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.

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%.

This study is part of the Flow Boiling and Condensation Experiment (FBCE) and utilizes flow boiling data collected in both microgravity onboard the International Space Station (ISS) and Earth gravity at different channel orientations. The goal is to develop a prediction technique for heat transfer and critical heat flux (CHF) for flow boiling in both microgravity and Earth gravity using artificial neural networks (ANNs). The working fluid, n-perfluorohexane or FC-72, flows through a rectangular channel of 114.6 mm heated length, 2.5 mm heated width, and 5.0 mm unheated height with either one or two walls heated. The consolidated FBCE database for heat transfer coefficient comprises 29,226 datapoints spanning a mass velocity of 173 - 3200 kg/m2s, pressure of 102 - 238 kPa, subcooling of 0 - 44 degrees C, and thermodynamic equilibrium quality of -0.60 - 0.95 (spanning from highly subcooled to high-quality saturated boiling). Following a statistical analysis of various input parameters relevant to flow boiling and optimization of key model parameters, a fully connected feedforward ANN is developed to predict Nutp. It predicts the entire test database with an overall mean absolute error (MAE) of just 7.99% with consistent and accurate predictions in each subset. Similarly, 641 CHF datapoints from FBCE were consolidated into a database spanning a mass velocity of 99 - 3212 kg/m2s, inlet pressure of 97 - 239 kPa, inlet subcooling of 0 - 46 degrees C, inlet thermodynamic equilibrium quality of -0.61 - 0.86, and CHF values of 4 - 54 W/cm2. A separate ANN is developed by following the same methodology as heat transfer, and it predicts dimensionless CHF, BoCHF, with an overall MAE of just 12.05%. Existing seminal correlations are assessed for subsets of the two consolidated FBCE databases, and the ANNs are shown to have better accuracies in each subset of the database. The ANNs' high prediction accuracy, in conjunction with their ability to predict physical parametric trends in previously unseen data, shows their potential as prediction tools for both heat transfer and CHF for flow boiling in microgravity and Earth gravity.

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).

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.

This study examines the critical heat flux data obtained during the "Flow Boiling and Condensation Ex-periment (FBCE)" which was launched to the International Space Station (ISS) in August 2021. The overar-ching goals of FBCE are to obtain flow boiling and flow condensation data in high quality, long-duration microgravity, and investigate gravitational effects on two-phase flow physics. The first stage of FBCE com-pleted features the Flow Boiling Module (FBM), which collected flow boiling heat transfer data and flow visualization images in a highly accurate and steady microgravity environment from February 2022 un-til July 2022. Flow boiling experiments were performed with subcooled inlet of n-Perfluorohexane in a rectangular channel of dimensions 114.6-mm heated length, 2.5-mm heated width, and 5.0-mm height with either one or two, opposite, heated walls. The long-duration microgravity database encompasses a broad range of operating conditions: mass velocity of 199 - 3200 kg/m2s, inlet subcooling of 2.6 - 45.6 & DEG;C, and inlet pressure of 124.8 - 176.7 kPa. Image sequences leading up to and at CHF are presented to both illustrate the physical mechanism triggering CHF and explain the experimental trends observed in mi-crogravity. Experimental results show CHF is strongly dependent on mass velocity and higher degrees of inlet subcooling, but less dependent on inlet pressure in the tested ranges. Examination of the relation-ships and parametric trends between dimensionless groups governing CHF reinforces conventional trends and reveals, for subcooled CHF, a dependence of Boiling number at CHF on outlet thermodynamic equi-librium quality. Comparison of the new microgravity CHF data with Earth-gravity CHF data reveals, for single-sided heating, CHF in microgravity is degraded up to & SIM;38% at low mass velocity, with diminishing differences as mass velocity is increased, and for double-sided heating, less significant differences in CHF between the two gravitational environments. Experimental data are compared to predictions of various flow boiling CHF correlations which previously demonstrated their merit, and the most suitable one for the entire database is recommended. Flow visualization reveals a wavy vapor layer with wetting fronts described in the Interfacial Lift-off Model , which is used to predict CHF values with good accuracy.& COPY; 2023 Elsevier Ltd. All rights reserved.

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.

Much of published literature addressing flow instabilities in thermal management systems employing micro-channel modules has focused on the instability characteristics of the module alone, and far fewer studies have aimed at understanding the relationship between these characteristics and the compressible volume in the flow loop external to the module. From a practical point of view, developers of micro -channel thermal management systems for many modern applications are in pursuit of practical remedies that would greatly mitigate instabilities and their impact on cooling performance. The present study ex-perimentally examines the effects of compressible volume location in a closed pump-driven flow loop designed to deliver FC-72 to a micro-channel test module having 38 channels with 315-mu m hydraulic di-ameter. Three accumulator locations are investigated: upstream of the test module, downstream of the test module, and between the condenser and the pump. Both high-frequency temporal parameter data and high-speed video records are analyzed for ranges of mass velocity and heat flux, with inlet subcooling held constant at similar to 14.5 degrees C. Pressure Drop Oscillation (PDO) is shown to dominate when the accumulator is situated upstream, whereas Parallel Channel Instability (PCI) is dominant for the other two locations. PDO shows severe pressure oscillations across the micro-channel heat sink, with rapid bubble growth and confinement, elongated bubble expansion in both directions, flow stagnation, and flow reversal (includ-ing vapor backflow to the inlet plenum) constituting the principal sequence of events characterizing the instability. Spectral analysis of pressure signals is performed using Fast Fourier Transform, which shows PDO extending the inlet pressure fluctuations with the same dominant frequency to other upstream flow loop components, with higher amplitudes closer to the pump exit. From a practical system operation point of view, throttling the flow upstream of the heat sink effectively eliminates PDO but renders PCI dominant, and placing the accumulator in the liquid flow segment of the loop between the condenser and the pump ensures the most stable operation. (c) 2022 Elsevier Ltd. All rights reserved.

This study investigates critical heat flux (CHF) of n-Perfluorohexane flowing in a partially heated rectan-gular channel of 2.5-mm x 5.0-mm cross-section, within NASA's Flow Boiling and Condensation Exper-iment's (FBCE) Flow Boiling Module (FBM). A consolidated FBCE-CHF database is formed by compiling datasets from prior years of testing the FBM both at different orientations in Earth gravity (horizontal flow, vertical upflow, and vertical downflow) and on parabolic flights until it was launched to the In-ternational Space Station (ISS) in August 2021. This database encompasses a wide range of operating conditions (both subcooled liquid inlet of different inlet subcoolings and saturated two-phase inlet of different inlet qualities at different mass velocities and system pressures), heating configurations (single -and double-sided inlet), and different gravitational environments. The database is further categorized into three based on the type of CHF: subcooled CHF, saturated CHF with single-phase inlet, and saturated CHF with two-phase inlet. An exhaustive literature search is conducted to identify almost all flow boiling CHF correlations, which are then utilized to make predictions of the database and their accuracies assessed for each small subset of the database. Some correlations are capable of providing adequate CHF predic-tions for large portions of the database, while some provide very good predictions for very small subsets of operating conditions, typically those for which they were developed for. No single existing correlation is capable of predicting the entire database with good accuracy. Many correlations do not consider the effects of gravity on CHF, which is important for the different orientations tested, and even the few that do, are unsuccessful in predicting the microgravity data. A new simple CHF correlation is developed to address the drawbacks of the existing ones and is easy to use. This new correlation predicts the entire consolidated database with a mean absolute error of 17.44% with good accuracies for each subset of the database.(c) 2022 Elsevier Ltd. All rights reserved.

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.

This study explores subcooled flow boiling of n-Perfluorohexane in a rectangular channel of 5.0 mm height, 2.5 mm width (heated), and 114.6 mm heated length. This Flow Boiling Module (FBM) is part of the Flow Boiling and Condensation Experiment (FBCE), which is a long-term collaborative effort to study the effects of gravity on flow boiling and condensation for their implementation in future space missions besides other applications. Datasets obtained from subcooled-inlet experiments performed using the FBM prior to its launch to the International Space Station (ISS) are examined and local subcooled flow boiling datapoints compiled to form a consolidated database. The consolidated 2589 datapoints cover broad ranges of operating conditions (mass velocity: 172.79 - 3200.00 kg/m(2)s, pressure: 102.16 - 238.44 kPa, subcooling: 0.13 - 34.93 degrees C, quality:-0.560 - 0.000, heat flux: 1.80 - 49.99 W/cm(2)), flow orientations (vertical upflow, vertical downflow, and horizontal flow in Earth gravity), and partial heating con -figurations (single-and double-sided heating). Prior seminal correlations for subcooled flow boiling heat transfer are assessed for their predictive performance. Each local flow boiling curve is analyzed and manually demarcated into three regimes: partially developed boiling (PDB), fully developed boiling (FDB), and nucleate boiling degradation (NBD). A simple, yet very effective correlation based on dimensionless groups and having a fully explicit functional form, is developed. The new correlation well predicts both the subcooled flow PDB and FDB regimes with overall mean absolute errors (MAEs) of 9.59% and 6.91%, respectively. Although predictions for the NBD regime are much higher than observed with an overall MAE of 41.71%, they can be treated as upper limits to heat transfer here. The correlation is independent of both flow orientation and heating configuration. Overall, the new correlation clearly outperforms all prior seminal correlations for the entire consolidated database, with excellent predictions for both partially and fully developed subcooled flow boiling, i.e., for heat fluxes ranging from onset of nucleate boiling to onset of nucleate boiling degradation. (C) 2022 Elsevier Ltd. All rights reserved.

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.

Transportation industry is presently in fast track to transition from Internal Combustion Engine Vehicles (ICEVs) to Electrical Vehicles (EVs). One of the most pressing challenges to full adoption of EVs is very slow charging at the networks of charging stations proposed worldwide. Despite many recent so-called ‘ultra-fast’ charging methods, which capitalize on a variety of single-phase liquid schemes to cool the charging cable, thermal constraints limit the electrical current carrying capacity of the fastest commercial chargers to about 500 A. Achieving the faster charging time required for the anticipated proliferation of EVs will require increasing this current capacity to at least 2000 A, which poses formidable thermal challenges in design of the charging cable. This study explores the development of a vastly more powerful charging cable thermal management scheme to achieve this higher current threshold. Subcooled flow boiling is proposed as the primary means to dissipating the larger amounts of heat generated at higher currents. Experiments are performed by pumping highly subcooled dielectric liquid HFE-7100 though a concentric circular annulus mimicking a segment of an actual cable, with a uniformly heated 6.35-mm-diameter inner surface representing the electrical conductor and adiabatic 23.62-mm-diameter outer surface the external conduit. All experimental cases considered are conFig.d to ensure subcooled fluid conditions throughout the test module. It is shown the proposed cooling scheme is capable of tackling currents up to 2438 A, around four times higher than the present-day commercial maximum. With appropriate batteries and other ancillary components, this technology is expected to bring EV charging times down to less than 5 minutes. Aside from demonstrating this potential, an assessment of available subcooled boiling heat transfer coefficient correlations identified Moles and Shaw's to predict the new experimental data with an overall mean absolute error of only 11.68%. The flow and heat transfer physics are also explained in detail.

This study examines the advantages and disadvantages of micro- and macro-channel flow boiling for high-heat-flux cooling applications using both computational and theoretical/empirical methods. The computational simulations are conducted in ANSYS FLUENT using the Volume of Fluid (VOF) method along with the Lee phase change model, and accounting for both shear lift force and conjugate heat transfer along the channel walls. Computational results for both channel sizes are compared with theoretical/empirical results obtained using the Homogeneous Equilibrium Model (HEM) and Separated Flow Model (SFM), and both HEM and the Homogenous Frozen Model (HFM) are used to assess the potential for two-phase choking. The computational results show bubbles in micro-channels are highly confined and tend to grow longer in the flow direction. The two methods show good agreement in predicting wall temperatures. Overall, micro-channel heat sinks are shown to fare much better than macro-channels in terms of heat transfer performance, evidenced by both significantly higher heat transfer coefficients and lower wall temperatures, but this comes at the cost of significantly higher pressure drop and pumping power requirements. It is also shown micro-channels are prone to choking due to high two-phase Mach number.

An experimental investigation was conducted to study the effect of relative waviness (amplitude-to-wavelength ratio) on heat transfer and pressure drop characteristics of wavy minichannel array heat sinks. Hydrodynamically fully developed flows of de-ionized (DI) water, and 0.5% and 0.8% concentrations of Al2O3/water nanofluid were introduced into two heat sinks, each with an array of 15 rectangular wavy minichannels of amplitude-to-wavelength ratio of 0.100 and 0.133. The minichannels had a width of 0.9 mm and a depth of 1.3 mm and were machined on a 30 × 30 mm2 Aluminum substrate of 11 mm thickness. Reynolds number was varied from 700 to 2300 and a constant heat flux of 45,000 W/m2 was applied. Analysis of experimental results suggests increases in both Nusselt number and pressure drop with an increase in relative waviness in the laminar and transitional flow regimes. A maximum performance factor (PF) of 2.6 is achieved for a Reynolds number of 1900 using DI water in the wavy minichannels of higher relative waviness. Temperature distributions and new Nusselt number correlations for both geometries are also presented.

This study involves experimental investigation of key parameters influencing CHF for confined round single jets and jet arrays impinging normally onto square heated surfaces. The experiments are performed using R-134a, a fluid widely used for thermal management of electronic and power devices, especially in aerospace applications. A comprehensive R-134a CHF database is acquired that considers the effects of various geometrical parameters and operating conditions. Close examination of the data trends reveals several strategies to augment CHF, such as increasing jet velocity and/or total mass flow rate and employing larger jet diameters for a fixed velocity or smaller diameters for a fixed flow rate. Higher CHF is also achieved by increasing saturation pressure for a fixed inlet fluid temperature (i.e., higher saturation pressures combined with higher inlet subcooling). Fluid exit qualities point to two different CHF mechanisms: subcooled CHF at high flow rates and saturated CHF at low flow rates. Underlying mechanisms are also propounded for two types of CHF transients: a sudden sharp temperature escalation at lower flow rates and a mild gradual increase at higher flow rates. Close inspection of the heating surface following CHF tests shows localized burnout patterns which provide significant insight into both the flow characteristics within the confinement region and the spatial distribution of surface temperature resulting from jet interactions. Statistical inference techniques are used in conjunction with the new understanding of fluid flow and heat transfer physics to formulate a new correlation form for CHF. The resulting correlation, which is based on a consolidated database of the present R-134a and previous FC-72 data, shows good prediction accuracy, evidenced by a mean absolute error of 16.66% for both fluids and over broad ranges of geometrical parameters and operating conditions. (C) 2020 Elsevier Ltd. All rights reserved.

Jet impingement boiling is a popular thermal management technique that caters to applications demanding very high heat dissipation rates. Like other boiling schemes, critical heat flux (CHF) is arguably the most important safety parameter for two-phase jet cooling, and determining CHF is often the starting point in a system's thermal design process. This article presents a systematized review of articles addressing jet impingement CHF. A very comprehensive search for studies is conducted, which includes research spanning over five decades and jets of various fluids, operating conditions, and geometrical configurations. Comprehensive lists of experimental jet CHF studies are given with detailed information on fluid, geometrical parameters, operating conditions, and notable conclusions. Parametric effects on CHF are also discussed in a systematic manner. Various techniques for increasing CHF, such as surface modification (including surface curvature, extended surface structures, surface coatings, or combinations thereof), specialized spent fluid removal schemes, and nanofluids, are discussed. An exhaustive list of CHF correlations is provided, along with the development rationale and applicability range for each. Also discussed are CHF trends for hybrid cooling schemes which combine jet impingement with other boiling schemes such as channel flow. The review is concluded with major conclusions and recommendations for future work. (C) 2021 Elsevier Ltd. All rights reserved.

•This study explores two-phase heat transfer characteristics for large length-to-diameter ratio micro-channels.•Transient flow patterns, instabilities, and dryout effects are examined using R-134a as working fluid.•Observations are summarized using a flow regime map and a dryout map.•Predictive tools are proposed for both subcooled and saturated boiling regions.

An experimental investigation was conducted to study the heat transfer and pressure drop characteristics of an array of wavy divergent minichannels and the results were compared with wavy minichannels with constant cross-section. The experiment was conducted in hydro dynamically developed and thermally developing laminar and transient regimes. The minichannel heat sink array consisted of 15 rectangular channels machined on a 30 × 30 mm2 and 11 mm thick Aluminium substrate. Each minichannel was of 0.9 mm width, 1.8 mm pitch and the depth was varied from 1.3 mm at entrance to 3.3 mm at exit for the divergent channels. DI water and 0.5 and 0.8 % concentrations of Al2O3/water nanofluid were used as working fluids. The Reynolds number was varied from 700 to 3300 and the heat flux was maintained at 45 kW/m2. The heat transfer and pressure drop of these minichannels were analyzed based on the experimental results obtained. It was observed that the heat transfer performance of divergent wavy minichannels was 9 % higher and the pressure drop was 30–38 % lesser than that of the wavy minichannels with constant cross-section, in the laminar regime. Hence, divergent channel flows can be considered one of the better ways to reduce pressure drop. The performance factor of divergent wavy minichannels was 115–126 % for water and 110–113 % for nanofluids.

An experimental investigation on the heat transfer performance and pressure drop characteristics of thermally developing and hydrodynamically developed laminar flow of deionized (DI) water and 0.1%, 0.5%, and 0.8% concentrations of Al2O3/water nanofluid in wavy and straight minichannels was conducted. Reynolds number was varied from 700 to 1900 and two different heat fluxes of 45 kW/m(2) and 65 kW/m(2) were applied. The performance factor (PF) of water in wavy minichannels over their straight counterparts was higher than the nanofluids. Temperature distributions and general correlations of these minichannels are also presented.