Limits to Critical Heat Flux Enhancement in a Liquid Film Falling Over a Structured Surface That Simulates a Microelectronic Chip

Abstract

The combination of increasing power densities and stringent surface temperature constraints for microelectric components has stimulated interest in cooling by means of pool boiling in a dielectric liquid. However, although dielectric fluorocarbons are highly compatible with electronic components, small thermal conductivities and latent heats make them poor heat transfer fluids. Moreover, pool boiling studies concerned with critical heat flux (CHF) enhancement have shown that significant hysteresis (temperature overshoot) at the inception of boiling may violate component temperature limitations. More recently, the falling liquid film has been considered as a means of enhancing the performance of dielectric coolants by controlling boiling hysteresis and increasing the critical heat flux (CHF). From experimental studies performed for a gravity-driven liquid (FC-72) film flowing over a smooth surface (Mudawwar et al., 1987), the ability to suppress hysteresis was demonstrated, and the trend of increasing CHF with decreasing heater length was determined. However, even for the smallest heater length of the study (L = 12.7 mm), CHF values were not much higher than those associated with pool boiling. It is known that CHF in a falling liquid film may be enhanced by reducing the length of the heated surface, machining longitudinal grooves in the surface, shroudingmore » the surface with a louvered flow deflector, or subcooling the liquid. However, experiments have yet to be performed in which these effects are considered collectively in order to establish upper limits for CHF enhancement. Hence, the purpose of this study has been to determine boiling characteristics for a subcooled film falling over a short microfinned surface with an attached flow deflector. Experiments have been performed for a 12.7-mm-long by 25.4-mm-wide microfinned surface and for subcooling in the range from 0 to 14C. The length of the heater is representative of microelectronic chip sizes.« less