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This study investigates the flow boiling heat transfer characteristics of refrigerants R134a and R407c in horizontal micro-fin tubes. We conducted experiments across a range of heat flux (15–35 kW·m−2), mass flux (50–250 kg·m−2 s−1), and saturation temperature (15 °C-25°C) conditions. Our findings reveal that R134a consistently exhibits a higher heat transfer coefficient (HTC) compared to R407c, making it advantageous for applications requiring efficient heat transfer. However, R134a also shows a tendency towards premature dry-out flow patterns at higher heat fluxes, which could limit its effectiveness under certain conditions. In contrast, R407c demonstrates more stable flow regimes, maintaining wavy-annular and annular flows without early dry-out, which highlights its robustness at higher heat fluxes. Flow pattern maps indicate that higher heat fluxes promote the formation of wavy and annular flow patterns in both refrigerants, with R134a transitioning to dry-out flows more readily than R407c. The influence of saturation temperature on HTC was also significant, with lower temperatures resulting in higher HTCs for both refrigerants. This can be attributed to the increased thermodynamic driving force for phase change at lower temperatures. Our study aligns with previous research, corroborating the critical role of micro-fin tubes in enhancing HTC. The findings have practical implications for the design and optimization of refrigeration systems, emphasizing the need to carefully select refrigerants and operating conditions to achieve efficient and stable heat transfer performance. [ABSTRACT FROM AUTHOR]

In thermal management applications using two-phase flow boiling, rectangular microchannels hold significant promise due to their ease of manufacturing and effective heat transfer characteristics. In this work, we combined experimental and theoretical analyses to propose a theoretical model based on thin liquid film evaporation for predicting heat transfer performance in rectangular cross-sectional microchannels. The heat transfer model is segmented into five zones based on two-phase flow patterns and transient liquid film thickness. These zones represent different flow boiling heat transfer mechanisms over time in microchannels: the liquid slug zone, elongated bubble zone, long-side wall dryout zone, corner liquid evaporation zone, and full dryout zone. The new model comprehensively explains experimental phenomena observed, including long-side wall dryout and thinning of the liquid film on the short-side wall. To validate our model, numerical solutions were computed to study the spatial and temporal variations in heat transfer coefficients. The results exhibited a consistent trend with experimental data regarding average heat transfer coefficients. We also analyzed factors influencing flow boiling characteristics, such as microchannel aspect ratio, hydraulic diameter, measurement location, fluid mass flux, and wall heat flux. [ABSTRACT FROM AUTHOR]

The development of new and more effective cooling technologies is required for several high thermal power dissipation applications such as in electronics cooling or high concentrated photovoltaic panels. The present paper addresses an experimental study on the development of a microchannel based heat sink to cool photovoltaic panels. Experiments focus on the test of a microchannel, with geometry and dimensions optimized from previous work. The analysis performed here emphasizes the experimental characterization of flow boiling in the microchannel under different working conditions. The results include pressure drop and heat flux maps, obtained combining pressure sensors with high-speed imaging and time resolved thermography. The analysis performed was able to identify where nucleation sites were formed. Slug flow interfacial heat transfer could be observed and accurately described in the heat flux maps. Overall, results show the high potential of combining high-speed imaging with time resolved infrared thermography to characterize complex flows. These results also show that there is a good potential for this microchannel based flow cooling in removing the required heat fluxes for the application considered here, when compared to other liquid and air-cooling technologies. [ABSTRACT FROM AUTHOR]

• Flow boiling of R134a in a plate heat exchanger with wavy corrugations was studied. • Three major flow patterns were identified with the variation of vapor quality. • The variation of heat transfer coefficient and wall temperature were discussed. • A new piecewise Nu correlation was developed based on flow pattern transition. As a common component in various applications, the two-phase heat transfer of plate heat exchanger has been frequently studied. However, further investigations are still required on the flow pattern, especially for those with new plate structures brought with the emergence of new applications. This study presented an experimental investigation on the flow boiling of R134a in a plate heat exchanger with mini-wavy corrugations designed for thermal management system of electric vehicle (EV). The flow pattern transitions in the test section were captured and discussed, meanwhile the effects of vapor quality and mass flux on the variation of heat transfer coefficient (HTC) were analyzed. Based on the observation, the two-phase flow pattern developed from pulsating film flow, then rough stream flow and finally to thin stream flow with the increase of vapor quality. The flow pattern map was determined with the superficial momentum of the liquid and vapor phase refrigerant, while the empirical correlations were developed based on two-phase Weber number to predict the transitions between the flow patterns. The HTC showed an intermediate increase firstly under low vapor qualities, and rapidly decreased after reaching the peak value at the vapor quality of 0.45–0.6. In addition, the experimental data were compared with the present correlations, and a new Nu correlation was developed considering the transitions of flow patterns, showing an optimal prediction on the experimental data with the mean absolute error of 11.3%. [ABSTRACT FROM AUTHOR]

Excessive hydrofluorocarbons (HFCs) usage significantly contributes to the global warming. Therefore, there is a widespread shift toward natural refrigerants with low global warming potential (GWP). Among these alternative refrigerants, R290, R600a, and their mixtures have gained considerable attention. However, their flammability limits the charge. To decrease the equipment volume and reduce the refrigerant charge, internally threaded tubes have been introduced to enhance the heat transfer. Therefore, in this study, flow boiling experiments were conducted on R290, R600a, and mixtures inside an internally threaded tube, obtaining heat transfer data under a pressure of 0.216–0.416 MPa, heat flux of 10.5–73.0 kW m−2, mass flux of 70–190 kg m−2 s−1. To quantitatively discern the dominant heat transfer mechanism, a dimensionless number Nz was introduced. In the nucleate boiling dominant region, an increase in the heat flux markedly enhances the heat transfer, with pressure and mass flux playing minor roles. Furthermore, the heat transfer deterioration trend of the mixtures aligns with the variation trends of concentration slip and boiling range. In the convective heat transfer dominant region, a rise in the mass flux significantly increases the heat transfer coefficient, while the pressure and heat flux have minor effects. Additionally, at high R290 mole fraction and vapor quality, high surface tension maintains a continuous liquid film, enhancing the heat transfer. Furthermore, new heat transfer correlations for flow boiling inside the internally threaded tubes were established, which exhibit a mean relative deviation of −2.30 % and 8.80 % for the obtained experimental results and collected data, respectively. [ABSTRACT FROM AUTHOR]

In the pursuit of enhancing thermal management for miniaturized electronic devices, our study delves into the impact of entry effects on Ledinegg instability and flow maldistribution within parallel microchannels. Utilizing a coupled model that incorporates phase change and pressure drop dynamics in boiling flow, we examine microchannels characterized by a 50 length-to-diameter ratio and a 200 μm hydraulic diameter. Our findings unveil a significant influence of entry effects, which narrow the total flow excursion interval, thereby bolstering system stability. Specifically, as the heat flux escalates from 5 W/cm2 to 120 W/cm2, the entry effects increasingly mitigate flow instability and maldistribution in parallel channels, diminishing the total flow rate range susceptible to flow instability by 4.73% and 47.52%, while narrowing the total flow rate range corresponding to uneven flow distribution by 4.70% and 46.75%, respectively. Furthermore, entry effects expand the inlet subcooling range necessary for stabilizing the parallel channel system by 38.89% and 1000%. This research not only underscores the importance of considering entry effects in microchannel design but also opens avenues for further exploration into enhancing thermal management solutions. [ABSTRACT FROM AUTHOR]

The article presents experimental results of boiling heat transfer during FC-770 flow in a group of five minichannels with a common heated wall. The flow orientation was changed from 0° to 180°, with a 15° increment. During the experiments, the temperature of its outer heated wall surface was measured by an infrared camera. At the same time, flow patterns were captured through the glass plate opposite the heated wall using a high-speed camera. The purpose of the calculations was to determine local heat transfer coefficients on the contact surface between the working fluid and the heated surface in the central minichannel, using a simplified 1D calculation method. The results in the form of dependences of the temperature of the heated wall and the heat transfer coefficient as a function of the distance from the channel inlet for various flow orientations were analysed. Furthermore, typical boiling curves and two-phase flow patterns were presented. The mean relative error of the heat transfer coefficient was determined for various flow orientation. The dependence of the void fraction as a function of heat flux was illustrated for various angles of minichannel inclination to the horizontal plane. It was observed that the void fraction increased with heat flux and with increasing angle of inclination of the minichannel to the horizontal plane. [ABSTRACT FROM AUTHOR]

Interconnected microchannels (IMCs) in flow boiling have the advantages of optimized heat transfer performance, energy savings and high efficiency, compact size, and strong customizability. They provide new solutions for thermal management and heat transfer at the microscale and have broad application prospects. To further investigate the effect of microchannels with different numbers of transverse sections on the flow boiling heat transfer, we performed numerical simulations on a rectangular microchannel (RMC) and IMCs with 3, 5, and 7 transverse microchannels at high and low mass flux. It was found that fluid experiences similar bubble and slug flow in different numbers of IMCs and the RMC at low mass flux. At a heat flux of q = 90 W/cm2, the downstream regions of the IMCs produce vapor films that span the channels, obstructing the cross-section and weakening the flow exchange between the channels, which lead the heat transfer performance factor of IMC-3, reaching 148.43%, 110.04%, and 116.92% of the RMC, IMC-5, and IMC-7. Under high-quality flux, as the heat flux increases, the heat transfer coefficient increases and the pressure drop decreases due to the existence of lateral microchannels introduced in the interconnected microchannels. Whether at high or low mass flux, structural reasons pertaining to the RMC can easily lead to the accumulation of bubbles and the occurrence of slugs, and the flow boiling instability increases with the increase of heat flux, which leads to a pressure drop and heat transfer performance generally lower than that of IMCs under the same conditions. At q = 120 W/cm2, IMC-7 showed the best heat transfer enhancement. Its heat transfer performance factor was 129.37%, 120.594% and 107.98% of the RMC, IMC-3, and IMC-5, respectively. This article provides theoretical support for the design of interconnected microchannels in thermal management. [ABSTRACT FROM AUTHOR]

To solve the problem of high heat flux heat dissipation in microelectronic devices, a manifold micro-channel heat sink with corrugated bottom (CB-MMC) is proposed on the basis of the manifold microchannel heat sink (MMC). The flow and heat transfer characteristics of HFE-7100 in MMC and CB-MMC are investigated numerically. The results show that CB-MMC reduces the pressure loss and enhances the heat transfer performance in single-phase flow. The orthogonal test method is used to obtain structural design solutions with optimal thermal performance. It is observed that the temperature reduction is always at the expense of the increase of the pressure drop. In addition, the optimization parameters combination obtained through comprehensive evaluation of temperature and pressure drop through weight matrix - optimized solution 19 (wavelength A = 800 µm, amplitude B = 40 µm, channel depth C = 180 µm, outlet width D = 300 µm, channel width E = 25 µm). Its Tave has decreased by 6.89 °C, ΔP decreased by 10.27 kPa. Moreover, the subcooled boiling flow and heat transfer performance in MMC and CB-MMC are comparatively studied. The results demonstrate that the dynamic behavior of vapor bubbles causes large pressure fluctuations, which further leads to small temperature fluctuations, and thus reduces the stability of the flow and boiling heat transfer. Compared with MMC, CB-MMC exhibits more stable two-phase flow and better boiling heat transfer. [ABSTRACT FROM AUTHOR]

It is widely established that flow boiling, being a direct cooling technique also employing the latent heat of the fluid, has the potential to be more efficient than being useful in single-phase conventional cooling methods. This results in considerable potential for thermal management in many fields like microelectronics, space technology, thermal power plants, etc. The increasing demand for heat dissipation, consequent to component miniaturization, has pushed the development of new strategies for enhancing heat transfer efficiency, such as employment of functionalized surfaces. This review aims to describe in detail the current status of technology related to flow boiling heat transfer enhancement via micro/nanoscale surface functionalization. Key objectives are an increased nucleation site density and enhanced bubble dynamics. The vast majority of findings show favorable heat transfer performance, evidenced by an earlier onset of boiling (ONB), an improved flow boiling heat transfer coefficient (HTC), and an ameliorated critical heat flux (CHF). Increased pressure drop is a serious concern in certain application cases. Nanoscale textures mainly enhance capillary wicking to nucleation sites, thus being more effective in combination with microscale textures that define fixed nucleation sites. Degradation effects need to be more thoroughly and systematically characterized for application cases. Extra effects related to the manufacturing process can be easily overlooked, but one should be aware of their possible existence when drawing conclusions. Finally, the implementation of enhanced surfaces in mainstream applications is hindered by the absence of general predictive design tools for different channel configurations/materials, fluids, and operating conditions. A more universal understanding of the basic mechanisms involving texture geometry is needed in this aspect. [ABSTRACT FROM AUTHOR]

• Flow boiling characteristics of the non-azeotropic mixture R448A in a microchannel tube were measured in one pass. • As mass flux or heat flux increased, HTC of R448A increased; when the saturation pressure increased, HTC of R448A decreased. • R448A had a higher HTC than the HFOs and R134a at low quality, but lower HTC at high quality. • The correlations for predicting pressure gradient and heat transfer coefficient were proposed or recommended. R448A is a mixture to replace R404A in the thermodynamic cycle applications. R448A has a temperature glide of about 5°C and might exhibit thermal non-equilibrium in the microchannel tube. This study investigated the flow boiling heat transfer and pressure drop of R448A, a five-component non-azeotropic mixture, in a microchannel tube. Six heat transfer coefficients (HTC) and pressure gradients (dP/dz) of R448A were measured in one pass in a 24-port microchannel tube. The experimental conditions covered mass fluxes from 100 to 200 kg-m−2s−1, heat fluxes from 2 to 8 kW- m−2, and saturation temperatures from 10 to 30 °C. The study compared the experimental results of R448A with four of its component fluids (R134a, R1234yf, R1234ze(E), and R32). R448A had higher HTC and lower dP/dz than its component fluids, except for R32 which had higher HTC and dP/dz than R448A. Muller-Steinhagen and Heck was the best correlation for predicting the dP/dz of R448A, with a mean absolute error (MAE) of 8.78 % and a mean error (ME) of 7.47 %. A new correlation for predicting the HTC of R448A, based on modifying Liu and Winterton (Müller-Steinhagen and Heck, 1986), was proposed with an MAE of 6.01 % and an ME of 0.588 %. [ABSTRACT FROM AUTHOR]

• Flow boiling heat transfer and pressure drop data for R513A. • HTC shows both nucleative and convective behavior with operating conditions. • Pressure drop increases with mass flux and decreases with saturation temperature. • New composite prediction method (MAPE = 25.3%) for heat transfer coefficient. • Frictional pressure gradients well predicted by Friedel correlation (MAPE = 13.8%). This paper presents two-phase heat transfer coefficient and pressure drop data of refrigerant R513A, a new azeotropic mixture conceived as possible alternative to R134a for medium temperature small-size refrigeration systems. All the experiments were performed in a commercial horizontal stainless-steel tube having an internal diameter of 6.0 mm and an outer diameter of 8.0 mm. The channel heating was obtained through DC current and Joule effect. The effect of the main operating parameters in terms of mass flux (from 150 to 500 kg/m2s), heat flux (from 5.0 to 40 kW/m2), saturation temperature (from 30 to 50 °C) and vapor quality (from the onset of boiling up to the dry-out occurrence) was analyzed and discussed for the heat transfer coefficient values, finding that both convective and nucleate boiling were significant contributions. The same ranges of mass flux and saturation temperature were also applied to frictional pressure gradient trends with vapor quality, taken in adiabatic conditions. The thermo-hydraulic performances were then compared with those of R134a, obtaining lower heat transfer coefficients and very similar pressure gradients. Finally, the collected experiments were assessed with values predicted from the most quoted correlations available in literature. A new composite method for nucleate-dominant or convective-dominant mechanisms was proposed for the evaluation of the heat transfer coefficient, with a mean absolute error of 25.3%, whereas the frictional pressure gradient values were well predicted with the Friedel correlation, that provides a mean absolute error of 13.8%. [ABSTRACT FROM AUTHOR]

Insufficient thermal-hydraulic knowledge for analysis of a reactivity-initiated accident demands experiments of fast-transient flow boiling heat transfer from moderate- to high-pressure conditions. In this study, those experiments are conducted for vertical upward tube flows of pressurized water. The tube wall is joule heated by stepwise electric pulse power to achieve an abrupt wall heating condition. The applied pulse power is varied from 4.68 to 13.59 GW/m3, which is beyond the power required for steady-state critical heat flux (CHF) to occur. Rapid evolution of the boiling wall temperature is extracted from outer wall temperature data by solving an inverse heat conduction problem. As a result, with increasing the applied pulse power, the time to occurrence of departure from nucleate boiling gets shorter, and the corresponding peak heat flux increases over the steady-state CHF, which is evaluated at the same flow condition. A logarithmic relation between the wall heating rate and the CHF increment ratio is also demonstrated. The effects of pressure, inlet subcooling, and mass flux on the transient peak heat flux are also investigated. As the pressure increases, the nucleate boiling duration gets shorter with decreasing peak heat flux. On the other hand, as the inlet subcooling increases, the nucleate boiling duration gets longer, and the peak heat flux increases. Contrarily, the mass flux does not show any noticeable effects on the transient heat transfer evolution. [ABSTRACT FROM AUTHOR]

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]

In this study, the flow boiling heat transfer and pressure drop characteristics of refrigerant R134a in micro-channels were experimentally investigated. The tests were performed in circular horizontal micro-channels with inner diameters of 0.5 mm and 1 mm and a heating length of 300 mm. The mass velocities varied from 500 kg/m²s to 2500 kg/m²s, and the heat fluxes varied from 15 kW/m² to 147 kW/m². The heat transfer coefficient (HTC) and frictional pressure drop (FPD) were measured and discussed in detail. According to the results, HTC was significantly affected by heat flux, whereas it was independent of mass velocity. Nucleate boiling was the dominant heat transfer mechanism for R134a flow boiling in the micro-channels. In comparison to the 1 mm channel, the 0.5 mm channel shows better performance in heat transfer, with a maximum increase of approximately 22 %. In addition, FPD increased with increasing mass velocity and decreasing channel diameter. Finally, several existing correlations for HTC and FPD were evaluated by comparing them with the experimental values. Tran's correlation (1996) presented a better agreement in terms of the average HTC, while for the FPD, the model of Kim and Mudawar (2013b) showed good prediction accuracy. [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]

Recent trends of miniaturization and densely packed circuits have necessitated the development of novel cooling methods, especially for emerging fields like flexible microelectronics, biotechnology, and nanotechnology. A polydimethylsiloxane (PDMS) based microchannel flow boiling heat sink can be used for these applications due to its unique properties as such as unique mechanical properties, biocompatibility, optical transparency, etc. PDMS microchannels are fabricated using soft lithography techniques, which involve using a mould to pattern the PDMS material. The mould is created using photolithography, and the PDMS is poured over the mould and cured to create the microchannels. In this study, an effort is made to compare the flow and heat transfer performance of a PDMS microchannel heat sink with different channel shapes, i.e. straight and diverging microchannel heat sinks for different heat flux and mass flux conditions. The PDMS microchannel was fabricated using the soft lithography technique and a flexible laser-induced graphene heater was fabricated and integrated with the developed PDMS microchannel to act as a heat source. Experimentally, the parameters like bubble pattern, pressure drop, exit vapour quality, and corresponding heat transfer coefficient were determined at varying mass and heat flux ranging from 19 to 114 kg m−2s−1 and from 13.3 to 156.6 kW m−2 respectively for both the microchannels. It was observed that the flow in the diverging microchannel is more stable and has a 13% higher heat transfer coefficient value compared to the straight microchannel under similar conditions. Further, a 3D numerical study was carried out to corroborate and elaborate on the results. The heat transfer, flow characteristics, and bubble pattern results show an excellent agreement with experimental results. This work has significant relevance in designing flexible microchannel heat sinks. [ABSTRACT FROM AUTHOR]

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Cooling methods for multiple heat sources with high heat flux have rarely been reported, but such situations threaten the stable operation of electronic devices. Therefore, in this paper, the use of two microchannel heat sinks is proposed, with and without grooves, labeled Type A and Type B, respectively. Experimental investigations on the flow boiling of two microchannel heat sinks connected in parallel and in series are carried out under different mass fluxes. In addition, a high-speed camera is used to observe flow patterns in the microchannels. The cold plate wall temperature (Tw), heat transfer coefficient (HTC), and pressure drop (PD) are obtained with the use of two microchannel heat sinks. The flow patterns of the bubbly flow and elongated bubbles in the microchannels are observed. The results of the analysis indicated that the Tw, HTC, and PD of the two microchannel heat sinks connected in parallel were degraded, especially when using the Type A-B parallel connection. Compared to the use of a single heat sink, the maximum decrease in HTC was 9.44 kW/(m2K) for Type A heat sinks connected in parallel, which represents a decrease of 45.95%. The influence of the series connection on the Tw, HTC, and PD of the two heat sinks is obvious. The Type A-A series connection exerted the greatest positive effect on the performance of the two heat sinks, especially in the case of the postposition heat sink. The maximum increase in HTC was 12.77 kW/(m2K) for the postposition Type A heat sink, representing an increase of 72.88%. These results could provide a reference for a two-phase flow-cooling complex for multiple heat sources with high heat flux. [ABSTRACT FROM AUTHOR]

This paper studies the evaporative heat transfer characteristics of R744 at low temperatures in a horizontal smooth tube as a cascade refrigeration system (CRS) among hybrid cascade refrigeration systems (HCRSs). There is a lack of research on the low-temperature evaporative heat transfer characteristics of R744 under the operating conditions of evaporators used in actual CRSs used in supermarkets. Therefore, this study aims to provide basic data on the evaporative heat transfer characteristics of R744 in the evaporators of refrigerators used in supermarkets. The tube used in the evaporation experiment conducted herein was a smooth horizontal copper tube with an inner diameter and length of 11.46 mm and 8000 mm, respectively. The experimental parameters were as follows: heat fluxes of 11.8–21.3 kW/m2, mass fluxes of 76.3–175.1 kg/(m2·s), and saturation temperatures of −49.8–−30.2 °C. The main results are summarized as follows: the evaporative heat transfer coefficient of R744 can be predicted well by using the correlation formula of Chen at the evaporation temperature of −40 °C in the CRS. [ABSTRACT FROM AUTHOR]

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• Experiments were conducted for flow boiling of zeotropic mixture refrigerants in horizontal tube. • Experimental data are obtained under high pressure and high heat flux conditions. • Local temperature and composition shift in vapor and liquid are analysed from the data. • Thermal resistance and mass transfer resistance are discussed. The paper reports an experimental investigation of local saturated flow boiling heat transfer of two environmentally-friendly zeotropic mixture refrigerants – R454B and R449A – in a horizontal 8.7 mm ID smooth tube, under high pressure and high heat-flux conditions. The test section is divided into five subsections to obtain quasi-local heat transfer and the refrigerants are heated by hot water in counter-current flow. For R454B, mass flux varied from 201 kg m−2 s−1 to 269 kg m−2 s−1 and the inlet pressure varied from 1.691 MPa to 3.282 MPa. For R449A, mass flux varied from 178 kg m−2 s−1 to 328 kg m−2 s−1 and the inlet pressure varied from 1.449 MPa to 3.244 MPa. The temperature glide and composition shift during flow boiling due to volatility difference between components were investigated. R454B and R449A displayed a temperature glide respectively from 1.12 K to 1.33 K and from 3.04 K to 4.61 K at the inlet of the test section in the present tests. During the process of saturated boiling, for R454B, mass fraction of the less volatile component, R1234yf, in liquid phase was over 7% more than that in vapor phase; For R449A, mass fractions of the less volatile components, R134a and R1234yf, in liquid phase were around 6% and 4% more than those in vapor phase. Thermal resistance and mass transfer resistance were discussed. Before the occurrence of partial dry-out, heat-transfer coefficients of R454B were mainly located between 6.5 kW m−2 K −1 and 10 kW m−2 K −1 while most of data of R449A lay in the range of 3.5 – 8 kW m−2 K −1. The results of heat-transfer coefficient indicate that R454B exhibited greater heat transfer than R449A under close operating conditions. For both refrigerants, the heat transfer performance was locally enhanced in the region of near-zero vapor quality and gradually declines before intermediate vapor quality. After intermediate vapor quality, it tended to rise with vapor quality until a dramatic fall due to partial dry-out. [ABSTRACT FROM AUTHOR]

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This study aims to explore the heat transfer characteristics of flow boiling in the minichannels under the action of phase separation structure. A test section of parallel counterflow minichannel was also fabricated with the different structures of phase separation. Furthermore, the pressure difference between the downstream and counterflow channel confined the bubbles to discharge the gas phase from the high- to the low-pressure channel through the phase separation film. The gas phase separation was realized under the high- and low-pressure switching between adjacent channels. Two kinds of structure channels were fabricated: Type 1 phase separation structure channel (SPS1 channel) (Structure of Phase Separation, SPS) with few vents, and type 2 phase separation structure channel (SPS2 channel) with multiple vents, compared with the SPS3 channel without phase separation structure. The aqueous glycerol solution with a mass fraction of 30% was used as the test working medium. The flow boiling test was performed on the rectangular mini channel with a cross-section of 2 mm×2 mm under the effective heat flux density is 151.43 kW/m², the mass flow rate of 121.25 kg/(m² ·s), and inlet temperature of 70°C. A systematic investigation was made to clarify the effects of high- and low-pressure switching cycles on the comprehensive performance and phase separation structures on the flow boiling heat transfer and temperature uniformity in mini channels. A high-speed camera was used to determine the length-to-diameter ratio of confined bubbles and the gas phase separation. An analysis was made to explore the heat transfer enhancement of the mini channel flow boiling under the action of the phase separation structure. The results show that the local saturated boiling heat transfer coefficient was the highest, and the total pressure drop was the lowest when the high- and low-pressure switching cycle was 120 s under the experimental conditions. An optimal value was achieved in the high- and low-pressure switching cycle. There was little difference in the boiling curve before the ONB point, while after the ONB point, the wall superheat of SPS2 and SPS3 channels was lower at the same heat flux. The maximum coefficients of local saturation boiling heat transfer in the SPS2 and SPS3 channels increased by 18.87% and 26.65%, respectively, compared with the SPS3. More importantly, the temperature uniformity of the SPS2 channel was the best in the two-phase region, SPS1 was the second, and SPS3 was the worst. The wall temperature standard deviations were reduced by 10.81% and 18.91%, respectively, along the SPS1 and SPS2 channels. The visual analysis results show that the phase separation structure reduced the length-to-diameter ratio of the confined bubbles, leading to the flow pattern transformation in the channel. Enhanced heat transfer was achieved in the first half cycle of the high- and low-pressure switching cycle. The length-to-diameter ratios of confined bubbles in the SPS1, SPS2, and SPS3 downstream channels were −118.71%, −158.16%, and 122.45% in the unit time, respectively. The phase separation structure can be expected to effectively enhance the heat transfer performance of flow boiling, and then improve the temperature uniformity of minichannels. The finding can provide new ideas for the application of the phase separation structure in minichannel heat exchangers.annel heat exchangers. [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. [ABSTRACT FROM AUTHOR]

• The pressure drop is measured for refrigerant R-600a. • Effects of mass flux, heat flux and vapour quality conditions are investigated. • The influence of flow patterns on the pressure drop is observed. • The best performing correlations for the data set are indicated. This study experimentally investigated the flow boiling pressure drop and flow patterns of R-600a in a horizontal multiport extruded tube with 1.47 mm ID. The experiments were carried out with mass flux between 35 and 165 kgm−2s−1 and heat flux ranging from 4.5 to 18.5 kWm−2 at a saturation temperature of 24 °C. Flow patterns and void fraction distribution amongst channels were observed with a high-speed camera in a visualization section. The influence of mass flux and vapour quality on pressure drop was verified. The flow patterns identified during tests were plug, slug churn and wavy-annular, differently distributed along the channels. The results were also compared with eight different correlations proposed in the literature. Results indicate the dependence of pressure drop on both mass and heat fluxes. [ABSTRACT FROM AUTHOR]

Experimental investigation of refrigerant R245fa flowing inside horizontal 1.62 mm and 2.43 mm tubes under different gravity levels were carried out to study effects of gravity level, tube diameter, and heat flux on flow boiling heat transfer and pressure drop. The experimental parameter range is the mass flux of 351 kg m−2 s−1, heat fluxes of 32 and 64 kW m−2, system pressure of 217.8 kPa, average vapor qualities of 0.007–0.915, and gravity levels from 1 to 2.77 g. The flow boiling heat transfer and pressure drop of R245fa in different tube sizes under hypergravity are first reported. The results show that the heat transfer was enhanced significantly for both tubes when the turntable starts to rotate (from 1 to 1.04 g), which attributes to the fluid disturbance and secondary flow generated by the Coriolis force. With the further increase in hypergravity (from 1.04 to 2.77 g), the heat transfer coefficient may be enhanced or deteriorated. The reason is that the effect of centripetal acceleration, which increases much faster with increasing rotational speed of the turntable than the Coriolis acceleration, offsets the contributions made by the Coriolis force. The effect of heat flux on flow boiling heat transfer interacts with other factors. It is possible for the heat transfer coefficient at lower heat flux to be greater than that at higher heat flux due to the compound flow and heat transfer mechanisms. Under hypergravity, the pressure drop in the small tube is slightly larger than that under normal gravity, while in the large tube it may be smaller than that under normal gravity. The experimental data are compared with the predictions of existing correlations of saturated flow boiling heat transfer and frictional pressure drop, and those which can provide the most agreeable predictions are identified. [ABSTRACT FROM AUTHOR]

The acquirement of the flow information in plate-fin heat exchanger (PFHE) is limited by its metal structure and complex flow condition. This work develops a new distributed optical measurement system to obtain flow information and boiling intensity. The system utilizes numerous optical fibers installed at the surface of the PFHE to detect optical signals. The attenuation and fluctuation of the signals reflect the variation of the gas-liquid interfaces and can be further used to estimate the boiling intensity. Practical experiments of flow boiling in PFHE with different heating fluxes have been carried out. The results verify that the measurement system can obtain the flow condition. Meanwhile, according to the results, the boiling development in PFHE can be divided into four stages with the increase in the heating flux, including the unboiling stage, initiation stage, boiling developing stage, and fully developed stage. [ABSTRACT FROM AUTHOR]

During the operation of ADN (ammonium dinitramide, (NH4+N(NO2)2−))-based thrusters, the ADN-based liquid propellant, a non-toxic green energetic material, tends to flow boil in the capillary tube due to heat transfer from the wall. A three-dimensional transient numerical simulation of the flow boiling of ADN-based liquid propellant in the capillary tube was carried out using the VOF (Volume of Fluid) coupled Lee model. The flow-solid temperature and the gas–liquid two-phase distribution and the wall heat flux at different heat reflux temperatures were analyzed. The results show that the magnitude of the mass transfer coefficient of the Lee model significantly influences the gas–liquid distribution in the capillary tube. The total bubble volume increased from 0 mm3 to 957.4 mm3 when the heat reflux temperature was increased from 400 K to 800 K. The bubble formation position moves upwards along the inner wall surface of the capillary tube. Increasing the heat reflux temperature intensifies the boiling phenomenon. When the outlet temperature exceeded 700 K, the transient liquid mass flow rate in the capillary tube was already reduced by more than 50%. The results of the study can be used as a reference for the design of ADN-based thruster. [ABSTRACT FROM AUTHOR]

Two-phase flow boiling is the preferred method for efficient heat dissipation in electronic equipment, so the bubble dynamics and heat transfer characteristics of flow boiling in a single pentagonal rib channel are numerically analyzed by the volume of fluid (VOF) model. Three typical rib structures are obtained, whose block ratios are 0.4, 0.75, and 0.25, and the corresponding length-diameter ratios are 1.72, 0.59, and 12.70, respectively. The Reynolds number is 14,000 and 28,000. The heat flux is set from 50 to 150 kW/m². The results show that the starting position of the bubble is at the cone and sides of the rib, and its moving direction is related to the configuration of the rib, the heat flux, and fluid velocity. Additionally, the bubbles are shedding at the corner of the rib, and the short rib does not appear to be the vortex shedding phenomenon. The wall temperature and temperature fluctuation increase with the increase of heat flux and with the decrease of fluid velocity. The heat transfer coefficient decreases with the increase of heat flux and with the decrease of fluid velocity. The short rib channel has a higher heat transfer coefficient, approximately 14% and 50% higher than that of the median and long rib channels, respectively. Compared with the single-phase flow, the main frequency of the lift coefficient increases with the increase of the fluid velocity while decreasing with the increase of heat flux. The main frequency of the median rib without harmonic signal keeps constant, and that for long rib decreases by 38%. The study of the instantaneous characteristics of bubble growth in pentagonal rib channels is instructive to the efficient heat dissipation in electronic equipment. [ABSTRACT FROM AUTHOR]

In recent years, the open microchannel has drawn increasing interest, but severe local dryout limited the heat transfer capability of flow boiling. It was anticipated that nanostructures with exceptional capillary wicking abilities would overcome this problem. In this study, blade-like CuO nanostructures were created in the copper open microchannels to experimentally investigate water flow boiling. Experiments were carried out in nanostructured open microchannels (NMCs), and smooth-surface open microchannels (SMCs), as a comparison, were examined under identical operating conditions. Four main flow patterns, including bubbly flow, slug flow, and two kinds of stratified flow, dominated successively in NMCs and SMCs. Although the flow patterns were similar in NMCs and SMCs, the heat transfer coefficient (HTC) of flow boiling was greatly enhanced by nanostructures under conditions of medium and high heat flux, while the nanostructures' influence on HTC was unnoticeable at low heat flux. At medium and high heat fluxes, the dependence of HTC on heat flux and flow rate indicated the joint contribution of nucleate boiling mechanism and convective evaporation mechanism to heat transfer. The enhanced effect of nanostructures on nucleate boiling and convective evaporation became more prominent as heat flux increased, leading to a higher HTC in NMCs than in SMCs at higher heat flux conditions. [ABSTRACT FROM AUTHOR]

Three‐dimensional simulations are performed to study the bubble breakup and boiling heat transfer in Y‐shaped bifurcating microchannels with different heat fluxes and bifurcating angles. Results show that the breakup regime for continuously growing bubble changes from tunnel breakup to obstructed breakup as the heat flux increases. Interestingly, the pinch‐off stage of bubble breakup becomes inconspicuous in small acute‐angle bifurcating microchannel. The flow structure changes from smooth mode to twining mode as the bifurcating angle increases. The bubble transit leads to the shrink or disappearance of vortex. The heat transfer is tightly associated with bubble dynamics. The negative heat transfer enhancement is observed at low heat flux and it is eliminated at high heat flux. The heat transfer is enhanced with the increase in heat flux, while the effect of bifurcating angle is relatively complex. The present study provides new insights into the two‐phase flow in bifurcating structures. [ABSTRACT FROM AUTHOR]

Traditional microchannel needs to face the flow-reversal difficulty in high heat fluxes due to limited space. It results in large pressure and temperature fluctuation. Porous microchannels arouse more interest to provide a new solution to this problem. Flow boiling experiments in porous microchannels with PFA were investigated. Porous microchannels were sintered by 10 μm (or 30 μm) spherical copper particles with pore-forming agent (Na2CO3, 60–90 μm). Porous microchannels were composed of 23 parallel porous microchannels with 600 μm in width and 1200 μm in depth.The addition of PFA (pore-forming agent) could increase the sample porosity. For Q10 series, sample porosities increase from 20.4% to 52.9% with the PFA percentage change from 0% to 40%, while for the Q30 series they increase from 26.6% to 47.5%. Experimental results showed the boiling heat transfer coefficient (HTC) reached the maximum at the moderate porosity for both Q10 and Q30 series. Too large or too small porosity would degrade boiling heat transfer performance. It demonstrated that there existed an optimal range of PFA content for sintered microchannels. PFA content has a minor effect on the average pressure drop and would not cause the rapid increase in flow resistance. Visual observation disclosed that the sample porosity would affect the pressure instability significantly. The sample with moderate porosity showed periodic pressure fluctuation and could establish rhythmical boiling. Particle size also exerted a certain influence on the boiling heat transfer performance. Q30 series could achieve higher HTC and CHF (Critical heat flux) than Q10 series. This is attributed to the larger ratio of layer-thickness-to-particle-size (δ/d) for Q10-series samples. [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]

A comparative analysis of the flow boiling characteristics of R450A, R515B, and R1234ze(E) inside a 7.0 mm OD microfin tube was performed. The mass velocity was explored from low values, starting from 50 kg m−2 s−1, up to 400 kg m−2 s−1, and the heat flux was tested in the range of 10–50 kW m−2, keeping a constant saturation temperature of 30 °C at the inlet of the test section. R515B and R1234ze(E) showed similar values of the heat transfer coefficient and frictional pressure gradient under all the investigated working conditions. R450A showed lower values of the heat transfer coefficient, especially at low heat flux and high mass velocity, compared to R515B and R1234ze(E), but the gap of the heat transfer performance between the three fluids reduced at high heat flux. The frictional pressure drops of R450A were generally lower than those of R515B and R1234ze(E). In the end, some correlations for the evaluation of both the heat transfer coefficient and the pressure drop were selected, and the estimated values were compared against the experimental counterpart. Furthermore, the effects of nucleate boiling and of convective boiling, as well as of the temperature glide for R450A, were analyzed and estimated. [ABSTRACT FROM AUTHOR]

• Boiling heat transfer of R1234yf/R32 and R1234ze(E)/R32 was investigated. • Flow patterns for adiabatic and boiling flows were visualized. • Transition boundary for flow patterns inside horizontal minichannels was proposed. • Effects of mass flux, quality, heat flux, and composition were investigated. • Heat transfer correlation for refrigerant mixtures inside minichannel was proposed. This study experimentally investigates the flow patterns and boiling heat transfer of binary mixtures R1234yf/R32 and R1234ze(E)/R32 in a horizontal circular minichannel with an inner diameter of 2.0 mm. The effects of mass flux, heat flux, vapor quality, and mass composition are evaluated. In the adiabatic two-phase flow, the observed flow patterns included slug/plug (intermittent), wavy, churn, and annular flows. A simple flow pattern transition boundary is proposed to distinguish between two types of flow regimes: one controlled by shear stress and one controlled by surface tension and gravity. The results showed that heat flux significantly affects slug/plug flows, but it has minimal impact on other flow patterns. The heat transfer coefficient increased as the mass flux increased, particularly at higher qualities. At higher mass fluxes, the heat transfer coefficient increased as the transition to annular flow occurred with increasing quality. Conversely, at lower mass fluxes, the heat transfer coefficient either slightly decreased or remained almost constant as the quality increased. Heat transfer degradation decreased as the mass flux increased, and the effect of degradation in annular flows was minimal. R1234ze(E)/R32 (80/20 mass%), which had the largest temperature glide, exhibited the lowest heat transfer coefficients; however, this degradation decreased as the mass flux increased. The heat transfer coefficients for both the pure and mixed refrigerants increased in proportion to the heat flux raised to the power of 0.7. The previous correlation predicted the heat transfer coefficients of the pure refrigerants with a mean absolute percentage error (MAPE) of 12.6 %; however, it overestimated for mixtures. The proposed correlation predicted the heat transfer coefficient for mixtures with an MAPE of 13.4 %. [ABSTRACT FROM AUTHOR]

• 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]

Microchannel evaporator is one of the key components in many thermal systems such as heat exchanger of HAVC and refrigeration system. The evaporation heat transfer performance of microchannel evaporator, which usually has many flow paths, is highly affected by the distribution of liquid flow that will seriously deteriorate the flow boiling performance due to the lack of liquid supply, local dryout, etc. In this study, a novel two-pass microchannel evaporator (L × W × H = 103 × 64 × 20 mm) is designed and fabricated. The width, height and length of each individual channel are 0.6, 1 and 65 mm, respectively. To significantly improve liquid redistribution after the first-pass, evenly distributed spacers with different diameter holes were placed in the connecting flow path. Additionally, the channel number ratio between the first and second pass plays an important role. Therefore, three different channel ratios of 1:1, 1:2, and 3:5 were selected in this study. The heat transfer and flow characteristics of the two-pass microchannel evaporator using working fluid HFE-7100 were experimentally studied for flow rate varying from 30 ml/min to 110 ml/min. The inlet subcooling keeps about 30 K. Also, visualization studies were conducted for the explanations. The experimental results indicate that the thermal performance is superior in microchannel evaporator with channel ratio of 1:2. By considering Reynolds number of each channel in the first pass, microchannel evaporator with channel ratio of 1:1 outperforms other two evaporators. Furthermore, the wall temperature uniformity of this evaporator is better than others. The achieved heat flux of 20 W/cm2 is much higher than the reported studies. The overall two-phase HTC of this microchannel evaporator is about 4.5 kW/m2K. • A novel two-pass microchannel evaporator is designed with spacers to improve liquid distribution. • The effect of channel number ratio between the first and second pass of the evaporator in the performance was investigated. • Thermal performance is superior in microchannel evaporator with channel number ratio of 1:2. • A high heat flux of 20 W/cm2 is achieved using HFE7100. [ABSTRACT FROM AUTHOR]

Microchannel flow boiling can dissipate high heat fluxes, but it may face issues such as excessive pressure drops and flow instability due to the rapid bubble expansion in microchannels. In this study, we developed two types of membrane-venting heat sinks featuring either open microchannels (MV-OMC) or closed microchannels (MV-CMC). Experiment revealed that MV-OMC not only effectively reduced pressure drop but also mitigated flow instability contrasted with MV-CMC. Additionally, the impact of system pressure, inlet temperature, and flow rate on MV-OMC's performance had been discussed in detail. Reducing system pressure enhanced boiling heat transfer but aggravated pressure drop and flow instability. Moreover, increasing the inlet temperature significantly suppressed flow instability at high heat flux. At an inlet temperature of 50 °C and a flow rate of 947.9 kg/(m2·s), the MV-OMC heat sink in negative pressure operating status stably dissipated a heat flux of 488.6 W/cm2 with small fluctuations of ± 0.2 °C in solid temperature and ± 0.2 kPa in pressure drop. In all, this investigation revealed the potential of membrane-venting microchannel heat sinks in high heat flux scenarios where stable dissipation was required. [ABSTRACT FROM AUTHOR]

• Flow boiling under low pressure is used to deal with instantaneous high heat flux. • Thermal performance of flow boiling in minichannel under low pressure is studied. • The wall temperature of minichannel is significantly decreased under low pressure. To cope with the challenge of instantaneous high heat flux of high-speed aircraft, a novel minichannel flow boiling thermal management method based on open system is proposed, in which the heat is dissipated by directly draining the high temperature medium to low pressure environment. To test its feasibility, the experiment on the thermal performance is conducted based on a long heat sink consisting of four long parallel mini channels. The evolution mechanism of flow boiling pattern under various back pressure conditions is investigated. The heat transfer coefficient (HTC) changes along the channel is compared, and the flow pattern evolution is obtained by visualization method with different pressure. The results indicate that flow boiling exhibits different heat transfer characteristics at different heat flux densities, and the decrease in pressure has a certain effect on the evolution of convective two-phase flow patterns. With the increase in heat flux, the wall thermal uniformity does not increase monotonically. In addition, we are surprised to discover that under the same heat flux, the wall temperature of the channel wall significantly decreases under low pressure. In our experiments, the obtained maximum temperature drop reached 20 K. Accompanied by the advantages of simple in structure, avoiding the introduction of large volume and weight cooling equipment, the proposed thermal management method provides a new idea to deal with the short-time high heat flux problem that may occur in hypersonic aircraft. [ABSTRACT FROM AUTHOR]

• Temperature overshoot of up to 40 °C was observed on the TTC during startup. • Gradual, transitional and overshoot startup processes were categorized based on dynamic temperature response characteristic. • Effects of flow rate and inlet condition on startup characteristic were investigated. This study presents an experimental analysis of the startup behavior of a manifold microchannel heat sink within a mechanically pumped flow loop. A simulated thermal test chip, embedded with temperature sensor, acted as the heat source, facilitating rapid and precise temperature measurements in the heating zone. Using ammonia as the coolant, heat flux of 634 W/cm2 over the chip area of 6 × 6 mm2 was effectively dissipated. The junction temperature surged to 86 °C during the startup and finally stabilized at 75.1 °C, highlighting the importance of assessing the transient thermal performance. Within the scope of this study, the transient maximum chip temperature during startup was observed to be capable of exceeding the ultimate steady-state temperature by nearly 40 °C. Based on the dynamic response characteristics of the chip temperature, the startup processes were categorized into three types: gradual, transitional, and overshoot. And the variations in chip temperature across these startup categories were investigated in conjunction with crucial parameters such as flow rate and pressure. Furthermore, the effects of flow rate and inlet condition on the startup processes were also examined. [ABSTRACT FROM AUTHOR]

• The effect of non-uniform metal foam inserts on R134a flow boiling was studied. • The flow characteristics depend on metal foams pore density and configuration. • The heat transfer coefficient can be enhanced up to 190% by gradient foams. • Gradient configurations are available to compromise between heat transfer enhancement with pressure drop increment according to requirements. • New correlations were proposed for boiling heat transfer coefficient and pressure drop in metal foam filled tubes. The present study investigates influences of using copper foam inserts on the heat transfer coefficient and pressure drop of boiling refrigerant in tubes with 8 mm internal diameter. R134a is selected as working fluid, which is still a popular refrigerant due to its low price and various applications in different subjects of HVAC, refrigeration, and electronics cooling, despite the new policies regarding phasing down fluorinated refrigerants. The tests are conducted for vapor quality of 0.15–0.85, mass velocity of 50–130 kg/m2s, and a 5.0 kW/m2 uniform heat flux. Under the mentioned conditions, five different simple and gradient arrangements of metal foams with 10 and 20 PPI pore density and same porosity of 0.95 are investigated. According to the research findings, mass velocity and foam PPI directly impact heat transfer coefficient and pressure drop. Moreover, increase of the vapor quality leads to an increased pressure drop. However, the manner in which the heat transfer coefficient varies with changes in vapor quality is contingent upon the mass velocity in the test section. By incorporating metal foams, a remarkable enhancement in the heat transfer coefficient of over 200 % has been demonstrated, which underscores the heat transfer augmentation potential of these materials. Despite this enhancement feature, the pressure drop caused by metal foam inserts limits their application to cases where heat transfer enhancement is in priority. Gradient metal foams show compromising increment of the imposed pressure drop along with enhancements in heat transfer coefficient; so, they can be utilized according to the design demands. Based on the results, approximately 190 % improvement is attained by using these materials for the heat transfer coefficient. New correlations are developed for predicting pressure drop and boiling heat transfer coefficient of R134a, concerning the foam arrangement effect. The results exhibit strong consistency with the present empirical data by average absolute errors of 8.86 % for pressure drop and 4.32 % for heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Experimental investigations on the flow boiling heat transfer characteristics of high-pressure water are carried out in parallel twin narrow rectangular channels. An integrated structure was utilized to define the geometry of the test section in this study. The cross-sectional size of a single channel measures 60 × 2 mm, with an effective heating and flow length of 1450 mm. Experiments encompass a range of pressures 10–16 MPa, mass flux 500–1500 kg m−2·s−1 and heat flux 100–300 kW m−2. First, the heat transfer characteristics in the parallel twin channels are determined. Results show the heat transfer and flow distribution between the twin channels are generally balanced, with only a slight fluctuation observed near the saturation point. Besides, for one narrow rectangular channel, the boiling heat transfer coefficient is found to be essentially affected by the system parameters. The increases in mass flux, heat flux, and pressure will all exert the promotion of boiling enhancement. Among these factors, pressure and heat flux have a relatively more pronounced impact. In addition, heat transfer mechanisms in narrow rectangular channel with a large aspect ratio under high-pressure conditions are analyzed in detail. The transverse buoyancy effect has been identified as a significant factor in the regions of single-phase forced convection and partial subcooled boiling. Once the heat transfer reaches the stage dominated by nucleate boiling, the bubble nucleation and motion behaviors constitute the primary factors of heat transfer enhancement, under the coupling effects of channel structure and high-pressure conditions. • Boiling heat transfer in parallel twin narrow rectangular channels is investigated. • Developed test section geometry is designed and applied to this study. • Buoyancy effect was observed in transverse direction of the channel. • High pressure improved the heat transfer performance in narrow rectangular channel. [ABSTRACT FROM AUTHOR]

• Flow boiling HTC and dP/dz of R410A and R452B are measured. • HTC of both fluids increases when mass flux or heat flux increases. • Saturation conditions have small effect on HTC of both fluids. • Both mass flux and saturation conditions have strong effects on dP/dz. • R410A has higher HTC than R452B at high mass flux or heat flux. This study investigated the flow boiling performance of the refrigerants R410A and its low Global Warming Potential (GWP) alternative R452B in a multiport microchannel tube. Heat transfer coefficient (HTC) and pressure gradient (dP/dz) for the two refrigerants were measured. The experiments covered a range of mass flux from 100 to 250 kg m−2s−1, heat flux from 2 to 8 kW m−2, and inlet temperatures from 0 to 20 °C. This study analyzed the effect of vapor quality, mass flux, heat flux, and saturation pressure on HTC and dP/dz. Additionally, the study also employed a Pearson correlation coefficient analysis to reveal the influence of these conditions on refrigerant performance. The Pearson method provided insights into the nonlinear relationships between the tested parameters and their impact on the heat transfer and flow characteristics. Experimental results were compared with predictive models to validate their applicability in practical scenarios. The comparison led to a recommendation of models. Kim and Mudawar and Bertsch et al. models for predicting the pressure gradient and heat transfer coefficient, respectively, were highlighted for their predictive accuracy. [ABSTRACT FROM AUTHOR]

• The hybrid of fin and porous-coating structure was fabricated to enhance flow boiling heat transfer in micro-channel. • Heat transfer coefficients, inlet pressure fluctuations, and pressure drops were measured. • Flow boiling instability is mitigated by the hybrid of fin and porous-coating structure. • Nucleation sites are increased by the hybrid of fin and porous-coating structure. • The generation of large size bubbles is resisted in the hybrid of fin and porous-coating micro-channel. A hybrid of fin and porous-coating (HFP) structure was manufactured to enhance the flow boiling heat transfer in a micro-channel heat sink. The heat transfer coefficients, inlet pressure fluctuations, and pressure drops were measured in a variety of mass fluxes and heat fluxes. The flow regimes were visualized to explain the enhancement of heat transfer coefficient in the two-phase flow region. The flow-boiling heat transfer characteristic in the HFP micro-channel was compared with three referenced micro-channels, i.e. a smooth micro-channel, a finned micro-channel, and a hybrid of fin and smooth-surface (HFSS) micro-channel. The results showed that the onset of nucleate boiling is initiated earlier and the critical heat flux is postponed in the HFP micro-channel compared to the three referenced micro-channels. With the porous-coating structure located downstream of the fins (i.e. HFP micro-channel), the nucleation sites are increased and the generation of large size bubbles is resisted especially in high heat flux conditions. Due to the strong hydrophilicity and liquid storage capacity in the HFP micro-channel, the maximum average heat transfer coefficient has been increased by 137.7 %, 207.4 % and 150.7 % in the two-phase region as compared to the smooth micro-channel, finned micro-channel and HFSS micro-channel, respectively. [ABSTRACT FROM AUTHOR]

• The use of a hybrid elliptical tube bundle alternative to a conventional circular tube bundle is demonstrated. • The heat transfer performance of the hybrid elliptical tube bundle is higher and limited to 12 % in comparison to the conventional circular tube bundle. • The total pressure drop is reduced up to 120 % by using a hybrid elliptical tube bundle. • The trade-off between the heat transfer and total pressure drop of a hybrid elliptical tube bundle is better and can lead to lowering the pumping power. In the current experimental study, the thermal-hydraulic behaviour of a novel smooth hybrid elliptical tube bundle (TB) is compared with the performance of the corresponding conventional smooth circular TB. The smooth hybrid elliptical tube, made up of AISI-304, is in the form of a conventional elliptical shape from the outside but a circular shape from the inside. The operating parameters encountered in the current investigation are Heat Flux (10–75 kW/m2), Mass Flux (19–100 kg/m2s), and pitch-to-diameter ratio (P/D) (1.25 - 1.95). The outcomes show that the thermal-hydraulics performance in the case of a smooth hybrid elliptical TB is higher than a conventional smooth circular TB. The augmentation in the heat transfer coefficient obtained with a smooth hybrid elliptical TB is limited to 12 % in the entire range. The pressure drop improvement is found to be 48 % to 120 % equated to the conventional smooth circular TB. However, the effective performance of the smooth hybrid elliptical TB measured in terms of H/P ratio (H/P = heat transfer coefficient/total pressure drop) is found to be in the range of 0.3–2.5 compared to the conventional smooth circular TB which is in the range of 0.2–1.1. This is evident from the present experimental study that the employment of the elliptical TB will lead to lower pumping power than a conventional smooth circular TB owing to its lower pressure because of the streamlined surface. The obtained two-phase heat transfer enhancement efficiency based on enhancement factor and pressure drop penalty factor showed a higher gap TB (P/D = 1.95) has higher performance. Moreover, for the first time, an effort has been made to analyze the physics associated with vapor bubble dynamics and interaction along the height of the smooth hybrid elliptical TB. [Display omitted] [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]

This study presents an extensive evaluation of heat transfer characteristics, flow patterns, and pressure drop for saturation pressures ranging from 460–660 kPa in a horizontal smooth tube of 5 mm internal diameter using R134a as the working fluid. The effect of saturation pressures for mass fluxes of 150–300 kg/m2s and heat fluxes of 8.26–23.3 kW/m2 which are typical of refrigeration and air conditioning applications are also investigated. Flow patterns observed during the study are predicted with a well-known flow pattern map of Wojtan et al. The experimental results are compared with seven (7) correlations developed based on different theories to find which correlation best predicts the experimental data. The results show that, at low mass flux, increasing saturation pressure results in an increased heat transfer coefficient. This effect is more pronounced in the low vapor quality region and the dominant mechanism is nucleate boiling. At high mass flux, increasing saturation pressure leads to an insignificant increase in the heat transfer coefficient. At this high mass flux but low heat flux, the heat transfer coefficient increases with vapor quality, indicating convective boiling dominance. However, for high heat flux, the heat transfer coefficient is linear over vapor quality, indicating nucleate boiling dominance. Pressure drop is observed to decrease with increasing saturation pressure. Increasing saturation pressure increases the vapor quality at which the flow pattern transitions from intermittent flow to annular flow. The flow patterns predicted are a mixture of slug and stratified wavy and purely stratified wavy for low mass fluxes. For increased mass fluxes, the flow patterns predicted are slug, intermittent, annular, and dryout. Cooper's model was the best predictor of the experimental data and the trend of heat transfer coefficient. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model is established to study the flow boiling heat transfer characteristics of the liquid film in a rotating pipe, and the effectiveness of the model is verified by a comparison between the numerical results and the experimental results. The effects of rotational speed, heat flux, and Coriolis force on the characteristics of heat transfer of the rotating liquid film are investigated. The conclusions are drawn as follows: (1) The convection of the rotating liquid film is enhanced while the nucleate boiling in the rotating liquid film is inhibited by the increase in the rotational speed; (2) With the influence of these two factors, the heat transfer coefficient increases with centrifugal acceleration increasing from 20 g to 40 g, then decreases with centrifugal acceleration increasing from 40 g to 120 g; (3) The turbulent intensity of the flow with Coriolis force is obviously increased compared to that without Coriolis force when the centrifugal acceleration ranges from 20 g to 80 g, which shows no increase at higher centrifugal accelerations when the turbulence is sufficiently strong. The Coriolis force also has an impact on the heat transfer coefficient of the liquid film, which should not be ignored when studying the boiling heat transfer of a rotating liquid film. [ABSTRACT FROM AUTHOR]

Volume of fluid（VOF） model and user-defined function were applied to simulate the flow boiling of water in microchannel with V-shaped, trapezoidal, square and dovetail-shaped cavities on the heating wall. The effects of cavity shape on the bubble behaviors of bubble nucleation, growth, detachment and coalescence were analyzed. The results show that when the heat flux on the wall is 300 kW· m-2, all cavities are activated as boiling nucleation sites. Compared with the other cavities with V, trapezoidal and square shapes, the onset of boiling time in microchannel with dovetailed cavities is relatively earlier when flow boiling occurs. In the nucleate boiling stage, compared with the trapezoidal cavity, the growth and detachment times of the bubble in the microchannel with dovetailed cavity are decreased by 7.50 ms and 6.70 ms, respectively, while the detachment frequency of the bubble is increased from 33.8 s-1 to 66.7 s-1, which is helpful to enhance the flow boiling heat transfer of water in microchannel. The coalescence and elongation of the detached bubbles in microchannel can increase the evaporation area of liquid film near the heating wall and improve the disturbance of the liquid phase simultaneously. However, it can cause local drying on the heating wall and reduce the stability and reliability of flow boiling heat transfer in microchannel. [ABSTRACT FROM AUTHOR]