Unified mechanistic multiscale mapping of two-phase flow patterns in microchannels

To better understand the underlying two-phase phenomena and thus better predict transitions between the various two-phase flow patterns, it is necessary to update our way of thinking from one-of-a-kind flow pattern maps of limited applicability to a generalized approach based on first principles, mechanistic analysis and multi-scale characterization and representation of the important features of these complex flows. While in macro-sized channels and pipes this need is typically addressed by the use of empirically-validated flow regime maps, there is - as yet - no consensus on two-phase flow regime maps for microchannels and miniature pipes. This study presents a set of recommendations for the development of a new comprehensive type of flow pattern map that not only covers adiabatic, evaporating and condensing flows in one seamless flow pattern identification tool, but also includes multiscale information about the flow itself, and furthermore contains embedded mechanistic methods for the principal two-phase phenomena for use in developing unified models for pressure gradients, heat transfer, void fraction, CHF, etc., all in one coherent global method. (C) 2012 Elsevier Inc. All rights reserved.

Convective boiling heat transfer in a single micro-channel

Lorenzo Consolini

The current industrial need for compact high heat density cooling devices has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes, their heat transfer characteristics are still unclear and require investigation. The present study aims at providing further insight on two-phase single micro-channel heat transfer, through a sensitivity analysis on the effect of the different operational parameters, fluid properties, and channel size on thermal performance. The current database includes results for three different refrigerants, R-134a, R-236fa and R-245fa, two channel diameters, 510 and 790µm, and a multitude of heat fluxes and mass velocities, for a total of over 1800 data points. Two-phase flow instability, which represents one of the factors that has prevented a well-established understanding of the micro-scale phenomenon, has been also investigated, with results showing the potential error it may induce on the estimation of the heat transfer coefficients. Finally, a one-dimensional analysis based on the effect of interfacial shear on the temporal evolution of film thickness is proposed for a heat transfer model for slug flow, which has been shown to be the dominant flow mode at low and intermediate vapor qualities in micro-channel evaporators.

EPFL2008

Numerical Modeling of Annular Laminar Film Condensation in Circular and Non-Circular Micro-Channels under Normal and Micro-Gravity

Stefano Nebuloni

A theoretical and numerical model to predict film condensation heat transfer in mini, micro and ultra micro-channels of different internal shapes is presented in this thesis. The model is based on a finite volume formulation of the Navier-Stokes and energy equations and it includes the contributions of the unsteady terms, surface tension, axial shear stresses, gravitational forces and wall thermal conduction. Notably, interphase mass transfer and near-to-wall effects (disjoining pressure) are also included. This model has been validated versus various benchmark cases and versus published experimental results from three different laboratories, predicting micro-channel heat transfer data with an average error of 20 % or better. The conjugate heat transfer problem arising from the coupling between the thin film fluid dynamics, the heat transfer in the condensing fluid and the heat conduction in the channel wall has been studied and analyzed. The work has focused on the effects of three external wall boundary conditions: a uniform wall temperature, a non uniform wall heat flux and single-phase convective cooling. The thermal axial and peripheral conduction occurring in the wall of the channel can affect the behavior of the condensate film, not only because it redistributes the heat, but also because the annular laminar film condensation process is dependent on the local saturation to wall temperature difference. When moving from mini to micro and ultra-micro channels, the results shows that the axial conduction effects can become very important in the prediction of the wall temperature profile and they can not be ignored. Under these conditions, the overall performances of the heat exchanger become dependent not only on the fluid properties and the operative conditions but also on the geometry and wall material. Results obtained for steady state conditions are presented for circular, elliptical and flattened shape cross sections for R-134a and ammonia, for hydraulic diameters between 10 µm and 3 mm. Microscale condensation finds applications in heat pipes and compact heat exchangers for electronic equipment or spacecraft thermal control, in automotive condensers, in residential air conditioning and in refrigeration applications: the influence of the steady or unsteady gravitational field and of the inertia forces on the flow field and consequently on the heat transfer performances is investigated allowing the model to be applied as a design and optimization tool for enhanced heat exchangers.

EPFL2010

Numerical Modeling of Annular Laminar Film Condensation in Circular and Non-Circular Micro-Channels under Normal and Micro-Gravity

Stefano Nebuloni

EPFL2010