An Experimental Method for Controlled Generation and Characterization of Microchannel Slug Flow Boiling

This study uses high-speed imaging to characterize microchannel slug flow boiling using a novel experimental test facility that generates an archetypal flow regime suitable for high-fidelity characterization of key hydrodynamic and heat transfer parameters. Vapor and liquid phases of the fluorinated dielectric fluid HFE-7100 are independently injected into a T-junction to create a saturated two-phase slug flow, thereby eliminating the flow instabilities and flow-regime transitions that would otherwise result from stochastic generation of vapor bubbles by nucleation from a superheated channel wall. Slug flow boiling is characterized in a heated, 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 uniform heat flux via Joule heating. 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 under heat fluxes ranging from 30 W/m2 to 5160 W/m2. 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. The experimental approach demonstrated in this study provides a unique platform for the investigation of microchannel slug flow boiling transport under controlled, stable conditions suitable for model validation

High‐Frequency Thermal‐Fluidic Characterization of Dynamic Microchannel Flow Boiling Instabilities: Part 2 ‐ Impact of Operating Conditions on Instability Type and Severity

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 boil- ing 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 mi- crochannel 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 condi- tions 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 en- abling 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 performance is assessed. Two predominant dynamic instabilities are observed: a time-periodic series of rapid-bubble- growth instabilities, and the pressure drop instability. A spectral analysis of the time-periodic data is performed to determine the characteristic oscillation frequencies. 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 be- tween single-phase flow and flow boiling that cause large-amplitude wall temperature oscillations due to 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

Time-Resolved Characterization of Microchannel Flow Boiling During Transient Heating: Part 1 – Dynamic Response to a Single Heat Flux Pulse

Microchannel flow boiling is an attractive approach for the thermal management of high-heat-flux elec- tronic 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 con- stant heating conditions are used: a low heat flux corresponding to single-phase flow (15 kW/m 2 ), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m 2 ), and a very high heat flux which exceeds critical heat flux and would cause dryout if applied continuously (150 kW/m 2 ). 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 synchro- nized 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 boil- ing, 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

High‐Frequency Thermal‐Fluidic Characterization of Dynamic Microchannel Flow Boiling Instabilities: Part 1 ‐ Rapid‐Bubble‐Growth Instability at the Onset of Boiling

Dynamic flow boiling instabilities are studied experimentally in a single, 500 μm-diameter glass mi- crochannel subjected to a uniform heat flux. Fluid flow is driven through the microchannel in an open- loop test facility by maintaining a constant pressure difference between a pressurized upstream reservoir and a reservoir at the exit that is open to the ambient; the working fluid is HFE-7100. This hydrodynamic boundary condition resembles that of an individual channel in a parallel-channel heat sink where the channel mass flux can vary in time. 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. Part 1 of this study investigates the rapid-bubble-growth instability at the onset of boiling; the effect of flow inertia and inlet liquid subcooling is assessed. 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. Low flow inertia exacerbates the rapid-bubble-growth instability by starving the heated channel of liquid replenishment for longer durations and results in severe temperature increases. In the case of high flow inertia or high inlet liquid subcooling, flow reversal is still observed at the onset of boiling, but results in a minimal wall temperature rise because liquid quickly replenishes the heated channel. A companion paper (Part 2) investigates the effect of flow inertia, inlet liquid subcooling, as well as heat flux on the thermal-fluidic oscillations during time-periodic flow boiling that follows the initial incipience at the onset of boiling considered here

Time-Resolved Characterization of Microchannel Flow Boiling During Transient Heating: Part 2 – Dynamic Response to Time-Periodic Heat Flux Pulses

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

Quantifying the Variation in Protein Content in White Clover (\u3cem\u3eTrifolium Repens\u3c/em\u3e L.)

White clover (Trifolium repens L.) is the main legume in temperate pastures. It has relatively low levels of water-soluble carbohydrate but produces forage of high quality with a high crude protein (CP) content and dry-matter digestibility (Beever, 1993). Some studies have suggested that the forage quality of white clover can be problematic because its high CP content may contribute to inefficient use of nitrogen in the rumen and exacerbate diffuse pollution via excreta (Waghorn & Caradus, 1994). The development of white clover germplasm with lower CP content would potentially benefit forage production and grassland management. A study was carried out to quantify the variation in CP content within an existing gene pool and develop high throughput techniques for protein determination appropriate to a plant breeding programme