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• 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]

• Designed superhydrophilic micro-ribbed surfaces with strong capillary force. • The HTC of hydrocarbon fuel has increased by 43.3% compared to deionized water. • The larger width, lower height, smaller spacing can enhance flow boiling. Due to the high heat transfer coefficient, flow boiling has applied in various industrial fields. Previous literature mainly focuses on the enhancement methods of flow boiling for water or refrigerants, with few reports on the enhancement of flow boiling for hydrocarbon fuels. Therefore, in this study, we design different types of micro-finned surface with strong capillary force to increases the nucleate sites and keep the wall in a superwetting state during the flow boiling to postpone the appearance of "annular bubble" flow. The influence of fin width, fin height, fin spacing and coolant type on boiling heat transfer through experiments and numerical simulation. The results show that the wider the fin width, the lower the fin height, the smaller the fin spacing, the stronger the heat transfer enhancement effect, and the more stable the flow boiling. The best heat transfer performance of the finned surface is achieved with the average wall temperature 27 °C lower and heat transfer coefficient 1.9 times higher than the smooth surface. Moreover, the heat transfer coefficient of hydrocarbon fuel is 43.3% larger than that of deionized water. [ABSTRACT FROM AUTHOR]

• A theoretical model of liquid film thickness during flow boiling for non-circular microchannels has been proposed. • This new model considers the effects of evaporation, shear force and transversal flow. • Numerical calculations are performed to investigate the temporal variations of transient liquid film thickness. Liquid film thickness is a dominant feature for understanding boiling heat transfer mechanism in microscale slug flow. Flow boiling in circular microchannels has been extensively studied. Microchannels with non-circular cross-section are more common in industrial applications, but there have been few studies on such complex cross-sections. In the present study, the transient liquid film thickness during flow boiling in non-circular microchannels was experimentally investigated by a laser confocal displacement meter. Non-circular tubes with inner dimension of 0.39 × 0.39, 0.5 × 0.5, 0.6 × 0.6, 0.7 × 0.7 and 0.3 × 0.8 mm2 were used for the test section, and water and ethanol were used as working fluids. The variation of liquid film thickness under adiabatic condition in non-circular microchannels was analyzed and an empirical correlation was proposed for predicting initial liquid film thickness. On this basis, a new theoretical model for liquid film thickness variation under flow boiling in non-circular microchannels was developed, considering the effects of evaporation, shear force and transversal flow. [ABSTRACT FROM AUTHOR]

• Studied flow-boiling in square channels partially filled with gradient porous media. • Proposed a new solver GPMPhaseChangeFoam embedded in open-source platform OpenFOAM. • Flow boiling heat transfer rate depends on filling ratio and porosity gradient. • The negative GPM offers higher heat transfer performance than HPM and positive GPM. Boiling heat transfer is widely used in industry and daily life and can be enhanced by porous media structure. Previously HPM have been shown to significantly improve heat transfer performance, but how the porosity gradient affects boiling heat transfer remains unknown. In this paper, the subcooled flow boiling in vertical square channels partially filled with gradient porous media was numerically investigated by using a newly developed solver named GPMPhaseChangeFoam embedded in the open-source platform OpenFOAM. The results indicate that in positive GPM inserted channels, when z ≤ 0.16 m Nu z increases with increasing H p *, and when z ≥ 0.16 m Nu z increases with increasing H p * from 0.2 to 0.8 but deceases with H p * rises from 0.8 to 1.0, and the effect of H p * on Nu z is more pronounced in the entrance region over the exit region. In negative GPM inserted channels, Nu z increases with increasing H p * throughout the channel length in positive GPM inserted channels, and the effect of H p * on Nu z is less pronounced in the entrance region than the exit region. Flow resistance increases with H p *, but changes little with Δ ε , and Nu fd increases with H p * in both positive and negative GPM inserted channels. However, Nu fd decreases with an increase in ∆ ε from 0 to 0.3 in positive GPM inserted channels, from -0.3 to 0 in negative GPM inserted channels, and ∆ ε = -0.3 at H p * = 1.0 is 13.4 % higher than that of ∆ ε = 0.3. Furthermore, the negative GPM inserted channel behaves higher flow boiling heat transfer performance than the HPM and positive GPM. [Display omitted] [ABSTRACT FROM AUTHOR]

• The heat transfer characteristics of MMC with various configurations are studied. • The groove wall configuration reduces T max of the heated wall by 6.4 K. • Improved structural configurations can accelerate the bubble growth process. • Groove wall configurations reduce pressure drop by up to 12 %. Manifold microchannels (MMC) are regarded as an efficient thermal management technique for electronic devices. However, research on optimizing the channel configuration of heat transfer in MMC has been limited. This study focuses on the design of MMC with micro fins and groove wall configurations, based on plain wall configurations. The subcooling boiling process of MMC at various heat and mass fluxes was investigated utilizing the fluid volume method (VOF) model, and parameters including average temperature, overall temperature distribution, local heat transfer coefficients, vapor-phase volume fraction, and pressure drop were thoroughly analyzed. The results demonstrate that the groove wall configuration decreases the average and maximum temperatures by up to 5.3 K and 6.4 K, respectively, compared to the plain wall and micro fins configurations. In addition, the improved configurations reduce the pressure drop by up to 12.9 % (micro fins) and 12 % (groove wall), respectively, with an increase in the fluctuation of pressure drop to varying degrees. Both improved configurations modify the overall temperature distribution, enhance the local heat transfer coefficient, and accelerate the bubble growth and detachment. These findings are not only significant for the thermal management of MMC-based electronic micro-components but also offer guidance for the design and optimization of MMC. [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]

The thermal log-law Θ + (y +) = β + 2. 12 log (y +) is valid in flow boiling with a value of β that evolves as the flow develops. Using a multiphase flow cross-literature database, this constant is shown to be β O S V = − 7 at the point of onset of significant void (OSV). This means that at the OSV the liquid is at saturation temperature up to y + ≃ 30. The OSV predictions using this model have a similar mean average error as the Saha and Zuber 1974 correlation for one less fitted constant for channel, pipe and annular flows for pressure from 1 to 147 bar and Peclet numbers from 3. 5 ⋅ 1 0 3 to 4 ⋅ 1 0 5 . This model is used to build a heat flux partitioning (HFP) inspired from system-scale codes (Lahey 1978). It predicts the distribution of the heat flux between the liquid phase and the evaporation term when the total heat flux is known. It does not give information on the total heat flux as a function of wall temperature and cannot be used to draw a boiling curve. In imposed flux conditions, this partition provides more coherent flux distribution between the evaporation and liquid terms than Kurul and Podowski (1990) based approaches and improves void fraction predictions in high-subcooling regions on the DEBORA database (Garnier et al. 2001) and on experiments by Bartolomei and Chanturiya (1967) and Bartolomei et al. (1982). When the wall temperature is imposed, it must be coupled with an empirical boiling total heat flux correlation to replace a traditional HFP. The prediction of the total heat flux is then as good as that of the correlation. [Display omitted] • The temperature profile logarithmic law holds in flow boiling with a modified constant. • Onset of significant void occurs when the near-wall liquid layer, for y + ≲ 30, is saturated. • We propose a model to distribute heat between evaporation and the liquid for a given heat flux. • It can be used as a heat flux partition in simulations where the wall heat flux is imposed. • When the wall temperature is imposed it can be coupled with a heat transfer correlation. [ABSTRACT FROM AUTHOR]

• New data on flow boiling pipe flows in microgravity are presented: flow pattern, heat transfer coefficient. • A criteria for dependence of heat transfer coefficient on gravity is proposed. • A correlation for the prediction of heat transfer coefficient in microgravity is based on the model of Kim and Mudawar (2013) including heat transfer due to nucleate boiling and convection. The development of long-term space thermal management systems has informed research into the influence of gravity on boiling. This work explored the influence of gravity on the hydrodynamics and heat transfer of boiling flow. Experiments were carried out using two test loops each consisting of a 6 mmID transparent cylindrical test section. Upward (+ 1 ░ g) and downward (− 1 ░ g) flow boiling experiments were carried out in the laboratory while microgravity (μ g) experiments were carried out during a parabolic flight campaign. The results of flow visualisation showed significant influence of gravity on the flow patterns and the influence of gravity was generally limited to mass flux, G ≤ 400 k g / m 2 s and/or vapor quality, x ≤ 0.35. In all three gravity conditions, the measured heat transfer coefficient was influenced by heat flux, mass flux and/or vapor quality. For liquid Reynolds number, R e l o ≤ 2000 (G ≤ 150 k g / m 2 s) and boiling number B o < 0.002 the measured heat transfer coefficient was highest in − 1 g flow and lowest in μ g flow but becomes comparable at B o > 0.002. A correlation for predicting microgravity heat transfer coefficient was proposed in this work and the proposed correlation predicted 100 % of the μ g data in the current work within ± 20 % , predicted nearly 100 % of the μ g data of Ohta et al. (2013) within ± 30 % and around 85 % of the μ g data of Narcy (2014) within -20 % to +50 %. A correlation for predicting the gravity dependent regime as it relates to heat transfer coefficient in + 1 g and μ g flows was also proposed in this work. The proposed criterium correctly predicted over 85 % of the gravity-dependent heat transfer coefficient in the current work and the works of Lebon et al. (2019) , Narcy (2014) , Ohta et al. (2013). Gravity dependence Map: B o < 0.002 − 0.002 1 + e [ − 2 (F r m − 2) ] Prediction of heat transfer coefficient in microgravity h 2 ∅ _ μ g = [ (1.15 h n b _ + 1 g ) 2 + (F. h c b _ + 1 g ) 2 ] [Display omitted] [ABSTRACT FROM AUTHOR]

Microchannel flat-tube heat exchangers are known for their high heat transfer efficiency and compact size. In practical applications, the predominantly adverse influence of insoluble impurities on the condensation process necessitates further exploration into the underlying mechanisms affecting the flow boiling process. The effect of common insoluble impurities on the flow boiling of R410A refrigerant in a microchannel flat-tube heat exchanger has been investigated, and variation of the impurity volume fraction on heat transfer, mass transfer, pressure drop, and bubble formation are analyzed. Additionally, an adaptive modification method for the mass transfer factor is proposed. The numerical model of flow boiling heat transfer in a microchannel is validated through experimental data and empirical correlations. Thermo-hydraulic performance evaluation incorporates efficiency index and area goodness ratio metrics. Results reveal that the presence of insoluble impurities complicates the boiling heat transfer mechanism by influencing bubble evolution and multiphase flow patterns. Air impurities at a volume fraction not exceeding 5% prove beneficial for enhancing flow boiling efficiency in microchannels; however, a 38.5% decrease in efficiency index is observed with 1% oil impurities present. Optimal thermo-hydraulic performance is achieved with approximately 2%–3% water impurities. [Display omitted] • Mechanisms of air, water, and oil impurities on flow boiling heat transfer are explored. • Differences across various microchannel locations and surfaces are revealed. • A new self-correcting phase-change-factor numerical model is proposed. • An experimental platform for microchannel flow boiling heat transfer is introduced. [ABSTRACT FROM AUTHOR]

This study intends to address the inquiry of finding the practical ranges of flow conditions during flow boiling of R134a in the presence of copper foam inserts based on analyzing the second law of thermodynamics. The experiments are exercised under mass velocities from 50 kg/m2s to 130 kg/m2s within copper tubes of 3/8 in., while the uniform boundary condition of 5.0 kW/m2 heat flux is fixed for all tests. Metal foam pieces consisting of 10 and 20 PPI pore densities are employed in diverse arrangements. The results reveal that the metal foam inserts systemically affect the dominant entropy generation mechanism during the flow boiling, i.e., heat transfer or friction. The total rate of generated entropy in metal foams is directly affected by vapor quality and mass velocity, especially at high fluid velocities, when frictional pressure drop is the determining mechanism of entropy generation. In addition, at a fixed quality, the number of pores in copper foam inserts affects the rate of total entropy generation, except for the lowest mass velocity. The performance assessment is conducted by computing the entropy generation number. According to this criterion, metal foams show a promising performance under low vapor quality and mass velocity conditions, where the flow patterns are most likely to be slug or stratified–wavy. Eventually, the development of two correlations is undertaken for the estimation of friction and heat transfer contributions to the rate of entropy generation with mean errors of 9.40 % and 5.54 %, respectively. [Display omitted] • The effect of metal foams on flow boiling was studied from the entropy generation point of view. • The heat transfer contribution to entropy generation was dominant for plain tubes. • For metal foams, frictional entropy generation was the primary irreversibility source. • Metal foam PPI increases the frictional and total entropy generation. • Metal foam inserts are recommended for low mass velocity and vapor quality. [ABSTRACT FROM AUTHOR]

• An experimental study of ammonia flow boiling has been provided for the high-pressure conditions. • Horizontal, upflow and downflow configurations have been considered in smooth, inner-finned and annuli tubes. • The qualitative and quantitative insight into the difference in the boiling mechanism has been proposed. The manuscript introduces an experimental study of the ammonia flow boiling in the different tube profiles and gravity orientation, enhancing existing ammonia test data with high saturation temperature cases (45 to 65°C). The mass velocity of the study varies from 60 to 140 kg·m−2·s−1, vapour quality from 0.1 to 0.9, and heat fluxes from 1.1 to 7.7 W·cm−2. Four aluminium tube profiles were tested: ID 11 mm smooth tube, ID 11.5 mm inner-finned tube and derived designs of two annuli channels. It has been demonstrated that Zurcher et al. and Seiner models accurately predict stratified-wavy to annular/intermittent transition in the horizontal smooth tube at high saturation temperatures. Kattan et al. evaporation HTC correlation modified by Wojtan et al. with the suppression factor of nucleation boiling constituent (0.8) precisely corresponds to the new ammonia data. However, a new phenomenological recommendation for suppression factor determination has been proposed. At the same time, the Kattan et al. evaporation HTC model without suppression factor provides the best estimates for the upflow cases of smooth tube, horizontal and upflow in annuli tubes. A specific evaporation HTC behaviour has been observed in the finned line and finned annuli at the horizontal and upflow configuration, which necessitates much deeper theoretical and experimental investigations of the inter-fin flow boiling mechanism and phase distribution between the core and inter-fin parts of the cross-section. In particular, the gravity-independent two-phase pattern has been revealed at the mass velocity 120 kg·m−2·s−1 and saturation temperature of 45°C, which is characterised by the independence of the evaporation HTC on heat fluxes at vapour quality in the range of 0.1 to 0.7. [ABSTRACT FROM AUTHOR]

• A dynamic two-phase flow lumped model is developed for parallel microchannels • Channel-to-channel thermal conductance (CCllllll) mitigates the flow maldistribution • Linear stability shows diminishing effect on maldistribution with increasing CCllllll • A single eigenvalue revealed to govern flow maldistribution in thermally coupled channels • In the limit of very high CCllllll , this eigenvalue only depends on fluid properties Phase change heat transfer processes are attractive for the efficient thermal management of advanced semiconductor devices, with flow boiling in parallel microchannel heat sinks being one promising solution. However, there are several implementation challenges associated with two-phase flow in parallel microchannels, particularly flow maldistribution, which can adversely affect performance and reliability. Two-phase flow models can be useful in predicting and understanding the instability mechanisms that leads to maldistribution, such that parametric performance trends and safe regions of operation can be identified. Channel-to-channel thermal coupling has been shown to alleviate maldistribution, but it is not yet understood why this effect is limited to some critical lateral thermal conductance above which there is no further benefit. In the current work, a two-phase flow model with a lumped thermal capacitance representation of the heat sink wall is developed to identify the mechanism for this asymptotic maldistribution stability boundary for thermally coupled microchannels. The lumped model allows the nature of stability boundary to be explained via a scaling analysis of the eigenvalues. In such systems, it is identified that a single eigenvalue determines the occurrence of flow maldistribution. Scaling analysis shows that, for small values of thermal conductance, this eigenvalue becomes a quadratic function of thermal coupling, which enhances the stability of microchannels and moves the stability boundary. However, for large values of thermal conductance, the eigenvalue is dependent only on the fluid properties, leading to a limit at which thermal coupling can no further improve stability or affect the stability boundary. The lumped model developed in this work enables mechanistic understanding of the role of channel-to-channel thermal coupling on mitigating flow maldistribution and therefore may offer important guidance on the design of flow boiling systems and heat sinks. [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]

• R134a flow boiling 3D simulations were performed in smooth tubes using ANSYS fluent. • Effect of saturation condition, heat and mass flux was studied at varying diameter. • Heat transfer and pressure drop results are in dimensional and non-dimensional forms. • Results agree well with relevant correlations and experiments with low deviations. • Findings contribute to understanding flow boiling and optimizing heat exchangers. Three-dimensional transient simulations using ANSYS Fluentʼs mixture multiphase model was conducted to investigate the impact of heat flux, mass flux, and saturation temperature variations on pressure drop behaviour and heat transfer characteristics during flow boiling of R134a refrigerant in smooth horizontal tubes of different diameters. The simulations employ an implicit numerical method to solve the governing equations, with relevant source terms for mass and energy transfer, incorporating the continuum surface force (CSF) and shear stress transport (SST) κ-ω models to account for surface tension and flow turbulence respectively. The results are presented in both dimensional (heat transfer coefficient and pressure drop) and non-dimensional (Nusselt number and two-phase frictional factor) forms. The Nusselt number shows a direct relationship with the Reynolds number, while the two-phase friction factor exhibits different behaviour. At higher vapor quality, mass flux significantly affects heat transfer performance and pressure drop. Heat flux plays a crucial role in heat transfer, but its impact on pressure drop is negligible. Higher saturation temperatures enhance heat transfer while reducing pressure gradients. The obtained results demonstrate good agreement with existing correlations, with mean absolute deviations (MAD) as low as 1.2 % for the heat transfer coefficient and 23.7 % for the pressure drop behaviour. This highlights the model's accuracy in predicting flow boiling behaviour of R134a in smooth horizontal channels, providing valuable insights for thermal analysis. The findings contribute to the understanding of flow boiling processes and aid in designing and optimizing efficient heat exchangers for various engineering applications. [ABSTRACT FROM AUTHOR]

• The distribution of vapor and liquid in the cross-section of inclined tubes is demonstrated. • The velocity distribution of vapor and liquid in the cross-section of inclined tubes is analyzed. • Types of boiling crisis in inclined tubes is revealed. • The inclination angle affects the wall temperature rise and the location of boiling crisis. The tube of marine heat exchangers is inclined under the action of wind, wave and current, which changes the flow and heat transfer characteristics and thus poses a threat to the safe and stable operation of the heat exchanger equipment. In this study, the Eulerian-Eulerian two-fluid model is applied to simulate the flow boiling and boiling crisis process in 5 upward flowing inclined tubes with inclination angles ranging from 0° to 90° with 14.9 mm inner diameter. The study uses water as the working fluid at 7 MPa with 500 kg/m2s inlet mass flux and 410 kW/m2 uniformly distributed heat flux. The results show that the distribution of vapor-liquid phase along the radial direction in the inclined tubes is highly non-uniform, with the vapor phase accumulating at the top of the tubes. This phenomenon is particularly obvious in the low vapor quality region. In this region, the axial flow velocity at the top part of inclined tubes is 3 times higher than that at the bottom part of the tubes. As the vapor quality increases, the non-uniformity in vapor-liquid phase and flow velocity distribution decreases. In the inclined tubes, two types of boiling crisis occur: Departure from Nucleate Boiling (DNB) at the top of the tube and dryout in the region with high vapor quality. The wall temperature distribution along the circumferential direction is non-uniform, with a localized overheating region at the top of the wall near the inlet. The inclination angle affects both the location and the magnitude of the wall temperature increase after the onset of boiling crisis. The frictional pressure drop increases as the vapor quality increases and decreases as the inclination angle increases. [ABSTRACT FROM AUTHOR]

• Maldistribution in parallel channel systems can impede performance significantly. • Modeling maldistribution is challenging due to non-linearity and multiple solutions. • Entropy analysis allows predicting flow maldistribution in parallel channels. • Maldistribution takes place since it is thermodynamically favored over uniform flow. • Flow maldistribution is associated with the maximization of entropy production. Flow across heated parallel channel systems exists in many applications. The performance of such systems experiencing multiphase flow could suffer from the deleterious effects of flow non-uniformity or maldistribution. Modeling the behavior of such systems is challenging due to the inherent non-linearity associated with the multiphase flow and the difficulty in determining the actual flow among several possible flow distributions. This study addresses the challenge by analyzing the entropy production in such systems. Using experiments on two thermally isolated, nominally identical, and externally heated parallel channels, we quantify irreversibility in the resulting multiphase flow by evaluating the entropy generation rate. Our experiments reveal that certain flow conditions result in severe maldistribution (flow ratio > 10) in the channels, associated with a sharp rise in entropy production. Such an increase is not predicted for uniform flow distribution across parallel channels, making maldistributed flow a thermodynamically favored state over equally distributed flow. We extend this understanding to non-identical parallel channels as well. With entropy analysis providing additional insight besides the fundamental equations governing mass, momentum, and energy conservation, this approach is valuable in predicting and controlling flow distribution in parallel channel systems. [ABSTRACT FROM AUTHOR]

This paper presents experiments to investigate flow boiling and pressure gradients of R600a. The study employed microfin and smooth copper tubes simulating conditions in a domestic refrigerator, using low mass fluxes (MFs) and saturation temperatures (T sat). The test sections featured a tube length of (400 mm), an inner diameter of (4.3 mm), and an outer diameter of (5 mm). Test conditions varied, including MFs of (25, 35, and 45 kg m−2 s−1), vapor quality from (0.18–1), T sat of (−15, −9, and −5 °C), and HFs of (10, 14, and 18 kW m−2). Both tubes shared identical dimensions, clearly comparing microfin impact on flow boiling characteristics. The study evaluated enhancement factor (EF), penalty factor (PF), and overall efficiency (η) by comparing heat transfer coefficients (HTC) and pressure gradients in the microfin tube versus the smooth tube. Results indicate a significantly boosted HTC in the microfin tube compared to the smooth tube, with a reasonable increase in pressure drop. Under specific conditions (HF of 10 kW m−2, T sat of −9 °C, and MFs of 25 kg m−2 s−1), the average HTC is enhanced by 325%, while the pressure drop is increased by 150%. Experimental HTC and frictional pressure gradient comparisons aligned well with values predicted by existing literature correlations. [ABSTRACT FROM AUTHOR]

This work presents for the first time an innovative solution for measuring small pressure changes as a pressure front moves along a surface at the micro-scale, targeting applications in microfluidics. Pump-less flow boiling is an attractive method of improving heat removal from electronic devices for its lack of additional pump and the efficiency gained from phase-change cooling over conventional pumped single-phase cooling. Efforts to characterize the mechanisms of boiling at the micro-scale to effect pump-less flow has primarily focused on taking pressure measurements at the entry and exit of the channel, not within it, limiting the ability of current computer models to scale this technology. A pressure sensor array designed to operate in boiling conductive fluids is proposed in this paper to create a map of the pressure front on channel surfaces from nucleating gas-phase bubbles within a tapered micro-gap. 8 200 mm piezoresistive transducers are fabricated on a 0.9 mm by 20 mm chip using a novel hybrid bulk- and surface-micromachining process to create 240 nm thick diaphragms targeting a 1 µV output for 20 Pa of applied pressure, or a sensitivity of 50 nV Pa-1. Characterization of the fabricated devices yields a sensitivity of 155 nV Pa-1 with R2 = 0.90 at 5 V input, and a high correlation with the diaphragm's physical deformation. [Display omitted] • Fluid mechanics of flow boiling in tapered micro-channels are not well-modeled. • A MEMS pressure sensor array is designed to operate in boiling conductive fluids. • Piezoresistive design scaled to 20 Pa pressure detection. • Innovative solution for measuring micro-scale pressure fronts. [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]

• Heat transfer enhancement of zeotropic mixtures was studied. • Regulating mass transfer resistance was conducted by constructing surface structure. • Multi-cone staggered structure reduces mass transfer resistance and improve heat transfer. The mass transfer resistance is a significant element leading to heat transfer degradation of zeotropic mixtures in the flow boiling. In this paper, the heat transfer enhancement of zeotropic mixtures in the flow boiling was investigated through regulating the mass transfer resistance by constructing reasonable surface structure. The heat and mass transfer, pressure drop characteristics of stratified bubble flow for R134a/R245fa zeotropic mixtures in a rectangular channel with multi-cone staggered spoiler porous structure were studied experimentally. The bottom of channel was sintered with multi-cone staggered porous layer with thickness/height/diameter of 1/3/2 mm, respectively. The experiments were carried out under an inlet evaporation temperature of 20 ℃, and the ranges for heat and mass fluxes were 10–150 kW/m2 and 100–150 kg/(m2·s), individually. Compared to the rectangular channel with smooth porous structure, liquid mass transfer resistance of zeotropic mixtures in the rectangular channel with multi-cone staggered porous structure was decreased obviously. The decrease degree for Mixture 2 was largest, while the maximum average decrease was 89.61 % at G = 100 kg/(m2·s). The heat transfer coefficients were obviously improved and heat transfer enhancement increased gradually especially under conditions of high heat flux. The enhancement degree for Mixture 2 was the largest, while the maximum average increase was 26.05 % at G = 100 kg/(m2·s). The pressure drop increased significantly, and the increasing degree for Mixture 2 was smallest, while the minimum average increase was 38.18 % at G = 150 kg/(m2·s). Multi-cone staggered structure was helpful to reduce liquid mass transfer resistance and improve heat transfer performance for zeotropic mixture. The zeotropic mixture with higher liquid mass transfer resistance would have better heat transfer intensify and smaller increase of pressure drop with the help of multi-cone staggered structure. [ABSTRACT FROM AUTHOR]

• The influence of operating parameters on heat transfer performance and instability was studied. • The dissipated heat flux up to 117 W/cm2 for a large heating area of 30 × 40 mm2 was achieved. • Four flow patterns were observed and the flow reversal occurred when the R is higher than unity. • An improved predictive model for heat transfer coefficient was proposed. Flow boiling in parallel microchannels is an efficient method to deal with the high heat flux removal challenge for electronic modules. In this work, the visualized experiments on the R134a flow boiling inside a high-heat-flux microchannels heat sink were performed. The heat sink contains 27 parallel microchannels with a hydraulic diameter of 0.89 mm and a length of 30 mm. The experiments were conducted over a base heat flux between 2 and 117 W/cm2 (based on the footprint area), mass velocity of 282–1033 kg/m2 s, the saturation temperature ranged from 27.2 to 35.5 °C, exit vapor quality up to 0.45 and an inlet sub-cooling approximately 3 °C. The effects of mass velocity, saturation temperature, and heat flux, on the flow boiling characteristics were analyzed and discussed. Results demonstrated that the heat transfer coefficient (HTC) rise with the increase of mass flux under middle and high heat flux. At the maximum mass velocity of 1033 kg/m2 s, the flow boiling HTC reached 51456 W/m2 K. Moreover, the pressure drop increased with higher mass and heat flux, while it decreased with increasing saturation temperature. Additionally, under high heat flux conditions, the two-phase flow became more stable, and the wall surface temperatures exhibited greater uniformity when higher mass velocity or saturation temperature was applied. The visualization results illustrated that there were four flow patterns and reverse flow occurred when R exceeded unity. Finally, the experimental results were evaluated with six previous heat transfer correlations. An improved predictive model for flow boiling HTC was developed and good agreement with present data was obtained with MAE of 9.08 %. A two-phase micro-channels cold plate for high-power density electronic components could be rationally designed and optimized with reference to this work. [ABSTRACT FROM AUTHOR]

• Liquid nitrogen flow boiling heat transfer at low pressure is studied. • Reveals heat transfer mechanisms based on the flow patterns. • The effects of parameters on the heat transfer coefficient (HTC) are analyzed. • Decreasing the pressure improves the convective evaporation. Nitrogen is the most important cryogen in superconductivity. This study investigates the heat transfer characteristics of flow boiling in a horizontal macro-tube with an inner diameter of 10 mm, particularly focusing on flow boiling at negative gauge pressure. The experiments cover the following ranges: inlet pressure from −31.0 to 2.5 kPa, mass flux from 27.0 to 71.3 kg/(m2·s), and heat flux from 0 to 28.68 kW/m2. The effects of operating parameters on the heat transfer coefficient (HTC) are examined, and the HTC data are compared with the predicted value from four correlations. At the bottom wall, the HTC distribution exhibits a "two-peak" shape. The primary peak is dominated by nucleate boiling in bubbly, plug, and slug flow with low vapor quality, while the secondary peak is dominated by convective evaporation with higher vapor quality. These two regions are roughly divided by the vapor quality of 0.3 to 0.4. In stratified flow, the HTC at the top wall is generally lower than that at the bottom. The HTC increases as the pressure decreases, particularly in the convective evaporation region. This is attributed to the high liquid–vapor density ratio, extensive thermal conductivity, and substantial surface tension at lower pressure. The effect of heat flux or mass flux on the HTC at the bottom wall differs between the nucleate boiling and convective evaporation regions. Increasing the heat flux improves the HTC where nucleate boiling dominates, while increasing the mass flux significantly improves convective evaporation, especially at higher heat flux. All selected correlations overestimate the HTC at the top wall. The Shah correlation demonstrates the best prediction accuracy for the HTC at the bottom wall, both at near atmospheric pressure (Mean Relative Error, MRE = 21 %) and at negative pressure (MRE = 19 %). [ABSTRACT FROM AUTHOR]

• LN 2 flow boiling in a 10 mm horizontal tube at low pressure is studied. • Stratified and wavy flow and its transition to annular flow were observed. • Oscillations of parameters in plug and slug flow are studied. • Thermal oscillation shows a strong relationship with the flow patterns. Liquid nitrogen holds significant importance in high-temperature superconductivity. This study experimentally investigates the saturated flow boiling of liquid nitrogen in a horizontal macro-tube with an inner diameter of 10 mm. The experiments cover the following ranges: inlet gauge pressure from −31.0 to 2.5 kPa, mass flux from 27.0 to 71.3 kg/(m2·s), heat flux from 0 to 28.68 kW/m2, and vapor quality from 0 to 1. In the initial segment of the study, this paper concentrates on flow patterns, two-phase flow instability, and pressure drop. Flow patterns are identified by analyzing temperature distributions along the tube and oscillations of operating parameters. The two-phase flow patterns mainly comprise bubbly flow, plug and slug flow, stratified flow and wavy flow, annular flow, mist and vapor flow. Annular flow tends to occur under negative pressure due to the increased vapor velocity resulting from decreased vapor density. Oscillations are observed in plug and slug flow, with the pressure drop and wall temperature displaying a small period and a strong relationship. The oscillations in pressure drop and wall temperature are linked to the periodic formation and release of large bubbles in plug and slug flow. Thermal oscillation exhibits a strong correlation with flow patterns. For vapor quality less than 0.2, the amplitude of wall temperature oscillations is larger, attributed to the occurrence of plug and slug flow. The experimental data of the total pressure drop are compared with predicted values from the homogeneous and separated flow models. The separated flow model provides a more accurate prediction, with a Mean Relative Error (MRE) of 39 % within the present experimental range. [ABSTRACT FROM AUTHOR]

• HFE-7100 boiling curves from the onset of boiling to dry-out, in a vertical and rectangular mini-channel. • Local heat transfer coefficient (HTC) calculated with an inverse heat conduction method. • Influence of subcooling and pressure on heat transfer, pressure drop and flow patterns. • Increasing pressure improves the critical heat flux (CHF) and HTC, delays the dry-out, and reduces pressure drops. • Increasing subcooling improves CHF and HTC, delays the dry-out, and modifies the flow pattern. The background of this study is the problem of thermal runaway in the battery cells of electric vehicles. Increasingly powerful and compact battery packs require highly efficient thermal management. One potential solution is the direct liquid cooling of lithium-ion battery cells using a dielectric liquid which boils over a safety limit temperature. This approach intends to prevent in a first step the cell thermal runaway and if it fails in a second step its propagation to its neighbors: the liquid boiling helps to mitigate the cell temperature rise and to transport and spread the heat. The arrangement of the cells in the pack and their close spacing induce a scientific problem of convective boiling in a mini-channel. To enhance designs using this solution, a comprehensive understanding of flow boiling and a precise local heat transfer quantification is essential. The HFE-7100 has been selected as a working fluid for its attractive thermophysical properties and a suitable saturation temperature. The present paper introduces new experimental results of flow boiling dielectric fluid, the HFE-7100, in a vertical rectangular mini-channel. Measurements are carried out from start of boiling to dry-out. Boiling curves are obtained for a mass flux of 391 kg/(m².s), at different pressures (0.7, 1, 1.5 bar) and at different subcoolings (Δ T s u b = 0 ; 15 ; 30 ; 45 ∘ C). The test section is an aluminum block instrumented with three lines of five K-type thermocouples located all along the flow. The visualization of the flow is achieved through a glass pane with a high-speed camera to identify different flow patterns. The local heat transfer coefficient is calculated by a resolution of a 2D inverse heat conduction method. The experiment mainly aims at determining the heat transfer coefficient, the critical heat flux (CHF) and the pressure drop and at characterizing both the flow regimes and the dry-out phenomenon. The influence of subcooling and pressure on flow boiling is analyzed. An increase in subcooling enhances the CHF and the heat transfer coefficient, and delays the dry-out. Subcooling has an impact on the flow pattern and modifies the type of CHF. In the operating conditions tested, a decrease in pressure enhances CHF and delays dry-out. On the other hand, the heat transfer coefficient increases with rising pressure. Similarly, pressure drops are lower with higher pressure. [ABSTRACT FROM AUTHOR]

• A "Synchronous backflow" phenomenon is observed in foam fin microchannels (FFMCs) and it is the cause of heat transfer deterioration. • An HTC correlation is developed for FFMCs with different porosities and fin-channel width ratios with a MAPE of 9.05 %. • HTC increases as porosity decreases in low-quality region while the variation is opposite in high-quality range due to the balance of heat transfer area and transverse flow resistance. • Fin area utilization efficiency drops with wider fins and optimal fin-channel width ratio is 1. • FFMC with 0.78 porosity and the same fin and channel widths shows the best flow boiling performance due to higher HTCs and lower pumping powers. Flow boiling in copper foam fin microchannels (FFMC) has demonstrated better heat transfer performance and flow stability. In this work, systematic experiments are conducted to investigate the effects of structural parameters on flow boiling in five FFMCs with porosities ranging from 0.78 to 0.82 and ratios of fin width to channel width ranging from 0.5 to 2. Five mass fluxes ranging from 68 kg/(m2s) to 408 kg/(m2s) and effective heat fluxes ranging from 2 W/cm2 to 297 W/cm2 are tested. The heat transfer, pressure drop, and flow instability characteristics are analyzed. Generally, the heat transfer in FFMCs follows the "three-zone" model. The heat transfer coefficient first increases with increasing heat flux and then decreases. A "synchronous backflow" phenomenon is observed in FFMCs, which is the direct cause of heat transfer deterioration. Due to the interaction of heat transfer area and the transverse flow resistance through the fins, the heat transfer coefficient increases with decreasing porosity in the low-quality region, while the opposite trend is observed in the high-quality region. The utilization efficiency of the fin area decreases as the fin width increases. An optimal ratio of fin width to channel width exists corresponding to the best heat transfer performance, which equals one in this work. A correlation is developed based on 583 data points considering heat transfer mechanisms in channels and fins. The new correlation predicts the experimental values with a mean absolute percentage error of 9 %, and over 99 % of the data are predicted within a 30 % deviation. A performance evaluation criterion is defined to comprehensively compare the FFMCs, and the FFMC with the porosity of 0.78 demonstrates the best flow boiling performance due to higher heat transfer coefficients and lower pumping powers when the ratio of fin to channel width is equal to one. [ABSTRACT FROM AUTHOR]

• Flow boiling heat transfer characteristics of R1234ze(E) (a low global warming potential (GWP) refrigerant) at moderate and high saturation temperatures is investigated. • Heat transfer characteristics and flow pattern visualization through high-speed camera were investigated for saturation temperatures ranging from 60 to 95 °C, corresponding to reduced pressures from 0.35 to 0.75. • Pre-dryout data measured were compared against different prediction methods developed under different theories. • Saturation temperature has a significant effect on heat transfer coefficient and the nucleate boiling dominates the heat transfer. • The higher the saturation temperature, the earlier the vapor quality at which dryout incipience occurs. This study reports experimental results of flow boiling heat transfer characteristics of R1234ze(E) (a low global warming potential refrigerant) at moderate and high saturation temperatures. The study was performed in a single horizontal circular smooth tube of 2.07 mm internal diameter. Heat transfer characteristics and flow pattern visualization through high-speed camera were investigated for saturation temperatures ranging from 60 to 95 °C, corresponding to reduced pressures from 0.35 to 0.75, mass velocities from 300 to 500 kg/m2s, heat fluxes from 30 to 50 kW/m2 and vapor qualities from 0 to 1. The pre-dryout data (214 data points) measured were compared against different prediction methods from the literature developed under different theories. The study revealed that the saturation temperature has a significant effect on heat transfer coefficient whereas the mass velocity presented a less pronounced or negligible effect. Furthermore, the heat transfer coefficient was observed to decrease slightly to a minimum in the low vapor quality region and then levels off or increases slightly with increasing vapor quality until dryout occurs. As a result, nucleate boiling was observed to be the dominant mechanism controlling the heat transfer. It was observed that the higher the saturation temperature, the earlier the vapor quality at which dryout incipience occurs. Considering the 214 pre-dryout data points analyzed, the methods proposed by Kew and Cornwell (1997), Lazarek and Black (1982) and Choi et al. (2007) were the most accurate, predicting the entire pre-dryout database with mean absolute errors (MAEs) of 13.7%, 13.9 % and 14.4 %, respectively. [ABSTRACT FROM AUTHOR]

• 1 Flow and boiling heat transfer in a 5 mm tube were studied experimentally. • 2 Heat transfer enhancement is obviously founded in the region where x e < 0.05. • 3 New empirical correlations are developed based on the experimental data. • 4 New correlations have higher prediction accuracy than the existing correlations. Experimental investigation on flow boiling heat transfer in a small tube under high system pressure has been conducted in this study. The test-section is constructed with a 316L stainless steel tube. The inner diameter and heated length of the test-section are 5 mm and 930 mm, respectively. The working fluid is deionized water. The operating conditions of the experiments are as follows, the mass flux from 548 to 978 kg/(m2·s), the wall heat flux from 136 to 238 kW/m2, and the system pressure from 1.0 to 2.9 MPa. The experimental results showed that the inlet subcooling mainly affects the onset of nucleate boiling, but its impact on the heat transfer coefficient at the same vapor quality is almost negligible. The heat transfer coefficient increases with the increase of mass flux and wall heat flux, especially in the region where x e < 0.02. The average heat transfer coefficient is higher under high system pressure owing to the variation of the thermophysical properties. Additionally, the existing flow and heat transfer correlations cannot predict the flow boiling heat transfer coefficient and frictional pressure drop in the experiments accurately enough. Therefore, the new flow and heat transfer correlations are developed based on the experimental data in this paper, and the mean absolute deviation of the flow boiling heat transfer correlation and frictional pressure drop correlation are 13.3% and 12.1%, respectively. [ABSTRACT FROM AUTHOR]

The study of bubble dynamics is important in various engineering applications as it can provide a fundamental understanding of the bubbly flow behavior. This paper presents a novel method for tracking bubble trajectories and classifying bubble behaviors, including formation, extinction, continuous movement, fragmentation, and coalescence. The method is based on the theory of perfect matching in bipartite graphs. By establishing the perfect matching between bubbles identified at adjacent time instants, judging the continuity of bubble motion, and simply calculating the number of bubbles in the neighbor of matching bubbles at adjacent time instants, the method can deal with the challenging situation where the bubble position and volume significantly change between two adjacent time instants. The method is validated in a simulated vertical pipe boiling flow, showing that the tracking accuracy is 100% and classification accuracies for continuous movement, fragmentation, and coalescence events are estimated to be larger than 90% in a wide range of time steps between two adjacent time instants. • A Kuhn-Munkres-based algorithm for precise multi-bubble tracking is proposed. • Multi-bubble behavior through changes in bubble position and quantity is classified accurately. • A quantitative analysis of tracking accuracy in a 2D pipe bubble flow by LBM is conducted. • 100% tracking accuracy and >90% classification accuracy over wide time steps are achieved. [ABSTRACT FROM AUTHOR]

• Influence of thermophysical properties on flow dynamics and heat transfer performance were experimentally investigated. • The relative effects of forces governing the liquid - vapor interface are profoundly analyzed with quantitative evaluation of the non-dimensional parameters. • A new heat transfer correlation that is valid for vastly different working fluids is proposed and compared with some of the existing correlations. • Drastic difference presented in bubble dynamics, flow pattern transition, heat transfer and pressure drop for working fluids with vastly different thermophysical properties. A thorough understanding of the governing interfacial interactions during flow boiling in a microchannel would provide important insights into the underlying physics of fluid dynamics and heat transfer mechanisms. In this study, flow-boiling experiments were performed in a single rectangular microchannel with hydraulic diameter of 0.18 mm. Three working fluids (water, ethanol, and HFE7100) with significantly different thermo-physical properties were tested under an inlet subcooling of 25 °C, mass flux of 50–330 kg·m−2·s−1, and effective heat fluxes up to 400 kW·m−2. Non-dimensional groups were used to quantify the relative effects of the forces governing the liquid–vapor interface. The bubble dynamics, flow pattern transition, heat transfer, and pressure drop were compared for the three different working fluids, and a mechanistic explanation of the flow boiling performance was proposed based on a force scaling analysis. Meanwhile, through investigating the influence of dimensionless parameters on heat transfer coefficient, a new basic heat transfer correlation that applies to a wide range of working fluids was developed, with 92.5 % of data points falling within ± 30 % error bands and with a mean absolute error of 14.7 %. HFE7100 exhibited the minimum increase in pressure drop with heat flux owing to its smallest liquid-to-vapor density ratio, low viscosity, and lowest interfacial tension effect. This study lays the foundation for the development of more reliable theoretical correlations and provides important insights into optimal design principles. [Display omitted] [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]

Electrohydrodynamic can significantly improve flow boiling heat transfer. However, pin electrode configuration has not been investigated comprehensively. This study explores the flow boiling performance of vertical minichannels in non-uniform electric field formed by pin electrode. The influences of pin electrode spacing and array region on flow pattern and heat transfer under electric field are studied. Flow patterns of bubbly, slug and confined bubble flow were captured and analyzed by using visualization techniques. The results demonstrate that the volume of bubbles decreases and the number increases under electric field. Besides, electric field mitigates bubble coalescence and delays the flow pattern transition. The electric field enhancement ratio is up to 1.64. The enhancement effect of these tested electrode configurations is related to the length to diameter ratio for the bubble. Under present experimental conditions, when the average length-to-diameter ratio exceeds 3.12, the electric field enhancement effect begins to diminish. In the theoretical aspect, we perform numerical simulations of the detachment behavior of the bubbles under different electrode arrangements. Furthermore, simulations of the bubble detachment process under an electric field are carried out to investigate the enhanced mechanism of pin electrode on minichannel flow boiling. [ABSTRACT FROM AUTHOR]

Pumped two-phase loop (P2PL) capable of handling high heat fluxes at low thermal resistance while consuming minimal pumping power offers a promising cooling solution for emerging high-power electronics. However, existing research primarily focuses on standalone evaporators with fixed boundary conditions, neglecting the interactions between the evaporator and other components within the P2PL. To address this gap, in this study, high-speed visualization with simultaneous temperature and pressure measurements is used to investigate the flow boiling regimes and their impact on the thermal characteristics of R134a in parallel microchannels. The ranges of parameters are: mass flux from 135.7 to 339.3 kg/m2-s, heat flux from 0 to 33.7 W/cm2, a fixed inlet temperature of 10 °C, inlet subcooling from 0.6 to 3.5 °C, and vapor quality from subcooled to dryout. Flow separation at the microchannel inlet induced both pressure-driven flashing of the subcooled liquid and thermal-driven nucleation on the heated walls, leading to rapid bubble formation. At low heat fluxes, bubbly flow dominates the inlet, swiftly transitioning to slug flow. Increased heat fluxes induce a unique jet flow regime featuring a wavy vapor jet. The local heat transfer coefficient was used to track the influence of flow regime transitions on microchannel thermal characteristics. • Flow separation at the channel inlet created an ideal condition for nucleation. • Strong nucleation of side walls with no nucleation on bottom walls was observed. • High-speed flow visualization reveals a unique jet flow regime at high heat fluxes. • An early transition to annular flow regime with enhanced heat transfer was observed. • Local heat transfer coefficient captures the effect of flow regime transitions. [ABSTRACT FROM AUTHOR]

In this work, a microchannel heat sink with trapezoidal rib walls (TRAM), in which walls are partially made of trapezoidal ribs by MEMS (Microelectrical Mechanical System) technique are put forward. The influences of mass flux, heat flux and exit vapor quality on flow pattern, boiling curving, heat transfer coefficient (HTC), two-phase pressure drop, and wall temperature characteristics of TRAM are investigated and the traditional rectangular microchannel heat sink (RECM) is as the reference. Experiments are carried out using pure acetone liquid with the mass flux from 255 to 843 kg/(m2·s) and heat flux from 10 to 132 W/cm2. The results show that the onset of nucleate boiling (ONB) of TRAM increase with the increase of mass flux. The HTC of TRAM is larger than that of RECM whether at low or high heat flux. The pressure drop of TRAM increases more significantly with increasing heat flux and vapor quality. At the same time, the pressure drop of TRAM is lower than that of RECM. The flow instability is more moderate at a larger mass flux and heat flux owing to the more uniform bubble size and bubble distribution. • Novel microchannel heat sink with trapezoidal ribs in sidewalls is proposed. • The sharp angles, concave pits, and narrow gaps formed between the ribs provide conditions for bubble nucleation. • Heat transfer is enhanced and pressure drop is reduced for mostly microchannel heat sink with trapezoidal-ribbed walls. • The uniformity and stability of wall temperature are analyzed in microchannel heat sink with trapezoidal rib walls. [ABSTRACT FROM AUTHOR]

The development of compact and efficient corrugated heat exchangers for refrigeration and air conditioning systems is reliant upon thermal-hydraulic performance, which is quantifiable in terms of entropy generation. Hence, this study presents the generation of entropy analysis for flow boiling evaporation with an environment-friendly refrigerant (R600a) in the novel design of corrugated (outward circular ribs corrugated, inward circular ribs corrugated, outward triangular ribs corrugated, and inward triangular ribs corrugated) U-bend tube exchangers. The performance of the corrugated tube heat exchangers has been reported in comparison with the smooth U-bend tube heat exchanger. The numerical calculations were executed using CFD code for various masses ranging from 100 to 400 kg/m2s at the saturation temperature and vapor quality of 20 °C and 0.2 respectively while 15 kW/m2 heat flux was applied on the external surface of the straight tubes. The results revealed that the entropy generation increases as the mass flux increases with the inward circular and triangular ribs corrugated tube having the highest generated entropy followed by the outward circular and triangular ribs corrugated and then the smooth tube with the least entropy generation. In reference to smooth tube for the mass flux of 100 kg/m2s, the entropy generation for inward circular and triangular ribs corrugated increased by 46.88% and 130.93% respectively while the outward circular and triangular ribs corrugated increased by 18.35% and 39.26% respectively. Lower dimensionless Bejan numbers were obtained in the corrugated tubes with respect to mass fluxes compared to the smooth tube. The fluid flow irreversibility contribution to the overall entropy generated was higher in the corrugated tubes due to the corrugation surface which causes swirl mixing. The results of the numerical calculations were confirmed and corresponded with those established in the literature. [ABSTRACT FROM AUTHOR]

Addressing the cooling challenges posed by multi-heat sources with high heat flux in densely integrated electronic devices remains a pressing concern. A study was conducted to explore the performance of two distinct microchannel heat sinks, with featuring grooves or not, in a series connection. This investigative series connection mode exhibited notable enhancements in performance. Bubble behavior flow patterns within microchannels were observed utilizing a high-speed camera, followed by an analysis of the associated heat transfer mechanisms. The study encompassed the acquisition and detailed analysis of three key parameters, wall temperature, heat transfer coefficient (HTC), and pressure drop, of both heat sinks across diverse series connection configurations. Flow patterns were varied and including bubble flow, elongated bubble formations, and the presence of thin liquid films with steam channels in different positional heat sinks. Remarkably, the series connection exerted a substantial influence on postposition heat sinks compared to their preposition counterparts. Among the investigated series connection modes, with featuring grooves ones, exhibited the most favorable impact on heat sink performance. Notably, the postposition heat sink in this configuration demonstrated a maximum HTC increase of 12.77 kW/(m2K). The distinctive heat transfer mechanisms led to higher HTC and lower wall temperature in postposition heat sinks compared to their preposition counterparts. Findings in this study offer crucial insights into addressing cooling challenges associated with multi-heat sources featuring high heat flux, serving as valuable groundwork for potential solutions in this critical domain. [ABSTRACT FROM AUTHOR]

• The steady-state operation of high-temperature water flow boiling was investigated within 100–250 °C for space heat dissipation. • Four parameters of heat exchanger efficiency (HEE), heat transfer power (HTP), liquid-side outlet quality (LSOQ), and two-phase heat transfer coefficient (HTC) were analyzed under different operating pressures and mass flow rates. • The parameter values were calculated and variation rules were summarized. With the development of space nuclear reactors, the transmission and emission of a large amount of waste heat in space are inevitable. Because of its large heat transfer, clean nature, and safe working medium, flow boiling of high-temperature water is ideal for space applications. This study focuses on high-temperature heat dissipation and experiments on the flow boiling heat transfer of high-temperature water. The steady-state operation characteristics of high-temperature water flow boiling heat transfer in the operating temperature range of 100–250 °C are studied with an emphasis on the variations in heat exchanger efficiency (HEE), heat transfer power (HTP), liquid-side outlet quality (LSOQ) of the high-temperature heat exchanger, and two-phase heat transfer coefficient (HTC). The HTP can be improved to a peak of 16 kW under a moderate air mass flow rate, and the LSOQ can be increased to approximately 0.95. The maximum HTC in this system could reach approximately 2100 W/(m2*K), which offers significant design guidance for the high-temperature water flow boiling system. [ABSTRACT FROM AUTHOR]

• A new wall boiling heat transfer model is developed to offer a fresh approach for CFD simulations within the Eulerian two-fluid framework. • Each parameter within the new model is calculated based on local physical properties and macroscopic parameters at cell level. • The new model reasonably predicts the wall temperature in flow boiling cases. • The sensitivity of the coefficients introduced in the proposed model are analyzed. Accurate prediction of flow boiling heat transfer is prominently dependent on the modeling of wall heat flux partitioning. In this paper, a new wall boiling heat transfer model was developed for three-dimensional Computational Fluid Dynamics (CFD) code to predict the wall heat flux and wall temperature. The proposed model partitioned the wall heat flux into convective heat flux and nucleate boiling heat flux, which were further modified by two correction factors. The key feature is that the new wall boiling heat transfer model was derived from bubble growth mechanism, incorporating reasonable assumptions, and each parameter within the model was calculated based on local physical properties and macroscopic parameters at the cell level. On this basis, the new wall boiling heat transfer model was coupled into ANSYS-Fluent and validated against various public experiments as well as the KIMOF experiments conducted under different conditions. Simulation results indicated that the proposed model could predict reasonable results for wall temperature and cross-section average void fraction. Finally, a comprehensive investigation was carried out to assess the sensitivity of the computational grids and the coefficients introduced in the new model. [ABSTRACT FROM AUTHOR]

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

The trend of miniaturisation in recent decades has led to the development of compact electronic devices. The reduction in the required dimension leads to the exponential rise in the heat flux dissipated from such a system. A proper thermal management system is necessary to keep the temperature of a computer chip’s junction within acceptable limits and maintain its performance. Flow boiling modification using straight fins in microchannels has proven to be an effective passive enhancement of the cooling system. The core interest of this research is figuring out the optimal configuration of the fin shapes and configurations. Hence, it is crucial to gain a comprehensive understanding of the flow boiling phenomenon to establish a more general approach. In this study, the boiling heat transfer performance of fin microchannels with various shapes and dimensions is investigated experimentally. The study has shown that the choice of fin geometry has a significant impact on the thermal performance of a heat transfer system. Specifically, the results indicate that a rectangular cross-section fin performs better than a trapezoidal one with the same fin gap. The rectangular cross-section fin exhibits the highest heat transfer coefficient of 5066.84 W/m2∙K, outperforming the trapezoidal fin in terms of heat transfer capability. As the hydraulic diameter reduces, the thermal boundary layer becomes denser, providing a more distributed saturated region. This leads to the increase in the heat transfer coefficient up to 22.5% and 17.1% for rectangular and trapezoidal fins, respectively. Additionally, the efficiency analysis shows that, albeit increasing the mass flux and reducing the gap increase the average cooling performance, but the pressure drop jumps up to 48%, reducing the efficiency of the heat removal system.

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.

The effects of input parameters such as pressure, mass flux, and inlet subcooling on bubble departure diameter, frequencies, and quadrupole magnetic field have been investigated. It was observed that process of changes in diameter and bubble departure frequency was similar with and without magnetic field, and the bubbles proved a regulated growth in the presence of a magnetic field. Compared to alike working conditions without a magnetic field, with subcooled, mass flux and pressure increase, the bubble departure diameters decreased by an average of 9.55%, 15.18% and 15%, and the bubble departure frequencies increased by 45.45%, 30.38% and 21.71%, respectively. [ABSTRACT FROM AUTHOR]

Pressure drop during flow boiling of R410A was experimentally investigated at a saturation temperature of 279 K over a mass flux range of 50–80 kg/(m2s) and a vapor quality range of 0.2–0.8. The inner tubes with the same outside diameter of 19 mm contain a smooth tube and three enhanced tubes: a micro-porous (No. 1) tube, a ring-shaped finned (No. 2) tube, and an independent small-bosses surface (No. 3) tube. It was observed that the pressure drop in single-phase and flow boiling conditions increased with the increase of the mass flux, and the ring-shaped finned tube showed the highest pressure drop (more than 1.35 times and 1.31 times that of the smooth tube, respectively) due to the effect of the ring-shaped fins. The pressure drop in the micro-porous tube and the smooth tube increased with the increase of outlet vapor quality in the range of the mass fluxes tested. Results from correlation analysis showed that two existing correlations can be implemented to predict the frictional pressure drop in a smooth tube within a deviation of ± 10%. Improved correlations to predict pressure drop in the enhanced tubes were presented, and 94% of the data points of the micro-porous tube were predicted within a ± 10% error band. [ABSTRACT FROM AUTHOR]

Flow boiling heat transfer coefficient of R410A in horizontal annular channels were evaluated with a 279 K saturation temperature over a mass flux range of 50–80 kg/(m2s) and a vapor quality range of 0.2–0.8. All the four tested annulus have a 19 mm outer diameter of the flow channel containing three types of enhanced tubes (a micro-porous tube, a ring-shaped finned tube, and an independent small-bosses surface tube) and a smooth tube. The micro-porous tube benefitted by its surface shows the highest heat transfer coefficient (the enhancement factor exceeds 2.52) in flow boiling. The smooth tube shows a higher heat transfer coefficient than the independent small bosses tube due to its special surface structure. For changing average vapor quality of outlet and inlet, the heat transfer coefficients of the micro-porous tube and smooth tube increase with the increase of the average vapor quality. Considering the heat transfer coefficient and the data of pressure drop in all of these annular channels reveals that the micro-porous tube can be a great choice for flow boiling on the annular side for the best performance. The results from correlation analysis of smooth tube predict experimental data with a ± 10% error band. The improved correlations for enhanced tubes were presented, and the experimental values are predicted within a ± 2% error band. [ABSTRACT FROM AUTHOR]

To improve the heat transfer performance (HTP) of stainless steel, the micro-nano concave chromium (Cr) coating was fabricated on the surface of 304 stainless steel by the combination technology of electrodeposition and anodic oxidation. The effects of electroplating and anodizing time on the morphology, contact angle (CA) and heat transfer coefficient (HTC) of the coating were studied. The results show that the samples with large surface area and good wettability have higher HTC. Besides, the Cr coating obtained after 2 h electroplating has silver white appearance, smooth surface and a thickness of about 37 μm. The 2-h electroplated Cr coating after anodizing for 30 min presents a concave pore structure with a pore diameter of 7.4 μm and the best wettability (a CA of 41.8°), which is 20° lower than that of the stainless steel substrate. The two-phase heat transfer efficiency of the sample with the concave Cr coating was increased by 22.84 % when the fluid mass flux is 33.3 kg m−2 s−1 and the inlet temperature is 40 °C and the fluctuations of pressure drop in the micro-channel was effectively reduced. The enhanced HTP of concave Cr coating is owing to the larger specific surface area and better wettability caused by the unique rough concave structure. [ABSTRACT FROM AUTHOR]

Entropy generation analysis of the flow boiling in microgravity field is conducted in this paper. A new entropy generation model based on the flow pattern and the phase change process is developed in this study. The velocity ranges from 1 m/s to 4 m/s, and the heat flux ranges from 10,000 W/m2 to 50,000 W/m2, so as to investigate their influence on irreversibility during flow boiling in the tunnel. A phase–change model verified by the Stefan problem is employed in this paper to simulate the phase–change process in boiling. The numerical simulations are carried out on ANSYS-FLUENT. The entropy generation produced by the heat transfer, viscous dissipation, turbulent dissipation, and phase change are observed at different working conditions. Moreover, the Be number and a new evaluation number, EP, are introduced in this paper to investigate the performance of the boiling phenomenon. The following conclusions are obtained: (1) a high local entropy generation will be obtained when only heat conduction in vapor occurs near the hot wall, whereas a low local entropy generation will be obtained when heat conduction in water or evaporation occurs near the hot wall; (2) the entropy generation and the Be number are positively correlated with the heat flux, which indicates that the heat transfer entropy generation becomes the major contributor of the total entropy generation with the increase of the heat flux; (3) the transition of the boiling status shows different trends at different velocities, which affects the irreversibility in the tunnel; (4) the critical heat flux (CHF) is the optimal choice under the comprehensive consideration of the first law and the second law of the thermodynamics. [ABSTRACT FROM AUTHOR]

The rapid advances in performance and miniaturization of electronic devices require a cooling technology that can remove the produced heat at a high rate with small temperature variations, as is obtained in flow boiling. To obtain insight in flow boiling, we performed numerical simulations in a 200 μm square microchannel using the local front reconstruction method. Besides validation with literature results, a parametric study shows an increasing heat removal rate and bubble growth rate with increasing wall temperature, liquid mass density, and liquid heat capacity and decreasing inlet velocity indicating the importance of phase change compared to convective transport. Finally, the heat transfer in the liquid film is studied using a Nusselt number defined with the film thickness, which is comparable to Nusselt number for falling films on hot surfaces. It is observed that convective effects are more pronounced at the bubble rear compared to the bubble front. [ABSTRACT FROM AUTHOR]

In this study, the evaporation heat transfer characteristics of an intermediate temperature of R744 in a smooth horizontal tube, when operating as an indirect refrigeration system (IRS) among hybrid cascade refrigeration systems (HCRSs), are evaluated. Studies on the characteristics of intermediate-temperature evaporation heat transfer under the operating conditions of evaporators used in actual refrigeration systems, such as IRS, cascade refrigeration systems, and HCRS, used in supermarkets are lacking. Thus, this study provides basic data on the characteristics of evaporation heat transfer of R744 in the evaporators of refrigerators used in supermarkets. The tube employed to the evaporation experiment in this study was a horizontal smooth copper tube with a length of 8000 mm and an inner diameter of 11.46 mm. The experimental variables were measured over a wide range of mass flux of 200–500 kg/(m2·s), heat flux of 10–40 kW/m2, and saturation temperature of −40–0 °C. The main results are summarized as follows: (1) The application of Kandlikar's correlation formula at an evaporation temperature of −20 °C in an IRS helps in a good prediction of the R744 evaporation heat transfer coefficient. (2) The pressure drop according to the heat and mass flux showed the same heat transfer coefficient trend, but the pressure drop at saturation temperature was different from the trend of heat transfer coefficient. [ABSTRACT FROM AUTHOR]