Experimental Study on the Effect of Stabilization on Flow Boiling Heat Transfer in Microchannels

The effect of flow instabilities on flow boiling heat transfer in microchannels is investigated using water as the working fluid. The experimental test section has six parallel rectangular microchannels with each having a cross sectional area of 1054 × 197 microns. Flow restrictors are introduced at the inlet of each microchannel to stabilize the flow boiling process and avoid the backflow phenomena. The mass flow rate, inlet temperature of water, and the electric current supplied to the resistive cartridge heater are controlled to provide quantitative heat transfer information. The results are compared with the unrestricted flow configuration. INTRODUCTION The advancements in microprocessors and other high power electronics have resulted in increased heat dissipation from those devices. In addition, to reduce cost, the functionality of microprocessor per unit area has been increasing. The increase in functionality accompanied by reduction in chip size has caused its thermal management to be challenging. In order to dissipate the increase in heat generation, the size of conventional fin-type heat sinks has to be increased. As a result, the performance of these high heat flux generating electronics is often limited by the available cooling technology and space to accommodate the larger conventional air-cooled heat sinks. One way to enhance heat transfer from electronics without sacrificing its performance is the use of heat sink with many microchannels and liquid water passing through it. Because of the small size of microchannel heat sink, the performance of a computer system can also be increased by incorporating additional microprocessors at a given space without the issue of over-heated or burned-out chips. The present paper involves cooling of electronic devices using two-phase flow in microchannel heat sink. Two-phase heat transfer has significant advantages over single-phase heat transfer because flow rates are smaller through the use of the latent heat of vaporization, approximately uniform fluid and solid temperatures can be obtained, and it can also be directly coupled with a refrigerant system to provide a lower coolant temperature. There are, however, complexities associated with vaporization in multiple narrow channel arrays that are not completely understood. The phenomenon characterized by vapor expansion in both the upstream and downstream directions causing flow reversal was observed by Kandlikar et al. [1] and also by Kandlikar and Balasubramanian [2]. Both employed a high-speed digital video camera to observe this behavior in minichannels and microchannels. Similar flow instabilities were observed by Li et al. [3] and Peles [4], among other investigators. Kandlikar [5] reported that flow instabilities in microchannels are due to rapid bubble expansion and occasional flow reversal, and they cause a major concern in implementing flow boiling in microchannels. Because of this, research on obtaining experimental heat transfer and pressure drop data for flow boiling of water in microchannels is immediately warranted. Focusing on flow instabilities, Kandlikar et al. [6] found that the heat transfer performance in microchannels can be improved by using flow restrictors to stabilize the flow in microchannels, and partially stabilized flow was observed for the first time. Further research is needed to study the effect of different pressure drop elements as flow restrictors. Hence, the objective of the present work is to experimentally investigate the effect of pressure drop elements on microchannel heat transfer performance. 1 Copyright © 2006 by ASME