The 4th International Conference on Heat Transfer and Fluid Flow in Microscale
September 4-9, 2011  Fukuoka, Japan
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  Prof. Kenneth S. Breuer, Brown University, USA
   Contact line dynamics at the nanoscale with and without evaporation

The liquid-solid-vapor contact line has been a topic of intense research for many years and represents a point at wich the continuum description of a fluid breaks down, and need to be modified by molecular description. The physics can be made more difficult when complex interactions, such as surface hydrophobicity and/or volatile liquids are introduced to the problem. In this talk we present some recent research on the microscale behavior of moving contact lines with both evaporating and non-volatile fluids and on both hydrophilic and hydrophobic surfaces.


Prof. Stephane Colin, University of Toulouse, France
  Experimental techniques for the analysis of gaseous microflows
Gas microflows must be accurately controlled for a lot of MEMS applications (micro-heat exchangers, pressure gauges, fluidic micro-actuators for active control of aerodynamic flows, mass flow and temperature micro-sensors, micropumps and microsystems for mixing or separation for local gas analysis, mass spectrometers, vacuum and dosing valves…). Advanced models have recently been developed for the analysis of hydrodynamics and heat transfer of gas microflows. They cover the different regimes of these rarefied flows: slip flow, transition and free molecular regimes. These models, however, are generally based on simplifying assumptions, the validity of which requires an experimental validation. This paper presents the techniques available for the global and local experimental analysis of gas microflows. It is underlined that only few accurate experimental data are available in the literature. The current limits of flowrate, pressure, velocity or temperature measurements are detailed and future developments of experimental techniques are discussed.

Prof. Tomoaki Kunugi, Kyoto University, Japan
This study focuses on the clarification of the bubble behaviors of the subcooled pool boiling via visualization by using a high-speed camera, the discussion on its mechanism, and finally the establishment of a boiling and condensation model for numerical simulation on the subcooled pool boiling phenomena. The boiling and condensation model has been improved by introducing the following models based on the quasi-thermal equilibrium hypothesis; (1) a modified phase-change model based on the enthalpy method for the water-vapor system, (2) a relaxation time derived by considering unsteady heat conduction. Resulting from the numerical simulations on the subcooled pool boiling based on the MARS (Multi-interface Advection and Reconstruction Solver) with improved boiling and condensation model, the numerical results regarding the bubble growth process of the subcooled pool boiling show in good agreement with the experimental observation results and the existing analytical equations. Moreover, In order to further progress the numerical simulations of bubble growth process, the patch-work calculations with changing grid size have been performed.

Dr. Khellil Sefiane, University of Edinburgh, UK
  Fundamental studies of nanofluid solutions two phase heat transfer and phase change
Nanofluid are stable suspensions of nanometre scale particles in a base fluid. Many indicate that adding even a low concentration of nanoparticles can significantly enhance the thermal conductivity, so nanofluids have attracted a lot of interest with regards to possible heat transfer application.
Many works have reported impressive enhancement of heat transfer in single and two phase system when nanofluids are used. Some contradictory results have also been reported about the effect of nanoparticles in two phase heat transfer. Indeed in some experimental investigation a degradation of heat transfer is reported rather than enhancement. The underlying machanisms through which nanoparticles act when used in two phase heat transfer application is still an open question.
In this talk we present the results of experimental investigations on the effect of nanoparticles during the evaporation of sessile droplets and vapour bubbles. The observed effects of these latter on the rate of evaporation as well the three phase cantact line dynamics could give some clues to the underlying mechanisms when attempting to understand fro example boiling of nanofluids.
We present the data about the effect the presence of nanoparticle have on the evaporation rate of droplets on heated substrates as well as in a educed pressure environment. The presence of nanoparticles is found to promote pinning of the three phase contact line hence delaying the departure of vapour bubbles.
These fundamental findings can be useful in clarifying the controlling mechanisms when nanofluids are used for heat transfer applications.

Prof. Naoki Shikazono, The University of Tokyo, Japan
  Liquid Film in Micro Tube Two Phase Flow Systems
Liquid film thickness is a very important parameter which dominates two phase flow heat transfer in micro tubes.  It is investigated that liquid film thickness under steady condition depends basically on capillary number, but it also shows strong dependence on Reynolds number even in micro flows.  Under flow boiling or condensation conditions, bubble velocity is not constant but accelerated or decelerated due to phase change.  It is thus very important to investigate the effects of acceleration and deceleration on the formation of liquid film.  In the present paper, experimental investigation of liquid film thickness using confocal method is reported.  Finally, some numerical studies in micro tube heat exchangers using proposed correlations are conducted.

Prof. Peter Stephan, Darmstadt University of Technology, Germany
  Characteristics of evaporative heat transfer in the vicinity of a 3-phase contact line
In many applications such as e.g. nucleate or flow boiling, drop impact on a hot wall, or evaporation from porous structures 3-phase contact lines occur where the evaporative heat transfer characteristics differ from that in the bulk fluid. Experimental and numerical investigations are presented and analysed with a focus on the special local heat and fluid transport phenomena. The scales of interest in these investigations range from a nanometre scale to millimetre scale, approximately. Advancing and receding contact lines show rather different heat transfer characteristics than static contact lines do. Thus, the evaporative heat transfer might be strongly influenced by contact line velocity and apparent contact angle. Vice versa, the heat flux and wall temperature superheat influence the apparent contact angle. The interaction of these parameters will be discussed qualitatively in a more general sense and quantitatively for some typical examples.

Prof. Xing Zhang, Tsinghua University, China
  Novel heat and charge transport properties of nanometallic films and the potential applications
In view that electron is the mutual carrier of charge and heat in metal, the transport properties of these two should exhibit the same changes in nanometallic films. However, we have found several novel heat and charge transport characteristics of nanometallic films. One significant difference is that the decrease ranges of the in-plane thermal and electrical conductivities of nanometallic films are different from simultaneous measurement of these two on suspended nanofilms. This indicates that the thermal and electrical transports do not obey the Wiedemann-Franz law, which is famous classical in solid physics. Another difference is that the electron–phonon relaxation is nearly the same as that of bulk metal while the electrical resistivity greatly increases and exhibit significant size effect. These two reveals that due to the different effect of surface and grain boundary scatterings on thermal and electrical transport, the nanometallic films exhibit several novel transport properties, which will provide potential applications, i.e., new thermoelectric materials. And hence, we also developed a new self-heating 2-omega method used to study and evaluate new thermoelectric materials. Feeding an ac electric current at the frequency of 1-omega into the specimen creates a temperature fluctuation at the frequency of 2-omega and according to the Seebeck effect, a Seebeck voltage fluctuation is generated at 2-omega. The Seebeck coefficient can be steadily extracted from the relationship between 2-omega voltage and frequency omega or between 2-omega and 3-omega voltages.