Abstract
Microchannel flow boiling is an attractive approach for the thermal management of high-heat-flux elec- tronic devices that are often operated in transient modes. In Part 1 of this two-part study, the dynamic response of a heated 500 μm channel undergoing flow boiling of HFE-7100 is experimentally investigated for a single heat flux pulse. Three heat flux levels exhibiting highly contrasting flow behavior under con- stant heating conditions are used: a low heat flux corresponding to single-phase flow (15 kW/m 2 ), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m 2 ), and a very high heat flux which exceeds critical heat flux and would cause dryout if applied continuously (150 kW/m 2 ). Transient testing is conducted by pulsing between these three heat flux levels and varying the pulse duration. High-frequency measurements of heat flux, wall temperature, pressure drop, and mass flux are synchro- nized to high-speed flow visualizations to characterize the boiling dynamics during the pulses. At the onset of boiling, the dynamic response resembles that of an underdamped mass-spring-damper system subjected to a unit step input. During transitions between single-phase flow and time-periodic flow boil- ing, the wall temperature temporarily over/under-shoots the eventual steady operating temperature ( e.g. , by up to 20 °C) thus demonstrating that transient performance can extend beyond the bounds of steady performance. It is shown that longer duration high-heat-flux pulses (up to ~50% longer in some cases) can be withstood when the fluid in the microchannel is initial boiling, relative to if it is initially in the single-phase flow regime, despite being at an initially higher heat flux and wall temperature prior to the pulse.
Keywords
Dynamic response, Flow boiling, Microchannel, Heat flux pulse, Transient heating
Date of this Version
2020
Published in:
doi.org/10.1016/j.ijheatmasstransfer.2020.119643