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

Abstract

Dynamic instabilities during flow boiling in a uniformly heated microchannel are investigated. The focus of this Part 2 of the study is on the effect of operating conditions on the instability type and the resulting time-periodic hydrodynamic and thermal oscillations, which have been established after the initial boiling incipience event. Part 1 of this study investigated the rapid-bubble-growth instability at the onset of boiling in the same experimental facility. Fluid is driven through the single 500 μm-diameter glass microchannel by maintaining a constant pressure difference between a pressurized upstream reservoir and a reservoir downstream that is open to the ambient, so as to resemble the hydrodynamic boundary conditions of an individual channel in a parallel-channel heat sink. Simultaneous high-frequency measurement of pressure drop, mass flux, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior. The effect of flow inertia, inlet liquid subcooling, and heat flux on the hydrodynamic and thermal oscillations and time-averaged

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

“…A subsequent increase in the wall temperature is observed (Figure 4b). This cyclical process, namely, single-phase flow, boiling incipience, rapid-bubble-growth instability, and return to single-phase flow, marks the start of time-periodic flow boiling, which is discussed in Part 2 (Kingston et al, 2017) of this two-part study.…”

“…This deliberate choice of boundary conditions in the present study enables the rapid-bubble-growth instability and the pressure drop instability to be studied in the absence of the confounding influence of the parallel channel instability. Part 1 of this two-part study focuses on the rapid-bubble-growth instability at the onset of boiling while Part 2 (Kingston et al, 2017)…”

Flow Boiling of Water in Square Cross Section Microchannel at Different Inlet Subcooling Conditions

“…This stage appears to be driven by intense evaporation from the bottom and the sides of the channel. Flow reversal was observed at almost all heat fluxes greater than 155 kW/m 2 [10], Kandlikar and Steinke [11] and Diaz and Schmidt [4], using HFE 7100, water and nhexane respectively.…”

“…Fig. 7 shows the effect of heat flux on two-phase pressure drop of the channel for the mass flux of 200 kg /m 2 s at =50 K and 5 K. Increase in two-phase pressure drop with heat flux occurred due to increase in vapour generation and therefore increase in acceleration pressure drop component [10].…”

Ledinegg instability-induced temperature excursion between thermally isolated, heated parallel microchannels

“…The custom-built experimental facility, schematically illustrated in Figure 1, is adapted from our previous studies of flow boiling instabilities in a single microchannel [25,26], and uses a pressurized reservoir to deliver degassed, dielectric HFE-7100 liquid (Novec Engineered Fluid,…”