On the nature of critical heat flux in microchannels
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.