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

Enabling Highly Effective Boiling from Superhydrophobic Surfaces

Avariety of industrial applications such as power generation, water distillation, and high-density cooling rely on heat transfer processes involving boiling. Enhancements to the boiling process can improve the energy efficiency and performance across multiple industries. Highly wetting textured surfaces have shown promise in boiling applications since capillary wicking increases the maximum heat flux that can be dissipated. Conversely, highly nonwetting textured (superhydrophobic) surfaces have been largely dismissed for these applications as they have been shown to promote formation of an insulating vapor film that greatly diminishes heat transfer efficiency. The current Letter shows that boiling from a superhydrophobic surface in an initial Wenzel state, in which the surface texture is infiltrated with liquid, results in remarkably low surface superheat with nucleate boiling sustained up to a critical heat flux typical of hydrophilic wetting surfaces, and thus upends this conventional wisdom. Two distinct boiling behaviors are demonstrated on both micro- and nanostructured superhydrophobic surfaces based on the initial wetting state. For an initial surface condition in which vapor occupies the interstices of the surface texture (Cassie- Baxter state), premature film boiling occurs, as has been commonly observed in the literature. However, if the surface texture is infiltrated with liquid (Wenzel state) prior to boiling, drastically improved thermal performance is observed; in this wetting state, the three-phase contact line is pinned during vapor bubble growth, which prevents the development of a vapor film over the surface and maintains efficient nucleate boiling behavior

A Free-Particles-Based Technique for Boiling Heat Transfer Enhancement in a Wetting Liquid

An easy-to-implement technique for pool boiling heat transfer enhancement is proposed and evaluated through an experimental investigation. This free-particle technique brings about nucleate boiling at a low degree of superheat by means of metal particles that are not fixed to the heated surface, but rather are free to move with respect to the surface. The effects of copper particles with sizes ranging from tens of nanometers to 9 mm on nucleate boiling heat transfer and critical heat flux (CHF) of the wetting dielectric fluid FC-72 are investigated. Visualizations of the bubble nucleation characteristics due to the free particles are presented. Experimental results show that the introduction of microscale free particles onto a superheated surface effectively facilitates bubble nucleation and thus increases the nucleate boiling heat transfer coefficients. Millimeter-sized as well as nanoscale free particles do not have a strong effect on the boiling heat transfer performance of this wetting fluid. Introduction of a large quantity of microscale free particles reduces CHF by increasing the resistance to liquid replenishment and vapor departure; however, by properly selecting particle size and quantity, an improvement in both nucleate boiling heat transfer and CHF is observed. For the case where 0.2 g of 10 μm-diameter free particles are placed on a polished copper surface, corresponding to a particle layer thickness of approximately 67 μm, the average nucleate boiling heat transfer coefficient is enhanced by 76.3% over the heat flux range of 10 to 159 kW/m2, while CHF is increased by 10%

Predicting Two-Phase Flow Distribution and Stability in Systems with Many Parallel Heated Channels

Two-phase heat exchangers are used in a variety of industrial processes in which the boiling fluid flows through a network of parallel channels. In some situations, the fluid may not be uniformly distributed through all the channels, causing a degradation in the thermal performance of the system. Amethodology for modeling two-phase flow distributions in parallel-channel systems is developed. The methodology combines a pressure-drop model for individual parallel channels with a pump curve into a system flow network. Due to the non-monotonicity of the pressure drop as a function of flow rate for boiling channels, many steady-state solutions exist for the system flow equations. A new numerical approach is proposed to analyze the stability of these solutions, based on a generalized eigenvalue problem. The method is specifically designed for analyzing systems with hundreds of identical parallel channels. The method is first applied to analyze the flow distribution and stability behavior in two-channel and five-channel systems. The asymptotic behavior of the flow stability is then analyzed for increasing numbers of channels, and it is shown that the stability behavior of a system with a constant flow-rate pump curve simplifies to the stability behavior for a constant pressure-drop pump curve. A parametric study is conducted to assess the influence of inlet temperature, heat flux, and flow rate on the stability of the uniform flow distribution solution as well as on the severity of flow maldistribution. Below some critical inlet subcooling, uniform flow distribution is always stable and maldistribution does not occur, regardless of heat flux and flow rate. Above this critical inlet subcooling, there is a range of operating parameters for which uniform flow distribution is unstable. With increasing inlet subcooling, this range broadens and the severity of the associated maldistribution increases