On the Nature of Critical Heat Flux in Microchannels

TLDR

The critical heat flux limit is a key design constraint in flow boiling, and understanding its mechanism in microchannels—different from conventional dryout and potentially higher with microfabrication—has become essential for cooling high‑heat‑flux devices. The study aims to develop a comprehensive CHF database for microchannels across diverse fluids, configurations, and operating conditions to support their adoption in high‑heat‑flux cooling. The authors conduct extensive experiments to map CHF, tackling challenges like flow distribution, conjugate effects, and instrumentation in parallel microchannels. CHF in parallel microchannels is governed by upstream compressible volume or excursive instabilities, not dryout, and flow stabilization via an entrance orifice is expected to increase CHF.

Abstract

The critical heat flux (CHF) limit is an important consideration in the design of most flow boiling systems. Before the use of microchannels under saturated flow boiling conditions becomes widely accepted in cooling of high-heat-flux devices, such as electronics and laser diodes, it is essential to have a clear understanding of the CHF mechanism. This must be coupled with an extensive database covering a wide range of fluids, channel configurations, and operating conditions. The experiments required to obtain this information pose unique challenges. Among other issues, flow distribution among parallel channels, conjugate effects, and instrumentation need to be considered. An examination of the limited CHF data indicates that CHF in parallel microchannels seems to be the result of either an upstream compressible volume instability or an excursive instability rather than the conventional dryout mechanism. It is expected that the CHF in parallel microchannels would be higher if the flow is stabilized by an orifice at the entrance of each channel. The nature of CHF in microchannels is thus different than anticipated, but recent advances in microelectronic fabrication may make it possible to realize the higher power levels.