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AbstractThis study experimentally investigated the flow boiling critical heat flux (CHF) under heaving conditions pertinent to floating nuclear power plants (FNPPs). Experiments were conducted using R134a to simulate the operational pressure of pressurized water reactors, extending the CHF database to a pressure range of 5 to 18 MPa. The tested heaving motion reached up to 0.6 g of maximum acceleration, with a period that varied from 3 to 5.3 s. A parametric trend involving mass flux, critical quality, and motion conditions was analyzed through extensive tests encompassing static and heaving conditions.The results showed that CHF reduction was observed due to the effect of heaving acceleration, even in the absence of oscillations in other thermal-hydraulic factors. The CHF reduction was up to 9% in the present experimental conditions. The heaving motion effect was prominent under two specific thermal-hydraulic conditions: when the critical quality was close to zero and when it exceeded the value of 0.6. Furthermore, an examination of wall temperature responses suggested that a longer period of heaving motion can lead to an earlier occurrence of CHF. This fundamental effort was conducted to enhance the understanding of the CHF mechanism under motion conditions, with the aim of contributing to the safe and efficient design of FNPPs. [ABSTRACT FROM AUTHOR]

Understanding the nanofluid flow boiling phenomena is essential for many engineering applications including atomic reactors, cooling of electronic devices, synthetic buildings and spacecraft. Hybrid nanofluids, which are commonly defined as mixtures of two or more nanoparticles, are yet unknown to have improved features. When the coupled nanoparticles are suitably coated with one another, one on top of the other, this would be a successful technique as the usage of such nanofluids is hardly reported. The current research aims to investigate the effects of various novel hybrid ZnO coated Cu (Cu@ZnO) nanofluid concentrations on the critical heat flux (CHF) and heat transfer coefficient (HTC) for their potential real-life application. The Cu@ZnO nanoparticles with hybrid properties were selected because Cu increased the conductivity of heat whereas ZnO improved chemical durability. Efficient spark discharge in liquid nitrogen was employed in three stages to produce robust Cu@ZnO nanoparticles with hybrid properties. In order to create durable nanofluids, the nanoparticles with mixed properties were dispersed using an ultrasonication process at four different volumetric concentrations: 0.025%, 0.050%, 0.075% and 0.1%. At 45°C, the nanofluids' thermal conduction was 31% higher than that of base fluid. The flow boiling (subcooled) tests are carried out in minichannel with novel nanofluids. When the flow boiling investigation was being conducted, a greater concentration improved the HTC and CHF. In comparison to water, 0.1% of nanofluid caused the maximum rises in CHF and HTC, which were found to be 168.09% and 186.9%, respectively. It is possible for the fluid to utilize latent heat, create bubbles, distinct and delay the commencement of CHF because the thick Cu@ZnO-coated surface possesses strong thermal conductivity. [ABSTRACT FROM AUTHOR]

Reactor fuel assemblies may be biased and bowed as a whole due to factors such as manufacturing and installation deviations or external forces. This consequence may have an impact on heat transfer and criticality, and even the possibility of edge-rod criticality. In order to evaluate the impact size, a set of high-quality dynamic mesh generation technology was developed through CFD. The dynamic mesh technology was used for the base condition to make the overall displacement or bowing of the fuel rods. Using the Eulerian two-fluid model and the improved RPI wall boiling model, the CHF of the 5×5 rod bundle channel with different offset distances and bowing degrees was predicted, and the CFD prediction results were verified with the CHF look-up table value. The results show that the CHF predicted by the non-deformation and small deformation conditions are in good agreement with the values of the sub-channel parameter CHF look-up table, while the large deformation condition triggers the side rods critical and CHF look-up table is not applicable. Because the CHF table is made based on an 8 mm round tube, the non-8 mm round tube or rod bundle structure (such as the central sub-channel) can be corrected by the hydraulic diameter correction factor, and the prediction accuracy is often high. However, at the edge and corner channel position of the rod bundle assembly, the heated boundary and wet circumference are not uniform, and the offset or bowing of the fuel rods squeezes the corner channel, resulting in a more unbalanced distribution of the heated boundary and wet circumference at this location. As a result, the CHF look-up table is not applicable. The critical occurrence position and numerical value under different deformation conditions were compared, the distribution of mass fluxes in different sub-channels at the critical height was also compared, and the sensitivity of various deformation types to CHF was summarized in this paper. For the calculation results of the undeformed fuel assembly, the flow rate of the central sub-channel is the highest, the side channel is the second, and the corner channel is the smallest. For the deformed fuel assembly, the offset and bowing change the flow distribution of the sub-channels, the flow of the corner channels decreases, and the flow of the central sub-channel increases. When the gap of the corner channel is so small that the coolant flow cannot effectively take away the bubbles on the surface of the fuel rods, causing local congestion of the bubbles, the side rods will be critical. The XY-type condition has the strongest influence on the corner channel flow, but the weakest effect on the overall flow distribution, and the C-type bowing condition has the strongest influence on the overall flow distribution. In the calculation condition, the axial power is uniformly biased and bowed by 0.5 mm without changing the critical axial position, while bowing by 0.8 mm and 1 mm affects the critical position. The larger the three deformation degrees, the smaller the predicted CHF value and the increase of the critical probability of the side rods. Among them, the XY-type deformation is the least sensitive to the CHF value, while the C-type bowing with a cosine axial heat flux has the greatest impact on the CHF value. Compared with the same degree of bowing, the critical heat flux density value of axial power cosine is smaller. When bowing 0.5 mm, the critical position of the axial power uniform condition occurs at the center, while the critical position of the axial power cosine condition occurs at the edge. That is, the cosine power may also be the cause of the criticality of the side rods.

There are several industrial applications where boiling is used, for example boilers, refrigeration systems, nuclear reactor cooling, and microelectronic chip cooling. Experimental research has been carried out to determine the flow boiling heat transfer capabilities of copper-alumina-coated surfaces for application in heat transfer equipment. De-ionized (DI) water is used as the coolant for experimentations in a minichannel with dimensions 10 × 1.5 × 10 mm. Copper surfaces coated with thin copper-alumina nanocomposite films are created using the electrodeposition process. The coated layer created using an electrochemical technique offers strong adhesiveness with the base copper and is therefore anticipated to be suitable for real-world heat transfer appliances as part of the ongoing scientific development in subcooled flow boiling. The electrochemical technique offers easier control over its various parameters, such as current density, duration and electrolyte composition, making it possible to easily achieve a variety of surface characteristics, such as crystallinity, wettability and porosity. as required in the coated surfaces. Additionally, the copper-alumina is a hydrothermally stable oxide material that is well suited for use in boiling heat transfer devices. The boiling (subcooled flow) heat transfer tests are carried out at various mass flows. The improvement in the two-phase heat transfer coefficient (HTC) and critical heat flux (CHF) can reach up to 90 % and 93 %, respectively. The coated surfaces have improved CHF and HTC because of improved wettability, increased surface roughness, and the existence of active nucleate sites in high-density. [ABSTRACT FROM AUTHOR]

The current study investigates the distribution of local heat transfer coefficient, measurement of pressure loss and CHF in a lobed horizontal channel for flow boiling. Lobed channels with different lobed depths of 2.3, 2.0 and 1.25 mm, each having three different channel lengths (400, 600, and 1000 mm), are used for the experiments. The working substance used is R-123 with different mass flux. The heat flux range used is from 14 to 375 kW/m2. Pressure drop, critical heat flux, and local heat transfer coefficients of the lobed channel are compared with corresponding smooth horizontal tubes. [ABSTRACT FROM AUTHOR]

Copyright of Atomic Energy Science & Technology is the property of Editorial Board of Atomic Energy Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Critical heat flux (CHF) is an essential parameter that plays a significant role in ensuring the safety and economic efficiency of nuclear power facilities. It imposes design and operational restrictions on nuclear power plants due to safety concerns. Therefore, accurate prediction of CHF using a hybrid framework can assist researchers in optimizing system performance, mitigating risk of equipment failure, and enhancing safety measures. Despite the existence of numerous prediction methods, there remains a lack of agreement regarding the underlying mechanism that gives rise to CHF. Hence, developing a precise and reliable CHF model is a crucial and challenging task. In this study, we proposed a hybrid model based on an artificial neural network (ANN) to improve the prediction accuracy of CHF. Our model leverages the available knowledge from a lookup table (LUT) and then employs ANN to further reduce the gap between actual and predicted outcomes. To develop and assess the accuracy of our model, we compiled a dataset of around 5877 data points from various sources in the literature. This dataset encompasses a diverse range of operating parameters for two-phase flow in vertical tubes. The results of this study demonstrate that the proposed hybrid model performs better than standalone machine learning models such as ANN, random forest, support vector machine, and data-driven lookup tables, with a relative root-mean-square error (rRMSE) of only 9.3%. We also evaluated the performance of the proposed hybrid model using holdout and cross-validation techniques, which demonstrated its robustness. Moreover, the proposed approach offers valuable insights into the significance of various input parameters in predicting CHF. Our proposed system can be utilized as a real-time monitoring tool for predicting extreme conditions in nuclear reactors, ensuring their safe and efficient operation. [ABSTRACT FROM AUTHOR]

This work aimed to study the intensification of flow boiling heat transfer and critical heat flux (CHF) under conditions of highly reduced pressures due to a modification of the inner wall surface of a mini-channel. Such research is relevant to the growing need of high-tech industries in the development of compact and energy-efficient heat exchange devices. We present experimental results of the surface modification effect on hydrodynamics and flow boiling heat transfer, including data on the CHF. A description of the experimental stand and method for modifying the test mini-channel is also presented. The studies were carried out with freon R-125 in a vertical mini-channel with a diameter of 1.1 mm and a length of 50 mm, in the range of mass flow rates from G = 200 to 1400 kg/(m2s) and reduced pressures between pr = p/pcr = 0.43 and 0.56. The maximum surface modification effect was achieved at a reduced pressure of pr = 0.43, the heat transfer coefficient increased up to 110%, and the CHF increased up to 22%. [ABSTRACT FROM AUTHOR]

To improve the heat transfer efficiency of the reactor fuel assembly, it is necessary to accurately calculate the two-phase flow boiling characteristics and the critical heat flux (CHF) in the fuel assembly. In this paper, a Eulerian two-fluid model combined with the extended wall boiling model was used to numerically simulate the 5 × 5 fuel rod bundle with spacer grids (four sets of mixing vane grids and four sets of simple support grids without mixing vanes). We calculated and analyzed 11 experimental conditions under different pressure, inlet temperature, and mass flux. After comparing the CHF and the location of departure from the nucleate boiling obtained by the numerical simulation with the experimental results, we confirmed the reliability of computational fluid dynamic analysis for the prediction of the CHF of the rod bundle and the boiling characteristics of the two-phase flow. Subsequently, we analyzed the influence of the spacer grid and mixing vanes on the void fraction, liquid temperature, and secondary flow distribution. The research in this article provides theoretical support for the design of fuel assemblies.

• Experimental CHF data for microgravity flow boiling with saturated two-phase inlet. • Flow patterns reveal high-density fronts rewet the wall, facilitating boiling. • CHF for flow boiling in microgravity is similar to vertical upflow in Earth gravity. • The Interfacial Lift-off Model predicts this CHF database well with 29.2 % MAE. • CHF correlation proposed by present authors is accurate at predicting with 22.4 % MAE. This study examines data collected as part of the "Flow Boiling and Condensation Experiment" (FBCE), which collected microgravity flow boiling data onboard the International Space Station (ISS) between February 2022 and July 2022. This study focusses on detailed analysis of critical heat flux (CHF) data for microgravity flow boiling experiments with two-phase mixture inlet, which is unavailable in the literature. n-Perfluorohexane is used in a rectangular channel with a heated length of 114.6 mm, heated width of 2.5 mm, and adiabatic height of 5.0 mm with either single- or double-sided heating. The database covers parametric ranges never before studied in long term microgravity: mass velocity of 249.8 – 1999.9 kg/m2s, inlet thermodynamic equilibrium quality of 0.02 – 0.86, and inlet pressure of 120.4 – 200.4 kPa. Image sequences recorded surrounding CHF reveal the periodic passing of a relatively high concentration of liquid, termed high-density fronts, plays a key role in rewetting the wall and facilitating boiling. Trends show CHF is weakly affected by inlet pressure and mass velocity at high mass velocity, and, at low mass velocity, strongly affected by both inlet quality and mass velocity. Upon comparing the new microgravity CHF data with prior Earth-gravity vertical-upflow CHF data, the relatively weak influence of gravity on CHF during flow boiling with two-phase inlet, contrary to subcooled inlet, is established. Intricate observations of flow features suggest a wavy liquid-vapor interface with a central vapor core and boiling within the liquid sub-layer is the primary mechanism of CHF, and the Interfacial Lift-off Model for flow boiling CHF well predicts the present unique database evidenced by 29.2 % mean absolute error. The predictive capability of select prior correlations for flow boiling CHF is assessed for the present operating conditions, and one previously proposed by the present authors performed the best with an overall 22.4 % mean absolute error, suggesting its applicability for this unique data. [ABSTRACT FROM AUTHOR]

• Autonomous vision tool is employed for capturing bubble behaviors in flow boiling. • Novel approach to feeding vision tool data into modeling frameworks investigated. • Void fraction models and correlations are validated by actual experimental data. • Heat transfer correlations are improved by including behaviors of wetting fronts. • Critical heat flux models have empiricism reduced with data input from the vision tool. Flow boiling is a highly efficient configuration for meeting the high heat dissipation demands of thermal management systems. However, the complex physics of two-phase flow has hindered its broader application, especially in terms of quantifying visual information. Recent advancements in machine learning vision tools have revolutionized the analysis of phase change phenomena by enabling the digitalization of physically meaningful features such as void fraction, vapor-liquid interfacial behaviors, and liquid-solid wall wetting front areas en masse. In this study, we systematically investigate two-phase models that compute void fractions, heat transfer coefficients, and critical heat flux using live bubble data streams under microgravity. The collected empirical bubble data is used to supplement and validate traditional control-volume-based theoretical modeling approaches. Void fraction data is first validated with analytical frameworks. This is followed by void fractions and wetting front areas being used to improve correlations predicting heat transfer coefficients. This work showcases the potential of using a new machine learning-based strategy to accelerate scientific formula discovery through the extraction of multi-level and physically meaningful features. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, experiments of downward-facing flow boiling were conducted to investigate material effects on CHF. Experiments were conducted using aluminum, copper, and carbon steel. It was found that different materials had different CHFs. Aluminum has the biggest CHF while copper has the lowest CHF for each mass flux. After experiment, surface wettability increased and surface became rougher, which was probably due to the oxidation process during nucleate boiling. The CHF difference is likely to be related to the surface wettability, roughness and thermal effusivity, which influences the bubble behavior and in turn affects CHF. Further studies are needed to determine which factor is dominant.

Copyright of Experimental Technology & Management is the property of Experimental Technology & Management Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• Two different mechanisms of triggering CHF in micro/mini-channels are revealed. • A simple model is developed to validate a CHF mechanism Ⅰ: liquid layer dry-out. • The helmholtz instability theory can predict well the subcooled boiling data. • A CHF database is collected to verify the previous and new proposed model. A new flow pattern map has been developed and presented in Part Ⅰ of this paper for R134a, HFE-7100 and Deionized water inside micro/mini-channels. To further explain the phenomenon and understand the mechanism of triggering critical heat flux (CHF), the theoretical model to predict the CHF of saturated/subcooled flow boiling inside the micro/mini-channels is proposed based on the liquid layer dry-out and Helmholtz instability (intermittent and continuous vapor blanket) theories. Both the effect of the dry-out flows and the liquid sublayer under the vapor blanket were taken into consideration. For saturated and subcooled boiling, the two mechanisms of triggering the CHF were revealed based on different flow patterns and experimental phenomena, respectively. The boiling heat transfer performance has been experimentally investigated under different liquid subcoolings, mass fluxes and geometry parameters. By analyzing the previous models and establishing a database, the comparison of new model predictions with present experimental data shows good agreement within a standard deviation of ± 10 %. Besides, prior CHF data were used to verify this new model, which also presents varying degrees of success in predicting the CHF. [ABSTRACT FROM AUTHOR]

• The number of publications focused on the flow boiling instabilities is increasing in the last decades. • Most of the studies are based on microchannel and pin fin structures. • The integration of combined geometries is a promising approach to further improve the efficiency and reliability of flow boiling technology. Flow boiling technology is considered one of the most promising cooling solutions for high power microelectronics. However, flow boiling instabilities cause serious reliability problems that hinder large scale commercial use of this technology. Extensive experimental research has been carried out in the last years to mitigate these issues, focusing on geometrical aspects, surface modification, wettability, working fluid, or active control strategies, among others. Nevertheless, despite the relevance of geometrical aspects, none of the studies conducted a comprehensive review on geometrical structures and their instabilities mitigation effect. This review aims to provide constructive insights into relevant studies about experimental flow boiling geometrical enhancements and their impact on instabilities mitigation. It focuses on pin-fin arrays, most of the microchannel structures, inlet restrictors, and combined geometries. Firstly, every geometrical option is analysed in detail, summarizing experimental works in terms of geometry tested, operating parameters, and instabilities improvement. Secondly, a graphical overview is performed aimed at identifying research gaps and tendencies. This study found that significant progress has been made in geometric mitigation of flow boiling instabilities, and several advanced methodologies have been proposed to improve the reliability of flow boiling technology. The integration of various geometric configurations is a promising avenue, but further research is required to reach a widely accepted solution. [ABSTRACT FROM AUTHOR]

Critical heat flux (CHF) represents heat transfer limit for a heat sink working under flow boiling conditions. In the literature, for the first time, in micro-pin-fin heat sinks with expanding structure, this paper experimentally focusing on CHF and proposing a correlation for saturated flow boiling at quite low mass velocities. Parallel channel heat sink is evaluated as reference case. Three mass velocities (G = 45, 68, 90 kg m−2 s−1) are studied by beginning from 130 W up to boiling crisis. Test-range falls in the pure saturated boiling conditions, and inlet temperature is approximately kept constant at 60 °C. Flow mechanism is figured out over the flow images. The heat sink with expanding structure (T-2) clearly boosts CHF compared to parallel counterpart (T-1); such that, by successfully manipulating bubble dynamics, T-2 increases the CHF up to 32.9% at the lowest mass velocity (G = 45 kg m−2 s−1). For highest mass velocity (G = 90 kg m−2 s−1), T-2 provides lower wall superheat up to 89.5% against T-1. The CHF ranges from 225 to 443.31 kWm−2, even though quite low mass velocities. Regarding prediction of the experimental data, mean absolute error (MAE) of proposed correlation is 4.7%; and all those of predictions fall in ±12% error bands. [ABSTRACT FROM AUTHOR]

• Particle Image Velocimetry is investigated in the liquid phase during flow boiling. • Effect of mass flow rates and heat flux on liquid phase velocity up to CHF is captured. • Normal velocity magnitude is seen to increase near the two-phase interface. • Low mass velocities show a reduction in velocity magnitude as we approach the interface. • After a transition mass velocity, velocity magnitude does not reduce near the interface. Subcooled flow boiling is a highly efficient cooling systems for thermal management systems. This study explores the intricate dynamics of subcooled flow boiling within a horizontal channel, investigating the impact of vapor generation on liquid-phase velocity using Particle Image Velocimetry (PIV) and advanced image processing techniques. Four mass flow rates ranging from 5–20 g/s with subcooled inlet conditions are investigated in a rectangular channel with single-sided heating. Three regions of interest along the heated channel are investigated for instantaneous PIV analysis. The PIV system captures detailed velocity profiles, illustrating the impact of varying mass flow rates and heat flux levels on flow behavior. Vapor masking techniques are introduced to enhance the precision of PIV data by mitigating interference from the vapor phase. Results demonstrate the influence of vapor bubbles on flow resistance, revealing non-uniform velocity distributions and turbulence near the liquid–vapor interface. The study emphasizes the critical role of inertia and buoyancy forces in shaping the velocity profiles. Moreover, the investigation sheds light on the effects of flow rates on the interfacial behaviors, hinting at a transition point between 10 and 15 g/s. In summary, this research contributes valuable insights into the nuanced dynamics of flow boiling, laying the foundation for future studies on turbulence, heat transfer, and phase-change phenomena in two-phase thermal management systems. [ABSTRACT FROM AUTHOR]

• A comprehensive database of critical heat flux containing more than 15,000 experimental data points is generated. • Multiple machine learning (ML) algorithms are used to predict critical heat flux trained and tested on the generated database. • Artificial neural network (ANN) and extreme gradient boosting (XGBoost) algorithms outperform other counterpart techniques. • Critical heat flux for vertical flow systems is found to be highly dependent upon Weber number and boiling number. The critical heat flux (CHF) associated with the departure from nucleate boiling (DNB) determines the design and safety aspects of two-phase flow boiling systems. Despite the availability of several predictive tools, within the thermal engineering community, the pursuit of an accurate and robust CHF model remains a significant challenge. In this unique study, we extracted a substantial database from literature to develop machine-learning (ML) models to predict CHF for vertical flows commonly employed in the process industry. The extensive database encompasses 15,006 experimental data points gathered from diverse sources covering wide range of geometric and operational ranges of D, L, P, G, h, and x. This database is then employed to develop five ML – based models, namely Artificial Neural Networks (ANN), K-Nearest Neighbors (KNN), Adaptive Boosting (AdaBoost), Extreme Gradient Boosting (XGBoost), and Random Forests (RF). The selection of optimal input features is conducted by exploring various combinations of both dimensional and dimensionless parameters to identify the most influencing inputs for CHF prediction. The models incorporating features D, L, P, G, h, x, L/D, ρ r , We, and BO achieve CHF predictions with Mean Absolute Errors (MAEs) of 8.85 % for the ANN model and 10.39 % for the XGBoost model. The optimized ML models outperformed even the highly reliable CHF correlations and lookup tables. The feature importance analysis of input parameters highlights strong dependence of CHF on dimensionless numbers such as Weber number and Boiling number. [ABSTRACT FROM AUTHOR]

Critical heat flux (CHF) is an essential parameter that plays a significant role in ensuring the safety and economic efficiency of nuclear power facilities. It imposes design and operational restrictions on nuclear power plants due to safety concerns. Therefore, accurate prediction of CHF using a hybrid framework can assist researchers in optimizing system performance, mitigating risk of equipment failure, and enhancing safety measures. Despite the existence of numerous prediction methods, there remains a lack of agreement regarding the underlying mechanism that gives rise to CHF. Hence, developing a precise and reliable CHF model is a crucial and challenging task. In this study, we proposed a hybrid model based on an artificial neural network (ANN) to improve the prediction accuracy of CHF. Our model leverages the available knowledge from a lookup table (LUT) and then employs ANN to further reduce the gap between actual and predicted outcomes. To develop and assess the accuracy of our model, we compiled a dataset of around 5877 data points from various sources in the literature. This dataset encompasses a diverse range of operating parameters for two-phase flow in vertical tubes. The results of this study demonstrate that the proposed hybrid model performs better than standalone machine learning models such as ANN, random forest, support vector machine, and data-driven lookup tables, with a relative root-mean-square error (rRMSE) of only 9.3%. We also evaluated the performance of the proposed hybrid model using holdout and cross-validation techniques, which demonstrated its robustness. Moreover, the proposed approach offers valuable insights into the significance of various input parameters in predicting CHF. Our proposed system can be utilized as a real-time monitoring tool for predicting extreme conditions in nuclear reactors, ensuring their safe and efficient operation.

Aim of this work was to study flow boiling in an annular channel at low pressure and low flow on a tube with modified surface roughness. The tube with the outer diameter of 9.14 mm and the heated length of 380 mm was made of Inconel 625 and was manually modified using 150 grit sandpaper. The tube was placed in a glass tube with an inner diameter of 14.8 mm. Outlet pressure was set to 120, 200 and 300 kPa with varying mass flow from 400 to 600 kg/(m2.s). A high speed camera was used to record several experiments to fully understand ongoing phenomena. Surface roughness was analysed using a confocal laser microscope and the effects of different mass flux and pressure on the CHF value were observed. Above all, the optimization of the flow parameters was done from the collected data and from the observed behaviour of the experimental loop.

Steady state flow boiling experiments were conducted on a technically smooth Inconel 625 tube with outer diameter 9.1 mm at inlet pressures 131, 220 and 323 kPa, inlet temperatures 62, 78 and 94 °C and approximately 400, 600 and 1000 kg/(m2.s) mass flow. Water of these parameters was entering into the vertically aligned annulus, where the uniformly heated tube was placed until the critical heat flux (CHF) appeared. The experimental data were compared to estimations of CHF by local PGT tube correlation and Groeneveld’s look-up tables for tubes. The results imply that in the region of low pressure and low mass flux, the differences between calculations and experiments are substantial (more than 50 % of CHF). The calculations further imply that look-up tables and tube correlations should be corrected to the annulus geometry. Here, the Doerffer’s approach was chosen and led to a substantial enhancement of CHF estimation. Yet, a new correlation for the region of low pressure and flow is needed.

As per the available literature, most of the techniques used for boiling surface modification utilize high operational temperature and longer production duration, ensuing in higher energy utilization with more safety worries for large scale of manufacturing. Again, the important criterion for boiling surfaces is to control the porous layer (porosity) to achieve high rate of cooling. Considering these aspects, a nature-inspired and simple technique for surface modification is proposed in the present work, which is single-step forced convection electrochemical deposition followed by single-step sintering. In this technique, the surface properties can be easily managed. Using this technique, the nanocomposite coatings of higher thermal conductive Cu–TiO2 (~ 300 W/mK) nanoparticles are fabricated on copper heating surface. The growth in nanograins is occurred during sintering process, which enhances the connectivity between the deposited nanograins. The porosity, thickness of porous layer, roughness, and wettability of the electrodeposited structured surfaces are raised with amplified current supply up to 75%, 42 µm, 1.32 µm, and 38°, respectively. The heat transfer performances of these developed surfaces are analyzed through flow boiling experiments. The maximum enhancement in critical heat flux (~ 143%) and heat transfer coefficient (~ 153%) is attained on developed coated surface at lower mass flux. These augmentations are attributed to better surface wettability and better surface morphological characteristics of the developed surfaces, which are due to the existence of sufficient liquid microlayer on them. The proposed electrodeposition technique can be employed for practical cooling applications where effective handling of high rate of heat flux is a challenge. [ABSTRACT FROM AUTHOR]

This work aimed to study the intensification of flow boiling heat transfer and critical heat flux (CHF) under conditions of highly reduced pressures due to a modification of the inner wall surface of a mini-channel. Such research is relevant to the growing need of high-tech industries in the development of compact and energy-efficient heat exchange devices. We present experimental results of the surface modification effect on hydrodynamics and flow boiling heat transfer, including data on the CHF. A description of the experimental stand and method for modifying the test mini-channel is also presented. The studies were carried out with freon R-125 in a vertical mini-channel with a diameter of 1.1 mm and a length of 50 mm, in the range of mass flow rates from G = 200 to 1400 kg/(m2s) and reduced pressures between pr = p/pcr = 0.43 and 0.56. The maximum surface modification effect was achieved at a reduced pressure of pr = 0.43, the heat transfer coefficient increased up to 110%, and the CHF increased up to 22%.

Flow boiling experiments in microchannels under low mass fluxes have rarely been reported in the literature. In this study, flow boiling critical heat flux (CHF) for a parallel straight copper microchannel heat sink with 41 channels and hydraulic diameter of 420 µm (width: 300 µm; height: 700 µm) is reported under low mass fluxes(< 100 kg/m2s). The flow patterns reveal that the inertial force will be too low to flush the bubble downstream. The heat transfer coefficient was observed to decrease with an increase in the heat flux or vapor quality, which pointed toward convective boiling dominancy inside the channels. When approaching the CHF, the vapor flowed back to the heat sink's inlet plenum, resulting in a flow blockage, which is the main factor for the CHF occurrence. [ABSTRACT FROM AUTHOR]

In the present study numerical simulation of flow boiling process has been conducted for evaluation of critical heat flux conditions under the effect of different parameters (mass flux, heat flux, channel length and surface roughness). Comparison between the results of the present study and previous researches were done. The comparison shows a good agreement between the present study and previous researches. The three different turbulence models (k-epsilon, k-omega and Reynolds Stress) are considered for simulation of boiling heat transfer and CHF phenomenon. The highest accuracy of simulation is obtained by k-epsilon model. The results express that the wall temperature value of tube with adiabatic and heated boundary conditions for first and second half of the tube is lower than the wall temperature when the fluid flows only in the heated wall section. Reduction of velocity value also leads to reduction of maximum wall temperature value and CHF value. Decreasing Roughness value as an effective parameter leads to an increase in wall temperature. Maximum value of the wall temperature, after CHF point, also increases with increase in heat flux value. CHF depends on the surface roughness and rises with increasing roughness value. [ABSTRACT FROM AUTHOR]

This paper presents new experimental data on pressure drop, heat transfer coefficient, and critical heat flux during flow boiling of low-pressure fluid R123 in microchannels. High mass velocities were used, above the values found in the literature in tests with this fluid. Experiments were conducted in a single smooth and horizontal microchannel, with an internal diameter of 1.1 mm and heating sections of 46 and 100 mm. The mass velocities used were between 1500 and 7500 kg/m2s. Saturation temperature ranged from 40.1 to 85.1 o C. The outlet vapor quality varied between -0.54 and 0.53 for the CHF results, between 0 and 0.5 for the pressure drop results, and between 0 and 0.35 for the HTC results, respectively. Record critical heat flux values for flow boiling of R123 in smooth microchannels were achieved, reaching 1.44 MW/m2. An analysis was made of the influence of an upstream pressure drop in avoiding instabilities during critical heat flux experiments. The highest heat transfer coefficient values during convective boiling of R123 in smooth microchannels were also obtained, with values close to 43.3 kW/m2K. The saturation temperatures used were among the highest for flow boiling of R123 in smooth microchannels found in the literature. Correlations for frictional pressure drop, heat transfer coefficient, and critical heat flux were evaluated in predicting the experimental data. An analysis was made on the influence of the void fraction correlation when estimating both the accelerational and frictional pressure drop. • Flow boiling tests at high mass fluxes for R123 in microchannel are presented. • New record Critical Heat Flux for R123 in smooth microchannels was obtained. • The highest value of heat transfer coefficient for R123 was measured. • This paper presents the first available diabatic pressure drop data for R123 in microchannels. • Experimental data are compared against predictive methods. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate the impact heater orientation and inlet conditions on heat transfer mechanisms during flow boiling, inside horizontally oriented rectangular channel using PF-5060. The experimental setup allows both upward and downward-facing single-sided heating configurations. Various inlet mass flux are examined, along with different levels of inlet subcooling. The impact of gravity is studied using flow visualization. During downward facing heating, vapor accumulates along the copper heater wall due to buoyancy effects resulting in notably low CHF and heat transfer values. In contrast, with upward facing heating, buoyancy assists in extraction of vapor from the copper heater wall, facilitating increased liquid contact and higher CHF and heat transfer values. CHF values begin to converge at high inlet mass flux for both orientations, which can be attributed to inertia dominating gravity. Inlet subcooling also influences CHF, with highly subcooled conditions yielding higher CHF. The impact of orientation and subcooled inlet on CHF and heat transfer coefficients are captured. Experimental CHF data was used to validate the Hydrodynamic Instability-Based Model for CHF prediction demonstrating a Mean Absolute Error of 10.8%. Effectively considering gravity forces, heated wall orientation, and flow regimes, the model demonstrates a proficiency in its comprehensive approach. • Visualization shows gravity is dominant below mass velocity of 1200 kg/m2s. • Flow starts to become inertia dominant above 1200 kg/m2s regardless of heating orientation. • CHF is affected by inlet subcooling with higher subcooling giving higher CHF. • CHF is lower in the downward facing configuration for lower mass velocities. • The Hydrodynamic Instability-based CHF model predicted data with 10.8% MAE. [ABSTRACT FROM AUTHOR]

• Artificial neural network models developed for flow boiling heat transfer and CHF. • Databases consolidated from experiments in microgravity (ISS) and earth gravity. • For 29,226 heat transfer datapoints, the final ANN has an overall MAE of 7.99%. • For 641 CHF datapoints, the final CHF ANN has an overall MAE of 12.05%. • The ANNs outperform seminal correlations and capture physical parametric trends. 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°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 feed-forward ANN is developed to predict Nu tp. 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°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, Bo CHF , 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. [ABSTRACT FROM AUTHOR]

This paper carried out an experimental study on the critical heat flux during flow boiling of R134a in a vertical helically coiled tube. The length, inner diameter, coil diameter, and pitch of the test tube were 1.85 m, 8 mm, 205 mm, and 25 mm, respectively. Experiments cover the mass flux range of 190–400 kg·m−2·s−1, heat flux of 15–55 kW·m−2, inlet pressure of 0.8–1.1 MPa, and inlet vapor quality of 0.01–0.35. The effects of critical heat flux identification method, mass flux, system pressure, and inlet vapor quality on critical heat flux were presented. The critical heat flux obtained by the wall temperature rise method was larger than that obtained by the wall temperature oscillation method. The deviation of the critical heat flux corresponding to two methods, including wall temperature rises sharply above 10 ℃ and wall temperature drastic oscillation, was about 20% under the present experimental conditions. The critical heat flux increased with mass flux while it decreased with the inlet vapor quality and pressure. The experiment data were compared with four existing empirical correlations. A new correlation is proposed for critical heat flux prediction in vertical helical tubes. [ABSTRACT FROM AUTHOR]

Steady state flow boiling experiments were conducted on a technically smooth Inconel 625 tube with outer diameter 9.1 mm at inlet pressures 131, 220 and 323 kPa, inlet temperatures 62, 78 and 94 °C and approximately 400, 600 and 1000 kg/(m2.s) mass flow. Water of these parameters was entering into the vertically aligned annulus, where the uniformly heated tube was placed until the critical heat flux (CHF) appeared. The experimental data were compared to estimations of CHF by local PGT tube correlation and Groeneveld's look-up tables for tubes. The results imply that in the region of low pressure and low mass flux, the differences between calculations and experiments are substantial (more than 50% of CHF). The calculations further imply that look-up tables and tube correlations should be corrected to the annulus geometry. Here, the Doerffer's approach was chosen and led to a substantial enhancement of CHF estimation. Yet, a new correlation for the region of low pressure and flow is needed. [ABSTRACT FROM AUTHOR]

Aim of this work was to study flow boiling in an annular channel at low pressure and low flow on a tube with modified surface roughness. The tube with the outer diameter of 9.14 mm and the heated length of 380 mm was made of Inconel 625 and was manually modified using 150 grit sandpaper. The tube was placed in a glass tube with an inner diameter of 14.8 mm. Outlet pressure was set to 120, 200 and 300 kPa with varying mass flow from 400 to 600 kg/(m2.s). A high speed camera was used to record several experiments to fully understand ongoing phenomena. Surface roughness was analysed using a confocal laser microscope and the effects of different mass flux and pressure on the CHF value were observed. Above all, the optimization of the flow parameters was done from the collected data and from the observed behaviour of the experimental loop. [ABSTRACT FROM AUTHOR]

Critical heat flux (CHF) of liquid hydrogen (LH2) flow boiling under microgravity is vital for designing space cryogenic propellant conveying pipe since the excursion of wall temperature may cause system failure. In this study, a two-dimensional axisymmetric model based on the wall heat flux partition (WHFP) model was proposed to predict the CHF condition under microgravity including the wall temperature and the CHF location. The proposed numerical model was validated to demonstrate a good agreement between the simulated and experimentally reported results. Then, the wall temperature distribution and the CHF location under different gravity conditions were compared. In addition, the WHFP and vapor-liquid distribution along the wall under microgravity were predicted and its difference with terrestrial gravity condition was also analysed and reported. Finally, the effects of flow velocity and inlet sub-cooling on the wall temperature distributions were analysed under microgravity and terrestrial gravity conditions, respectively. The results indicate that the CHF location moves upstream about 5.25 m from 1 g to 10−4 g since the void fraction near the wall reaches the breakpoint of CHF condition much earlier under the microgravity condition. Furthermore, the increase of the velocity and decrease of the sub-cooling have smaller effects on the CHF location during LH2 flow boiling under microgravity. • A CFD model is built to predict liquid hydrogen flow boiling CHF under microgravity. • Wall temperature and CHF location were compared for different gravity conditions. • Effects of inlet velocity and inlet sub-cooling on the CHF of LH2 are investigated. • The CHF location moves upstream with the decrease of the gravity-induced buoyancy. [ABSTRACT FROM AUTHOR]

Nanofluid flow boiling and critical heat flux (CHF) are interesting topics for many researchers and have industrial application for preventing potential damages. On this basis, numerous experimental studies have been performed on the determination of CHF; however, due to the weakness of the existing correlations for this purpose, an accurate numerical study on CHF is yet needed to be reported. In the present study, the subcooled flow boiling of nanofluid has been simulated to predict CHF. For estimating the nucleation site density (NSD), a correlation is used, into which the liquid properties, flow characteristics and surface properties contribute. The studies were performed at different concentrations of alumina nanoparticles. The simulation results indicated error percentages below 10% in CHF estimation, confirming the reliability of the results. No significant difference was observed between the CHF values calculated in the simulation studies considering constant thermal properties for the nanoparticles, as compared to those obtained with variable thermal properties for the nanoparticles. Taking the tube in a horizontal position, rather than the vertical position, increased the CHF value. However, the increase in CHF was negligible. [ABSTRACT FROM AUTHOR]

Steady-state internal flow boiling experiments were conducted on various materials, including accident tolerant fuel cladding material Fe–12Cr–6Al (C26M2) alloy, Zircaloy, and metal-based materials, at atmospheric pressure (84 kPa), 10°C inlet subcooling, and 200 kg/m2‧s mass flow entering the test tubes until critical heat flux (CHF) was reached. The clad thickness effects on flow boiling CHF were evaluated showing a negative relation between CHF and clad thickness up to 0.711 mm. An approach was established to mechanistically understand the measured CHF differences among the tested materials using thermal effusivity, activity, diffusivity, and surface thermal economy. No clear relations were observed within the range of thermal properties of the tested materials. Compared to past CHF data for a mass flux of 300 kg/m2‧s, the CHF data for 200 kg/m2‧s showed increased relative differences among materials. This result implies that higher mass flux may further decrease apparent material sensitivity to CHF. [ABSTRACT FROM AUTHOR]

The present study investigates chilldown characteristics of a horizontal copper transfer line with 7.94 mm outer diameter, 0.81 mm wall thickness and 500 mm length. Data presented in this paper is for the experiments conducted with different mass fluxes (66 kg (m2.s)−1 to 102 kg (m2.s)−1) in a horizontal copper transfer line under terrestrial gravity conditions. Temperature measurements were recorded at six equidistant points to a distance of 330 mm from an inlet. Inverse problem solving method is utilized to calculate corresponding heat flux and heat transfer coefficients. Considering the thermal properties of the quenched wall, an empirical relation was developed. It is found that while employing copper transfer lines instead of stainless steel, thermal mass of the section is reduced by a factor of 100, thereby encountering a reduction of 50% in critical heat flux. [ABSTRACT FROM AUTHOR]

Due to the confinement effect, vapor backflow is ubiquitous in flow boiling process in microscale confined domains. This, in turn, severely deteriorate the flow boiling performances. To solve this dilemma, many microchannel configurations have been exploited to regulate two-phase flows such as inlet orifices and auxiliary channels. However, the inlet orifices would lead to significant increase of flow resistance, scarifying the pumping power. In this study, periodic symmetric saw-tooth structure is designed and fabricated along sidewalls of parallel copper microchannels to effectively suppress the vapor backflow without the cost of pressure drop. The effects of this periodic symmetric saw-tooth structure on suppression of vapor backflow and two-phase flow mixing are studied in both forward and reverse directions. The flow boiling performances of this saw-tooth microchannel configuration was experimentally investigated for total inlet flow rates ranging from 30 to 70 ml/min (mass flux ranging from 183.3 kg/m2·s to 427.7 kg/m2·s). The experimental results and visualization study demonstrate that the successful suppression of vapor backflow in the forward direction and enhancement of two-phase flow mixing in the backward direction can dramatically enhance flow boiling performance. Significant boost on both HTC and CHF is demonstrated. A CHF of 217 W/cm2 is demonstrated at 70 ml/min in the forward direction. Also, significant difference in CHF is observed between the two flow directions as the further increase of flow rates. Compared to conventional plain wall microchannels, the CHF and HTC of this study can be significantly enhanced by ∼77.4 % and ∼250 %, respectively. Note that this improved boiling performance is achieved without sacrificing pressure drops. • A new microchannel configuration with periodic saw-tooth structures was explored to effectively inhibited the unwanted two-phase backflow. • Flow boiling performances (regarding HTC, CHF and pressure drops) of this saw-tooth copper microchannel were experimentally studied on DI water for total inlet flow rates varying from 30 to 70 ml/min. • Suppression of vapor backflow in the forward direction and enhancement of two-phase flow mixing in the backward direction can dramatically enhance flow boiling performance. • Significant boost on both HTC and CHF is demonstrated. A CHF of 217 W/cm2 is demonstrated at 70 ml/min. CHF and overall HTC are increased by ∼77.4 % and ∼250 %, respectively. • These enhancements of thermal performances are achieved without the additional pumping power. [ABSTRACT FROM AUTHOR]

• The geometry-metrics-dependent flow boiling characteristics have been experimentally explored in the microchannels of various width. • The critical heat flux in microchannel is strongly affected by microchannel geometries. • An improved correlation has been established to enable more versatile, accurate estimates of the critical heat flux in microchannels with a modest deviation of 11.2 %. Microchannel flow boiling has been regarded as one of the most effective ways to remove high heat flux. The microchannel geometry plays a pivotal role in the flow boiling characteristics while its relevance with the flow boiling has not been quantitatively finalized yet. In this work, we designed three sets of microchannels of various widths to probe the geometry-metrics-dependent flow boiling heat transfer. It has been found that the flow patterns differed by the microchannel widths, and so did the average heat transfer coefficient (HTC) and critical heat flux (CHF). Particularly, the narrow microchannel of 50 μm in width was highly susceptible to triggering the CHF due to the in-channel confinement of the bubble. The flow instability was found with less dependence on the microchannels but with strong relevance to the mass flux. Finally, we tailed a correlation by including the geometry metrics of the microchannel to enable more versatile and accurate CHF estimates with a small deviation of 11.2 %. The present study contributes to valuable experimental trials to probe the geometric effect of microchannels and further sheds light on geometry-dependent flow boiling characteristics. [Display omitted] Flow boiling characteristics in microchannel is intimately correlated to the channel geometry metrics. [ABSTRACT FROM AUTHOR]

• A wall boiling model was coupled into Fluent to test its performance. • Prediction accuracy was assessed with the data in both tubes and fuel assembly. • The effects of boundary conditions on the prediction accuracy were analyzed. • The sensitive parameters of the Demarly's model on CHF prediction were identified. With the development of observations on micro bubble behaviors, a wall boiling model based on dry-patch mechanism was proposed by Demarly (2020). To qualify its performance in CFD simulations, the model was implemented into Fluent by User Defined Function in this paper and coupled with the multi-scale interface model. The model prediction accuracy was tested by the data collected from the 2006 CHF look-up-table and the Large Water Loop experiments. The predicting deviations were less than ±30% in both vertical tube and fuel assembly simulations, which showed the models could perform well under the selected conditions. Different boundary conditions were discussed to analyze the effects on CHF prediction. In addition, the most sensitive parameters of Demarly's model were identified. It was shown that the diameter ratio, bubble departure diameter and departure frequency had a significant impact on the predicted results, which may could support a further model development in CFD. [ABSTRACT FROM AUTHOR]

• A novel design of opposed wall jet subcooled boiling is proposed. • Maximum standard deviation of chip temperature fluctuations is below 0.3 K. • Maximum standard deviation of inlet/outlet pressure fluctuations is below 0.1 kPa. • Pressure drop characteristics independent of heat flux are achieved. • Dependence of acoustic power on heat flux shows an inverse "N" shape feature. Although flow boiling is well recognized as an effective method for cooling high-power density electronic devices, the instability of flow boiling is still a great challenge owing to the bubble confinement in flow passage. In the present work, a novel design of opposed wall jet subcooled boiling is proposed to improve the instability of flow boiling. A small chamber is adopted as boiling space instead of limited flow channels to achieve high stability of flow boiling. Flow boiling experiments using deionized water are conducted for effective heat flux varying from 25.7 to 1112.6 W/cm2 at different flow rates (150–350 ml/min), inlet subcooling degrees (20–50 K), and outlet pressures (12.4–31.2 kPa). The heat transfer, pressure drop and acoustic characteristics, as well as parameter transient fluctuations, are analyzed. High-speed visualization is performed to observe bubble behavior. The boiling chamber can provide sufficient space for bubble expansion and the detached bubbles rapidly collapse into microbubbles in the subcooled liquid, preventing the blockage of the flow passage by large bubbles. Therefore, the significantly smaller transient fluctuations of temperature and pressure and the pressure drop characteristics independent of heat flux are obtained. Moreover, an inverse "N" shape dependence of acoustic power on heat flux is observed. As heat flux approaches critical heat flux (CHF), an increasing trend of the standard deviation of temperature fluctuations and a declining trend of acoustic power are also observed so that a real-time detection method is developed to predict the impending CHF. [ABSTRACT FROM AUTHOR]

Flow boiling experiments were conducted in an annular channel at very low flow rates and atmospheric pressure varying the inlet subcooling from 20 to 40. critical heat flux occurred during the oscillatory flow pattern transition from slug to churn regime. Comparison of low-pressure and low-flow critical heat flux correlations with the present data has shown that these correlations are not applicable for prediction of critical heat flux under oscillatory flow pattern transition from slug to churn. Power transient experiments were also performed at low-pressure and low-flow conditions. Flow regimes and the sequence of events leading to critical heat flux were visually observed using the high-speed camera. [ABSTRACT FROM AUTHOR]

• There exists a relevant two-phase sublayer where void morphology is different from the bubbly bulk flow and the flow close to the wall. • A fundamental change in flow morphology close to the wall during the transition to film boiling was observed. • The void phase is subjected to increased acceleration close to critical heat flux in the immediate vicinity of the heater surface. • An exponential increase of the size of void structures and dry times at the heater surface was observed as critical heat flux occurred. • A phenomenological extension of the dry spot critical heat flux mechanism to subcooled flow boiling is presented. Local measurements from optical fiber micro probes are presented on vertical subcooled flow boiling of refrigerant Novec 649. Experiments were conducted in a rectangular flow channel with a cross section of 40 mm by 40 mm at a mass flux density of 1000 kg m − 2 s − 1 and a subcooling of 13 K. Boiling regimes ranged from single-phase convection to fully developed film boiling. Detailed description of the measurement methods and in particular of the fiber optical micro probes is given. Average boiling curves are provided for three measurement positions along the direction of flow. Void profiles above the copper heater surface are presented for void fraction, void ligament velocity, void ligament size and detection frequency. Time-resolved data of the void phase at critical heat flux was obtained at a distance of 100 µm away from the heater surface. The dataset is discussed with respect to a previously developed conceptualization for the critical heat flux process in flow boiling and the recent consensus on dry spot formation and re-wettability being the main cause of critical heat flux in pool boiling. A phenomenological extension to flow boiling is presented. [ABSTRACT FROM AUTHOR]

• Flow boiling CHF data in microchannels for 3 different DI-water based nanofluids. • Different deposition patterns were observed on the surface images. • Wettability/wickability evaluation of the surfaces at the CHF location. • None effect of the wettability/wickability on the CHF value. • The Methods of Shah [38] and Zhang et al. [39] were ranked as the most accurate. This work concerns an experimental evaluation of the flow boiling critical heat flux of DI-water and nanofluids inside a 1.1 mm ID channel. Results were obtained for DI-water and solutions of Al 2 O 3 (40–80 nm)/DI-water, Al 2 O 3 (10 nm)/DI-water and SiO 2 (80 nm)/DI-water for volumetric concentrations of 0.1% and 0.01%. Additional experiments were performed for pure DI-water on surfaces previously covered with nanoparticles through the flow boiling process. Experiments were performed for mass velocities ranging from 133 to 494 kg/m2s, saturation temperature of 130 °C and vapor qualities at the CHF location ranging from 0.65 to 0.84. The CHF results for DI-water in the tube before covering its surface with nanoparticles were compared with well reputed CHF prediction methods from literature and a reasonable agreement was found. Despite of the large augmentation of the wettability/wickability of the surfaces covered by a layer of nanoparticles, the differences between the CHF values for DI-water for the surfaces with and without the nanoporous covering were within the uncertainty of the CHF measurements. This behavior suggests a negligible effect of the nanoporous layer and, consequently, of wettability/wickability on the saturated CHF in microchannels for high vapor qualities and the range of conditions evaluated in present study. Based on an analysis of the CHF behaviors observed in this study, the saturated CHF mechanism was associated to periodical lumps of liquid that promotes intermittently the surface dryout and its re-wetting. The CHF is established when the time between two consecutives liquid lumps is low enough, so that the surface reaches a temperature for which its re-wetting is not possible. [ABSTRACT FROM AUTHOR]

• A new theoretical model for predicting the trigger mechanism for flow boiling CHF is presented. • Flow boiling on the wall is sustained mostly by the wetting front with discrete vapor bubbles. • CHF is triggered by parallel-to-flow and perpendicular-to-wall combined instability analysis. • The model predictions show good agreement with experimental data, evidenced by an MAE of 15.36%. CHF is an important design parameter in two-phase thermal management systems utilizing flow boiling schemes. In this study, a new theoretical model for predicting the trigger mechanism for flow boiling CHF is presented. Flow visualization shows that flow boiling on a heated wall is sustained mostly by the region containing discrete vapor bubbles called the wetting front because it allows the heated wall continuous access to the bulk liquid. CHF is triggered when intense evaporation of the liquid in the wetting front causes combined effects of parallel-to-flow direction interfacial stability and perpendicular-to-wall direction Helmholtz instability generating a continuous vapor patch on the wetting front. The new mechanistic CHF model incorporates three sub-models: a separated flow model to calculate two-phase flow parameters, an interfacial instability model to calculate the critical wavelength of parallel-to-flow vapor wave, and a Helmholtz instability model to calculate the critical wavelength of perpendicular-to-wall liquid-vapor interface. The CHF model is used to predict experimental data for a rectangular channel with single-sided heating in Earth gravity and microgravity for a variety of operating conditions. The model predictions show good agreement with experimental data, evidenced by an MAE of 15.36%. [ABSTRACT FROM AUTHOR]

Highlights • A corresponding principle of CHF based on molecular thermodynamic is developed. • Two difficulties in micro scale and macro scale are solved. • The novel principle is able to connect the CHF of various fluids. Abstract An equation of state is a thermodynamic equation relating fluid properties which describe the state of matter under a given set of physical conditions. Considering there are some connections between the properties of different fluids, we believe there are some corresponding relations between the critical heat flux (CHF) of different fluids. A corresponding principle was developed to predict the critical heat flux with the experimental data obtained with modeling fluids. A series of scaling criteria were developed on the basis of fluid similarity and molecular thermodynamics, which is totally different from the conventional scaling methods based on experimental data and empirical correlations. The prediction error of this principle is much lower than conventional prediction methods. The average and maximum error of the corresponding principle is about 10%. [ABSTRACT FROM AUTHOR]

Highlights • Low CHF values are observed in flow boiling of water under subatmospheric pressure. • New data of CHF is obtained across flow rates, subcooling and pressure conditions. • Performance of existing models is shown to vary significantly. • A new model is developed and shown to capture the range of condition considered. Abstract Critical Heat Flux (CHF) is the maximal limit of heat flux in two-phase nucleate boiling heat transfer; therefore, an understanding of CHF under a wide range of conditions is important for safe system operation. In this work, CHF experiments are conducted over a range of subatmospheric system pressures in a vertical square channel that is heated on one side. The experimental conditions cover a pressure range of 20 kPa to 108 kPa, a mass flux range of 45–190 kg/m2-s, and an inlet subcooling range of 0–14 K. Heat flux is gradually increased until an excursion of the wall temperature occurs, indicating CHF. For the experimental conditions considered, CHF increases with rising system pressure, mass flux, and inlet subcooling, although the effects of mass flux and inlet subcooling are weak. A new correlation for CHF is developed and found to predict the data with an average error of ±15.6%. [ABSTRACT FROM AUTHOR]

Advanced nuclear reactors employ External Reactor Vessel Cooling (ERVC) as a crucial measure to safeguard the structural integrity of the Reactor Pressure Vessel's (RPV) lower head and to confine molten material within it during severe accidents. The ERVC technique effectively mitigates severe accidents by efficiently dissipating decay heat from the molten pool located within the lower head, utilizing coolant flow boiling. The key determinant of the upper limit of ERVC cooling capability is primarily contingent on the Critical Heat Flux (CHF). In this study, flow boiling in downward heating curved flow channel under forced convection is numerically investigated using the Eulerian-Eulerian two-fluid model and realizable k-ε turbulence model. Compared with the experiment, the maximum deviation of numerically predicted CHF is less than 5%, indicating that the numerical method is reliable. A thorough analysis is conducted on the impact of variable structure and thermal-hydraulic operating conditions on CHF. The results indicate that as the coolant mass flow rate, system pressure, and inlet subcooling increase, the CHF exhibits a corresponding increase. The CHF increases by 56.6% when the mass flow rate increases from 400 to 1000 kg/(m2·s). The CHF at the pressure of 0.3 MPa is 16% higher than that of 0.1 MPa. The CHF increases from 1.14 to 1.84 MW/m2 as the inlet subcooling escalates from 10 to 20 K, with a 60% growth rate, while the CHF increases to 2.03 MW/m2 when the inlet subcooling increases from 20 to 50 K, with a 10.33% increase. The maximum difference in CHF for the three flow channel widths (120, 150, and 170 mm) is only 1.7%, indicating that the influence of flow channel widths is insignificant. This study can provide theoretical guidance for the design and optimization of ERVC in advanced nuclear reactors. [ABSTRACT FROM AUTHOR]

• A heat flux model based on probability analysis of bubbles' interaction is developed. • The boiling crisis is indicated based on the development of irreversible dry spot. • The model well predicts the boiling curve and CHF. • The model can predict the pressure, inlet subcooling, flow mass flux and wall wettability effects on boiling curves and CHF. • The model can be applicated to the flow boiling in inclined downward heated channel. The in-vessel retention system and ex-vessel retention system are very important to the safety of nuclear power plants under severe accidents. While the success of such safety systems relies on well understanding the corresponding physical mechanisms of boiling heat transfer and critical heat flux (CHF). Challenges till remain in accurately predicting the subcooled flow boiling curve especially in the low-pressure and low-flow conditions due to its complex boiling phenomenon. The present study introduces a theoretical model to predict the boiling curve and critical heat flux for subcooled flow boiling in inclined downward heated rectangular channel. The proposed model well estimates the transition from forced convection, isolated bubble nucleate boiling to fully developed boiling regime by considering the growth and interaction of bubbles. Through probability analysis of bubbles' interaction, the proportion of heat flux in different boiling regimes is determined. In addition, the flow boiling CHF is predicted based on the probability analysis of dry spots. The new model is validated by the subcooled flow boiling experiments with vertical single-side heated channel under low-pressure and low-flow conditions. The predicted boiling curves are consistent with experimental results corresponding to different thermal-hydraulic parameters, such as pressure, mass flux, inlet subcooling and wall wettability (hydrophilic and hydrophobic), and the prediction error of CHF is within ±15%. Furthermore, the inclination effect on CHF is validated by the subcooled flow boiling experiments in inclined channel with the inclination angle varying from 0° to 90°, which shows the good applicability of the developed model. [ABSTRACT FROM AUTHOR]