The transition from single-phase to two-phase thermal systems in future space vehicles demands a thorough understanding of flow boiling critical heat flux (CHF) in reduced gravity, including microgravity. This study is a comprehensive, consolidated investigation of the complex trends of flow boiling CHF in a rectangular channel in both microgravity and for different orientations in Earth gravity. It is shown that the Interfacial Lift-off Model provides good predictions of CHF data for both gravitational environments and both single-sided and double-sided heating. CHF mechanism in Earth gravity is shown to be highly sensitive to flow orientation at very low velocities, but is consistent with the wavy vapor layer depiction of the Interfacial Lift-off Model at high velocities. The model predicts a stable vapor-liquid interface for downflow with a downward-facing heated wall at lower velocities, and wavy interface with a critical wavelength that decreases with increasing velocity at higher velocities. Predicted CHF values for microgravity fall about midway between the maxima and minima for Earth gravity. Overall, predicted values of CHF and key interfacial parameters for all orientations in Earth gravity and for microgravity converge above similar to 1.5 m/s, which points to a velocity threshold above which inertia begins to effectively negate gravity effects. (C) 2015 Elsevier Ltd. All rights reserved.

In a previous study by the corresponding author and co-worker Mita and Qu (2015) (J. Mita, W. Qu, Pressure drop of water flow across a micro-pin-fin array part 2: adiabatic liquid–vapor two-phase flow, International Journal of Heat and Mass Transfer 89 (2015) 1007–1015), a modified Martinelli–Chisholm type correlation was developed to predict adiabatic water liquid–vapor two-phase frictional pressure drop across an array of staggered circular micro-pin-fins. This work expands on the study in ref. Mita and Qu (2015) and examines whether the correlation is geometry specific or it can be extended to describe a different array configuration. A square micro-pin-fin array were prepared, and adiabatic water liquid–vapor two-phase frictional pressure drop across the array was experimentally investigated. The square pin-fins were 200 microns in side length, 670 microns in height, and 400 microns in both longitudinal and transverse pitches. Two-phase friction multiplier and Martinelli parameter were calculated based on the measured pressure drop as well as a single-phase friction factor correlation developed for the same square array. An excellent agreement was found between the experimental data and correlation predictions despite the distinctive geometrical features possessed by the two sets of micro-pin-fin arrays. The result points to the possibility of establishing the correlation as a generalized one applicable to a broad range of staggered micro-pin-fin array configurations.

This study is the second part of a two-part investigation of flow boiling critical heat flux (CHF) in microgravity, which is simulated in parabolic flight experiments. Using FC-72 as working fluid, flow boiling experiments are conducted in a rectangular channel fitted with two opposite heated walls, allowing either one or both heated walls to be activated during a test. While the first part explored flow boiling conditions leading to CHF, this part addresses events just before CHF, during the CHF transient, and immediately following CHF. For both single-sided and double-sided heating, interfacial behavior just before CHF is characterized by dominant wavy vapor layers covering the heated walls, where liquid is able to access the walls only in wetting fronts corresponding to the wave troughs. CHF is associated with successive lift-off of wetting fronts from the walls, consistent with the Interfacial Lift-off Model, which has been validated extensively in past studies using single-sided heating in both μge and 1−ge. It is shown this model predicts μge double-sided flow boiling CHF with excellent accuracy. Additionally, the model points to convergence of CHF values for μge and 1−ge for inlet velocities greater than about 1m/s. Therefore, by maintaining velocities above this threshold allows designers of space systems to achieve inertia-dominated performance as well as to adopt prior data and correlations developed from terrestrial studies.

Space agencies worldwide are actively exploring the implementation of two-phase thermal management systems to support astronaut life onboard future space vehicles and planetary bases. Key motivations for these efforts are to increase the efficiency of power utilization and reduce overall weight and volume. These advantages are realized by orders of magnitude enhancement in heat transfer coefficient achieved with flow boiling and condensation compared to single-phase systems. This study will review published literature concerning two-phase flow and heat transfer in reduced gravity. Discussed are the different methods and platforms dedicated to exploring the influence of reduced gravity, including ground flow boiling experiments performed at different orientations relative to Earth gravity, as well as reduced gravity adiabatic two-phase flow, pool boiling, flow boiling and CHF experiments. Despite the extensive data and flow visualization results available in the literature, it is shown that there is a severe shortage of useful correlations, mechanistic models and computational models, which compromises readiness to adopt flow boiling in future space systems. Key recommendations are provided concerning platform, heater design, and operating conditions for future studies to expedite the deployment of two-phase thermal management in future space missions.

Space agencies worldwide are being confronted with the challenges of more distant manned space missions, which will demand greater energy efficiency and reduced weight and volume. One method being considered to reduce the weight and volume of a long duration mission spacecraft is to replace present single-phase Thermal Control Systems (TCSs) with ones that rely on flow boiling and condensation. This transition will require a thorough understanding of the influence of reduced gravity on flow boiling and condensation, and the development of predictive tools for both. This study is the first part of a two-part study investigating flow boiling of FC-72 in microgravity, which is simulated in a series of parabolic flight maneuvers. Flow boiling experiments are conducted in a rectangular channel fitted with two opposite heating walls. The operating conditions include liquid inlet velocities of 0.1–1.9m/s, liquid mass velocities of 224.2–3347.5kg/m2s, and inlet subcoolings ranging from 2.8 to 8.1°C. The study includes both high-speed video analysis of interfacial features and heat transfer measurements. A dominant wavy vapor layer behavior is encountered for most operating conditions. Boiling is sustained mostly in ‘wetting fronts’ corresponding to contact regions between the wave troughs and the wall, and abated near the wave peaks. During a flight parabola, the heated wall temperatures decrease slightly as the aircraft enters the hypergravity ascent phase, then increase slightly during the microgravity phase, and decrease once again during the hypergravity descent. These temperature variations point to enhancement in flow boiling heat transfer with increasing gravity, and conversely a reduction with microgravity.

Future manned space missions are expected to greatly increase the space vehicle's size, weight, and heat dissipation requirements. An effective means to reducing both size and weight is to replace single-phase thermal management systems with two-phase counterparts that capitalize upon both latent and sensible heat of the coolant rather than sensible heat alone. This shift is expected to yield orders of magnitude enhancements in flow boiling and condensation heat transfer coefficients. A major challenge to this shift is a lack of reliable tools for accurate prediction of two-phase pressure drop and heat transfer coefficient in reduced gravity. Developing such tools will require a sophisticated experimental facility to enable investigators to perform both flow boiling and condensation experiments in microgravity in pursuit of reliable databases. This study will discuss the development of the Flow Boiling and Condensation Experiment (FBCE) for the International Space Station (ISS), which was initiated in 2012 in collaboration between Purdue University and NASA Glenn Research Center. This facility was recently tested in parabolic flight to acquire condensation data for FC-72 in microgravity, aided by high-speed video analysis of interfacial structure of the condensation film. The condensation is achieved by rejecting heat to a counter flow of water, and experiments were performed at different mass velocities of FC-72 and water and different FC-72 inlet qualities. It is shown that the film flow varies from smooth-laminar to wavy-laminar and ultimately turbulent with increasing FC-72 mass velocity. The heat transfer coefficient is highest near the inlet of the condensation tube, where the film is thinnest, and decreases monotonically along the tube, except for high FC-72 mass velocities, where the heat transfer coefficient is enhanced downstream. This enhancement is attributed to both turbulence and increased interfacial waviness. One-g(e) correlations are shown to predict the average condensation heat transfer coefficient with varying degrees of success, and a recent correlation is identified for its superior predictive capability, evidenced by a mean absolute error of 21.7%.

This study explores the complex flow boiling CHF mechanisms encountered at different orientations relative to Earth’s gravity when the fluid is supplied as a two-phase mixture. Using FC-72 as working fluid, different CHF regimes are identified for different orientations, mass velocities and inlet qualities. Low mass velocities are shown to produce the greatest sensitivity to orientation, while high mass velocities greatly reduce this influence, especially for high inlet qualities. It is also shown that the influence of orientation can be negated by simultaneously satisfying three separate criteria: overcoming the influence of gravity perpendicular to the heated wall, overcoming the influence of gravity parallel to the heated wall, and ensuring that the heated wall is sufficiently long to ensure liquid contact. These criteria are combined to determine the minimum mass velocity required to negate gravity effects in both terrestrial and space applications. Exceeding this minimum is of paramount importance to space systems since it enables the implementation of the vast body of published CHF data, correlations and models developed from terrestrial studies for design of thermal management systems for space applications.

Determining flow boiling critical heat flux (CHF) using mechanistic models or empirical correlations requires careful validation with the aid of reliable databases. But, while many new databases are being made available in the literature, the methods used to detect CHF vary greatly, producing different CHF estimates for the same fluid and operating conditions. The variations in detection method are the result of both heated wall design and criteria used to terminate an experiment in response to wall temperature excursions. This study investigates the interfacial phenomena preceding the occurrence of CHF for flow boiling with a finite inlet vapor void. Experiments are conducted with FC-72 in a rectangular channel that is heated along one side. Temporal records of the heated wall temperatures are used to track the complex transient response of the heated wall, and identify differences between temperature excursions associated with momentary localized dryout and those with true CHF. It is shown that the flow enters the channel fully separated, with a liquid layer sheathing all four channel walls surrounding a central vapor core. At high heat fluxes, a wavy vapor layer begins to form beneath the liquid layer adjacent to the heated wall, and cooling is provided mostly through wetting fronts associated with the wave troughs in accordance with the Interfacial Lift-off Model. However, depending on mass velocity, inlet quality and flow orientation, conditions may arise that cause breakup of the heated wall liquid layer into ligaments that are entrained in the vapor core. This phenomenon causes localized dryout and wall temperature excursions at heat fluxes well below CHF, but the wall is able to recover from these excursions by a combination of reattachment of ligaments with the heated wall and lateral heat conduction within the wall itself. Recommendations are made concerning construction of the heated wall and CHF detection in pursuit of reliable CHF data.

This study explores the mechanism of flow boiling critical heat flux (CHF) for FC-72 in a rectangular channel fitted along one side with a heated wall. The flow is supplied as a two-phase mixture and the channel is tested at different orientations relative to Earth’s gravity. High-speed video imaging is used to identify the CHF trigger mechanism for different orientations, mass velocities and inlet qualities. It is shown that orientation has a significant influence on CHF for low mass velocities and small inlet qualities, with the orientations surrounding horizontal flow with downward-facing heated wall causing stratification of the vapor towards the heated wall and yielding very small CHF values. High mass velocities cause appreciable diminution in the influence of orientation on CHF, which is evidenced by similar flow patterns and CHF trigger mechanism regardless of orientation. The interfacial lift-off model is shown to predict the influence of orientation on CHF with good accuracy. Overall, this study points to the effectiveness of high mass velocities at combating buoyancy effects and helping produce CHF values insensitive to orientation.

This study investigates the hydraulic performance of a copper micro-pin-fin array subjected to water liquid single-phase flow conditions. The test section contains an array of 1950 staggered square micro-pin-fins with 200 micron x 200 micron cross-section by 670 micron height. The ratios of longitudinal pitch and transverse pitch to pin-fin equivalent diameter are equal to 2. Seven water inlet temperatures from 22 degrees C to 80 degrees C, and seventeen maximum mass velocities for each inlet temperature, ranging from 181 to 1649 kg/m(2)s, were tested. The test module was well insulated to maintain adiabatic conditions. Comparison of predictions of eleven existing friction factor correlations with the experimental data show relatively large discrepancies. The experimental study was complemented with a numerical analysis of single-phase flow in the micro-pin-fin array. Numerical results show excellent agreement with experimental data for Reynolds numbers below 700.

This study concerns pressure drop of adiabatic water liquid-vapor two-phase flow across an array of 1950 staggered square micropin-fins having a 200×200 μm cross section by 670 μm height. The ratios of longitudinal pitch and transverse pitch to pin-fin equivalent diameter are equal to 2. An inline immersion heater upstream of the micropin-fin test module was employed to produce liquid-vapor two-phase mixture, which flowed across the micropin-fin array. The test module was well insulated to maintain adiabatic condition. Four maximum mass velocities of 184 kg/m2 s, 235 kg/m2 s, 337 kg/m2 s, and 391 kg/m2 s, and a range of vapor qualities for each maximum mass velocity were tested. Measured pressure drop increases drastically with increasing vapor quality. Nine existing two-phase pressure drop models and correlations were assessed. The Lockhart–Martinelli correlation for laminar liquid-laminar vapor combination in conjunction with a single-phase friction factor correlation proposed for the present micropin-fin array provided the best agreement with the data.