Increased rate of heat dissipation from electronic chips was explored by the application of flow boiling in mini-channel (D = 2.54 mm) and micro-channel (D — 510 \\x,m) heat sinks with special emphasis on reducing pressure drop and coolant flow rate. A pressure drop model was developed that accounts for the single-phase inlet region, the singleand two-phase heated region, and the two-phase unheated outlet region. Inlet and outlet losses associated with the abrupt contraction and expansion, respectively, were also accounted for, and so were the effects of compressibility and flashing within the two-phase region. Overall, the major contributor to pressure drop was the acceleration caused by evaporation in the channels; however, compressibility effects proved significant for the micro-channel geometiy. Based upon practical considerations such as pressure drop, erosion, choking, clogging, and manufacturing ease, the mini-channel geometiy was determined to offer inherent advantages over the microchannel geometiy. The latter is preferred only in situations calling for dissipation of high heat fluxes where minimizing weight and liquid inventory is a must.

Two-Phase Electronic Cooling Using Mini-Channel and Micro-Channel Heat Sinks: Part 2—Flow Rate and Pressure Drop Constraints

A comparison of the performance of 52 void fraction correlations was made based on an unbiased experimental data set of 1208 data points. A comprehensive literature search was undertaken for the available void fraction correlations and experimental void fraction data for upward vertical two-phase flow. The performance of the correlations in correctly predicting the diverse data set was evaluated. Comparisons between the correlations were made and appropriate recommendations were drawn. The analysis showed that most of the correlations developed are very restricted in terms of handling a wide variety of data sets. Based on this analysis, void fraction correlations with the best predictive capability are highlighted.

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Comparison of Void Fraction Correlations for Different Flow Patterns in Upward Vertical Two-Phase Flow

In gas–liquid two-phase flow, the prediction of two-phase density and hence the hydrostatic pressure drop relies on the void fraction and is sensitive to the error in prediction of void fraction. The objectives of this study are to analyze dependence of two-phase density on void fraction and to examine slip ratio and drift flux model-based correlations for their performance in prediction of void fraction and two-phase densities for the two extremes of two-phase flow conditions, that is, bubbly and annular flow or, alternatively, the low and high region of the void fraction. It is shown that the drift flux model-based correlations perform better than the slip ratio model-based correlations in prediction of void fraction and hence the two-phase mixture density. Another objective of this study is to verify performance of different two-phase dynamic viscosity models in prediction of two-phase frictional pressure drop. Fourteen two-phase dynamic viscosity models are assessed for their performance against 616 d...

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Effect of Void Fraction and Two-Phase Dynamic Viscosity Models on Prediction of Hydrostatic and Frictional Pressure Drop in Vertical Upward Gas–Liquid Two-Phase Flow

Resume On a etabli des correlations en vue de la prevision du coefficient de transfert de chaleur local au cours de l'ebullition en convection forcee de melange de frigorigenes pour 4 compositions de melanges binaires de R13 et R12. Parmi les correlations examinees, a peu pres egalement prometteuses, on a trouve que le type de Lavin-Young etait le meilleur. Des correlations generalisees ont aussi ete etablies pour toutes les compositions de melange. L'influence du facteur de concentration a permis une amelioration marginale des correlations. On a trouve aussi que les nombres de Prandtl des phases liquide et vapeur avaient une influence sur les correlations generalisees pour la prevision du coefficient de transfert de chaleur local.

Heat transfer correlations for the forced convection boiling of R12-R13 mixtures

Experimental determination of void fraction in two-phase flows can be achieved in a number of different ways, including: the quick closing of valves at each end of a test section, impedance measurement techniques, gamma or x-ray absorption, and analysis of visualization results. This paper presents a method of determining void fraction in separated, two-phase refrigerant flows through analysis of images obtained through high speed visualization. A statistical technique known as Change Point Analysis was used to automate the determination of the location of the liquid/vapor interface in stratified and intermittent type flows. This determination allows for a calculation of the void fraction in these separated flows that can be examined as a function both of time (different images) and length (location within the image). The developed technique was applied to visualization results of separated R134a flows in transparent tubes with diameters of 7.2mm, 8.7mm, and 15.3mm.

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H.K. Varma

Papers

Prediction of pressure drop during forced convection boiling of mixture of refrigerants 13 and 12