Experimental Study on Saturated Flow Boiling Critical Heat Flux in Microchannels

The saturated flow boiling critical heat flux in microchannels is investigated using water as the working fluid. The experimental test section has six parallel microchannels with each having a cross sectional area of 1054 × 157 micrometers. The mass flow rate, inlet temperature of the working fluids, and the electric current supplied to the resistive cartridge heater are controlled to provide quantitative information near the CHF condition in microchannels. The trends in present CHF with mass flux and quality are similar to those obtained by Qu and Mudawar [1] and earlier investigators. 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 more microprocessors at a given space without the issue of overheated or burned-out chips. Flow boiling in microchannels is being studied worldwide because of its potential in high heat flux cooling. Comparing to single-phase flow, flow boiling is advantageous because it utilizes the heat of vaporization of a working fluid. Because of that, given a mass flow rate, the heat flux in flow boiling is much higher than that of single phase. In addition, flow boiling in microchannel heat sink can provide approximately uniform fluid and solid temperatures, and it can also be directly coupled with a refrigerant system to provide a lower coolant temperature. In designing a two-phase microchannel heat sink, it is necessary to know its critical heat flux (CHF). This is because CHF determines the upper thermal limit on the microchannel operation, and the rapid rise in operating temperature after CHF is detrimental to electronics. That is why CHF data and a good understanding of CHF in microchannels are needed before the application of two-phase microchannel heat sink can be implemented. Furthermore, very few experimental CHF data have been reported in microchannels. Hence, the objective of the present work is to experimentally investigate the CHF of saturated flow boiling in microchannels using water as the working fluid. The present experiment involves the collecting of CHF data over the ranges of mass flux and heat flux supplied to the microchannels. The CHF data is then compared to those obtained by Qu and Mudawar [1]. In their experiment, Qu and Mudawar obtained the CHF data using 21 parallel channels with each channel having a cross-sectional area of 215 × 821 microns. The operating conditions from Qu and Mudawar’s and present experiments can be found in Table 1. 1 Copyright © 2006 by ASME Table 1 Operating conditions CHF Data by Fluid Operating conditions