Keynote Speakers

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Professor Yasuyuki Takata

A Challenge of Lowering Wall Superheating at Onset of Nucleate Boiling

  • International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Japan
  • School of Engineering, Institute for Multiscale Thermofluids, University of Edinburgh, UK
  • Professor Yasuyuki Takata Keynote (PDF)
 

Abstract

Onset of nucleate boiling (ONB) always needs wall superheating, dTsat, to activate bubble nucleation sites. In general, dTsat, ONB ranges from a few degrees to several tens of degrees depending on the type of fluids, wettability, surface structures and concentration of non-condensable gasses. From a viewpoint of electronic cooling, low dTsat, ONB is desirable to avoid thermal damages to electronic chips and to ensure stable boiling heat transfer. We have been studying the effects of wettability and solubility of air on the ONB and obtained some important findings for water. Regarding the wettability effect, hydrophobic area of the surface attracts dissolved air and works as an excellent nucleation site. Therefore, by making use of biphilic surfaces nucleate boiling is significantly enhanced. The boiling performance of biphilic surfaces is of about 7 times larger than that of mirror copper surface. This enhancement technique is very effective in subatmospheric conditions. Presence of both hydrophobicity and dissolved air drastically reduces the dTsat, ONB. In the case of subcooled boiling, the dTsat, ONB sometimes can become negative. We have unveiled the mechanism of early onset of nucleation by making use of a special experimental apparatus so to remove any initial dissolved air from the boiling water.

Challenges in lowering dTsat, ONB also concern other fluids. We succeeded to lower the dTsat, ONB for ethanol by halloysite nanotube (HNT) coating where ethanol displays contact angles higher than 90°. Nucleate boiling for ethanol is also enhanced on the biphilic surface by about 3 to 4 times when compared with bare copper surface. Our recent challenge is to lower the dTsat, ONB and enhance nucleate boiling for HFE7100 by making use of anodized aluminum surface, which has nanopores of 20-200 nm in size. The present talk will also report on some other experimental findings related to this recent study.

Biography

Professor Yasuyuki TAKATA is a Research Professor at International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University. He is also Professor Emeritus at Kyushu University and Honorary Professor at the University of Edinburgh. He was a Professor in the Department of Mechanical Engineering, Kyushu University until March 2022. His research interests include two-phase flow and heat transfer, thermophysical properties of hydrogen at ultra-high pressure, micro refrigerator and micro heat transfer device and numerical simulation of thermal and fluid flow. He was the Presidents of Heat Transfer Society of Japan (HTSJ) from 2019 to 2020 and Japan Society of Thermophysical Properties in 2016. He served as the President of the Asian Union of Thermal Science and Engineering (AUTSE) from October 2020 to September 2022. He received numerous awards including the JSME Thermal Engineering Achievement Award in 2010, and ASME ICNMM2018 Outstanding Leadership Award in 2018 and Heat Transfer Society Award for Scientific Contribution in 2022. He is a Council Member of Science Council of Japan since October 2020.

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Professor Tassos G. Karayiannis

Aspects of Flow Boiling in Small to Micro Scale Heat Exchangers

  • Department of Mechanical and Aerospace Engineering, College of Engineering, Design and Physical Sciences, Brunel University London, UK
  • Centre for Energy Efficient and Sustainable Technologies, Brunel University London, UK
  • Professor Tassos Karayiannis (PDF)
 

Abstract

The high heat fluxes generated by modern electronic equipment necessitate a new approach to cooling these devices, as the dissipation of such high thermal loads using single-phase air or liquid heat sinks is no longer possible.  The use of flow boiling in small to micro scale heat exchangers is consider as one of the most viable methods to help alleviate this thermal bottle neck – a few megawatts per meter square on average and reaching significantly higher values at the hot spots – allowing proper operation of these devices and new developments in the area. Other applications include possible use in small scale refrigeration systems, cooling of fuel cells, batteries and vehicle power electronics, solar photovoltaic panels and radar systems.  The advantage of flow boiling in such systems is due to the possible small temperature difference of the substrate to be cooled reducing thermo-mechanical stresses and early failure plus small flow rates due to the high heat transfer coefficients resulting in smaller pumps and power consumption by the thermal management system.  Fundamental issues that are currently being investigated in order to facilitate adoption of these small to micro scale evaporators include the definition of the macro to micro scale dimensions, the prevailing flow regimes and the effect of mass flux, heat flux, channel aspect ratio and length plus material and surface characteristics. These heat exchangers form part of a thermal system and the return temperature from the condenser and hence the possible degree of subcooling at the inlet of the evaporator is also a critical factor, bearing in mind the short lengths of the heat exchangers and the desire to achieve uniform substrate temperatures along the flow direction. The presentation will cover research in flow boiling in single tubes and channels and in multichannel heat exchangers with rectangular passages. Results for a microgap heat exchanger (single wide channel) of the same height and base area as the multi-channel heat exchangers will form benchmark comparative data. The effect of the parameters mentioned above plus the effect of coatings on the flow patterns, pressure drop and heat transfer rates will be presented. Flow instabilities will be discussed, along with ways to reduce their impact on the thermo-fluid characteristics of the evaporator. The development of correlations predicting the flow pattern boundaries, heat transfer rates and pressure drop will then be presented based on an analytical/statistical approach plus machine learning techniques. Finally, the integration of the micro evaporators in thermal management systems, which requires also the design of small-scale condensers will be discussed. 

Biography

Professor Tassos Karayiannis studied at the City University London and the University of Western Ontario. He started his career as a researcher at Southampton University and later as a British Technology Group Researcher at City University. Subsequently he worked at London South Bank University and joined Brunel University London in 2005, where he is now Professor of Thermal Engineering Leader of the Two-Phase Flow and Heat Transfer Group and Director of the Energy Efficient and Sustainable Technologies Research Centre. Professor Karayiannis has carried out fundamental and applied research in a number of single-and two-phase heat transfer areas. Initially he worked on convective heat transfer and subsequently on the enhancement of pool boiling and condensation processes using high intensity electric fields. In parallel, he carried out extensive experimental work in pool boiling heat transfer with plane and enhanced surfaces. Professor Karayiannis has also been very actively involved with research in flow boiling in small to micro tubes and micro-multi-channels. This work involves fundamental studies as well as research leading to the design of high heat flux integrated thermal management systems. He has published more than 260 chapters in books, papers and industrial reports. He chairs the Committee of the International Conference Series on Micro and Nanoscale Flows now in its 8th edition. He is a Fellow of the EI and the IMechE, Member of the Assembly for International Heat Transfer Conferences and the Chairman of the UK National Heat Transfer Committee.

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Professor Ruina Xu

Simultaneous 2c-PLIF and μPIV measurements of droplets impingement on heated substrates and heat transfer mechanisms

  • Institute of Engineering Thermophysics, Department of Energy and Power Engineering, Tsinghua University, Beijing, China
  • Key Laboratory for CO2 Utilization and Reduction Technology of Beijing, China
  • Professor Ruina Xu Keynote (PDF)
 

Abstract

Droplet impact on solid is a critical process in many technologies or industries including spray cooling, spray coating, inkjet printing, etc. Meanwhile, it inherently demonstrates the beauty and complexity of nature by including the interplay among liquid-solid interaction, interfacial phenomenon and heat transfer in a microscale transient process. For decades, researchers have been investigating phenomena, mechanisms and applications of fluid dynamics, heat transfer and phase change of droplet impingement in various droplet, substrate and environment conditions. Most of them utilized high-speed photography or interferometry to study droplets’ morphological outcomes. In recent years, experimental methods i.e. such as IR imaging, LIF thermometry, and PIV are introduced to study the temperature and velocity distribution inside a droplet, providing us with new insights into microscale droplet heat transfer and fluid flow mechanisms.

However, simultaneous acquiring of internal velocity and temperature fields during droplet impingement, which can help us understand the interactions among droplet morphology, microscale heat transfer and microscale fluid dynamics, is rarely investigated. Two main challenges are simultaneous no-contact measurement of two fields in small time and length scales as well as reducing the influence of unneglectable reflection and refraction at varying droplet-gas interface.

In present research, we simultaneously applied μPIV and 2c-PLIF methods to study single and successive droplet impingement on heated smooth and engineered substrates. Our investigations revealed detailed droplet-wall heat transfer and fluid flow phenomena and microscale origins of different droplet impact outcomes at different substrate conditions. We found that droplet temperature distribution was strongly substrate-related: droplet was more likely to be heated from several near-wall points on hydrophilic substrates while more likely to be heated uniformly on hydrophobic substrates. The difference of droplet temperature caused differences in surface energy and surface wave motions which affected droplet morphologies. Vapor bubbles beneath the droplet affected the internal flow vorticity and droplet heating by bringing extra momentum and heat transfer resistance, resulting in a non- monotonic droplet heating trend with increasing wall temperature. The simultaneous droplet internal velocity and temperature field measurement method proposed in present study can help us understand the classic droplet impact process in a more detailed, real physical perspective in further investigation.

Biography

Ruina Xu is a professor (tenured) of the Department of Energy and Power Engineering at Tsinghua University and Deputy Director of the Key Laboratory for CO2 Utilization and Reduction Technology of Beijing. She obtained a B.S. in 2002 and PhD in 2007 from Tsinghua University. She has been researching convection heat/mass transfer, multiphase flow physics, prediction, control, and modeling to develop low carbon and carbon neutral solutions and spacecraft thermal protection applications, such CCUS, CO2 enhanced unconventional gas/oil exploitation, and next generation of solar-thermal and geothermal systems. Her research aims to answer fundamental questions about the dynamics of multiphase flow, heat and mass transfer in micro-/nano-scale and complex porous networks. In her lab, her team develop in-situ high pressure visualization experiments and numerical models from atom-, molecular-, pore-, core-scale to field scale, such as: in-situ high-pressure, high-temperature core flooding system using Magnetic Resonance Image (MRI), micro-model experiments using microscope and designing 1D nano-porous structure, Computational Fluid Dynamics (CFD), Lattice Boltzmann Method (LBM), Molecular Dynamics (MD), etc. She has been in charge of several grants from NSFC, MOST, and international cooperation as the Principal Investigator. She has over 80 journal articles, as well as 27 authorized international and national invention patents.

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Dr. Matteo Bucci

The Percolation Law of the Boiling Crisis

  • Nuclear Science and Engineering, Massachusetts Institute of Technology MIT, Cambridge, MA, USA
  • Dr Matteo Bucci (PDF)
 

Abstract

Nucleate boiling is an exceptionally effective heat transfer process. However, a boiling crisis suddenly occurs when the heat flux to remove from a heated surface is too high. The maximum heat flux that can be sustained by nucleate boiling depends on surface properties and operating conditions, and it is an important operational limit in many scientific and industrial applications. Many scientists and engineers have attempted to describe this phenomenon and predict this limit mechanistically. However, a universal theory has eluded the thermal science community for almost a century.

Here, we reveal theoretically and experimentally the presence of a unifying law of the boiling crisis. This law emerges from an instability in the near-wall bubble interaction phenomenon, described as a percolation process driven by three fundamental boiling parameters: nucleation site density, average bubble radius and product of bubble growth time and detachment frequency. Our analysis demonstrates that the boiling crisis occurs on a well-defined critical boundary in the multidimensional space of these parameters for a wide variety of boiling surfaces and operating conditions. We anticipate that this fundamental property of the boiling process can inspire the design of engineered surfaces that enhance the nucleate boiling limit, as well as mechanistic modelling criteria for the design of advanced two-phase heat transfer system. 

Biography

Dr. Matteo Bucci is Associate Professor of Nuclear Science and Engineering at MIT. He has joined the MIT faculty in 2016, where he teaches undergraduate and graduate courses in nuclear reactor engineering and design, and two-phase heat transfer.  His thermal-hydraulics group at MIT focuses on two major research axes related to nuclear reactor safety and design: (1) New understanding of heat transfer mechanisms in nuclear reactors, (2) Engineered surfaces and coatings to enhance two-phase heat transfer. His group also develops and uses advanced diagnostics, such as high-speed infrared thermometry and phase-detection, and post-processing algorithms to perform unique heat transfer experiments. Matteo has published over 40 articles in the areas of two-phase flow and heat transfer, and surface engineering technology. For his research work and his teaching, he won several awards, among which the MIT Ruth and Joel Spira Award for Excellence in Teaching (2020), ANS/PAI Outstanding Faculty Award (2018), the UIT-Fluent Award (2006), the European Nuclear Education Network Award (2010), and the 2012 ANS Thermal-Hydraulics Division Best Paper Award (2012). In 2022, Matteo received the inaugural DOE Early Career Award for Nuclear Energy. Matteo is Editor of Applied Thermal Engineering and a consultant for the nuclear industry.

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Professor Nenad Miljkovic

Tailoring Surface Chemistry and Surface Roughness to enable the Long-Term Stable Dropwise Condensation of Steam and Refrigerant Working Fluids

  • Department of Mechanical Science and Engineering, University of Illinois at Urbana – Champaign, USA
  • Department of Electrical and Computer Engineering, University of Illinois at Urbana – Champaign, USA
  • Material Research Laboratory, University of Illinois at Urbana – Champaign, USA
  • International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Japan
  • Professor Nenad Miljkovic Keynote (PDF)
 

Abstract

Almost a century ago, dropwise condensation of steam on a hydrophobic surface was shown to have a 10X higher condensation heat transfer coefficient when compared to filmwise condensation on hydrophilic surface. The resulting overall heat transfer enhancement has the potential to result in a 2% overall energy efficiency increase for steam-based power plants, which are responsible for the overwhelming majority of global electricity production. The potential of dropwise condensation has driven researchers to design thin (≈100 nm-thick) hydrophobic coating materials. However, the lack of long-term (> 3 year) durability has been the main hindrance to coating utilization over the past century. In this talk, I will present our recent progress in designing thin and durable hydrophobic coating materials that enable stable dropwise condensation. First, I will discuss our fundamental studies probing the origin of hydrophobic coating degradation. We show that nanoscale pinhole defects in the coating are the source of steam penetration during condensation, where the condensate forms water blisters that pressurize and delaminate the coating. The understanding of the mechanics of water blister formation and growth enables us to develop quantitative guidelines for rational coating design and selection. Next, I will present the design of self-healing vitrimer thin film (dyn-PDMS) that actively eliminate coating defects to prevent the initiation of blisters. The dyn-PDMS thin film maintains excellent hydrophobicity after scratching, cutting, and indenting due to the dynamic exchange of its network strands. In addition to dyn-PDMS, I will show how alternate coating solutions such as fluorinated-diamond like carbon (F-DLC) with polymer-like low surface energy and metal-like exceptional mechanical properties can enhance dropwise condensation durability. We show experimentally that the high bending stiffness and coating adhesion makes F-DLC durable to 5,000 cycles of mechanical abrasion and enables more than 3 years of continual stable dropwise condensation. I end my talk by discussing our recent exciting demonstration of the stable dropwise condensation of two commercial low surface tension refrigerants (HFO-1336mzz(E) and HFO-1233zd(E)), which are novel, non-flammable, low global warming potential (GWP) Hydrofluoroolefins (HFO).

Biography

Professor Nenad Miljkovic is a Professor of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign (UIUC). He has courtesy appointments in Electrical and Computer Engineering, and the Materials Research Laboratory. He is the Co-Director of the Air Conditioning and Refrigeration Center (ACRC), which is supported by 25 industrial partners. His group’s research intersects the multidisciplinary fields of thermo-fluid science, interfacial phenomena, scalable nanomanufacturing, and renewable energy. He is a recipient of the NSF CAREER Award, the ACS PRF DNI Award, the ONR YIP Award, the ASME ICNMM Young Faculty Award, the ASME Pi Tau Sigma Gold Medal, the CERL R&D Technical Achievement Award, the US Army Corps of Engineers ERDC R&D Achievement Award, the SME Young Faculty Award, the Bergles-Rohsenow Young Investigator Award in Heat Transfer, the ASME EPPD Early Career Award, and is an ASME Fellow.

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Professor David Quéré

The quest for dew-repellent materials

 

Abstract

Dew forms when a cold material is placed in a humid atmosphere. We discuss how a few textured, hydrophobic materials can self-evacuate the dew forming at their surface. We focus in particular on the case of materials decorated with conical nanostructures, for which it is found that water droplets can remain quasi-spherical down to micrometric diameters – leading to remarkable anti-dew properties.

Biography

David Quéré is a Senior Researcher at CNRS and ESPCI-Paris and a Professor at École Polytechnique. He is engaged in experimental research in Soft Matter Physics and Fluid Mechanics, with a strong interest in interfacial hydrodynamics (drops, films, bubbles, coating, wicking) as well as in aerodynamics, morphogenesis and biomimetics. He is on the Editorial Board of Physical Review Fluids, Soft Matter, Advances in Colloid & Interface Science and Droplet. He received the 2014 Silver Medal of CNRS and the 2021 Fluid Dynamics Prize of APS, and he became a Distinguished Professor at ESPCI in 2016.

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