Flow Regime Identification in Micro-thermal Systems

Francis A. Kulacki

Department of Mechanical Engineering

University of Minnesota

Minneapolis, MN 55455

T: 01-612-625-3807

F. 01-612-624-5230

E. kulacki@me.umn.edu

ABSTRACT

Ultra compact heat exchangers will be more the rule in future as cooling requirements of electrical and electronic systems rise with more power per footprint area being the rule.  Advanced systems and devices with multi-layer architectures and steady heat dissipation in excess of 100 W/cm² with peak rates up to 5 kW/m² have been proposed.   Air cooled heat sinks have limits well below predicted this peak heat, flux and single-phase forced convection systems scale such that the pumping penalty may be restrictive on system design.  It appears that two-phase heat transfer in pumped liquid systems is the only feasible path to higher dissipation rate owing to the large enhancement possible afford when the liquid to vapor transition occurs.  When the flow passages of the heat exchangers are reduced to sub-millimeter diameters – to the “microchannel scale – two-phase flow regimes appear to change dramatically from those traditionally expected in large diameters pipes.  Flow regime maps are altered because of the interplay of surface tension, viscosity, and the ratio of the heat of phase change to convective transport, and heat transfer coefficients are expected to scale uniquely to flow regime.  Despite some 25 years of research on microchannel heat transfer, the dynamics of bubble formation and its relation to convective to heat transfer coefficient remains an open field for basic and applied research.  Key to obtaining reliable heat transfer correlations for thermal engineering is the identification and codification of flow regimes.  This seminar will discuss recent progress toward understanding of flow regimes in microchannel two-phase flow with heat transfer.  Results of an experimental nature will be emphasized with an emphasis on visualization of the flow.  New results on the use of multi-component, multi-phase flow will be presented as well.

The Speaker: Dr. Frank Kulacki is Professor of Mechanical Engineering at the University of Minnesota.  His research lies in the area of thermal science and engineering, with focal interests being convective heat transfer, transport in porous media, nuclear safety, nuclear waste storage and disposal, catalytic combustion, electrohydrodynamic augmentation of heat and mass transfer, and composite materials.  His current research and scholarly interests include coupled heat and mass transfer in porous media, two-phase flow in micro-channels, natural convection heat transfer with solar energy applications, heat transfer in metal foams, hybrid renewable energy systems, energy policy, management of technology, and the adaptation of computer-based technologies in engineering education.  He is a Fellow of ASME and AAAS and over the years has held positions of department head and dean of engineering.