Numerical investigation of hydrodynamics and heat transfer of elongated bubbles during flow boiling in a microchannel

Flow boiling within microchannels has been explored intensively in the last decade due to their capability to remove high heat fluxes from microelectronic devices. However, the contribution of experiments to the understanding of the local features of the flow is still severely limited by the small scales involved. Instead, multiphase CFD simulations with appropriate modeling of interfacial effects overcome the current limitations in experimental techniques. Presently, numerical simulations of single elongated bubbles in flow boiling conditions within circular microchannels were performed. The numerical framework is the commercial CFD code ANSYS Fluent 12 with a Volume Of Fluid interface capturing method, which was improved here by implementing, as external functions, a Height Function method to better estimate the local capillary effects and an evaporation model to compute the local rates of mass and energy exchange at the interface. A detailed insight on bubble dynamics and local patterns enhancing the wall heat transfer is achievable utilizing this improved solver. The numerical results show that, under operating conditions typical for flow boiling experiments in microchannels, the bubble accelerates downstream following an exponential time-law, in good agreement with theoretical models. Thin-film evaporation is proved to be the dominant heat transfer mechanism in the liquid film region between the wall and the elongated bubble, while transient heat convection is found to strongly enhance the heat transfer performance in the bubble wake in the liquid slug between two bubbles. A transient-heat-conduction-based boiling heat transfer model for the liquid film region, which is an extension of a widely quoted mechanistic model, is proposed here. It provides estimations of the local heat transfer coefficient that are in excellent agreement with simulations and it might be included in next-generation predictive methods.

Numerical investigation of the influence of leading and sequential bubbles on slug flow boiling within a microchannel

Mirco Magnini, John Richard Thome

Multiphase CFD simulations are presently employed to investigate the flow boiling of multiple sequential elongated bubbles in a horizontal microchannel. Most of the computational studies published so far explored the features of boiling flows within microchannels by simulating the fluid-dynamics of a single evaporating bubble, but the present work shows that multiple bubble simulations are necessary to capture the essential features of the heat transfer process of a slug flow. In particular, it is shown that leading and sequential bubbles interact thermally and hydrodynamically due to the evaporation process, thus possessing different growth rates, velocities and thicknesses of the thin liquid films trapped between the bubbles interfaces and the channel wall. The evaporation of this thin liquid film is the dominant heat transfer mechanism in the vapor bubble region and the transit of trailing bubbles strongly enhances the time-averaged heat transfer coefficient of the bubble-liquid slug unit, by as much as 60% higher relative to the leading bubble under the operating conditions presently set. Furthermore, the presence of a recirculating vortex just after the tail of the bubble in the liquid slug trapped between the bubbles was found in the simulations, significantly improving the heat transfer between the wall and the bulk liquid, thus maintaining the heat transfer coefficient much higher than otherwise expected in the liquid slug region as well. Finally, a new multiple bubble heat transfer model is proposed to predict the local variation of the heat transfer coefficient, which might prove to be useful to improve the current boiling heat transfer methods, such as the three-zone model of Thome et al. [1,2]. The numerical framework employed to perform this study was the commercial CFD solver ANSYS Fluent 12 with a Volume Of Fluid interface capturing method, which was improved here by implementing external functions, in particular a Height Function method to better estimate the surface tension force and an evaporation model to compute the phase change. (C) 2013 Elsevier Masson SAS. All rights reserved.

Elsevier2013

Contribution to the heat transfer analysis of natural and substitute refrigerants evaporated in a horizontal tube

Olivier Zürcher

The substitution of CFC refrigerants in refrigeration systems, heat pumps and organic Rankine cycles, requests a good knowledge of the heat transfer and pressure drop properties of substitute fluids. A contribution to this international effort is proposed with the study of two hydrofluorocarbon refrigerants (HFC-134a and the zeotropic mixture HFC-407C) and the study of the natural refrigerant ammonia. The HFCs have been experimentally tested on the first test rig developed in the Laboratory of Industrial Energy Systems (LENI) which was substantially modified to cope with ammonia, both in terms of safety requirements and operational conditions. The experimental test section is composed of two concentric tubes, with in tube evaporation of the refrigerant inside the inner tube of counter-current Annular water heating. A new database of the local heat transfer coefficients for the refrigerants HFC-134a, HFC-407C and ammonia together with pressure drop measurements has been collected and used to define and calibrate a new and more general heat transfer model. The two HFC refrigerants have been tested on the test rig developed by Kattan [36]; due to the chemical and the thermophysical properties of ammonia, a new test rig has been designed for the ammonia tests and a new calculation procedure based on the mean temperature measurement on the tube wall has been used. The experimental database covers evaporation tests on plain and on microfin tubes, and with nominal oil concentrations varying from 0 to 5 [%] (by wt.). Detailed modeling is concentrated on the tests on plain tube without oil. Each of the three refrigerants has been evaporated at 4[°C]. The other experimental conditions for the HFCs refrigerants were an inside tube diameter of 10.92[mm], a heat flux range of 2 – 5 [kW/m2] and three mass velocities of G = 100, 200, 300 [kg/(m2s)] visualized as Stratified-Wavy flows (G = 100[kg/(m2s)]), and mainly Annular for the higher mass velocities (G = 200, 300[kg/(m2s)]). The other global conditions for ammonia were an internal tube diameter of 14.00 [mm], a heat flux range of 5 – 70[kW/m2] and eleven mass velocities of G = 10,20,30,40,45,50,55,60,80,120,140[kg/(m2s)], corresponding to Stratified, Stratified-Wavy, Intermittent and Annular flow patterns; complementary tests, varying the mass velocity at constant vapor qualities (x = 20,50,80[%]), have been made in particular to better characterize boundaries between flow patterns. The flow patterns are visualized through glass sections at both ends of the 3.1 [m] long test tube. An extensive review of void fraction models with sensitivity analysis on the actual available flow pattern map has been made. From the 17 correlations reviewed, the models of Taitel-Dukler [71] and Rouhani [79] have been used. Other improvements to the threshold criteria of the Stratified-Wavy to Annular transition, lead to an accurate diabatic map to predict the flow patterns and their transition for very different families of fluids like HFC's and ammonia. A new approach in the prediction of two-phase flow heat transfer has been proposed through the study of each flow pattern separately, according to a new criterion defining the onset of nucleate boiling as a function of the critical convective heat transfer coefficient representative of the location where nucleate boiling might occur. A function based on a pseudo Biot number allows, from the mean heat flux around the tube periphery, to base the model on two different mean heat fluxes applied respectively to the parts of the perimeter in contact with the liquid and the vapor in stratified types of flow. Considering pure convective heat transfer, or mixed convective and nucleate heat transfer, this division allows the use of a common criterion to be applied to each flow pattern. The two-phase flow heat transfer coefficient has finally been obtained as a weighted mean function of the vapor and the liquid heat transfer coefficient with respect to their contact surface with the heated tube. Based on the database of refrigerants HFC-134a and ammonia, the standard deviation of the new heat transfer model is of σ = 27.9[%]. Even if the database showed that the flow conditions were close to, or in the turbulent to laminar flow transition, and even if the major part of the experimental points were purposely obtained close to the various flow pattern transitions, the new model showed very good agreement with the experimental database. Due to the precision of the new flow pattern map and the effectiveness of the onset on nucleate boiling criterion, this new heat transfer model accurately predicts the heat transfer conditions during evaporation. Finally, a new method to model separated flows is proposed, based on partial hydraulic diameters and a mean interface velocity.

EPFL2000

Numerical Modeling of the Conjugate Heat Transfer Problem for Annular Laminar Film Condensation in Microchannels

Stefano Nebuloni, John Richard Thome

This paper presents numerical simulations of annular laminar film condensation heat transfer in microchannels of different internal shapes. The model, which is based on a finite volume formulation of the Navier-Stokes and energy equations for the liquid phase only, importantly accounts for the effects of axial and peripheral wall conduction and nonuniform heat flux not included in other models so far in the literature. The contributions of the surface tension, axial shear stresses, and gravitational forces are included. This model has so far been validated versus various benchmark cases and versus experimental data available in literature, predicting microchannel heat transfer data with an average error of 20% or better. It is well known that the thinning of the condensate film induced by surface tension due to gravity forces and shape of the surface, also known as the "Gregorig" effect, has a strong consequence on the local heat transfer coefficient in condensation. Thus, the present model accounts for these effects on the heat transfer and pressure drop for a wide variety of geometrical shapes, sizes, wall materials, and working fluid properties. In this paper, 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. In particular, the work has focused on three external channel wall boundary conditions: a uniform wall temperature, a nonuniform wall heat flux, and single-phase convective cooling are presented. As the scale of the problem is reduced, i.e., when moving from mini- to microchannels, the results show that the axial conduction effects can become very important in the prediction of the wall temperature profile and the magnitude of the heat transfer coefficient and its distribution along the channel. [DOI: 10.1115/1.4005712]

2012