"Experiment of Marangoni convection in a liquid bridge on the International Space Station and related computational studies"|
Prof. Hiroshi KAWAMURA
Suwa University of Science, President
Tokyo University of Science, Professor Emeritus
A series of Marangoni convection experiment in a liquid bridge started on ISS (International Space Station) in August 2008 as the first science experiment in KIBO, the Japanese experiment module on ISS. This series of experiment named MEIS (Marangoni Experiment in Space) was selected in 1993 as a candidate of the first group of the space experiment in KIBO. MEIS aimed to perform a series of experiment on Marangoni convection in a liquid bridge in the microgravity.
It took 15 years up to the start of the space experiment because of the delay in the construction of ISS. These 15 years, however, allowed us better understanding of the phenomena through more detailed terrestrial experiments and computational studies.
In this presentation, an overview of the experiment and its outcomes will be mentioned. Since MEIS was the first sceince experiment in KIBO, its performance itself was a new trial. The experiment had to be performed in close collaboration with operation teams of JAXA as well as NASA. The way of cooperation itself had to became established gradually through mutual discussions and understandings. One of the major difficulties encountered in the performance of the experiment was appearance and growth of gas bubbles in the liquid bridge, which were not experienced in the terrestrial experiments. A way of their elimination was explored through rather long and patient trials.
The critical conditions for the transition from steady to oscillatory flow were obtained for various conditioned and their theoretical and computational reproductions were also performed. Flow velocity was measured through the three-dimensional particle tracking velocimetry and compared with numerical calculations.
One of the special features found in the terrestrial experiment was PAS, Particle Accumulation Structure, in which dispersed tracer particles tended to accumulate along a three-dimensional closed loop azimuthally moving in the liquid bridge. This peculiar phenomenon was successfully represented through numerical simulation. In the microgravity, too, the PAS was found to form with a much larger particle size than in the terrestrial experiment, indicating that the diameter ratio of the particle to the liquid bride is essential in its appearance. Reason of the formation of the PAS and its possible application will be discussed in the presentation.
"Numerical modelling of transport phenomena in SOFC systems"|
Prof. Janusz Szmyd
Eurotherm Committee, President
Head of Dept. of Fundamental Research in Energy Engineering,
AGH University of Science and Technology
At the current level of energy consumption, the industrial-scale reserves of fossil fuels around the globe are sufficient for around 100 years more. The international projects involving fusion (ITER) or the exploitation of hydrates may yield answers allowing for practical, applicable solutions within 50 years, with actual implementation then following by the end of the 21st century. In the meantime, if global energy security is to be assured and levels of pollution reduced, it will be advisable to bring “third-generation” power-supply technologies into full(er) use in the upcoming years and Solid Oxide Fuel Cell (SOFC) has the potential to become one of the most important types of an energy conversion device.
However, high-temperature operation, thermal management of the SOFC system becomes an important issue. The temperature distribution is a critical factor in terms of cell lifespan, including degradation of electrodes microstructure. Proper thermal management requires detailed modelling, including numerical analysis of the transport phenomena within an SOFC system. Various mathematical models have been developed to solve transport equations coupled with electrochemical processes to describe the reaction kinetics accounting for internal reforming chemistry in SOFCs. In recent years, the SOFC technology has made significant progress – notably, in considering electrodes’ microstructure morphology, as well as in providing miniaturized and efficient methane steam reforming. A number of contributions on these critical topics will be discussed. The objective of this talk is to summarize the present status of the SOFC modelling efforts and their impact on understanding and optimization of SOFC systems.
Prof. Mamoru Tanahashi
Tokyo Institute of Technology, Japan
"Designing thermal-functional nanostructures by materials informatics"|
Prof. Junichiro Shiomi
The University of Tokyo, Japan
To achieve the development goals related to energy, whether for sustainability or smart society, further hardware innovation is indispensable as well as software. Here, the innovation of hardware requires creation of new functional materials. Specific requirements for materials may change depending on future technological trends, but it is certain that the demands will become diverse and complex, therefore, new foundation for material development that can respond to it is urgently needed. Thermal functional materials are not an exception, where materials with larger controllability of heat transfer are strongly needed for better thermal management and harvesting. For this, advancement in phonon engineering, which is to control thermal properties by understanding and engineering the state and transport of phonons (or their interaction or correlation with other quasi-particles) is indispensable. While there has been a great progress in the last decades particularly in terms of nanostructures, the above demands require further controllability and disignability. Over the last years, we in Thermal Energy Engineering Lab at the University of Tokyo together with the collaborators have grained the controllability by pushing the limit of phonon scattering, strain engineering, and phonon coherence. In addition, to realize better designability, we have developed frameworks to pursue materials informatics for heat transfer by combining thermal transport calculations and machine learning. In this talk, I will discuss some successful cases to design nanostructures to enhance/impede thermal transport, and to optimize the trade-off among multiple competing properties i.e. thermal conductance, electrical conductance, and Seebeck coefficient. Finally, I will introduce some cases where optimally-designed structures are experimentally realized.
"Transition to turbulence – a new perspective on transient turbulent flow"|
Prof. Shuisheng He
The University of Sheffield, U.K.
Ever since the publication of the pioneering work by Osborne Reynolds in 1883 defining the concept of laminar and turbulent flows, the subject has remained a central theme of research in fluid mechanics due to its fundamental importance to the subject and its relevance to engineering applications and the natural world. It is probably one of the most fundamental concepts that the flow in a pipe is laminar when the Reynolds number is below around 2300 and turbulent when the Re is higher. In this talk, we will present a radically new perspective of the laminar/turbulent flow and will show that many turbulent-to-turbulent transient processes are in fact laminar-turbulent transition. We will present experimental evidence as well as DNS simulations to demonstrate that, following an increase of flow rate from an initially turbulent flow, the new flow will re-establish itself as a result of laminar-turbulent transition due to instability, rather than progressively evolving from the initial turbulent flow to a new one. The transient process is characterised by the appearance of elongated streamwise streaks followed by generation of turbulent spots which are typical of bypass transition processes. More recently, it has been shown that three-dimensional transient flows are also characterised by transition. This new understanding leads to new interpretation of the statistics of transient flows and could have profound implications in the understanding and prediction of unsteady flows encountered in practice, and lead to the development of new strategies for turbulence control, all of which are yet to be explored.
"Scale-resolving simulation methods for industrial flows"|
Prof. Branislav Basara
AVL List GmbH, Graz, Austria & Chalmers University of Technology, Sweden
The Reynolds-Averaged Navier-Stokes (RANS) turbulence models are still the most widely used for simulating complex industrial flows. However, in many modern applications with targeted error re-quirements, irrespective of the type of RANS turbulence model used, the turbulence closure model is the largest source of error. The Large Eddy Simulation (LES) approach has been increasingly used in recent years but its use is mostly restricted to research studies due to high computational costs. Nevertheless, there are many research groups in the industry that are using LES to simulate more accurately the complex physics of industrial flows. However, by assessing the present LES performance, it is obvious that LES cannot be a tool for everyday use soon. This leaves the space to development of scale-resolving simulation (SRS) methods such as hybrid RANS/LES (HRL), which in principle, provide a more af-fordable solution than LES while being more accurate than RANS. The scale-resolving simulations can be broadly classified as zonal and bridging (also called non-zonal) approaches. Bridging models employ the same closure model form in the entire domain without requirements for the interface between dif-ferent modelling zones. The work presented here will show applicability of the bridging methods for very complex CFD application, e.g. external car aerodynamics, intake ports, engines etc. Beside the present status, the possibility on the further development of models dedicated for example, to heat transfer, spray and combustion modules in the framework of the hybrid RANS/LES, will be addressed and some development hints will be given.
"Numerical analysis of multiphase flows and its applications to high speed water entry problems"|
Prof. Warn-Gyu Park
Pusan National University, Korea
The numerical modeling of multiphase flows, especially free surface flow for water entry and water impact problems, is a vast topic, which is still receiving increasing attention, particularly for applications in many hydraulic and hydrodynamic problems. The important examples for hydraulic problems in civil engineering are the potential risks of failures of levees, dams, reservoirs, and ﬂood management. Free surface hydrodynamics are also very important in environmental, naval, and ocean engineering, and consist of a series of problems, such as water entries of objects, sloshing of liquids in tanks, in the design of ships, wave breaking in ships, ship maneuvering against waves, and green water on decks, offshore platforms, harbors, and coastal areas. In the present talk, the numerical analysis for multiphase flows for simulating 3D complex free surface and water entry problems will be introduced. The present in-house code adopted dual-time, pseudo-compressibility method in order to solve the RANS equations in a vector form. By implementing the combination of the equations and special treatments for the interfaces between phases, e.g., the VOF interface tracking method and interface sharpening method in a generalized curvilinear coordinate system, the proposed model is capable of predicting complex, arbitrary, free surface, and water impact flows in hydraulic and hydrodynamic fields that occur in geometrically complex domains. Several sets of example computations concerning important physical characteristics of complex free surface and water impact flows of structure and the use of Chimera grids and 6DOF motions model confirm that the numerical methods are promising for extensive applications to accurate and efﬁcient simulations of both simple and full 3D free surface, water impact ﬂows in practical problems.
"High-fidelity simulation of conjugate heat transfer in aircraft in-flight icing"|
Prof. Rho Shin Myong
Gyeongsang National University, Korea
In Earth’s atmosphere, supercooled water droplets can be observed in the airflow around aircraft (or wind turbine) moving inside clouds. Exposure to these water droplets may cause substantial ice accretion on the surfaces of wings, engine inlet, rotor and wind turbine blades, and air data systems, resulting in significant reduction of aerodynamic and propulsive performance. For example, ice accretion on the surface of an engine air intake can deteriorate the safety of aircraft due to the engine performance degradation. Thus, careful attention must be paid to how to protect the aircraft and rotorcraft from ice accumulation.
In order to tackle this problem, much effort has been put into the study of in-flight icing physics and its computational models. This talk presents recent advances in the high-fidelity computational simulation of aircraft in-flight icing—in particular, conjugate heat transfer associated with ice protection systems and effect of supercooled large droplet (SLD).
As the first example, ice accretion on the surface of an electro-thermal anti-icing system around a rotorcraft engine air intake was investigated on the basis of computational and experimental methods. Then application of the electro-thermal anti-icing system to Korean Utility Helicopter and lessons learned from the recent icing certification campaign (through Korea Aerospace Industries Ltd.) are briefly described.
Next, the unified computational solvers for clean air, large droplet impingement, ice accretion, conjugate heat transfer, and the aerodynamic analysis of ice effects were developed within a single unstructured upwind finite volume framework. The solvers were then applied to investigate ice accretion and the resulting aerodynamic effects on multi-element airfoils for near-freezing SLD icing conditions. Some non-intuitive results were found when compared with non-SLD case.
"Direct numerical simulations of incompressible multiphase magnetohydrodynamics with liquid-vapor phase change"|
Prof. Mingjiu Ni
University of Chinese Academy of Sciences, China
A new phase change model has been developed for the simulation of incompressible multi-phase magnetohydrodynamics(MHD), in which the interface is captured by the VOF(Volume-of-Fluid) method and the Lorentz force is calculated by the consistent and conservative method. Based on the reconstructed geometrical interface, one-field formulation is used to solve the flow, energy and even the species equations if phase change is induced due to the temperature or species concentration gradient at the liquid-vapor interface, and also note that when evaporation is considered, the vapor mass is only transported outside the liquid phase. The numerical approach is approved to be energy and mass conservative across the interface. After that, the method has been implemented in an incompressible multiphase MHD solver developed in our previous work (Zhang and Ni, J. Comp. Phys. 270 (2014) 345-365). Moreover, when computing the electromagnetic fields, a cut-cell approach is implemented to keep the sharpness of the interface, which is treated as an electrically insulating boundary as it translates and deforms with the fluid. The phase change model has been validated by simulating a series of one-dimensional, two-dimensional and three-dimensional benchmarks problems, while the numerical results agree well with either the theoretical solutions or the experimental data. After that, several numerical cases are simulated to investigate the MHD effect on the liquid-vapor phase change problems, which are always encountered in the metallurgy industry and fusion engineering. For instance, by simulating the three-dimensional film boiling simulations under the influence of magnetic field, the vapor bubble is observed to elongate along the field direction during its growth. In addition, the evaporation phenomena driven by either temperature or species gradient is also found to be suppressed when external magnetic fields are imposed.
"Pore-scale study of multiphase flow and reactive transport in electrodes of proton exchange membrane fuel cells"|
Prof. Li Chen
Xi’an Jiaotong University, China
Proton exchange membrane fuel cell (PEMFC) is a promising and attractive candidate for a wide variety of power applications such as fuel cell vehicles, due to its advantages including high energy density, high efficiency, low operating temperature, and quick start-up. Currently, there are several challenges remaining for commercialization of PEMFC including performance, durability and cost. Porous electrodes (gas diffusion layer, microscopic porous layer and catalyst layer) are key components in PEMFCs. A deep understanding of the coupled transport processes in electrodes is of great importance for improving cell performance and reducing system cost.
During the past 10 years in our group, advanced pore-scale numerical methods based on the lattice Boltzmann method have been developed to study the pore-scale gas-liquid two-phase flow, heat and mass transfer, proton and electron conduction, and electrochemical reactions in porous electrodes of PEMFCs. For the gas diffusion layer (GDL), carbon-fiber based porous structures of GDL were reconstructed. Permeability, effective diffusivity and thermal conductivity of the GDL were predicted. Effects of pore structures and surface wettability on liquid water and distributions in GDL were studied. Subsequently optimized GDL structures with perforated hydrophilic pores were proposed to enhance water management and mass transport. For the catalyst layer (CL), nanostructures of CL including carbon, Platinum, electrolyte and pores were reconstructed. Effects of porosity and ionomer/carbon ratio on effective diffusivity and conductivity were explored. Local transport resistance across the pore-ionomer interface was investigated. Effects of agglomerate degradation of Pt particles on CL performance were also studied. Finally, structures of CL were optimized to enhance mass transport and reduce cell cost.
"Effect of various factors on heat transfer and combustion process in micro combustors"|
Prof. Wenming Yang
National University of Singapore, Singapore
The past decade has witnessed a rapidly growing trend in the miniaturization of mechanical and electro-mechanical engineering devices. However, the miniaturization of these devices is limited by the weight of the available power systems (batteries) which occupy significant fractions of both mass and volume of the entire devices. Typical portable mechanical devices also suffer from short operation cycles between charges or replacement. To meet the growing demand for power sources that are compact, lightweight and powerful, various combustion-based micro power generators are being developed. Of which, micro-thermophotovoltaic (TPV) power generator is one of the most promising candidates which uses PV cells to convert radiation energy from combustion of fossil fuel into electricity. To maximize the output power of the system, a high and uniform temperature distribution along the external surface of the micro combustor is demanding, which, on the other hand, will affect the stability of flame in the micro combustor due to significant increased heat loss as a result of the high surface-to-volume ratio. So it is very important for us to find an optimal balance between maximizing heat radiation and maintaining a stable flame. In this work, an extensive study has been conducted to investigate the impact of various factors on the combustion process in micro combustors and the performance of the micro-TPV system. The results indicate that the performance of the micro combustor could be improved by optimizing the shape of micro combustor, design of the inlet and fuel, as well as employing heat recuperation, block insert and porous media. Very interestingly, we found that not only the porosity affects the flame stability and radiation performance, even at the same porosity but different pore size, the performance of the micro combustor could also be different.
"Thermal management of electronics using solid-liquid phase change material based heat sinks"|
Prof. Chakravarthy Balaji
Indian Institute of Technology Madras, India
Miniaturization of electronic components continues to pose challenges in thermal management. This has led to increased research focus and advancements in the field of PCM based heat sinks for use in high-heat-flux environments. These heat sinks are often not operated continuously over long periods and are invariably used in intermittent cycles. Under these conditions, the time taken for the PCM to solidify back plays a key role in the thermal performance. The heat transfer community has been trying hard to mimic real time scenarios of electronic cooling in their laboratories and is currently working on various techniques to maintain the device with safe operational temperature limits. Most of the PCMs usually have a very low thermal conductivity, as a consequence of which when heat is dissipated from the electronic equipment into the PCM, even before a significant quantity of the PCM melts, the components may reach unsafe temperatures. By using a high thermal conductivity base material, known as a thermal conductivity enhancer (TCE) in conjunction with a PCM, this challenge can be addressed. Based on our own experimental and numerical investigations in the last decade, geometric parameters of a heat sink and also the hotspots imposed by a spatially non-uniform heat flux have been observed to have a considerable effect on the melting and solidification cycle and hence on the thermal performance of the heat sink., in typical geometries. The diversity in the performance and conflicting nature of both the objectives of heat sink have motivated us to perform a multi-objective optimization using candidate multi objective algorithms to determine the optimum combination of the variables considered that stretches the charging period and minimizes the discharging period of the heat sink simultaneously. The solutions thus obtained are finally validated by conducting in house experiments for the optimized configurations. These studies have established that geometric parameters and discrete heating situations have to be necessarily considered in future optimization of heat sinks. Additionally, it was found that, that the thermal performance of the heat sink is a strong function of orientation and fill ratios. The thermal performance was found to monotonically increase with the fill ratio for a finned heat sink and the unfinned heat sink performed well at lower fill ratios. The multi objective optimization problem needs to be solved without simplifying assumptions in order to lead us to the “best” design that satisfies thermal management objectives. For problems of this class, non-dominated sorting genetic algorithm (NSGA-II) was seen to show superior performance.