High-frequency thermal-fluidic characterization of dynamic microchannel flow boiling instabilities: Part 1 – Rapid-bubble-growth instability at the onset of boiling

High-frequency thermal-fluidic characterization of dynamic microchannel flow boiling instabilities: Part 2 – Impact of operating conditions on instability type and severity

“…The rapid-bubble-growth instability is observed in microscale flow boiling systems because the small characteristic channel sizes confine vapor bubbles formed during boiling. This confinement causes the bubbles to significantly influence the operating characteristics of the system, and can cause pressure fluctuations and flow reversal (Barber et al, 2011); under certain operating conditions, this can lead to large wall temperature excursions at the onset of boiling (Kingston et al, 2018).…”

“…In Part 1 of this two-part study (Kingston et al, 2018), the rapid-bubble-growth instability at the initial onset of boiling was investigated, and under certain operating conditions, was shown to result in large wall temperature excursions. In Part 2 of this two-part study, highspeed flow visualizations are synchronized to high-frequency pressure drop, mass flux, and wall temperature measurements and are used to characterize the time-periodic dynamic flow boiling instabilities in a single heated microchannel with hydrodynamic boundary conditions that resemble an individual channel in a parallel-channel system.…”

“…The custom-built experimental facility used in this work is described in detail in Part 1 (Kingston et al, 2018) The microchannel outside wall temperature is measured at a single, fixed location using an infrared (IR) camera at 500 frames per second (fps), which is focused on a single black dot painted on the outside surface of the microchannel. The intensity measured by the IR camera is converted to a temperature using a calibration of the IR camera as detailed in Part 1 of this study (Kingston et al, 2018). The flow through the microchannel is visualized using a high-speed camera at 30,000 fps for a duration of 12 s. An exposure time of 19 μs was used with the highspeed camera to reduce blur.…”

“…At low power conditions, the flow remained a single-phase liquid; synchronized, high-frequency sensor data and IR images were recorded for 6 s once a steady-state condition was achieved. At the minimum power level required to first cause nucleation in the channel, data were recorded to capture this brief boiling onset event that was investigated in Part 1 of this study (Kingston et al, 2018). At all higher power levels, sensor and imaging data were recorded for 12 s once the flow became time-periodic; these results are analyzed in this Part 2 of the study.…”

“…This power loss is a function of the time-averaged wall temperature (over each data acquisition time), resulting in a different power loss for each operating condition; Ploss was between 9% -49% of Ptotal. Details of the procedure used to quantify the power loss to the ambient are provided in Kingston et al (2018). The power into the microchannel is calculated by subtracting the power loss from the total electric power supplied using Pin = Ptotal -Ploss.…”