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 synchro- nized 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/m 2 ). Three different time-periodic flow boiling fluctuations were observed for the heat flux lev- els 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/m 2 and heating pulse fre- quencies above 1 Hz, time-periodic flow regime transitions between single-phase and two-phase flow are reported. For heating profiles involving 150 kW/m 2 heat flux pulses, fluid in the microchannel is al- ways 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 char- acteristics, 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 sys- tem. For heating pulse frequencies above 25 Hz, the microchannel wall attenuates the transient heating profile and the fluid essentially experiences a constant heat flux.


Flow boiling, Microchannel, Pressure drop oscillations, Pulse frequency, Transient heating

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T.A. Kingston, J. A. Weibel, and S. V. Garimella, “Time-Resolved Characterization of Microchannel Flow Boiling During Transient Heating: Part 2 – Dynamic Response to Time-Periodic Heat Flux Pulses,” International Journal of Heat and Mass Transfer, Vol. 154, 119686, 2020.