The Critical Heat Flux Condition in Micro-Channels

Rensselaer Polytechnic Institute, Troy NY

Investigators

Abstract

Two-phase heat transfer has significant advantages over single-phase heat transfer, but a crucially important factor that must be taken into account in the design of micro-channel boiling heat transfer is the critical heat flux (CHF) condition, which sets the upper thermal limit on the micro-channel operation. Literature on the CHF condition in micro-channels is quite sparse and the applicability to micro-channels of existing CHF correlations for conventionally sized channels is unknown; therefore better capability to predict the CHF condition for micro-channels is needed. The objectives of this project are to develop a better quantitative and qualitative understanding of the CHF condition in single and multiple, parallel micro-channels, to assess the applicability of current CHF correlations to micro-channels, to develop a technique to minimize or eliminate flow instabilities and flow maldistribution in multiple micro-channels, and to develop a phenomenological model of the CHF condition that takes into account the confounding effects of conduction in the solid. To accomplish these objectives, both experimental and analytical investigations are performed. In the experimental study, circular tubes and rectangular channels heated on all sides are tested over a range of channel diameters, number of channels in parallel, flow rates, pressures, and power levels. The test section configurations include single circular tubes with varying tube-wall materials and wall thicknesses, multiple channels machined into large copper blocks, and microfluidic systems fabricated on silicon wafers, all instrumented with fast-responding arrays of temperature and pressure sensors. Two fluids, refrigerant R123 and water, are used. Because flow instabilities can be significant with boiling in multiple parallel channels, time-varying data are obtained and the channel geometry is modified in both on the microfluidic and copper block systems by the addition of upstream flow restrictions to reduce or eliminate instabilities; these same channel modifications are used to minimize or eliminate flow maldistribution. High-speed, microscopic flow-visualization studies are undertaken to complement the quantitative measurements. In the analytical study, a phenomenological model that includes the effects of local fluid and solid conditions and dynamic contact angles is produced by extending the previously developed CHF model for pool boiling to flow boiling in micro-channels. Effects due to wall thermal conductivity and thermal capacity (density-specific heat product) are incorporated while evaluating local wall temperature conditions. The model is tested against the single-tube CHF data. Differences between the CHF condition in single and multiple, parallel channels are clearly elucidated through experiments in the microfluidic and copper-block systems. Novel schemes of incorporating upstream individual flow restrictors in the microfluidic system and individual removable inserts in the copper-block system are used to study their effects on improving CHF in parallel-channel operation. Broader impact A better method for prediction of CHF will permit engineers to pursue ideas that cannot be implemented now because of uncertainties in thermal operating limits, such as higher-power electronics, computers, lasers, etc. The research findings are presented at national conferences, published in appropriate journals, and shared with industry and national laboratories during visits and seminars. To enhance the value of this research, an international short course on micro-channel heat transfer is held in conjunction with the ASME Rochester Heat Transfer Chapter. Also, the project enhances educational opportunities through the partnering of a PhD-granting institution (Rensselaer Polytechnic Institute) with one that does not grant PhD degrees (Rochester Institute of Technology). Minority graduate students are recruited from historically black colleges and universities. Because early exposure to research has been proven to be a significant spur for undergraduates to continue for a graduate degree, promising undergraduates are identified to participate in the project.