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Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m2 into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent at this condition. At a slightly lower constant heat flux of 60 kW/m2, boiling remains continuous in one of the channels, but the other channel experiences time-periodic flow regime oscillations between single-phase and two-phase flow. At this condition, extreme time-periodic wall temperature oscillations are observed in both channels with a long period (~7 s) due to oscillations in the severity of the flow maldistribution. For the transient heating conditions, square-wave heating profiles oscillating between different heat flux levels are applied to the channels. Because of their relatively high frequency, the heating transients are attenuated by the microchannel walls, resulting in effectively constant heating conditions and flow boiling characteristics like that of the aforementioned constant heating conditions. This study illustrates the susceptibility of parallel two-phase heat sinks to flow maldistribution, particularly when undergoing transient flow regime oscillations. [ABSTRACT FROM AUTHOR]

• Microchannel flow boiling under transient heating conditions is studied experimentally. • High-frequency sensor measurements are synchronized to flow visualizations. • Heat flux is pulsed between 15, 75, and 150 kW/m2 using a thin film heater. • The dynamic response to a transient pulse is qualitatively similar to that of a spring-mass-damper system. • Heat flux pulses that induce/arrest boiling cause a temporary wall temperature over/under-shoot. Microchannel flow boiling is an attractive approach for the thermal management of high-heat-flux electronic 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 constant heating conditions are used: a low heat flux corresponding to single-phase flow (15 kW/m2), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m2), and a very high heat flux which exceeds critical heat flux and would cause dryout if applied continuously (150 kW/m2). 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 synchronized 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 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. 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. [ABSTRACT FROM AUTHOR]

• The effects of heating pulse frequency on flow boiling are studied experimentally. • Flow boiling performance and transient heating characteristics are heavily coupled. • Time-periodic flow regime transitions can occur for heating pulse frequencies >1 Hz. • Pressure drop oscillations sync to the transient heating profile for 1 < f < 10 Hz. • Above a heating pulse frequency of 25 Hz, the fluid effectively experiences a constant heat flux. Flow boiling in microchannels is an effective method for dissipating high heat fluxes. However, two-phase heat sink operation during transient heating conditions remains relatively unexplored. In Part 1 of this two-part study, the dynamic response of flow boiling to a single heat flux pulse was experimentally studied. In this Part 2, the effect of heating pulse frequency on microchannel flow boiling is explored when a time-periodic series of pulses is applied to the channel. HFE-7100 is driven through a single 500 μm-diameter glass microchannel using a constant pressure reservoir. A thin indium tin oxide layer on the outside surface of the microchannel enables simultaneous transient heating and flow visualization. High-frequency measurements of heat flux, wall temperature, pressure drop, and mass flux are synchronized to the flow visualizations to characterize the boiling process. A square-wave heating profile is used with pulse frequencies ranging from 0.1 to 100 Hz and three different heat fluxes levels (15, 75, and 150 kW/m2). Three different time-periodic flow boiling fluctuations were observed for the heat flux levels and pulse frequencies investigated in this study: flow regime transitions, pressure drop oscillations, and heating pulse propagation. For heat flux pulses between 15 and 75 kW/m2 and heating pulse frequencies above 1 Hz, time-periodic flow regime transitions between single-phase and two-phase flow are reported. For heating profiles involving 150 kW/m2 heat flux pulses, fluid in the microchannel is always boiling and thus the flow regime transitions are eliminated. 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. Outside this heating pulse frequency range, the pressure drop instability occurs at the intrinsic frequency of the system. For heating pulse frequencies above 25 Hz, the microchannel wall attenuates the transient heating profile and the fluid essentially experiences a constant heat flux. [ABSTRACT FROM AUTHOR]