With the increase in gas content, more bubbles undergo chaotic oscillation, leading to smaller and more stable emulsion droplets, which explains the beneficial role of gas bubbles in USE. Violently oscillating microbubbles are, therefore, found to be the governing cavitation regime for emulsification process. These results provide new insights to the mechanisms of gas bubbles in oil–water emulsions, which may be useful towards the optimization of USE process in industry.
  

  Website 
• Subroto T, Eskin DG, Beckwith C, Skalicky I, Roberts D, Tzanakis I, Pericleous K, 'Structure refinement upon ultrasonic melt treatment in a DC casting launder'
  Journal of the Minerals, Metals and Materials Society 72 (2020) pp.4071-4081
  ISSN: 1047-4838 eISSN: 1543-1851
  Abstract

  This work focuses on ultrasonic melt treatment (UST) in a launder upon pilot-scale direct chill (DC) casting of 152-mm-diameter billets from an AA6XXX alloy with Zr addition. Two casting temperatures (650°C and 665°C) were used to assess their effect on the resulting microstructure (grain size, particle size, and number density). Structure refinement results show the feasibility of UST in the DC casting launder. This is quantified through the corresponding reduction of grain size by around 50% in the billet center, or more towards the billet surface, reduction of the average Al3Zr particle size, and increase in the particle number density. A higher Al3Zr particle density was obtained when the alloy was cast at 665°C. Numerical simulation results and suggestions on how to improve the treatment quality of UST in DC casting launder are also provided.
  

  Website 
• Priyadarshi A, Khavari M, Subroto T, Conte M, Prentice P, Pericleous K, Eskin D, Durodola J, Tzanakis I, 'On the governing fragmentation mechanism of primary intermetallics by induced cavitation'
  Ultrasonics Sonochemistry 70 (2020)
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract

  One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al3Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20–40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation.
  

  Website 
• Tyurnina AV, Tzanakis I, Morton J, Jiawei Mi, Porfyrakis K, Maciejewska BM, Grobert N, Eskin DG, 'Ultrasonic exfoliation of graphene in water: a key parameter study'
  Carbon 168 (2020) pp.737-747
  ISSN: 0008-6223
  Abstract

  Liquid Phase Exfoliation (LPE) is an efficient method for graphene flake exfoliation and considered to be compatible with industrial production requirements. However, most of available LPE methods require the use of harmful and expensive solvents for chemical exfoliation prior to mechanical dispersion of the flakes, and therefore an additional step is needed to remove the contamination caused by the added chemicals, making the process complex, costly, unsafe and detrimental to the environment.

  By studying the effects of key ultrasonic LPE parameters, our study demonstrates the possibility to control the production and quality of few-layer graphene flakes in pure water in a relatively short period of time. The driving frequency of an ultrasonic source, a higher acoustic cavitation intensity and uniform distribution of the cavitation events in the sonicated volume are the key parameters for controlling the thickness, surface area and production yield of few-layer graphene flakes. The results are discussed in the context of mechanical exfoliation. This opens a direction for developing LPE into a cost effective, clean, environmentally friendly, and scalable manufacturing process for the next generation of two-dimensional nanomaterials for industrial-scale applications.

  Website 
• Wang C, Connolley T, Tzanakis I, Eskin D, Mi J, 'Characterization of Ultrasonic Bubble Clouds in A Liquid Metal by Synchrotron X-ray High Speed Imaging and Statistical Analysis'
  Materials 13 (1) (2020)
  ISSN: 1996-1944
  Abstract

  Quantitative understanding of the interactions of ultrasonic waves with liquid and solidifying metals is essential for developing optimal processing strategies for ultrasound processing of metal alloys in the solidification processes. In this research, we used the synchrotron X-ray high-speed imaging facility at Beamline I12 of the Diamond Light Source, UK to study the dynamics of ultrasonic bubbles in a liquid Sn-30wt%Cu alloy. A new method based on the X-ray attenuation for a white X-ray beam was developed to extract quantitative information about the bubble clouds in the chaotic and quasi-static cavitation regions. Statistical analyses were made on the bubble size distribution, and velocity distribution. Such rich statistical data provide more quantitative information about the characteristics of ultrasonic bubble clouds and cavitation in opaque, high-temperature liquid metals.
  

  Website 
• Lebon B, Tzanakis I, Pericleous K, Eskin D, 'Numerical Modelling of the Ultrasonic Treatment of Aluminium Melts: An Overview of Recent Advances'
  Materials 12 (19) (2019)
  ISSN: 1996-1944
  Abstract

  The prediction of the acoustic pressure field and associated streaming is of paramount importance to ultrasonic melt processing. Hence, the last decade has witnessed the emergence of various numerical models for predicting acoustic pressures and velocity fields in liquid metals subject to ultrasonic excitation at large amplitudes. This paper summarizes recent research, arguably the state of the art, and suggests best practice guidelines in acoustic cavitation modelling as applied to aluminium melts. We also present the remaining challenges that are to be addressed to pave the way for a reliable and complete working numerical package that can assist in scaling up this promising technology.

  Website 
• Lebon GSB, Salloum-Abou-Jaoude G, Eskin D, Tzanakis I, Pericleous K, Jarry P, 'Numerical modelling of acoustic streaming during the ultrasonic melt treatment of direct-chill (DC) casting'
  Ultrasonics Sonochemistry 54 (2019) pp.171-182
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract

  Acoustic streaming and its attendant effects in the sump of a direct-chill (DC) casting process are successfully predicted under ultrasonic treatment for the first time. The proposed numerical model couples acoustic cavitation, fluid flow, heat and species transfer, and solidification to predict the flow pattern, acoustic pressure, and temperature fields in the sump. The model is numerically stable with time steps of the order of 0.01 s and therefore computationally attractive for optimization studies necessitating simulation times of the order of a minute. The sump profile is altered by acoustic streaming, with the slurry region depressed along the centreline of the billet by a strong central jet. The temperature gradient in the transition zone is increased, potentially interfering with grain refinement. The cooling rate in the sump is also altered, thereby modifying the dendrite arm spacing of the as-cast billet. The relative position of the sonotrode affects the sump profile, with the sump depth decreased by around 5 mm when the sonotrode is moved above the graphite ring level by 100 mm. The acoustic streaming jet penetrates into the slurry zone and, as a result, the growth direction of dendritic grains in the off-centre position is altered.
  

  Website 
• Lebon GSB, Tzanakis I, Pericleous K, Eskin D, Grant PS, 'Ultrasonic liquid metal processing: The essential role of cavitation bubbles in controlling acoustic streaming'
  Ultrasonics Sonochemistry 55 (2019) pp.243-255
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract

  The acoustic streaming behaviour below an ultrasonic sonotrode in water was predicted by numerical simulation and validated by experimental studies. The flow was calculated by solving the transient Reynolds-Averaged Navier-Stokes equations with a source term representing ultrasonic excitation implemented from the predictions of a nonlinear acoustic model. Comparisons with the measured flow field from Particle Image Velocimetry (PIV) water experiments revealed good agreement in both velocity magnitude and direction at two power settings, supporting the validity of the model for acoustic streaming in the presence of cavitating bubbles. Turbulent features measured by PIV were also recovered by the model. The model was then applied to the technologically important area of ultrasonic treatment of liquid aluminium, to achieve the prediction of acoustic streaming for the very first time that accounts for nonlinear pressure propagation in the presence of acoustic cavitation in the melt. Simulations show a strong dependence of the acoustic streaming flow direction on the cavitating bubble volume fraction, reflecting PIV observations. This has implications for the technological use of ultrasound in liquid metal processing.
  

  Website 
• Wang Y, Lebon B, Tzanakis I, Zhao Y, Wang K, Stella J, Poirier T, Darut G, Liao H, Planche M-P, 'Experimental and numerical investigation of cavitation-induced erosion in thermal sprayed single splats'
  Ultrasonics Sonochemistry 52 (2019) pp.336-343
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract

  Hydraulic components are coated by thermal spraying to protect them against cavitation erosion. These coatings are built up by successive deposition of single splats. The behavior of a single splat under mechanical loading is still very vaguely understood. Yttria-stabilized zirconia (YSZ) and stainless-steel splats were obtained by plasma spraying onto stainless steel substrates. The velocity and temperature of particles upon impact were measured and the samples were subsequently exposed to cavitation erosion tests. An acoustic cavitation simulation estimated the water jet velocity and hammer stresses exerted by bubble collapse on the surface of the specimen. Although the results suggested that high stress levels resulted from cavitation loading, it was clear that weak adhesion interfaces played a crucial role in the accelerated cavitation-induced degradation.
  

  Website 
• Eskin DG, Tzanakis I, Wang F, Lebon GSB, Subroto T, Pericleous K, Mi J, 'Fundamental studies of ultrasonic melt processing'
  Ultrasonics Sonochemistry 52 (2019) pp.455-467
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract

  Ultrasonic (cavitation) melt processing attracts considerable interest from both academic and industrial communities as a promising route to provide clean, environment friendly and energy efficient solutions for some of the core issues of the metal casting industry, such as improving melt quality and providing structure refinement. In the last 5 years, the authors undertook an extensive research programme into fundamental mechanisms of cavitation melt processing using state-of-the-art and unique facilities and methodologies. This overview summarises the recent results on the evaluation of acoustic pressure and melt flows in the treated melt, direct observations and quantitative analysis of cavitation in liquid aluminium alloys, in-situ and ex-situ studies of the nucleation, growth and fragmentation of intermetallics, and de-agglomeration of particles. These results provide valuable new insights and knowledge that are essential for upscaling ultrasonic melt processing to industrial level.
  

  Website 
• B.Pinedo, M.Hadfield, I.Tzanakis, M.Conte, M.Anand, 'Thermal analysis and tribological investigation on TPU and NBR elastomers applied to sealing applications'
  Tribology International 127 (2018) pp.24-36
  ISSN: 0301-679X
  Abstract This study investigates the contact temperatures reached due to frictional heating on TPU (Thermoplastic polyurethane) and NBR (Nitrile butadiene rubber) seal surfaces during operation. These elastomers present limited thermal resistance so an excessive temperature rise may affect their tribological performance. Sliding tests of the elastomers against steel cylinders were carried out and the surface temperature evolution was acquired during the tests using a high precision infrared camera. Frictional behaviour and temperature curves were analyzed. The influence of the experimental parameters, such as the sealing material, sliding velocity, applied load and steel surface conditions was examined. Experimental thermal results were compared with those calculated through well-established analytical models, in order to determine the advantages and limitations of the latter.Website 
• Lebon B, Tzanakis I, Pericleous K, Eskin D, 'Experimental and numerical investigation of acoustic pressures in different liquids'
  Ultrasonics Sonochemistry 42 (April 2018) (2018) pp.411-421
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract In an attempt to quantify the instantaneous pressure field in cavitating liquids at large forcing signals, pressures were measured in four different liquids contained in vessels with a frequency mode in resonance with the forcing signal. The pressure field in liquid metal was quantified for the first time, with maximum pressures of the order of 10–15 MPa measured in liquid aluminium. These high pressures are presumed to be responsible for deagglomeration and fragmentation of dendritic intermetallics and other inclusions. Numerical modelling showed that acoustic shielding attenuates pressure far from the sonotrode and it is prominent in the transparent liquids studied but less so in aluminium, suggesting that aluminium behaviour is different. Due to acoustic shielding, the numerical model presented cannot adequately capture the pressure field away from the intense cavitation zone, but gives a good qualitative description of the cavitation activity. The results obtained contribute to understanding the process of ultrasonic melt treatment (UST) of metal alloys, while facilitating further the guidelines formulation and reproducible protocols for controlling UST at industrial levels.Website 
• Wang F, Tzanakis I, Eskin D, Mi JW, Connolley T, 'In situ observation of ultrasonic cavitation-induced fragmentation of the primary crystals formed in Al alloys'
  Ultrasonics Sonochemistry 39 (2017) pp.66-76
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract The cavitation-induced fragmentation of primary crystals formed in Al alloys were investigated for the first time by high-speed imaging using a novel experimental approach. Three representative primary crystal types, Al3Ti, Si and Al3V with different morphologies and mechanical properties were first extracted by deep etching of the corresponding Al alloys and then subjected to ultrasonic cavitation processing in distilled water. The dynamic interaction between the cavitation bubbles and primary crystals was imaged in situ and in real time. Based on the recorded image sequences, the fragmentation mechanisms of primary crystals were studied. It was found that
  there are three major mechanisms by which the primary crystals were fragmented by cavitation bubbles. The first one was a slow process via fatigue-type failure. A cyclic pressure exerted by stationary pulsating bubbles caused the propagation of a crack pre-existing in the primary crystal to a critical length which led to fragmentation. The second mechanism was a sudden process due to the collapse of bubbles in a passing cavitation cloud. The pressure produced upon the collapse of the cloud promoted rapid monotonic crack growth and fast fracture in the primary crystals. The third observed mechanism was normal bending fracture as a result of the high pressure arising from the collapse of a bubble cloud and the crack formation at the branch connection points of dendritic primary crystals. The fragmentation of dendrite branches due to the interaction between two freely moving dendritic primary crystals was also observed. A simplified fracture analysis of the observed phenomena was performed. The specific fragmentation mechanism for the primary crystals depended on their morphology and mechanical properties.Website 
• Tzanakis I, Bolzoni L, Eskin G, Hadfield M, 'Evaluation of Cavitation Erosion Behavior of Commercial Steel Grades Used in the Design of Fluid Machinery'
  Metallurgical and Materials Transactions A 48 (5) (2017) pp.2193-2206
  ISSN: 1073-5623
  Abstract The erosion response under cavitation of different steel grades was assessed by studying the erosion rate, the volume removal, the roughness evolution, and the accumulated strain energy. A 20 kHz ultrasonic transducer with a probe diameter of 5 mm and peak-to-peak amplitude of 50 μm was deployed in distilled water to induce damage on the surface of commercial chromium and carbon steel samples. After a relatively short incubation period, cavitation induced the formation of pits, cracks, and craters whose features strongly depended on the hardness and composition of the tested steel. AISI 52100 chromium steel showed the best performance and is, therefore, a promising design candidate for replacing the existing fluid machinery materials that operate within potential cavitating environments.Website 
• Lebon GSB, Tzanakis I, Djambazov G, Pericleous K, Eskin DG, 'Numerical modelling of ultrasonic waves in a bubbly Newtonian liquid using a high-order acoustic cavitation model'
  Ultrasonics Sonochemistry 37 (2017) pp.660-668
  ISSN: 1350-4177
  Abstract To address difficulties in treating large volumes of liquid metal with ultrasound, a fundamental study of acoustic cavitation in liquid aluminium, expressed in an experimentally validated numerical model, is presented in this paper. To improve the understanding of the cavitation process, a non-linear acoustic model is validated against reference water pressure measurements from acoustic waves produced by an immersed horn. A high-order method is used to discretize the wave equation in both space and time. These discretized equations are coupled to the Rayleigh-Plesset equation using two different time scales to couple the bubble and flow scales, resulting in a stable, fast, and reasonably accurate method for the prediction of acoustic pressures in cavitating liquids. This method is then applied to the context of treatment of liquid aluminium, where it predicts that the most intense cavitation activity is localised below the vibrating horn and estimates the acoustic decay below the sonotrode with reasonable qualitative agreement with experimental data.Website 
• Tzanakis I, Lebon GSB, Eskin DG, Pericleous KA, 'Characterizing the cavitation development and acoustic spectrum in various liquids'
  Ultrasonics Sonochemistry 34 (2016) pp.651-662
  ISSN: 1350-4177
  Abstract A bespoke cavitometer that measures acoustic spectrum and is capable of operating in a range of temperatures (up to 750 °C) was used to study the cavitation behaviour in three transparent liquids and in molten aluminium. To relate these acoustic measurements to cavitation development, the dynamics of the cavitation bubble structures was observed in three Newtonian, optically transparent liquids with significantly different physical properties: water, ethanol, and glycerine. Each liquid was treated at 20 kHz with a piezoelectric ultrasonic transducer coupled to a titanium sonotrode with a tip diameter of 40 mm. Two different transducer power levels were deployed: 50% and 100%, with the maximum power corresponding to a peak-to-peak amplitude of 17 μm. The cavitation structures and the flow patterns were filmed with a digital camera. To investigate the effect of distance from the ultrasound source on the cavitation intensity, acoustic emissions were measured with the cavitometer at two points: below the sonotrode and near the edge of the experimental vessel. The behaviour of the three tested liquids was very different, implying that their physical parameters played a decisive role in the establishment of the cavitation regime. Non dimensional analysis revealed that water shares the closest cavitation behaviour with liquid aluminium and can therefore be used as its physical analogue in cavitation studies; this similarity was also confirmed when comparing the measured acoustic spectra of water and liquid aluminium.Website 
• Tzanakis I, Hodnett M, Lebon GSB, Dezhkunov N, Eskin DG, 'Calibration and performance assessment of an innovative high-temperature cavitometer'
  Sensors and Actuators A: Physical 240 (2016) pp.57-69
  ISSN: 0924-4247
  Abstract This paper describes a series of systematic experimental studies to evaluate the performance of a high temperature cavitometer under well-controlled conditions. The cavitometer was specifically designed for measurements in liquid metals: it operates through a long tungsten waveguide (probe), providing thermal protection to the piezo sensing elements placed outside the hot area, and with sufficient bandwidth to enable the monitoring of broadband acoustic emissions associated with cavitation activity. It was calibrated electrically, and acoustically, at kHz and MHz frequencies, and so can be used to estimate acoustic pressures (in Pa), providing physical, and consequently practical, meaning to cavitation measurements within liquid metals. Results obtained from ultrasonic sources in a cylindrical vessel using water showed that the cavitometer is a reliable and robust device for characterizing direct field acoustic pressures and broadband emissions from the resulting cavitation. Additionally, preliminary characterization of the real-time acoustic pressures during ultrasonic processing of liquid aluminium (Al) in a standard clay-graphite crucible were performed for the first time. The use of the calibrated cavitometer will establish a more generalized approach for measuring the actual acoustic pressures over a broad range of liquid temperatures within a sonicated medium, demonstrating its potential use as a tool for optimizing, controlling, and scaling-up processes.Website 
• Xu W, Tzanakis I, Srirangam P, Mirihanage W, Eskin D, Bodey A, Lee P, 'Synchrotron Quantification of Ultrasound Cavitation and Bubble Dynamics in Al-10Cu Melts'
  Ultrasonics Sonochemistry 31 (July 2016) (2016) pp.355-361
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract Knowledge of the kinetics of gas bubble formation and evolution under cavitation conditions in molten alloys is important for the control casting defects such as porosity and dissolved hydrogen. Using in situ synchrotron X-ray radiography, we studied the dynamic behaviour of ultrasonic cavitation gas bubbles in a molten Al–10 wt% Cu alloy. The size distribution, average radius and growth rate of cavitation gas bubbles were quantified under an acoustic intensity of 800 W/cm2 and a maximum acoustic pressure of 4.5 MPa (45 atm). Bubbles exhibited a log-normal size distribution with an average radius of 15.3 ± 0.5 μm. Under applied sonication conditions the growth rate of bubble radius, R(t), followed a power law with a form of R(t) = αtβ, and α = 0.0021 & β = 0.89. The observed tendencies were discussed in relation to bubble growth mechanisms of Al alloy melts.Website 
• Tzanakis I, Hodnett M, Lebon G, Eskin D, Pericleous K, 'Fundamental studies on cavitation melt processing'
  IOP Conference Series: Materials Science and Engineering 129 (1) (2016) pp.1-6
  ISSN: 1757-8981
  Abstract The application of ultrasound to industrial casting processes has attracted research interest during the last 50 years. However, the transfer and scale-up of this advanced and promising technology to industry has been hindered by difficulties in treating large volumes of liquid metal due to the lack of understanding of certain fundamentals. In the current study experimental results on ultrasonic processing in deionised water and in liquid aluminium (Al) are reported. Cavitation activity was determined in both liquid environments and acoustic pressures were successfully measured using an advanced high-temperature cavitometer sensor. Results showed that highest cavitation intensity in the liquid bulk is achieved at lower amplitudes of the sonotrode tip than the maximum available, suggesting nonlinearity in energy transfer to the liquid, while the location of the sonotrode is seen to substantially affect cavitation activity within the liquid. Estimation of real-time acoustic pressures distributed inside a crucible with liquid Al was performed for the first time.Website 
• Tzanakis I, Lebon GSB, Eskin DG, Pericleous K, 'Comparison of cavitation intensity in water and in molten aluminium using a high-temperature cavitometer'
  Journal of Physics: Conference Series 656 (2015)
  ISSN: 1742-6588
  Abstract The application of ultrasound to industrial casting processes has attracted research interest during the last 50 years. However, the transfer and scale-up of this advanced and promising technology to the industry have been hindered by difficulties in treating large volumes of liquid metal due to the lack of understanding of certain fundamentals. In the current study, experimental results on ultrasonic processing in deionised water and in liquid aluminium (Al) are reported. Cavitation activity was determined in both liquid environments using an advanced high-temperature cavitometer sensor. In water, the highest cavitation activity is obtained for the lowest sonotrode tip amplitudes. Below the sonotrode, the cavitation intensity in liquid aluminium is found to be four times higher than in water.Website 
• Tzanakis I, Lebon GSB, Eskin DG, Pericleous KA, 'Characterisation of the ultrasonic acoustic spectrum and pressure field in aluminium melt with an advanced cavitometer'
  Journal of Materials Processing Technology 229 (2016) (2015) pp.582-586
  ISSN: 0924-0136
  Abstract Currently, fundamental experimental studies in liquid metals are limited as there are very few available experimental tools for directly measuring acoustic cavitation in such extreme environments. In this work, a calibrated high temperature cavitometer was used for measuring acoustic emissions and acoustic pressure in sonicated liquid aluminium and in water. The extent of the cavitation zone was quantified in liquid aluminium and water. The differences between cavitation behaviour of water and liquid aluminium were explained in terms of acoustic shielding, attenuation, and bubble dynamics.Website 
• Lebon GS, Pericleous K, Tzanakis I, Eskin DG, 'Dynamics of two interacting hydrogen bubbles in liquid aluminum under the influence of a strong acoustic field'
  Physical Review E 92 (4) (2015) pp.043004-
  ISSN: 2470-0045 eISSN: 2470-0053
  Website 
• Tzanakis I, Xu W, Lebon GS, Eskin D, Pericleous K, Lee P, 'In situ synchrotron radiography and spectrum analysis of transient cavitation bubbles in molten aluminium alloy'
  Physics Procedia 70 (2015) pp.841-845
  ISSN: 1875-3892
  Abstract The melt processing of conventional and advanced metallic materials with high-intensity ultrasonic vibrations significantly improves the quality and properties of molten metals during their solidification. These improvements are primarily attributed to ultrasonic cavitation: the creation, growth, pulsation, and collapse of bubbles in the liquid. However, the development of practical applications is limited by the lack of fundamental knowledge on the dynamics of the cavitation bubbles; it is very difficult to directly observe ultrasonic cavitation using conventional techniques in molten metals due their high temperature and opaqueness.
   
  In this study, an in situ synchrotron radiography experiment was performed to investigate bubble dynamics in an Al-10 wt.% Cu alloy under an external ultrasound field at 30 kHz. Radiographs with an exposure time of 78 ms were collected continuously during the sonication of molten alloys at temperatures of 660±10 °C. To the best of our knowledge, this is the first time that transient cavitation bubbles have been observed in liquid aluminium. Quantification of bubble parameters such as average size and time of collapse were evaluated from radiographs using advanced image analysis. Additionally, broadband noise associated with the acoustic emissions from shock waves of transient cavitation bubbles and estimation of the real-time acoustic pressure at the driving frequency were assessed using an advanced high-temperature cavitometer in separate bulk experiments.
  Website 
• Tzanakis I, Lebon GS, Eskin D, Pericleous K, 'Effect of input power and temperature on the cavitation intensity during the ultrasonic treatment of molten aluminium'
  Transactions of the Indian Institute of Metals 68 (6) (2015) pp.1023-1026
  ISSN: 0972-2815
  Abstract Experimental results of ultrasonic processing of liquid aluminium with a 5 kW magnetostrictive transducer and a 20 mm titanium sonotrode excited at 17 kHz are reported in this study. A unique high-temperature cavitometer sensor, placed at various locations in the liquid melt, measured cavitation activity at various acoustic power levels and different temperature ranges. The highest cavitation intensity in the liquid bulk is achieved below the surface of the sonotrode, at the lowest temperature and with an applied power of 3.5 kW. This two-fold mechanism is related to (a) acoustic shielding and (b) the tendency of liquid aluminium to release hydrogen when the temperature drops, thus promoting multiple cavitation events. Understanding these mechanisms in liquid metals can result in a major breakthrough for the optimization of ultrasound applications to liquid metal processing.Website 
• Tzanakis I, Lebon GSB, Eskin DG, Pericleous K, 'Investigation of the factors influencing cavitation intensity during the ultrasonic treatment of molten aluminium'
  Materials & Design 90 (2016) (2015) pp.979-983
  ISSN: 0261-3069
  Abstract The application of ultrasound to casting processes is a subject of great interest: the resulting degassing, sonocrystallization, wetting, fragmentation, de-agglomeration and dispersion yield an improved cast material sonocrystallization,wetting, fragmentation, de-agglomeration and dispersion yield an improved cast material with fine grain structure. However, due to the lack of understanding of certain fundamentals involved in the process, the transfer and scale-up of this promising technology to industry has been hindered by difficulties in treating large volumes of liquid metal. Experimental results of ultrasonic processing of liquid aluminium with a 5-kW magnetostrictive transducer and a 20-mmniobiumsonotrode producing 17-kHz ultrasonic waves are reported in this study. A high-temperature cavitometer sensor that is placed at different locations in the liquid melt, measured cavitation activity at various acoustic power levels and in different temperature ranges. The highest cavitation intensity in the liquid bulk is achieved below the surface of the sonotrode, at the lowest temperature, and when the applied power was 3.5 kW. Understanding these ultrasonication mechanisms in liquid metals will result in a major breakthrough for the optimization of ultrasound applications in metal industries.Website 
• Eskin DG, Al-Helal K, Tzanakis I, 'Application of a plate sonotrode to ultrasonic degassing of aluminum melt: Acoustic measurements and feasibility study'
  Journal of Materials Processing Technology 222 (2015) (2015) pp.148-154
  ISSN: 0924-0136
  Abstract A flat plate sonotrode was used for ultrasonic melt processing (degassing) of aluminum melts. Calculations showed that this sonotrode should have several antinodes with maximum amplitude, spaced at 16.5 mm. The direct measurements of the amplitude in air and indirect measurements of foil cavitation erosion in water validated these calculations. Unique acoustic measurements of cavitation activity in water and a liquid aluminum alloy were performed using a cavitometer and confirmed that the cavitation conditions were met with this scheme. The melt degassing efficiency using the plate sonotrode was significantly higher (70-80%) than with a conventional cylindrical sonotrode (45-50%) in batch operation. The new scheme was also suitable for ultrasonic melt processing in the melt flow giving about 50% degassing efficiency, which opens the way to upscaling this technology to treat larger volumes of melt.Website 
• Lebon GSB, Tzanakis I, Pericleous KA, Eskin DG, 'Comparison between low-order and high-order acoustic pressure solvers for bubbly media computations'
  Journal of Physics: Conference Series 656 (2015) pp.1-4
  ISSN: 1742-6588
  Abstract Numerical modelling is a useful tool for the fundamental study of acoustic cavitation treatment in liquid metals. This treatment, also known as ultrasonic melt processing, significantly improves the properties and quality of metallic materials. However, the mechanisms leading to these observed improvements are still unclear and a fundamental study of cavitation treatment is required to understand this process. In this endeavour, this paper compares the use of high-order discretization schemes for solving acoustic pressures in cavitating liquids with its low-order counterpart. A fourth order scheme is shown to be more stable and accurate than a second order scheme when taking into account the acceleration of bubbles before their collapse, and is recommended for the full cavitation modelling of acoustic treatment of liquid metals.Website 
• Tzanakis I, Xu WW, Eskin DG, Lee PD, Kostovinos N, 'In situ observation and analysis of ultrasonic capillary effect in molten aluminium'
  Ultrasonics Sonochemistry 27 (2015) pp.72-80
  ISSN: 1350-4177 eISSN: 1873-2828
  Abstract An in situ synchrotron radiographic study of a molten Al-10 wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30 kHz, with a peak to peak amplitude at 140 microns, was used, producing a pressure output of 16.9 MPa at the radiation surface of the 1-mm diameter sonotrode. 
   
  This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density.
  Website 
• Lebon GSB, Pericleous K, Tzanakis I, Eskin D, 'Application of the "Full Cavitation Model" to the fundamental study of cavitation in liquid metal processing'
  IOP Conference Series: Materials Science and Engineering 72 (2015) (2015) pp.1-7
  ISSN: 1757-8981
  Abstract Ultrasonic cavitation treatment of melt significantly improves the downstream properties and quality of conventional and advanced metallic materials. However, the transfer of this technology has been hindered by difficulties in treating large volumes of liquid metal. To improve the understanding of cavitation processing efficiency, the Full Cavitation Model, which is derived from a reduced form of the Rayleigh-Plesset equation, is modified and applied to the two-phase problem of bubble propagation in liquid melt. Numerical simulations of the sound propagation are performed in the microsecond time scale to predict the maximum and minimum acoustic pressure amplitude fields in the domain. This field is applied to the source term of the bubble transport equation to predict the generation and destruction of cavitation bubbles in a time scale relevant to the fluid flow. The use of baffles to limit flow speed in a launder conduit is studied numerically, to determine the optimum configuration that maximizes the residence time of the liquid in high cavitation activity regions. With this configuration, it is then possible to convert the batch processing of liquid metal into a continuous process. The numerical simulations will be validated against water and aluminium alloy experiments, carried out at Brunel University.Website 
• Tzanakis I, Georgoulas A, Hadfield M, Kotsovinos N, 'Cavitation erosion damage of scroll steel plates by high-speed gas working fluid'
  International Journal of Computational Methods and Experimental Measurements 2 (2) (2014) pp.168-183
  ISSN: 2046-0546 eISSN: 2046-0554
  Abstract A steel plate is one of the critical components of a scroll expander system that can experience cavitation micro-pitting while in service. The content of the present paper consists of two distinct but interrelated parts. The first part aims to highlight that the use of computational fluid dynamics (CFD) simulations can constitute a potential tool for the prediction of cavitation erosion areas in scroll expander systems. For this purpose, a three-dimensional CFD, steady-state numerical simulation of the refrigerant working fluid is employed. Numerical results revealed the critical areas where cavitation bubbles are formed. These numerical critical areas are in direct qualitative agreement with the actual eroded regions by cavitation, which were found by microscopic observations across the steel plate on an after use, scroll expander system. The second part of the paper aims to further investigate the behaviour and the durability of the steel plate of the studied scroll expander system subjected to cavitation erosion by using an ultrasonic experimental test rig. Scanning electron microscopy and optical interferometer micrographs of the damaged surfaces were observed, showing the nature of the cavitation erosion mechanism and the morphological alterations of the steel plate samples. Experimental results are explained in terms of the cavitation erosion rates, roughness profile, accumulated strain energy, and hardness of the matrix. The experimental study can serve as a valuable input for future development of a CFD numerical model that predicts both cavitation bubbles formation as well as cavitation damage induced by the bubbles that implode on the steels plates.Website 
• Tzanakis I, Eskin D, Georgoulas A, Fytanidis D, 'Incubation pit analysis and calculation of the hydrodynamic impact pressure from the implosion of an acoustic cavitation bubble'
  Ultrasonics sonochemistry 21 (2) (2013) pp.866-878
  ISSN: 1350-4177 eISSN: 1873-2828
  Website 
• Tzanakis I, Conte M, Hadfield M, Stolarski T, 'Experimental and analytical thermal study of PTFE composite sliding against high carbon steel as a function of the surface roughness, sliding velocity and applied load'
  Wear 303 (1) (2013) pp.154-168
  ISSN: 0043-1648
  Website 
• Tzanakis I, Hadfield M, Henshaw I, 'Erratum to--‘Observations of acoustically generated cavitation bubbles within typical fluids applied to a scroll expander lubrication system’[Experimental Thermal and Fluid Science 35 (8)(2011) 1544--1554]'
  Experimental Thermal and Fluid Science 42 (2012) pp.265-270
  ISSN: 0894-1777
  Website 
• Tzanakis I, Hadfield M, Thomas B, Noya S, Henshaw I, Austen S, 'Future perspectives on sustainable tribology'
  Renewable and Sustainable Energy Reviews 16 (6) (2012) pp.4126-4140
  ISSN: 1364-0321
  Website 
• Tzanakis I, Georgoulas A, Hadfield M, Kotsovinos N, 'Cavitation erosion behaviour of the steel plate of a scroll expander system'
  Wit Transactions on Engineering Sciences 76 (2012) pp.129-137
  ISSN: 1746-4471 eISSN: 1743-3533
  Abstract A steel plate is one of the critical components of a scroll expander system that usually experiences cavitation in service. An experimental study is conducted to study the behaviour of the scroll’s steel plate subjected to cavitation erosion. For this purpose an ultrasonic transducer is utilised to produce cavitation bubbles. Micrographs of damaged surfaces were observed, showing the nature of the cavitation mechanism and the morphology alterations across the steel sample. Experimental results are explained in terms of the cavitation erosion rates, roughness profile, accumulated strain energy, and hardness of the matrix. Keywords: ultrasonic cavitation, erosion, bubbles, refrigerant, high carbon steel.Website 
• Tzanakis I, Hadfield M, Henshaw I, Garland N, Khan Z, 'Experimental sliding performance of composite tip seal with high-carbon steel plate under lubricated conditions applied to scroll expander systems'
  Tribology Transactions 54 (4) (2011) pp.505-513
  ISSN: 1040-2004 eISSN: 1547-397X
  Website 
• Tzanakis I, Hadfield M, 'Evaluation of flash temperatures of a composite elastomer tip seal with a dry sliding condition in contact with a high carbon steel plate'
  Surface Effects and Contact Mechanics X: Computational Methods and Experiments 71 (2011) pp.3-
  ISSN: 1746-4471 eISSN: 1743-3533
  Website 
• Tzanakis I, Hadfield M, Henshaw I, 'Observations of acoustically generated cavitation bubbles within typical fluids applied to a scroll expander lubrication system'
  Experimental Thermal and Fluid Science 35 (8) (2011) pp.1544-1554
  ISSN: 0894-1777
  Abstract An experimental study to evaluate the dynamic performance of three different types of cavitation bubbles is conducted. An ultrasonic transducer submerged into the working fluids of a scroll expander is utilised to produce cavitation bubbles and a high speed camera device is used to capture their behaviour. Three critical regions around the ultrasonic source, between the source and the solid boundary, and across the solid boundary were observed. Experimental results revealed that refrigerant bubbles sustain a continuous oscillatory movement, referenced as “wobbling effect”, without regularly collapsing. Analytical results indicate the influence of several factors such as surface tension/viscosity ratio, Reynolds number and Weber number which interpret that particular behaviour of the refrigerant bubbles. Within the refrigerant environment the bubbles obtain large Reynolds numbers and low Weber numbers. In contrast, within the lubricant and the water environment Weber number is significantly higher and Reynolds number substantially lower. The bubble radius and velocity alterations are accurately calculated during the cavitation process. Lubricant bubbles achieve the highest jet velocity while refrigerant bubbles having the lowest jet velocity are not considered as a destructive mean of cavitation for scroll expander systems.Website 
• Tzanakis I, Hadfield M, Garland N, 'Cavitation damage incubation with typical fluids applied to a scroll expander system'
  Tribology International 44 (12) (2011) pp.1668-1678
  ISSN: 0301-679X
  Abstract During the operation of a scroll expander system overpressure may occur resulting in cavitation damage. Impacts due to implosion of cavitation bubbles near to suction ports can result in damage to the scroll plate in the expander. The accumulation of cavitation pits across the scroll plate leads to cavitation erosion hence efficiency drop. An experimental analysis to identify the mechanical damage of the cavitation on various steel surfaces with different liquid environments was conducted.
   
  Three liquid environments and four steel grades were utilised experimentally. The liquids used for the tests were distilled water, used as a reference liquid, and the two working fluids of the scroll expander a synthetic lubricant and a high molecular refrigerant. The steel grades were a high carbon (AISI 1085) and low carbon (AISI 1010) martensitic steel with retained austenite, a chromium martensitic steel (AISI 52100) and a martensitic scroll plate (SP) sample. An ultrasonic transducer was utilised to produce cavitation conditions using a 5 mm diameter probe. The comparison of the results revealed the most hostile liquid environment according to the morphology evaluation of the incubation pits. The cavitation mechanisms are discussed and the cavitation resistance of the steel grades is evaluated. The best performing steel material against cavitation is determined for the conditions described.
  Website 
• Tzanakis I, Hadfield M, Georgoulas A, Kotsovinos N, 'Cavitation damage observations within scroll expander lubrication systems'
  WIT Transactions on Engineering Sciences 66 (2010) pp.261-272
  ISSN: 1746-4471 eISSN: 1743-3533
  Website 
• Tzanakis I, Hadfield M, Khan ZA, 'Durability of domestic scroll compressor systems'
  Surface Effects and Contact Mechanics IX/WIT Press (62) (2009) pp.229-240
  ISSN: 1746-4471 eISSN: 1743-3533
  Website 

Book chapters

• Eskin D, Tzanakis I, 'High-Frequency Vibration and Ultrasonic Processing' in Dmitry G. Eskin, Jiawei Mi (ed.), Solidification Processing of Metallic Alloys Under External Fields, Springer International Publishing (2018)
  ISBN: 9783319948416 eISBN: 9783319948423
  Abstract

  The application of ultrasound to the processing of liquids and slurries has a long history. This chapter considers the main mechanisms of ultrasonic processing of metallic alloys as well as principal applications of this technology to processing of liquid metals, casting of alloys and manufacturing of new materials. Some theoretical background is given as well, The text is illustrated with historical and new results including those obtained with most advanced techniques such as high-temperature cavitometry, high-speed in-situ observations and X-ray synchrotron imaging.
  

  Website 
• Tzanakis I, Lebon G, Eskin D, Pericleous K, 'Optimization of the Ultrasonic Processing in a Melt Flow' in Optimization of the Ultrasonic Processing in a Melt Flow, Wiley Online Library (2016)
  ISBN: 9781119225799 eISBN: 9781119274780
  Abstract Ultrasonic cavitation treatment of melt significantly improves the downstream properties and quality of conventional and advanced metallic materials. However, the transfer of this technology to treating large melt volumes has been hindered by a lack of fundamental knowledge, allowing for the ultrasonic processing in the melt flow. In this study, we present the results of experimental validation of an advanced numerical model applied to the acoustic cavitation treatment of liquid aluminum during continuous flow [1]. This was achieved by using a calibrated high-temperature cavitometer. The acoustic spectrum was analyzed at various points across the launder while acoustic pressures were calculated at the frequencies of interest.Website 
• Lebon G, Tzanakis I, Pericleous K, Eskin D, 'A High-Order Acoustic Cavitation Model for the Treatment of A Moving Liquid Metal Volume' in Nastac L, Zhang L, Thomas BG, Zhu M, Ludwig A, Sabau A, Pericleous A, Combeau H (ed.), A High-Order Acoustic Cavitation Model for the Treatment of A Moving Liquid Metal Volume, Wiley Online Library (2016)
  ISBN: 9781119225768 eISBN: 9781119274681
  Abstract Ultrasonic cavitation significantly improves the downstream properties and quality of metallic materials. However, this technology has not been successfully transferred to the industry due to difficulties in treating large volumes of liquid metal. To improve the understanding of cavitation efficiency so that it can be applied to a moving melt volume, an improved cavitation model consisting of a high-order acoustics solver is applied to the two-phase problem of bubble breakup and propagation in the melt. Results from numerical simulations of the ultrasonic field and flow pattern in a launder are presented. The use of baffles to modify the flow pattern and amplify sound waves in a launder conduit is examined to determine the optimum configuration that maximizes the acoustic pressure amplitude in high cavitation activity regions.Website 
• Xu W, Tzanakis I, Srirangam P, Terzi S, Mirihanage W, Eskin D, Mathiesen R, Horsfield A, Lee P, 'In Situ Synchrotron Radiography of Ultrasound Cavitation in a Molten Al-10Cu Alloy' in In Situ Synchrotron Radiography of Ultrasound Cavitation in a Molten Al-10Cu Alloy, Wiley Online Library (2015)
  ISBN: 9781119082415 eISBN: 9781119093466
  Abstract Chapter 9Website 
• Lebon G, Pericleous K, Tzanakis I, Eskin D, 'A model of cavitation for the treatment of a moving liquid metal volume' in Nastac L, Liu B, Fredriksson H, Lacaze J, Hong C-P, Catalina AV, Buhrig-Polaczek A, Monroe C, Sabau AS, Ruxanda REL, Luo A, Sen S Diószegi A (ed.), A model of cavitation for the treatment of a moving liquid metal volume, Wiley Online Library (2015)
  ISBN: 9781119082385 eISBN: 9781119093367
  Abstract Chapter 4Website 

Conference papers

• Beckwith C, Subroto T, Pericleous K, Djambazov G, Eskin DG, Tzanakis I, 'Ultrasonic Melt Treatment in a DC Casting Launder: The Role of Melt Processing Temperature'
  (2021)
  Abstract

  Ultrasonic melt treatment (UST) using a single sonotrode source in a launder is an efficient way to treat a large-volume melt. One key parameter is the melt processing temperature. Melt processing temperature affects the acoustic pressure generated by the sonotrode, which ultimately defines the cavitation development as well as the resulting acoustic streaming. Experimental results also show that processing temperature affects intermetallic number density and the final grain size. This work presents a numerical model covering acoustic cavitation, flow (including acoustic streaming), and heat transfer in direct-chill (DC) casting, to better understand this process. The UST effectiveness is quantified through the size of the high-pressure acoustic region and the melt residence time, a result reflected in experimental grain size data. The output of this work is useful for optimizing the selection of process parameters for UST DC casting.
  

  Website 
• Subroto T, Bruno Lebon GS, Eskin DG, Skalicky I, Roberts D, Tzanakis I, Pericleous K, 'Effect of ultrasonic melt treatment on the sump profile and microstructure of a direct-chill cast AA6008 Aluminum Alloy'
  (2021)
  Abstract

  This work focuses on the effects of ultrasonic melt treatment (UST) during direct-chill (DC) casting on the temperature distribution across the billet, sump profile, and the resulting microstructure. Two AA6008 billets were cast; one was treated with UST in the hot top while the other was not. To determine the temperature distribution along the billet, multi-point temperature measurements were made across the radii of both billets. The sump profile was also analyzed through macrostructure analysis, after Zn was poured into the sump, while structure refinement was quantified through grain-size measurements. A numerical model of ultrasound-assisted DC casting is validated using the temperature measurements. As an outcome, this study provides information on the extent to which UST affects the sump profile and the corresponding changes in the microstructure. The knowledge gained from this study paves the way towards optimization of UST parameters in DC casting.
  

  Website 
• Priyadarshi A, Subroto T, Conte M, Prentice P, Pericleous K, Eskin D, Durodola J, Tzanakis I, 'Ultrasound induced fragmentation of primary Al3Zr crystals'
  (2020)
  Abstract

  Ultrasonic cavitation melt treatment (UST) of aluminium alloys has received considerable attention in the metal industry due to its simple and effective processing response. The refined primary intermetallic phases formed in the treated alloys during controlled solidification, govern alloy structural and mechanical properties for applications in the automotive and aerospace industries. Since the UST is performed close to the liquidus temperatures of the alloys, understanding the refinement mechanism of the primary intermetallic phases has been beset by difficulties in imaging and handling of liquid metals. In this paper, the sonofragmentation behaviour of primary intermetallic Al3Zr crystals extracted from the matrix of an Al-3 wt% Zr alloy and fixed on a solid substrate was investigated. The intermetallics were exposed to cavitation action in deionized water at 24 kHz of ultrasound frequency. The fragmentation mechanism from the nearby collapsing cavitation bubbles was studied with in-situ high speed imaging. Results revealed that the main fragmentation mechanism is associated with the propagation of shock wave emissions from the collapsing bubble clouds in the vicinity of the crystal. The mechanical properties of the Al3Zr phase determined previously were used for the fracture analysis. It was found that an Al3Zr intermetallic undergoes low cycle fatigue fracture due to the continuous interaction with the shock wave pressure. The magnitude of the resulting shear stress that leads to intermetallic fragmentation was found to be in the range of 0.6 – 1 MPa.
  

  Website 
• Priyadarshi A, Subroto T, Conte M, Pericelous K, Eskin D, Prentice P, Tzanakis I, 'Nanoindentation and Cavitation-Induced Fragmentation Study of Primary Al3Zr Intermetallics Formed in Al Alloys'
  (2020)
  Abstract

  Mechanical properties of primary Al3Zr crystals and their in situ fragmentation behaviour under the influence of a single laser induced cavitation bubble have been investigated using nanoindentation and high-speed imaging techniques, respectively. Linear loading of 10 mN was applied to the intermetallics embedded in the Al matrix using a geometrically well-defined diamond nano-indenter to obtain the mechanical properties at room temperature conditions. Primary Al3Zr crystals were also extracted by dissolving the aluminium matrix of an Al-3wt% Zr alloy. The extracted primary crystals were also subjected to cavitation action in deionized water to image the fracture sequence in real time. Fragmentation of the studied intermetallics was recorded at 500,000 frames per second. Results showed that the intermetallic crystals fail by brittle fracture mode most likely due to the repeatedly-generated shock waves from the collapsing bubbles. The results were interpreted in terms of fracture mechanics using the nanoindentation results.
  

  Website 
• Subroto T, Eskin DG, Beckwith C, Tzanakis I, Djambazov G, Pericleous K, 'Improving Ultrasonic Melt Treatment Efficiency Through Flow Management: Acoustic Pressure Measurements and Numerical Simulations'
  (2020)
  Abstract

  The current challenge for upscaling the ultrasonic melt processing (USP) technology to industrial scale is in improving the treatment efficiency using a single-sonotrode setup. To achieve this, we suggest two innovative approaches: increasing the melt residence time and exploiting acoustic resonance. This can be achieved through flow management in a launder by partitions where the resonance length within the partitions is equal or at integer steps to the wavelength of the incident sound wave. This study focuses on acoustic pressure measurements at different partition configurations and flow conditions combined with numerical modelling of the process. The measurements are done both in liquid aluminum and in water as its transparent analogue. The acoustic pressure measurements are then used to assess melt treatment improvement through cavitation activity and pressure distribution in the launder as well as to verify and further develop the numerical model.
  

  Website 
• Subroto T, Eskin D, Tzanakis I , Lebon G, Miranda A, Pericleous K, 'Optimization of ultrasonic cavitation processing in the liquid melt flow'
  529 (1) (2019)
  ISSN: 1757-899X
  Abstract

  Abstract. Ultrasonic processing (USP) during direct-chill (DC) casting of light metal alloys is
  typically applied in the sump of a billet. This approach, though successful for structure
  refinement and modification, has two main drawbacks: (a) mixture of mechanisms that rely
  heavily on dendrite fragmentation and (b) a limited volume that can be processed by a single
  ultrasonic source. We suggest moving the location of USP from the sump to the launder and
  applying it to the melt flow for continuous treatment. The apparent benefits include: (a)
  degassing of the melt volume, (b) grain refinement through activation of non-metallic inclusions,
  fragmentation of primary crystals, and deagglomeration of grain refining substrates, and (c) a
  possibility to use a single ultrasonic source for processing large melt volumes. To optimize this
  process with regard to the acoustic intensity and melt residence time in the active cavitation zone,
  flow modification with baffles as well as informed location of the ultrasonic source are required.
  In this paper, we demonstrate the results of experimental trials where the degassing degree and
  grain refinement have been the indicators of the USP efficiency for two aluminium alloys, i.e.
  LM25 and AA7050. The results are supported by acoustic measurements and computer
  simulations.
  

  Website 
• Tzanakis I, Lebon GSB, Subroto T, Eskin D, Pericleous K, 'Acoustic Cavitation Measurements and Modeling in Liquid Aluminum'
  (2019) pp.1533-1538
  ISBN: 9783030058647
  Abstract

  The quantification of acoustic pressures in liquid metals is of paramount interest for the optimization of ultrasonic melt treatment (UST) of large volumes. Until recently, the measurements of acoustic pressure and cavitation intensity in a melt were cumbersome and unreliable due to the high temperatures and the lack of suitable instruments. These difficulties imposed strict limitations on the experimental and numerical investigation of cavitation and bubble dynamics within liquid metals. In recent years, our group used a unique calibrated high temperature cavitometer to measure cavitation activity and acoustic pressures in liquid aluminum. Phenomena such as acoustic attenuation, shielding, and cavitation intensity have been studied. These measurements were also used to validate a non-linear acoustic numerical model applicable to flow in bubbly liquids subject to acoustic cavitation. Both experimental and numerical characterization of the acoustic and flow fields provides a powerful tool to optimize cavitation processing in liquid metals.
  

  Website 
• Tzanakis I, Lebon GSB, Subroto T, Eskin D, Pericleous K, 'Acoustic Cavitation Measurements and Modeling in Liquid Aluminum'
  (2019)
  ISBN: 978-3-030-05864-7
  Abstract

  The quantification of acoustic pressures in liquid metals is of paramount interest for the optimization of ultrasonic melt treatment (UST) of large volumes. Until recently, the measurements of acoustic pressure and cavitation intensity in a melt were cumbersome and unreliable due to the high temperatures and the lack of suitable instruments. These difficulties imposed strict limitations on the experimental and numerical investigation of cavitation and bubble dynamics within liquid metals. In recent years, our group used a unique calibrated high temperature cavitometer to measure cavitation activity and acoustic pressures in liquid aluminum. Phenomena such as acoustic attenuation, shielding, and cavitation intensity have been studied. These measurements were also used to validate a non-linear acoustic numerical model applicable to flow in bubbly liquids subject to acoustic cavitation. Both experimental and numerical characterization of the acoustic and flow fields provides a powerful tool to optimize cavitation processing in liquid metals.
  

  Website 
• Tzanakis I, Lebon GSB, Eskin D, Hyde M, Grant PS, 'Investigation of acoustic streaming and cavitation intensity in water as an analogue for liquid metal'
  (2018) pp.591-596
  ISSN: 0025-6501 ISBN: 9780791861851
  Abstract

  This paper presents an investigation of the evolution of flow structures and cavitation intensity in water as an analogue for a liquid metal under ultrasonic excitation. Results are presented for 20 kHz high-power ultrasound. The input power ranged from 50% (8.5 μm p-p) to 100% (17 μm p-p). To identify the streaming structures and understand the recirculation flows for different vibrational amplitudes of the sonotrode, particle image velocimetry (PIV) measured the velocity field. Simultaneously, a calibrated cavitometer probe measured acoustic intensity in the fluid. The cavitation intensity away from the acoustic source decreased with increasing input acoustic power, but was relatively constant inside the cavitation zone (irrespective of the input power). PIV measurements showed that the direction of the flow pattern was strongly related to the vibrational amplitude of the sonotrode. These results are compared with the predictions of an acoustic cavitation model. The outcome of the present work will help to determine the efficient optimization of ultrasonic processing of liquid metals that is of increasing technological importance.
  

  Website 
• Eskin D, Tzanakis I, Wang F, Lebon G, Pericleous K, Lee P, Connolley T, Mi J, 'Fundamental Studies of Ultrasonic Melt Processing'
  (2017)
  ISBN: 9781908549297
  Abstract

  Ultrasonic (cavitation) melt processing is attracting considerable interest from both academia and industry as a promising technological route to provide clean, environment-friendly, and energy efficient solutions for some of the core issues of the metal casting industry, such as improved melt quality and structure refinement. Upscaling of this technology to industrial level is still hindered by insufficient understanding of the underlying mechanisms. In the last 5 years, we undertook an extensive programme of fundamental research into the mechanisms of cavitation melt processing using state-of-the-art techniques and facilities, including calibrated high-temperature cavitometry, high-speed filming, particle-image velocimetry, synchrotron X-rays imaging, and advanced electron microscopy. This paper gives an overview of recent results on measuring the acoustic pressure and melt flows in the treated melt, real-time observations of cavitation in liquid aluminium alloys, and in-situ and ex-situ studies of the nucleation and fragmentation of intermetallics. 
  

• Wang F, Tzanakis I , Eskin D, Mi J, Connolley T, 'In Situ Observation of Fragmentationof Primary Crystals by Ultrasonic Cavitationin Water'
  (2017)
  ISBN: 978-3-319-51541-0
  Abstract

  Ultrasonic melt processing is a promising technique for microstructural refinement in castings. Several mechanisms have been proposed for the observed effects, including cavitation-induced nucleation, activation of substrates and fragmentation. Until now, however, real-time experimental observations which could clarify any of the above mechanisms are very limited. For the first time we directly observed the fragmentation of primary crystals formed in alloys by ultrasonic cavitation. The primary crystals were extracted from real Al alloys and subjected to ultrasonic processing in water with in situ high-speed filming. The recordings of fragmentation of the primary crystals allowed us to observe the different mechanisms of fragmentation, depending on the mechanical properties and morphology of the primary crystals. The collapse of cavitation bubbles in water is less violent than that in liquid aluminum due to the lower cavitation threshold, viscosity and surface tension. Therefore the fragmentation mechanisms for the primary crystals observed in water should also be present for the same primary crystals in the more violent cavitation situation in liquid aluminum.
  

  Website 
• Wang F, Eskin D, Mi J, Connolley T, Tzanakis I, 'Refinement of Primary Al 3 Ti Intermetallic Particles in an Al-0.4 wt% Ti alloy by Ultrasonic Melt Processing'
  (2017)
  ISBN: 9781908549297
  Abstract

  The refining effect of ultrasonic melt processing on primary Al3Ti particles was investigated by applying ultrasound to an Al0.4 wt% Ti alloy over two temperature ranges: 810 to 770 oC (above liquidus) and 730 to 690 oC (below liquidus). In both cases, significant refinement of primary Al3Ti particles was observed. Based on the examination of the quenched samples taken before and after ultrasonication, it was proposed that cavitation-enhanced nucleation on indigenous particles plays the dominant role when the ultrasonication is applied above the liquidus, while cavitation-induced fragmentation prevails when the ultrasonication is carried out below the liquidus. Using scanning electron microscopy, energy dispersive X-ray spectroscopy, and transmission electron microscopy, the nucleating sites for the primary Al3Ti were identified as α-Al2O3
  particles. The refinement observed after ultrasonication above the liquidus is hence attributed to cavitation-enhanced nucleation on α-Al2O3 particles. In supplementary room temperature experiments, the cavitation-induced fragmentation of primary Al3Ti intermetallics was studied in-situ by high-speed imaging of the deeply-etched alloy samples immersed in distilled water. The dynamic interaction between the cavitation bubbles and primary Al3Ti particles was imaged in real time. A fragmentation process caused by oscillating bubble continuously interacting with a primary Al3Ti particle was clearly observed. 
  

• Wang F, Tzanakis I , Eskin D, Mi J, Connolley T, 'In Situ Observation of Fragmentationof Primary Crystals by Ultrasonic Cavitationin Water'
  (2017) pp.213-219
  ISBN: 978-3-319-51541-0
  Website 
• Tzanakis I, Hodnett M, Eskin D, Lebon B, 'Advanced calibration of a high-temperature cavitometer'
  (2014)
  
• Tzanakis I, Hadfield M, Khan ZA, Garland NP, 'An experimental study of refrigerant lubricated sliding contact between the main components of a scroll expander system'
  (2010)
  

Other publications

• Lebon GB, Pericleous K, Tzanakis I, Eskin D, 'UltraMELT: Numerical Modelling of the Ultrasonic Treatment of Liquid Aluminium'
  (2015) Abstract Ultrasonic cavitation treatment is a sustainable, economical, and pollution-free alternative for liquid metal processing. Beneficial effects of the treatment include degassing of dissolved gases by cavitation bubbles, enhancing nucleation, and refining the grain structure of the solidified sample. However, the transfer of this promising technology to the industry has been hindered by
  difficulties in treating large volumes of liquid metal, as is required by processes such as continuous casting. Fundamental research is required to answer this seemingly simple question: how long does it take to treat a certain volume of liquid with an ultrasonic source for minimum energy input, cost, and complexity? Answering this question will pave the way to widespread industrial use of ultrasonic melt processing with the benefit of improving the properties of lightweight structural alloys, simultaneously reducing the need for degassing with polluting (Cl, F) or expensive (Ar) gases or grain refinement additives (Ti, B). This research aims to answer the challenge of efficiently treating large liquid volumes by developing a comprehensive numerical model that couples the various multiscale and multi-physics phenomena involved in the treatment.
   
  Website 
• Tzanakis I, 'Sustainable design and durability of domestic micro combined heat and power scroll expander systems'
  (2010)