Computational Heat Transfer and Two-phase Flow in Miniature Tubes D. Lakehal and C. Narayanan ASCOMP GmbH, Zurich, Switzerland, lakehal@ascomp.ch ; chidu@ascomp.ch ABSTRACT Detailed computational microfluidics flow simulations have been performed to study the effect of two-phase flow regime on heat transfer in small pipes. We essentially show that only with interface tracking methods such as the Level Set employed here can the coupled heat-fluid flow problems well predicted, with detailed information about the physics of the transfer in tube confines. Overall the heat removal rate in two-phase flow is higher than in single phase. Subtle differences in thermal removal rates are revealed when flow-regime transition is triggered from bubbly to slug configuration. Keywords: Level sets, heat transfer, CFD, MEMS 1 INTRODUCTION Miniature pipes can now be exploited to make micro- cooling devices for electronic components (e.g. laptops, computer chips, cellular phones, etc.), radar and aerospace avionics. The physics of two-phase flow in tubes of this scale is multifaceted, featuring significant differences with macroscale transport phenomena. The differences concern pressure and temperature drop, friction coefficient, velocity profile, and heat removal rate. More subtle differences are rooted into the unbalance between surface forces, which dominate with decreasing tube size, and body forces. The high ratio of total surface area to volume, characterising microchannels, is useful in facilitating the removal of a large amount of heat from the tube confines. Also, the use of convective boiling is particularly desirable for increasing heat removal efficiency, as the latent heat of vaporization is appreciably higher than sensible heat changes for a set of temperature operating ranges. The ability to predict the physics of microfluidics and associated heat transfer in miniature tubes is essential for various emerging technologies, including ink-jet electro- thermal systems, MEMS design, chip cooling and medical diagnostics devices. In practical applications, the flow may involve phenomena acting at different time/length scales. At each level of the scale cascade, the physics of the flow is amenable to numerical prediction by scale-specific strategies. The simulation approach should typically be capable of predicting flow motion and topology; inter-phase transfer mechanisms; capillary forces; and other thermal effects (e.g. Marangoni). Only with such capabilities could the physics of microfluidics be accurately predicted. In this paper we report on the way this class of flow is tackled by use of the Level Set approach [1], in which we have incorporated phase-change capabilities [2], surface tension and triple-line dynamics models based on the Young’s unbalanced forces [3]. The focus here is on the role played by flow regime in controlling heat transfer. We will show that a tiny change in the gas-phase Reynolds number can trigger flow regime transition from bubbly-slug to a bubbly, which in turn leads to an increase in local heat transfer. The 2D axisymmetric simulations were performed in a 1mm diameter tube heated at the surface, in which air bubbles were injected into water stream. The computational strategy combines the unsteady Navier-Stokes equations for the flow and Level Sets for interface dynamics. The experimental data used as reference are extracted from [4]. 2 SIMULATION FRAMEWORK 2.1 TransAT © Microfluidics Code The CFD code TransAT © [2] is a macro/microfluidics, multi-physics, finite-volume code based on solving multi- fluid Navier-Stokes equations. The one-fluid formulation context on which TransAT © is built is such that the flow is supposed to involve in one fluid having variable material properties, which vary according to the color function as it is advected by the mean flow, identifying gas flow regions from the liquid phase. Specifically, both the Level-Set and the Volume of Fluid Interface Tracking Methods (ITM) [5] can be employed in the code to track evolving interfaces. 2.2 Interface Tracking Context When the exact shape of the interfaces separating two fluids is not known, or not relevant, one may resort to the averaged Two-Fluid approach, where separate conservation equations are required for each phase with appropriate interfacial exchange forces. ITM’s may be invoked when the identification of interfaces needs to be precise, as in the breakup of large bubbles, droplets or liquid jets. The key to the methods is the use of a single-fluid set of conservation equations with variable material properties and surface forces. The concept is attractive, since it offers the prospect of a more subtle strategy than that offered by the two-fluid formalism, while minimizing modeling assumptions. 2.3 Transport Equations The incompressible thermo-fluidics equations expressed within the single-fluid formalism take the following form 0 . = ∇ u (1) NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-7-3 Vol. 2, 2006 621