Abstract

In recent years, research in the field of flow boiling heat transfer at a microscale level has been constantly increasing due to rapid growth of technology applications that require the transfer of heat at high rates in a relatively small space and volume. The present study focuses on the experimental investigation and analytical modeling of flow boiling characteristics in silicon microchannel heat sinks. Two different designs of microchannel heat sinks were considered, namely, straight and cross-linked. The straight microchannel heat sink was composed of 45 parallel microchannels etched in a silicon wafer, for which each channel had a depth of 300 μm and a width of 242 μm. Three cross-link microchannels of width 500 μm were added to the straight microchannel design to create the cross-linked design. Experiments were carried out using FC-72 as the working fluid. Un-encapsulated Thermochromic Liquid Crystals were used, as full-field surface temperature sensors, to map the quantitative heat sink surface temperature and heat transfer coefficient measurements. In addition, the qualitative flow visualization was also considered to complement the experimental data for better understanding of two-phase flow characteristics.
In the straight microchannel heat sink, the results from the flow visualization study identified three major flow regimes: bubbly, slug, and annular. The frictional two-phase pressure drop was found to increase with exit quality for a constant mass flux. A new general correlation was developed to predict the two-phase pressure drop in microchannel heat sinks. The new correlation was verified with two extensive data sets from literature in addition to the experimental data presented in this study. The present study provides the first qualitative and quantitative local experimental data on the characteristics of boiling incipience in microchannels, including the effect of mass and heat fluxes. Flow boiling heat transfer measurements showed that the heat transfer coefficient decreases sharply for low exit quality and then remains almost constant as the exit quality increases. Two-phase heat transfer coefficient asymptotic models and correlations in microchannels were found to underpredict the data in the nucleate boiling regime.
Based on fundamental conservation principles, an analytical model was proposed to predict the flow boiling heat transfer coefficient in the annular flow regime in mini- and microchannel heat sinks. The model was validated against collected data sets from literature produced by different authors under different experimental conditions with a mean absolute error of 10%. The presented analytical model could correctly predict the different trends of the heat transfer coefficient reported in the literature as a function of the vapor exit quality.
To investigate the effect of cross-link design on flow boiling in microchannel arrays, flow boiling characteristics in a cross-linked microchannel heat sink were experimentally investigated. The flow visualization study indicated that the observed flow regime was primarily slug. Visual observations of flow patterns in cross-links also demonstrated that bubbles nucleate and grow rapidly on the cross-links’ surfaces and in the tangential direction at the microchannels’ entrance due to the effect of circulations generated in those regions. The two-phase pressure drop strongly increased with increasing exit quality as compared with that in the straight microchannel heat sink. Addition of cross-links resulted in a substantial increase in the boiling heat transfer coefficient due to the dominant nucleation boiling mechanism in the cross-link region. The flow boiling heat transfer coefficient showed a different trend in the cross-link design relative to the straight microchannel design. The flow boiling heat transfer coefficient increased with increasing exit quality at a constant mass flux.