Time-Resolved Characterization of Thermal and Flow Dynamics During Microchannel Flow Boiling
thesisposted on 14.05.2019 by Todd A. Kingston
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The continued miniaturization and demand for improved performance of electronic devices has resulted in the need for transformative thermal management strategies. Flow boiling is an attractive approach for the thermal management of devices generating high heat fluxes. However, designing heat sinks for two-phase operation and predicting their performance is difficult because of, in part, commonly encountered flow boiling instabilities and a lack of experimentally validated physics-based phase change models. This work aims to advance the state of the art by furthering our understanding of flow boiling instabilities and their implications on the operating characteristics of electronic devices. This is of particular interest under transient and non-uniform heating conditions because of recent advancements in embedded cooling techniques, which exacerbate spatial non-uniformities, and the demand for cooling solutions for next-generation electronic devices. Additionally, this work aims to provide a high-fidelity experimental characterization technique for slug flow boiling to enable the validation of physics-based phase change models.
To provide a foundation for which the effects of transient and non-uniform heating can be studied, flow boiling instabilities are first studied experimentally in a single, 500 μm-diameter borosilicate glass microchannel. A thin layer of optically transparent and electrically conductive indium tin oxide coated on the outside surface of the microchannel provides a spatially uniform and temporally constant heat flux via Joule heating. The working fluid is degassed, dielectric HFE-7100. Simultaneous high-frequency measurement of reservoir, inlet, and outlet pressures, pressure drop, mass flux, inlet and outlet fluid temperatures, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior.
The effect of flow inertia and inlet liquid subcooling on the rapid-bubble-growth instability at the onset of boiling is assessed first. The mechanisms underlying the rapid-bubble-growth instability, namely, a large liquid superheat and a large pressure spike, are quantified. This instability is shown to cause flow reversal and can result in large temperature spikes due to starving the heated channel of liquid, which is especially severe at low flow inertia.
Next, the effect of flow inertia, inlet liquid subcooling, and heat flux on the hydrodynamic and thermal oscillations and time-averaged performance is assessed. Two predominant dynamic instabilities are observed: a time-periodic series of rapid-bubble-growth instabilities and the pressure drop instability. The heat flux, ratio of flow inertia to upstream compressibility, and degree of inlet liquid subcooling significantly affect the thermal-fluidic characteristics. High inlet liquid subcoolings and low heat fluxes result in time-periodic transitions between single-phase flow and flow boiling that cause large-amplitude wall temperature oscillations and a time-periodic series of rapid-bubble-growth instabilities. Low inlet liquid subcoolings result in small-amplitude thermal-fluidic oscillations and the pressure drop instability. Low flow inertia exacerbates the pressure drop instability and results in large-amplitude thermal-fluidic oscillations whereas high flow inertia reduces their severity.
Flow boiling experiments are then performed in a parallel channel test section consisting of two thermally isolated, heated microchannels to study the Ledinegg instability. When the flow in both channels is in the single-phase regime, they have equal wall temperatures due to evenly distributed mass flux delivered to each channel. Boiling incipience in one of the channels triggers the Ledinegg instability which induces a temperature difference between the two channels due to flow maldistribution. The temperature difference between the two channels grows with increasing power. The experimentally observed temperature excursion between the channels due to the Ledinegg instability is reported here for the first time.
Time-resolved characterization of flow boiling in a single microchannel is then performed during transient heating conditions. For transient heating tests, three different heat flux levels are selected that exhibit highly contrasting flow behavior during constant heating conditions: 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 would cause critical heat flux if operated at this heat flux continuously (150 kW/m2). Transient testing is first conducted using a single heat flux pulse between these heat flux levels and varying the pulse time. It is observed that any step up/down in the heat flux level that induces/ceases boiling, causes the temperature to temporarily over/under-shoot the eventual steady temperature. Following the single heat flux pulse experiments, a time-periodic series of heat flux pulses is applied. 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 are 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. For heating pulse frequencies above 25 Hz, the microchannel wall attenuates the transient heating profile and the fluid essentially experiences a constant heat flux.
To improve our ability to predict the performance of heat sinks for two-phase operation, high-fidelity characterization of key hydrodynamic and heat transfer parameters during microchannel slug flow boiling is performed using a novel experimental test facility that generates an archetypal flow regime, devoid of flow instabilities and flow regime transitions. High-speed flow visualization images are analyzed to quantify the uniformity of the vapor bubbles and liquid slugs generated, as well as the growth of vapor bubbles over a range of heat fluxes. A method is demonstrated for measuring liquid film thickness from the visualizations using a ray-tracing procedure to correct for optical distortions. Characterization of the slug flow boiling regime that is generated demonstrates the unique ability of the facility to precisely control and quantify hydrodynamic and heat transfer characteristics.
This work has advanced state-of-the-art technologies for the thermal management of high-heat-flux-dissipation devices by providing an improved understanding on the effects of transient and non-uniform heating on flow boiling and an experimental method for the validation of physics-based flow boiling modeling.