This study presents the flow condensation heat transfer results of the Flow Boiling and Condensation Experiment (FBCE). The primary goal of FBCE is to obtain fundamental flow boiling and condensation heat transfer data in microgravity (mu ge) through experiments onboard the International Space Station. Experiments were performed with the Condensation Module for Heat Transfer (CM-HT), which is a tube-in-tube counterflow heat exchanger. Condensing nPFH flows through a stainless steel tube with an inner diameter of 7.24 mm and rejects heat to cooling water flowing in an annular channel (with inner and outer gap diameter of 7.94 and 12.70 mm, respectively) surrounding the tube. Experiments tested a broad range of nPFH mass velocities, G = 72.8 - 291.5 kg/m2s, inlet thermodynamic equilibrium qualities, xe,in = 0.28 - 1.19, inlet pressures, pin = 103.9 - 160.2 kPa, and water mass velocities, Gw = 129.4 - 324.7 kg/m2s. A parametric investigation shows local condensation heat transfer coefficient, h, is primarily dependent on G and local xe, which can be represented by the two-phase mixture Reynolds number, Retp. Channel averaged heat transfer coefficient in the saturated two-phase region, htp, increases with increasing G and xe,in. However, increasing inlet superheat does not affect htp, but does increase the heat transfer coefficient averaged over the entire channel, h. In the present experiments, G is sufficient to mitigate the effects of gravity, and htp in mu ge aligns with those for vertical down flow and horizontal flow in Earth gravity. Various correlations for htp were assessed, and the best performing correlation with a Mean Absolute Error (MAE) of 7.1% was that by Dorao and Fernandino, which is a function of Retp. Some correlations were shown to be overly dependent on the effect of gravity and were not applicable for the present mu ge database. A Separated Flow Model for annular condensation was employed to predict htp. The model's physical basis makes it seamlessly adaptable for mu ge, and it resulted in a MAE of 32.3%.

Mechanically pumped two-phase loops are strong candidates for future spacecraft thermal control, but condenser performance under long-duration microgravity is not yet well quantified. This work reports condensation heat-transfer measurements for n-perfluorohexane in a tube-in-tube annulus operated onboard the International Space Station. nPFH flows in a 7.24 mm inner tube with water coolant in a 12.70 mm annulus over a 574.5 mm active condensation length, with a water mass velocity of G(w) = 226.79 +/- 0.04 kg m(-2) s(-1). A 36-point matrix combines six nPFH mass velocities (G(nPFH) approximate to 60-182 kg m(-2) s(-1)) with two inlet categories: saturated two-phase inlets with x(e,in) approximate to 0.29, 0.67, and 0.96, and superheated vapor inlets with x(e,in) approximate to 1.07, 1.12, and 1.16. Axial wall and coolant temperatures are fitted with smooth profiles to reconstruct local heat flux, inner-wall temperature, thermodynamic equilibrium quality, and heat transfer coefficient. The saturated two-phase length L-tp increases strongly with G(nPFH) and is extended by higher inlet quality, ranging from roughly 90 to 550 mm and approaching the full test-section length at the highest mass velocities, where L-tp is truncated by the 574.5 mm active length. In both inlet modes, the largest local heat transfer coefficients occur near the upstream end of the saturated region, then decay monotonically as quality decreases, while the downstream subcooled segment exhibits much lower and weakly varying coefficients. When reorganized in terms of local thermodynamic equilibrium quality, the saturated-region data show that, at fixed GnPFH, superheated and saturated inlets follow common h(tp)(x(e)) and q(tp)(x(e)) trends, indicating that, for a given mass velocity, local condensation performance is controlled mainly by local quality rather than inlet quality. Seven Earth-gravity correlations are assessed against the two-phase heat transfer coefficients. The correlations of Boyko and Kruzhilin, Kim and Mudawar, and Cavallini and Zecchin reproduce the ISS data with mean absolute percentage errors of about 6%-8%, whereas Akers et al. and Wang et al. exhibit larger positive biases and Shah and Dobson-Chato yield intermediate errors. The results provide a well-characterized microgravity dataset for an annular condenser relevant to spacecraft thermal control and identify which existing condensation models offer the most reliable starting point for mechanically pumped two-phase loop design in low gravity.

For the development of orbital refueling stations, it is imperative to understand the cryogenic transfer line chilldown process and propose reliable modeling strategies. This work envisages to achieve this in a two-phase CFD modeling framework, improving the current understanding and modeling aspects of the chilldown process. A typical chilldown process is accompanied by various flow boiling regimes such as the film, transition and nucleate boiling regime which makes the model development challenging. In this paper, an unsteady 3D wall-coupled two-phase flow dispersed mixture model has been employed to predict the two-phase flow behavior during the chilldown process with the SST k-omega model to resolve the wall boundary layer for predicting accurate vapor film behavior in the film boiling regime. For estimating the mass transfer rates, the Lee phase model has been implemented, where the mass transfer coefficients which can be mesh dependent, are tuned to predict the boiling rates. The CFD results are validated against the vertical upflow liquid nitrogen chilldown experiments available in the literature for different inlet conditions. The 3D CFD predictions are also compared against the prior 2D axisymmetric simulations, and the effect of domain geometry and nature of computational meshes are discussed. In addition, predictions in different boiling regimes during the chilldown process are studied separately and the reasons for the agreements and disagreements are analyzed. Two-phase flow and thermal fields are studied along with the vapor film behavior to derive physical insights into the chilldown process and the wall quenching behavior.

This article is part of the multi-objective Flow Boiling and Condensation Experiment (FBCE) onboard the International Space Station, which utilized the Flow Boiling Module (FBM) for experiments during February - July 2022. This study investigates pressure drop characteristics of microgravity flow boiling of n-Perfluorohexane in FBM's rectangular channel of 5.0 x 2.5 mm(2) cross-sectional area and 114.6 mm heated length. Both subcooled and saturated inlet conditions are studied, while one or two opposite walls of the four are uniformly heated, to amass a large database of 3393 datapoints. Operating parameters explored include mass velocity (180.0 - 3200.1 kg/m(2)s), inlet quality (-0.62 - 0.87, corresponding to inlet subcooling of 46.0 - 0 degrees C), inlet pressure (119.6 - 200.4 kPa), wall heat flux (< 55.1 W/cm(2)), and heating configuration (one or two opposite walls heated). Pressure drop typically increases with increasing mass velocity, increasing inlet quality (for fixed mass velocity), and increasing heat flux (until a point after which it asymptotically reaches a plateau), and is higher for double-sided heating. Inlet pressure did not have an appreciable effect. The primary reason for most of these physical trends is flow acceleration increasing bulk flow velocities and both frictional and accelerational components of pressure drop. Only 2478 datapoints (with smaller experimental uncertainties) are considered for further analysis and assessment of prediction tools. 1099 purely saturated flow boiling datapoints are used to assess 7 mixture viscosity models used with the Homogeneous Equilibrium Model (HEM) and 17 empirical correlations used with the Separated Flow Model (SFM). Of these, the original SFM proposed by Lockhart and Martinelli (1949) is most accurate with a 17.1% mean absolute error (MAE). 1065 purely subcooled flow boiling datapoints are used to assess 9 seminal correlations, of which, the one by Hahne et al. (1993) is most accurate with 34.1% MAE. Finally, following a statistical analysis of input parameters, an artificial neural network with 6 hidden layers is developed and trained using the Adam algorithm. It accurately predicts the testing subset of the entire database with a 5.24% MAE, while conforming to expected physical trends in previously unseen data.

Understanding cryogenic chilldown process under reduced gravity conditions is essential for the advancement of the planned cryogenic fuel depots in space. The cryogenic chilldown process is accompanied by complex and intricate two-phase flow boiling regimes which makes the predictions difficult from a CFD perspective. The present work investigates the reduced gravity liquid nitrogen chilldown process using a 2D axisymmetric two-phase flow mixture model with Lee phase change modeling approach, which has been validated earlier for the terrestrial liquid nitrogen chilldown tests by the authors. The numerical results from the present study are compared with the recent parabolic flight experiments reported in the literature. The outer wall temperatures at different axial wall locations are compared with the experimental wall thermocouple measurements and a good agreement was observed. The contours of liquid volume fraction and temperature are plotted at different instants of time and the variations with terrestrial data are analyzed. The present numerical methodology will be validated for other test cases under reduced gravity conditions in the future to ensure reliable predictions.

During the cryogenic transfer line chilldown process, different two-phase flow boiling regimes are observed such as film, transition and nucleate boiling along with coupled heat transfer between the fluid and tube walls. To understand the chilldown process better, chilldown experiments are performed with a simulant fluid (PF-5060) through a 2.4 mm thick pyrex tube of 15.875 mm outer diameter. The chilldown experiments are performed in a horizontal configuration and wall temperatures at three different locations along the axis of the tube are recorded at top and bottom walls. To mimic the cryogenic chilldown process, the pyrex tube is heated to high initial wall temperatures by winding a Nichrome wire around the circumference of the tube and passing current through the high resistance wires. The complex two-phase flow regimes are captured using a high speed camera and the flow visualization images at different regimes are analyzed. The high speed images revealed interesting flow physics during the different boiling regimes at the top and bottom walls which needs further investigation.

In this study, closed loop chilldown experiments are performed on a 0.009525 m outer diameter SS-316 tube of length 0.6 m and thickness 0.001651 m in a horizontal flow configuration with PF-5060 as the working fluid. The challenges associated with the development of a closed loop chilldown test section in comparison with conventional open loop cryogenic chilldown experiments are discussed and the methods to overcome this are presented. The test section tube is heated to around 246–249 °C to perform the chilldown tests and the film, transition and nucleate boiling regimes along with the single phase liquid convective regimes are captured in the present experiments. To understand the effect of inlet mass flux on the chilldown characteristics of the tube, tests are performed at different inlet mass fluxes ranging from 132.83 kg/m2s – 1303.96 kg/m2s (Inlet Reynolds number ranging from 1,291–12,679) covering the entire regime of laminar to transition to turbulent inlet flows for inlet subcoolings of 33.3 – 41.9 °C. The effect of the inlet conditions on the behavior of chilldown curves, temperature of transition points (rewetting and onset of nucleate boiling), critical heat fluxes, heat flux curves, parasitic heat losses, regime-specific time-averaged heat fluxes and heat transfer coefficients are analyzed at different wall locations. The chilldown performance parameters such as chilldown time, liquid consumption and quench front propagation/rewetting velocities are compared at different inlet conditions and axial wall locations.

This study is an elaboration on flow instabilities observed during flow boiling experiments conducted onboard the International Space Station (ISS) as part of the Flow Boiling and Condensation Experiment (FBCE). During highly subcooled flow boiling, liquid backflow into the channel that rapidly condenses vapor within the channel was observed. Experiments were conducted with an enhanced sampling frequency of 30 Hz and an extended image sequence recording duration of at least 4 seconds at 500 frames per second to further investigate the instability. Identical experiments were performed both in microgravity onboard the International Space Station (ISS) and in Earth gravity during vertical upflow. Instabilities in microgravity are more severe than those in Earth gravity and, at their most severe, propagate to the channel's upstream region. Instabilities are observed within the channel when the intensity, I, which is dependent on inlet pressure fluctuations and mass velocity, exceeds 1.8 x 105 W/m2. Parametric trends of the frequency and amplitude of inlet pressure fluctuations during instability are examined, which reveal instabilities are most severe at low flow rates, high inlet subcoolings, and high heat fluxes. Various stability maps proposed in the literature are evaluated against the present database, and instabilities only manifest for subcooling numbers greater than 14. A subset of the database containing the Onset of Flow Instability (OFI) point is extracted to first evaluate correlations available in the literature and then develop a new correlation. The new correlation is applicable in both microgravity and Earth gravity conditions and predicts the database with a Mean Absolute Error (MAE) of 1.3%.

The paper presents a three-dimensional CFD simulation for predicting the liquid nitrogen chilldown process through a SS-304 tube. A dispersed mixture model has been employed to predict the two-phase flow behavior whereas the Lee phase change model has been used to predict the boiling process. Cryogenic chilldown process starts with a pure vapor region and proceeds to film, transition and nucleate boiling regimes before reaching the single phase liquid flow and complete chilldown of the transfer line. The present numerical model predicted the film boiling regime accurately and the slope of the chilldown curve deviates slightly from the experimental curve towards the transition and nucleate boiling regime, which needs further investigation. The contours of liquid volume fraction and temperature profiles has been presented to understand the inverted annular film flows in the film boiling regime in detail. In future, the present CFD model will be extended to a wide range of chilldown conditions and different cryogenic fluids to develop a robust methodology.

Understanding transfer line chilldown process under microgravity is important for the efficient transfer of cryogenic propellants in space fuel depots to facilitate future long duration space missions. The present work is part of the ongoing efforts to develop and test a two-phase flow chilldown test section to study the complete chilldown process under sustained microgravity conditions onboard the International Space Station. In this study, ground-based chilldown experiments are carried out on a 60 cm long SS-316 test section with PF-5060 as the working fluid. The complete chilldown curve was obtained including the film, transition and nucleate boiling regimes along with the temperature transition points. The effect of inlet liquid subcooling on the behavior of the chilldown curves are presented. Further, the chilldown and heat flux curves are analyzed to obtain re-wetting/Leidenfrost and onset of nucleate boiling temperature transition points as well as the critical heat flux values. The effect of inlet liquid subcooling on regime-specific heat flux and heat transfer coefficients are also examined.

This study is part of the Flow Boiling and Condensation Experiment (FBCE), a collaborative effort between the Purdue University Boiling and Two-Phase Flow Laboratory (PU-BTPFL) and the NASA Glenn Research Center. The FBCE fitted with the Flow Boiling Module (FBM) was launched to the International Space Station (ISS) in August 2021 and experiments were successfully performed from February to July 2022 to amass a large microgravity-flow-boiling database. This study is focused on heat transfer and flow visualization of microgravity flow boiling of n-Perfluorohexane in a rectangular channel of 5.0 mm height, 2.5 mm width (heated), and 114.6 mm length, with subcooled inlet conditions. High-speed-video photography is utilized to present flow patterns and temporal interfacial behavior. Heat transfer results are presented in the form of flow boiling curves and both parametric curves and streamwise profiles of wall temperature and heat transfer coefficient. Firstly, the parametric effects of mass velocity (199.4 - 3200.0 kg/m2s), inlet subcooling (0.2 - 46.0 degrees C), and inlet pressure (124.2 - 176.7 kPa), on the aforementioned aspects are assessed for double-sided heating to establish them for a microgravity environment. Of these three parameters, mass velocity and inlet subcooling mostly determine the microgravity flow boiling behavior, while inlet pressure plays an insignificant role. Flow patterns for doublesided heating are more complex than those for single-sided heating due to interaction between the two vapor layers. Vapor interaction is minimized at high subcoolings and high mass velocities due to strong condensation offered by the subcooled bulk liquid layer separating them. Despite the different flow patterns, both single- and double-sided heating generally result in similar parametric trends and local heat transfer coefficients for similar operating conditions. Flow instabilities manifest as temporal flow anomalies and temperature oscillations, and their severity increases with increasing boiling number. Secondly, the effects of heating configuration are analyzed by comparing and contrasting several aspects of single- and double-sided heating data. The heat fluxes at which onset of nucleate boiling degradation (ONBD) and critical heat flux (CHF) occur are distinctly different for single- and double-sided heating. There exists a threshold inlet subcooling demarcating the dominance of flow acceleration and condensation effects in vapor removal from the near-wall region and replenishment of fresh liquid for boiling. Above the threshold, condensation from the near-wall region is dominant and single-sided heating yields higher heat fluxes, and below it, acceleration is dominant and double-sided yields higher heat fluxes. At mass velocity in the range of 200 - 2400 kg/m2s, the threshold inlet subcooling lies in the approximate range of 20 - 30 degrees C (corresponding inlet quality of roughly -0.40 - -0.20).

As part of the ongoing efforts to develop and test a two-phase flow chilldown experimental setup onboard the International Space Station (ISS), experiments are conducted to analyze the line chilldown and flow boiling heat transfer characteristics of SS-316 using a simulant fluid PF-5060 in this study. The cryogenic transfer line chilldown process is pre-dominated by the film boiling regime for most of the duration followed by transition and nucleate boiling regimes. The line chilldown tests performed in this study are designed to mimic the cryogenic transfer line chilldown by heating the tubes to around 250 °C and then allowing the PF-5060 liquid flow to cool down the tubes. The test module consists of the main heated section, and a bypass section to maintain the flow rate while the test section preheating is undergoing. The main heating section is 60 cm long with wall temperature measurements at top and bottom sides of the tubes placed at equal distances. A fibre-glass insulated Nichrome tape heater achieves the desired initial temperature conditions, and a three-way valve activates to divert flow from the bypass section to the main test section when all the wall thermocouples read 250 °C or above. The present experimental design captured the entire chilldown curve from film boiling to single phase liquid regions.

This study is the culmination of a long-term collaborative effort between researchers from the Purdue University Boiling and Two-Phase Flow Laboratory (PU-BTPFL) and the NASA Glenn Research Center to investigate gravitational effects on flow boiling and flow condensation. The science and design concepts for this large-scale effort were initiated in 2011 and included several studies detailing various aspects of two-phase fluid physics in both Earth gravity and microgravity, culminating in construction of the largescale experimental facility named "Flow Boiling and Condensation Experiment (FBCE)". The experiment was launched to the International Space Station (ISS) in August 2021. Following the successful installation of FBCE, equipped with the Flow Boiling Module (FBM), onboard the ISS and completion of several safety checks, flow boiling experiments were performed for five months from February 2022 until July 2022. This resulted in a large flow boiling database covering broad ranges of operating parameters and heating configurations spanning several research objectives. This study investigates microgravity flow boiling of n-perfluorohexane with subcooled inlet in a single-side-heated rectangular channel of dimensions 114.6mm heated length, 2.5-mm heated width, and 5.0-mm height. Key operating parameters investigated are mass velocity (199.90 - 3200.13 kg/m 2 s), inlet subcooling (0.10 - 45.76 degrees C), and inlet pressure (113.30 - 164.29 kPa). Images and image sequences acquired via high-speed-video are presented to elucidate the interfacial flow physics. To analyze and explain the effects of various parameters in microgravity, heat transfer results are presented as flow boiling curves, streamwise profiles of wall temperature and heat transfer coefficient, and parametric trends of local and averaged heat transfer coefficient. Mass velocity and inlet subcooling significantly influenced most of the aforementioned aspects of flow boiling, whereas effects of inlet pressure were comparatively insignificant. Although the data and observed flow physics might be different, the parametric effects and trends in microgravity are similar to vertical upflow in Earth gravity. Some cases, especially low mass velocities, high heat fluxes, and large degrees of inlet subcooling, experienced temporally anomalous flow behaviors caused by two-phase flow instabilities manifesting as flow reversals and resulted in deviations in overall trends. Severe thermodynamic non-equilibrium is observed throughout the channel. Overall, FBCE's ISS experiments were successful for subcooled inlet with single-sided heating of rectangular channel, and the collected data well established the various effects on flow boiling physics in highly controlled long-term microgravity conditions. (c) 2023 Elsevier Ltd. All rights reserved.

Cryogenic fluid management plays a major role in refueling of spacecrafts while in space for NASA’s future human space exploration missions. Due to the low boiling points of cryogens, storage, transport and handling of these fluids becomes difficult and may result in inefficient operation of the space propulsion systems. For refueling applications in space, the cryogenic fluids have to be transported across different locations and hence, the transfer of cryogenic fluids through pipes become critical. The cryogenic chill-down process is characterized by different regimes of flow boiling, viz., film boiling, transition boiling and nucleate boiling. The prediction of these regimes in a single CFD framework available in the literature is challenging and the present work attempts to address this challenge by initially modeling the film boiling regime accurately and to incorporate an user-defined function for transition and nucleate boiling at a later stage. Hence, the aim of the present work is to numerically model and validate the film boiling regime of the chilldown curve for liquid nitrogen experiments available in the literature. The validations are carried out at different inlet mass fluxes to have a robust simulation methodology. A dispersed mixture model is used to predict the vapor-liquid interface dynamics with the phase change phenomena modeled using the Lee model.

This study investigates critical heat flux (CHF) for subcooled flow boiling of n-Perfluorohexane based on results of pre-launch Earth-gravity Mission Sequence Tests (MSTs) of the Flow Boiling and Condensation Experiment (FBCE), which was launched to the International Space Station (ISS) in August 2021. CHF mea-surements were made in a rectangular channel having a 2.5 mm by 5 mm cross-section and a 114.6-mm long heated segment. Both single-sided and double-sided heating were tested in vertical upflow in Earth gravity for a variety of inlet conditions. The inlet subcooling was varied in the range of 0.4 - 32.0 degrees C and encompassed both near-saturated and highly subcooled conditions. Experimental trends and high-speed video records were investigated to better understand the mechanism of CHF. Overall trends show CHF increases as flow rate and/or inlet subcooling are increased. Flow features from the events around CHF justify the applicability of the Interfacial Lift-off Model and the determination of limiting criteria for its application. The present experimental data are combined with prior databases for various flow orienta-tions with respect to Earth gravity and microgravity data collected on parabolic flights. Predictions are made using the Interfacial Lift-off Model for this consolidated subcooled-inlet FBCE-CHF database. A heat utility ratio was included in the model to capture the effects of subcooling and corresponding thermo-dynamic non-equilibrium. An overall mean absolute error of 19.04% indicates good predictive capability of the model for both heating configurations, different gravity environments, and a wide range of inlet subcooling. (c) 2021 Elsevier Ltd. All rights reserved.

This study explores use of Computational Fluid Dynamics (CFD) to predict near-saturated flow boiling of FC-72 in microgravity. The computational method employs transient analysis to predict detailed interfacial behavior and heat transfer characteristics along a rectangular channel heated along two opposite walls. Predicted results are validated against experimental temperature measurements and high-speed video images captured during a series of parabolic aircraft maneuvers for three sets of operating conditions which include variations of both mass velocity and wall heat flux. The computational method is based on the multi-phase volume of fluid (VOF) model, which is combined with appropriate phase change and turbulence models, and accounts for both shear-lift force on bubbles and conjugate heat transfer along the heating walls. A key advantage of the CFD method is ability to capture details that are very difficult to measure experimentally, including detailed spatial variations of bubble shape, void fraction, mixture fluid temperature, liquid velocity, and vapor velocity, results for which are presented for each of the three test cases. Different flow regimes predicted along the heated length exhibit a number of dominant mechanisms including bubble nucleation, bubble growth, coalescence, vapor blankets, interfacial waviness, and residual liquid sub-layer, all of which agree well with experiment. Vapor velocity is shown to increase appreciably along the heated length because of increased void fraction, while liquid velocity experiences large fluctuations. Non-equilibrium effects are accentuated with increasing mass velocity, contributing minor deviations of fluid temperature from simulations compared to those predicted by the analytical method. Predicted wall temperature is fairly uniform in the middle of the heated length but increases in the entrance region, due to sensible heat transfer in the subcooled liquid, and decreases toward the exit, mostly because of flow acceleration resulting from increased void fraction.

This article is part of a series of studies culminating from the multi-objective Flow Boiling and Condensation Experiment (FBCE) onboard the International Space Station, which utilized the Flow Boiling Module (FBM) for experiments between February and July 2022. This study investigates microgravity flow boiling of n-Perfluorohexane with liquid-vapor mixture (two-phase) inlet conditions to the FBM with either one or two opposite walls heated. The FBM's channel has a rectangular cross-sectional area of 5.0 × 2.5 mm2 and a heated length of 114.6 mm. Key parameters of interest include mass velocity (180 – 2400 kg/m2s), inlet quality (-0.01 – 0.87), inlet pressure (120 – 200 kPa), and heat flux (1.8 W/cm2 to critical heat flux), and a large database is amassed. The flow is visualized via a high-speed video camera and photographs are recorded at each heating increment to assess the periodic flow patterns within the channel and the near-wall interfacial behavior. Flow patterns are complex and mainly characterized by high- and low-density fronts alternately traversing the channel to yield high- and low-density-dominant periods of boiling. At all operating conditions, high-density fronts are faster during high-density-dominant periods. At low inlet qualities, the flow is annular near the channel inlet with a central vapor core surrounded by an annular liquid layer. Each high-density front having a high liquid fraction leaves a thin liquid layer sheared onto the heated walls. Boiling occurs within the liquid layer and a vapor layer is formed next to the heated wall. Inlet quality and mass velocity most dictate the overall flow patterns followed by heating configuration, and to a much lesser extent, heat flux and inlet pressure. Heat transfer characteristics are assessed via averaged boiling curves, streamwise profiles of wall temperature and heat transfer coefficient, and parametric curves of local and averaged heat transfer coefficients. Inlet pressure has an insignificant effect on heat transfer. At similar operating conditions, both the heating configurations yield similar trends and values of heat transfer coefficient and critical heat flux (CHF, slightly higher for single-sided) even though double-sided heating adds twice the heat to the fluid and doubly raises local quality. The heat fluxes required for both onset of nucleate boiling degradation and CHF are larger at high mass velocities and low inlet qualities. For a fixed inlet quality, high mass velocities yield higher average heat transfer coefficients at both lower and higher heat fluxes, while the nucleate boiling regime at intermediate heat fluxes is unaffected. For a fixed mass velocity, higher inlet qualities yield higher and lower average heat transfer coefficients at lower and higher heat fluxes, respectively.

Since 2012, researchers at the Purdue University Boiling and Two-Phase Flow Laboratory (PU-BTPFL) and NASA Glenn Research Center have been collaborating on a long-term effort to study flow boiling and condensation in microgravity. The ultimate goal has been to develop the Flow Boiling and Condensation Experiment (FBCE) for the International Space Station (ISS). Based on the findings from prior flow boil-ing experiments both at different orientations in Earth gravity and onboard parabolic flights simulating short durations of microgravity, a final refined experiment design, construction, and operating procedure have been arrived at for long-duration microgravity flow boiling experiments onboard the ISS. This study investigates flow boiling of n-Perfluorohexane with subcooled inlet in a rectangular channel of dimen-sions 114.6 mm heated length, 2.5 mm width, and 5 mm height. These pre-launch experiments (Mission Sequence Testing) were conducted in vertical upflow orientation in Earth gravity using the same exper-imental rig that was launched to the ISS in August 2021. The various operating parameters varied are heating configuration (single-and double-sided), mass velocity (180 - 3200 kg/m(2)s), inlet subcooling ( + 0 - 32 degrees C, encompassing both highly subcooled and near-saturated inlet conditions), and inlet pressure (119 - 191 kPa). High-speed video flow visualization images are presented to explain the two-phase in-terfacial physics within the channel's heated section. Heat transfer results in terms of flow boiling curves, streamwise profiles of wall temperature and heat transfer coefficient, and averaged heat transfer coef-ficients are analyzed and parametric effects elucidated. Severe temporal thermodynamic equilibrium is observed for near-saturated inlet at very low velocities. Nucleate boiling degradation starts at larger heat fluxes for single-sided heating than double-sided at low mass velocities with highly subcooled inlet, and conversely at high mass velocities with near-saturated inlet. Nucleate boiling degradation can be de-layed to higher heat fluxes by highly subcooling the inlet and increasing mass velocity. The entire local heat transfer coefficient profiles are degraded at higher heat fluxes for near-saturated inlet, but only the downstream part for highly subcooled inlet. This study also confirmed reliability of the upcoming ISS experimental data for subcooled inlet conditions and the collected Earth-gravity data will be used for comparison against the ISS data. (c) 2022 Elsevier Ltd. All rights reserved.

While many prior works in the field relied upon direct optical access to determine condensation flow regimes, the present work outlines a new methodology utilizing temperature and pressure measurements to identify condensation flow regimes. For vertical upflow condensation, amplitude of dynamic temperature and pressure oscillations are shown to clearly indicate transition from counter-current flow regimes (i.e., falling film, oscillating film, flooding) to annular, co-current flow (climbing film flow regime). In horizontal flow condensation, standard deviation between multiple thermocouple measurements distributed around the tube circumference was calculated at all axial (stream-wise) measurement locations. High values of standard deviation are present for stratified flow (stratified flow, wavy-stratified, plug flow), while axisymmetric flow regimes (i.e., slug flow, annular flow) yield significantly lower values. Successful development of this technique represents a valuable contribution to literature as it allows condensation flow regime to be identified without the often-costly restriction of designing a test section to allow optical access. Identified flow regimes in both vertical upflow and horizontal flow orientations are compared to regime maps commonly found in the literature in pursuit of optimum performing maps. (C) 2019 Elsevier Ltd. All rights reserved.