Microchannel flow boiling is an attractive approach for the thermal management of high-heat-flux electronic devices, many of which are operated under transient conditions.  In this study, the dynamic behavior of a heated 500 μm channel undergoing flow boiling of HFE-7100 is experimentally investigated for a single heat flux pulse and time-periodic heat flux pulses with varying pulse frequencies (0.1-100 Hz).  Three heat flux levels exhibiting highly contrasting flow behavior under constant heating conditions are used to conduct transient heating tests: 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^).  High-frequency measurements of heat flux, wall temperature, pressure drop, and mass flux are synchronized to high-speed flow visualizations to characterize the boiling dynamics.

At the onset of boiling during a single heat flux pulse, 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 boiling, 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.  For time-periodic heating conditions, three different flow boiling fluctuations were observed:  flow regime transitions, pressure drop oscillations, and heating pulse propagation.  For heating pulse frequencies between approximately 1 and 10 Hz, the thermal and flow fluctuations are heavily coupled to the heating characteristics, forcing the pressure drop instability frequency to match the heating frequency.  The observed dependence of flow instabilities and thermal performance on transient heating conditions indicates that flow boiling dynamics will significantly influence overall performance in various applications.


Time-Resolved Characterization of Microchannel Flow Boiling During Transient Heating

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

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