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• Investigated the application of electric currents to reduce thermal resistance. • Demonstrated the coupling effects between micro/nano-scale modified surfaces and electric currents. • At 5.1 mA/cm² current density, thermal resistance was reduced by 45.7% for smooth copper, 36.8% for ECD, and 38.1% for Ni-P-PTFE surfaces. The principle of saving energy and converting waste heat into valuable resources is a core concept. Heat exchange systems are significantly compromised by microbial fouling, which impedes heat transfer and increases energy consumption. This study investigated the coupling effects of micro/nano-scale surface modifications and the application of electric currents as a novel approach to inhibit microbial fouling. Smooth copper surface, electrochemical deposition (ECD) surface, and Nickel-Phosphorus-Polytetrafluoroethylene (Ni-P-PTFE) modified surface were tested and compared. Firstly, we analyzed the average heat transfer coefficient under clean conditions, taking into account different surface characteristics. Then, we employed a series of fouling experiments to evaluate the anti-biofouling performance of smooth copper surfaces, ECD surfaces, and Ni-P-PTFE surfaces. The assessment involved flow cytometry, Scanning Electron Microscopy (SEM) and measurements of thermal resistance. Finally, we investigated the effect of external electric current on microbial fouling mitigation. Results show that the ECD surface exhibited an enhanced heat transfer and a poor fouling resistance, while the Ni-P-PTFE surface demonstrated an excellent fouling resistance and a minimal increase in thermal resistance. The application of a low direct current density at 0.51 mA/cm² significantly reduced microbial viability on all surfaces. A higher electric current can further inhibit microbial adhesion and proliferation, enhancing the anti-fouling capability of the surfaces. Compared to surfaces without electric current, the smooth copper surface, ECD, and Ni-P-PTFE surfaces at 5.1 mA/cm² displayed a reduction in thermal resistance by 45.7%, 36.8%, and 38.1%, respectively. At 51 mA/cm², the reduction in thermal resistance for smooth copper surface, ECD, and Ni-P-PTFE surfaces was 75.7%, 78.2%, and 57.1%, respectively. This comprehensive study has validated the potential of combining micro/nano-scale surface modifications with electric current application as an effective strategy for enhancing heat transfer efficiency and anti-fouling properties of heat exchange systems. [ABSTRACT FROM AUTHOR]

The pinch-off dynamics during hydrogen bubble formation was experimentally investigated in a cross-junction microchannel. A binarization interface recognizing and key frame tracking method was established. By analyzing the breakup dynamics through spatial and time domains, the effects of interfacial tension and viscosity on hydrogen bubble pinch-off were revealed. A transitional stage between a liquid squeezing stage and a free pinch-off stage was newly observed and the transitional stage was named as the wave model stage because of the long-wave approximation of the interface at this stage. The time criteria between the three stages are proved to be around 1/10 of t cap (capillary time) and around t cap to the pinch-off moment, respectively. However, the power law exponents of the minimum radial radius R 0 for hydrogen - liquid flow, larger than those for nitrogen - liquid flow, are consistent with literature works in terms of both range and tendency. • Establish a binarization interface recognizing and key-frame tracking method. • Investigate the effect of viscosity on Hydrogen bubble pinch-off. • Realize the role of dynamic interfacial tension based on the mutation behavior. • Discover a wave model stage during hydrogen bubble pinch-off for the first time. • Reveal the time criteria of the wave model stage. [ABSTRACT FROM AUTHOR]

• The application of ammonia in two kinds of combustion mode were studied. • The difference of ignition method for ammonia under two modes is clarified. • HP engine can easier get higher power without geometric change. • LP mode requires attention to the treatment of NH 3 and N 2 O in tail gas. • Considering power and emission, ammonia in HP mode has a better effect. As a carbon-free hydrogen-carrying fuel, ammonia can effectively solve the problem of greenhouse gas emissions and has great application potential in marine engines. At present, low pressure and high pressure injection are the two main combustion modes of low-speed marine engine, and the former is closer to the Otto cycle, while the latter is closer to the Diesel cycle. In this study, a low pressure direct injection natural gas/diesel dual fuel engine was converted to an ammonia/diesel engine (LP). High pressure direct injection ammonia/diesel engine was converted by a diesel-only engine (HP). The application of ammonia was compared in above two combustion modes. Results show that using ammonia in HP mode can easier get high power, but LP's IMEP, efficiency performance is better. However, LP mode has a large risk of ammonia escape (2.79%) and unburning (5.49%), and N 2 O in the emission is also worthy of attention and further research. In addition, due to the difference in N 2 O, the greenhouse effect of LP mode is also greater. For ignition, LP mode is not only affected by diesel injection, but also in-cylinder swirl and equivalent ratio due to the influence of premix mode. Currently, the HP mode has more potential for marine engine. [ABSTRACT FROM AUTHOR]

• An experimental study of NOVEC-649 pool boiling on microporous surfaces. • 600% enhancement in heat transfer coefficient. • 55% enhancement in critical heat flux. • A heat transfer correlation was fitted. • A mechanistic model for critical heat flux was developed based on force analysis. Pool boiling performance of NOVEC-649 was experimentally studied on microporous surfaces prepared by an electrochemical deposition method. Microporous structures contribute to large surface roughness values and provide large quantities of cavities ranging from several hundreds of nanometers to several microns for bubble nucleation. The results show that a maximum enhancement of 600% in heat transfer coefficient and a maximum enhancement of 55% in critical heat flux are achieved on the deposited surfaces, compared with a smooth copper surface. Experimental heat transfer coefficients were compared with literature correlations, considering the effects of roughness and surface-liquid combination. Then a fitted Rohsenow correlation was discussed and developed to predict the present results. Experimental critical heat fluxes were compared with classical models. It was found that the critical heat flux on the smooth surface could be predicted by the lift-off model and the Kandlikar model, but these models cannot predict the critical heat fluxes on the deposited surfaces well. Following, the Kandlikar model was modified by further considering a wicking force and a roughness-factor-dependent surface tension force. The present modified CHF model was validated by comparing with present experimental data and the literature, with a deviation around ±30%. [ABSTRACT FROM AUTHOR]

• Modified porous surfaces enhance pool boiling performance of well-wetting liquids. • A dynamic force balance could predict bubble departure diameter. • A mechanistic heat transfer model was proposed for modified porous surfaces. • The heat flux contribution from micro-convection is significant. • Critical heat flux is linearly proportional to its capillary wickability. Boiling heat transfer intensification is of big relevance to energy conversion and conservation, materials and resources saving, and electronics cooling. This work aims to enhance saturated pool boiling of well-wetting liquids, i.e., acetone and HFE-7200 on nanoparticles-deposited surfaces by electrophoretic deposition and on microporous foam surfaces by electrochemical deposition. The electrophoretic-deposited surfaces enhance the heat transfer coefficient of acetone and HFE-7200 by up to 70% and 190%, respectively. However, the critical heat flux is not improved on electrophoretic-deposited surfaces. The electrochemical-deposited surfaces increase the boiling heat transfer coefficient by up to 370% and the critical heat flux by more than 30%. Bubble dynamics were visualized simultaneously. The bubble departure diameter from experiments can be predicted by a dynamic force balance model within a ±20% error band. A mechanistic heat transfer model was proposed for modified porous surfaces, including not only the heat fluxes from microlayer evaporation and transient conduction but also the heat flux from micro-convection due to liquid agitation and entrainment by growing and departing bubbles. The mechanistic heat transfer model can predict experimental pool boiling curves of acetone and HFE-7200 on electrophoretic-deposited and electrochemical-deposited surfaces relatively well, especially for the isolated bubble regime where most bubbles are isolated and bubble coalescence is not intensive. Besides, the critical heat flux of a modified surface can be estimated if the initial (maximum) wicked volume flux on the structured surface relative to the smooth surface is considered. [ABSTRACT FROM AUTHOR]

Highlights • Nanostructured surfaces. • Pool boiling experiments and heat transfer analysis. • Nucelate boiling and critical heat flux. • Enhanced heat transfer. • Analysis of mechanisms. Abstract In the present study, an electrophoretic deposition method was employed to modify copper surfaces with Cu-Zn (∼100 nm) nanoparticles. Pool boiling heat transfer of HFE-7200 on the modified surfaces was experimentally studied. The results showed that the heat transfer coefficient on the modified surfaces was significantly enhanced compared with that on a smooth surface, e.g., a maximum 100% enhancement, while the maximum superheat on the modified surfaces was around 20 K lower than that on the smooth surface. However, the critical heat flux (CHF) was not improved considerably, and supplementary tests indicated that the wickability of HFE-7200 was almost the same on the modified surfaces and the smooth surface. The departure diameters of bubbles were recorded by a high speed camera, which were compared with several models in literature. Active nucleation site sizes were evaluated by the Hsu nucleation theory and active nucleation site densities were estimated by appropriate correlations. In addition, a heat transfer model, considering natural convection, re-formation of thermal boundary layer and microlayer evaporation, was formulated to predict the heat transfer on the modified surfaces and the smooth surface. A relatively good prediction was achieved. [ABSTRACT FROM AUTHOR]

Highlights • Bubble departure radius correlations for saturated and subcooled boiling were proposed. • The dynamic advancing contact angle was used in this study. • The proposed correlations exhibit big improvement compared with existing correlations. • The correlations perform very well in different gravity levels. Abstract Based on a modified force balance model, new correlations were proposed for the prediction of vapor bubble departure radius in saturated and subcooled pool boiling under atmospheric pressure. To predict the departure radius, the wall temperature and contact angle are two important input parameters. Instead of the static contact angle, the present correlations use the dynamic advancing contact angle at root of the bubble base at the moment before bubble detachment (i.e., the maximum dynamic advancing contact angle) to calculate the bubble departure radius. The results show that for the bubble departure radius obtained in this study, the developed correlation can predict all the data points within a maximum error of 3.8% in both normal earth gravity and 0.01g e reduced gravity. Moreover, for data sets in the literature including 1g saturated boiling, 1g subcooled boiling, saturated boiling in reduced gravity, and subcooled boiling in reduced gravity, it is also demonstrated that compared with the thirteen existing correlations, the proposed correlations exhibit a big improvement in predicting bubble departure radius. [ABSTRACT FROM AUTHOR]

Highlights • Flow regimes were visualized at two locations for square microchannels. • Dimensionless numbers were used to map flow patterns. • Flow pattern evolutions were revealed along the microchannel. • A scaling relation was proposed based on flow rate ratio and capillary number. • Slug velocity was correlated with a new capillary number. Abstract Liquid-liquid flow regimes in three square microchannels were visualized simultaneously both at the cross-shaped junction and in the microchannel "far" from the junction in order to reveal flow regime evolutions along the microchannel. At the inlet junction, three major flow regimes including tubing/threading, dripping and jetting were mapped using the aqueous capillary number versus the organic Weber number. Correspondingly, in the main microchannel, annular, slug and droplet flow patterns were mapped using two dimensionless numbers (Weber number times Ohnesorge number) of both phases. Both dripping and jetting regimes at the inlet junction can evolve into either slug or droplet flows in the main microchannel. Besides, it was realized that as the organic flow rate increases, the transitional aqueous flow rate at the slug-droplet transition firstly increases, then decreases and then increases again. The droplet formation mechanism has transited from dripping to jetting, which causes the slug-droplet transition to occur at a much lower aqueous flow rate. Moreover, a scaling relation for the slug size in dripping was developed, which can predict the slug length for five different liquid-liquid systems. It applies for liquid-liquid microfluidic devices with a cross-shaped inlet junction in the dripping regime, for slug sizes longer than 1.5 times the channel depth. The slug velocity has been correlated as functions of the capillary number Ca j (µ c j / γ) by using the continuous phase viscosity and the bulk velocity. [ABSTRACT FROM AUTHOR]

Abstract Nucleate pool boiling performance of two well-wetting liquids, i.e., acetone and HFE7200, on three nanoparticle-coated surfaces were experimentally studied and compared with that of the smooth surface. Electrophoretic deposition was used to fabricate nano-porous surfaces. Surface roughness, static and advancing contact angles, capillarity of the smooth and coated surfaces were characterized. Compared to the smooth surface, the nanoparticle-coated surfaces decreased the wall superheat by more than 50% for acetone and 65% for HFE7200 at the same heat flux level, and accordingly enhanced the heat transfer coefficient by up to 85% for acetone and up to 200% for HFE7200. Bubble departure diameters were measured and correlated with the advancing contact angle, the capillary length and the Jacob number. A new mechanistic heat transfer model was proposed based on the heat flux partition method. The advancing contact angle was suggested to be used for calculation of the active nucleation site density. Based on the mechanistic model, transient heat conduction on and around nucleation sites over the whole bubble cycle contributes the most (>70%) to the total heat flux, while microlayer evaporation contributes around 10-30% to the total heat flux, with negligible natural convection. The critical heat flux was not enhanced for the two well-wetting liquids. [ABSTRACT FROM AUTHOR]

Abstract In the present study, copper surfaces were deposited with Cu-Zinc nanoparticles of 0.6 mg by an electrophoretic deposition method (EPD). Two deposition patterns were designed, i.e., fully deposition (EPD-F) and channel-pattern deposition (EPD-C). In the channel-pattern deposition, the smooth channel and the deposition channel occur alternatively, by keeping the width of the smooth channel as 3 mm, but the width of the deposition channel as 1 mm (EPD-C1) and 3 mm (EPD-C2), respectively. Pool boiling of HFE-7200 was studied on a smooth surface and the nanoparticle-coating surfaces. The results showed that the surface with fully deposition (EPD-F) had the highest heat transfer coefficient, around 100% enhancement compared with the smooth surface, while the surface with channel-pattern deposition (EPD-C2) had the highest critical heat flux, around 33.3% enhancement in comparison to the smooth surface. A high speed camera was used to study bubble dynamics, which indicated that the nanoparticle-coating surfaces had smaller bubble departure diameters and higher departure frequencies. A heat transfer model, considering natural convection, re-formation of thermal boundary layer and microlayer evaporation, was formulated to predict the heat transfer on the test surfaces, showing good prediction at low and moderate heat fluxes. CHF was analyzed from the perspective of the Rayleigh-Taylor instability. [ABSTRACT FROM AUTHOR]

As a novel coolant, the ethylene glycol-water (50 wt.%:50 wt.%) with graphene nanoplatelets nanofluids (GnP-EGW) were prepared at four weight concentrations (0.01, 0.1 0.5 and 1.0 wt.%), and heat transfer and pressure drop characteristics in a miniature plate heat exchanger (MPHE) were investigated. All nanofluid samples were prepared and diluted by ultrasonic vibration, and their thermal conductivity and dynamic viscosity were measured by a transient plane source method and a rotational rheometer, respectively. Firstly, the convective heat transfer coefficient (HTC) and pressure drop correlations were predicted under the condition that water was employed as working fluid in both the hot and cold sides of the MPHE. Then, the effects of GnP concentrations of nanofluids on the thermal and hydraulic performances have been determined for the MPHE with the nanofluid in hot side and the water in cold side. Parametric evaluation and performance comparison of the MPHE using GnP-EGW were analyzed via various operating conditions. Experimental analysis showed that: the proposed correlations from water can predict the experimental data of the base fluid and GnP-EGW nanofluids. In the proper concentration range from 0.01 to 0.1 wt.%, the GnP-EGW nanofluid has an acceptable pressure drop penalty but a higher heat transfer performance compared with the base fluid in the MPHE, which reveals that it might be a potential cooling medium. [ABSTRACT FROM AUTHOR]

Flow patterns for water-butanol, water-toluene, water-hexane, water-oil and water/glycerol (weight ratio 60:40) mixture-oil two-phase flows were visualized in the cross-shaped junctions of three square glass microchannels with hydraulic diameters of 200 µm, 400 µm and 600 µm. The aqueous phase is the continuous phase contacting the channel walls while the organic phase is the dispersed phase in the experiments. Three main flow pattern groups were observed, including the tubing/threading regime group, the dripping regime and the jetting regime. The flow regimes were mapped based on the Capillary number of the continuous phase and the Weber number of the dispersed phase. The flow rate ratio and the Capillary number of the dispersed phase were also employed to present flow patterns. The effects of hydraulic diameter of the square microchannels, flow rates, and physical properties, e.g., the interfacial tension and the viscosities of the aqueous and organic phases on flow pattern transitions were clarified. Besides, in the dripping regime, the dimensionless slug length can be scaled as a function of the Capillary number of the continuous phase for cross-shaped junctions. The slug velocity is linearly dependent on the average flow velocity in the dripping regime. [ABSTRACT FROM AUTHOR]

Heat transfer and pressure drop characteristics of a hybrid nanofluid mixture containing alumina nanoparticles and multi-walled carbon nanotubes (MWCNTs) were experimentally investigated in a chevron corrugated-plate heat exchanger. A MWCNT/water nanofluid with a volume concentration of 0.0111% and an Al 2 O 3 /water nanofluid with a volume concentration of 1.89% were mixed at a volume ratio of 1:2.5. A small amount of MWCNTs was added in order to increase the mixture thermal conductivity. Experiments with water used as both hot and cold fluids were carried out first to obtain a heat transfer correlation for fluids flowing in the chevron plate heat exchanger. The results of the nanofluid mixture were compared with those of the Al 2 O 3 /water nanofluid and water. Results show that the heat transfer coefficient of the hybrid nanofluid mixture is slightly larger than that of the Al 2 O 3 /water nanofluid and water, when comparison is based on the same flow velocity. The hybrid nanofluid mixture also exhibits the highest heat transfer coefficient at a given pumping power. The pressure drop of the hybrid nanofluid mixture is smaller than that of the Al 2 O 3 /water nanofluid and only slightly higher than that of water. Therefore, hybrid nanofluid mixtures might be promising in many heat transfer applications. [ABSTRACT FROM AUTHOR]

With the recent progress in nanotechnology, nanofluids are emerging as a new class of heat transfer fluids formed by adding nanometer-sized structures (e.g., particles, fibers, tubes) in conventional base fluids (e.g., water, ethylene glycol, engine oil). Due to attractive van der Waals forces, nanoparticles tend to agglomerate to form aggregates in nanofluids to form the so-called aggregate-laden nanofluids. Aggregation affects the nanofluid properties such as thermal conductivity and viscosity and further affects the heat transfer performance. The discrepancies regarding the influence of nanoparticles on thermophysical properties and heat transfer characteristics in the literature might arise due to nanoparticle aggregation. Firstly, three performance comparison criteria for nanofluids were proposed for thermally developing laminar flow, fully developed laminar flow and fully developed turbulent flow to evaluate the nanofluid efficiency as coolants. Secondly, parametric effects of aggregates on nanofluid viscosity and thermal conductivity were investigated. The cooling efficiency of the aggregate-laden nanofluids depends on aggregate parameters such as aggregate ratios, interfacial thermal resistance, volume fraction of aggregates in nanofluids and volume fraction of nanoparticles in the aggregates. One method to tailor the aggregate morphology is presented by dispersing nanoparticles of different size into a base fluid. By this method, the volume fraction of nanoparticles in the aggregates might increase, which thus enhances the nanofluid effectiveness due to reduction of viscosity. [ABSTRACT FROM AUTHOR]

This study presents a state-of-the-art overview on heat transfer characteristics of fluids (mainly water, carbon dioxide and hydrocarbon fuels) flowing in smooth tubes and enhanced tubes at supercritical pressures and tries to obtain a fundamental understanding of the unique characteristics. Heat transfer in enhanced tubes is much better than that in smooth tubes with a larger pressure drop penalty at supercritical conditions. Thermo-physical properties of fluids at supercritical pressures and relevant parametric effects (e.g., effects of mass flux, heat flux, pressure and flow direction) on heat transfer performance are outlined. Inconsistencies in the literature on heat transfer are emphasized and evaluated. Possible reasons are suggested to explain those inconsistencies. Moreover, the mechanisms for heat transfer deterioration at supercritical pressures are discussed and different correlations for predicting heat transfer deterioration are compared and assessed with experimental data. These predictive correlations based on one working fluid cannot be applied directly to other working fluids. Besides, several common buoyancy criteria proposed in the literature to distinguish forced convection and mixed convection are evaluated and show large discrepancies with experimental data. There is no buoyancy criterion developed for hydrocarbon fuels. Future research needs are warranted for heat transfer of near-critical and supercritical fluids. [ABSTRACT FROM AUTHOR]

Heat transfer and pressure drop characteristics of Al 2 O 3 /water and MWCNT/water nanofluids flowing in a chevron-type plate heat exchanger were experimentally investigated and compared with those of water. Results showed that heat transfer seemed to be improved by using nanofluids at constant Reynolds number. However, little heat transfer enhancement was observed based on a constant flow velocity. The heat transfer deterioration of MWCNT/water nanofluids was more intensive than Al 2 O 3 /water nanofluids due to the relatively large viscosity increase of MWCNT/water nanofluids. A new heat transfer correlation was proposed based on the experimental data of water and it predicts the experimental data of nanofluids accurately when the measured nanofluid properties (thermal conductivity and viscosity) were adopted for calculation. Besides, the pressure drop of nanofluid was reasonably higher than that of water and seemed to increase with increasing particle volume concentrations due to the increase in viscosity. However, there was not much difference between the pressure drop of nanofluids and that of water at low particle volume concentrations. A correlation for predicting the friction factor was obtained and it fitted the experimental data very well. [ABSTRACT FROM AUTHOR]

An experimental investigation was performed to evaluate convective condensation and evaporation of R22, R32 and R410A inside a smooth tube (inner diameter 11.43 mm), a herringbone tube (fin root diameter 11.43 mm) and a newly developed enhanced surface EHT tube (inner diameter 11.5 mm) at low mass fluxes. The inner surface of the EHT tube is enhanced by dimple/protrusion and secondary petal arrays. For condensation, the heat transfer coefficient of the herringbone tube is 2.0–3.0 times larger than a smooth tube and the EHT tube is 1.3–1.95 times that of the smooth tube. The heat transfer enhancement ratios of the herringbone tube and the EHT tube are larger than their respective inner surface area ratios. Mass flux has a non-monotonic relation with the condensation heat transfer coefficient in the herringbone microfin tubes; this was especially evident for R32 and R410A. For evaporation, the EHT tube provides the best evaporation heat transfer performance for all the three refrigerants; this is mainly due to the heat transfer enhancement produced from the larger number of nucleation sites, increased interfacial turbulence, boundary layer disruption, flow separation and secondary flow generation caused by the dimple and petal arrays. The evaporation heat transfer coefficient of the herringbone tube is only slightly higher than that of the smooth tube. Overall, the EHT tube provides increased heat transfer enhancement for both condensation and evaporation. [ABSTRACT FROM AUTHOR]

With fast growing power consumption and device miniaturization, micro/minichannels are superior to macrochannels or conventional channels for high heat-flux dissipation due to their large surface area to volume ratios and high heat transfer coefficients. However, the associated large pressure drop penalty and flow boiling instability of micro/minichannels hinder their advancement in many practical applications. Therefore, enhancement techniques are required to stabilize the flow and further augment the heat transfer performance in micro/minichannels. This work first presents the classification of micro/minichannels for single-phase flow and flow boiling and gives a general statement of heat transfer enhancement. Then a state-of-the-art overview of the most recent enhancement techniques is specifically provided for further sing-phase flow and flow boiling enhancement in micro/minichannels. Two promising enhancement techniques, i.e., interrupted microfins and engineered fluids with additives are discussed for single-phase flow. For flow boiling, the focus is given on several selected enhancement approaches which can effectively mitigate flow boiling instability and another hot research topic, i.e., nanoscale surface modification. Besides, effects of wettability on bubble dynamics are presented, and a concept of flow-pattern based heat transfer enhancement is proposed. For both single-phase flow and flow boiling enhancement, a special emphasis is on those enhancement techniques with high thermal performance and relatively low pressure drop penalty. [ABSTRACT FROM AUTHOR]

Abstract: An experimental investigation was performed for convective vaporization of R22 and R410A inside one smooth tube and five micro-fin tubes with the same outer diameter of 5mm. Data are for mass fluxes ranging from 100 to 620kg/m2 s at 279K saturation temperature. The results suggest that the tube with fin height of 0.15mm, apex angle of 25° and 38° starts has the best thermal performance for convective vaporization when mass velocity is less than 400kg/m2 s, while the tube with fin height of 0.12mm, apex angle of 25° and 58° starts has the best heat transfer performance at larger mass velocities, which is probably due to the relative size between fin height and liquid film thickness. Considering the effects of micro-fin on flow boiling, a new general semi-empirical model has been developed based on the present data and recent data from literature. The new model is applicable for intermittent and annular flow patterns, covering different fluids, nominal diameters from 2.1 to 14.8mm, mass fluxes from 100 to 650kg/m2 s, heat fluxes based on the total inner surface area from 0 to 30kW/m2, and reduced pressure from 0.08 to 0.69. The model predicts the parametric trends correctly and the average and local heat transfer coefficients accurately. The heat transfer mechanism can also be observed clearly by the new model. [Copyright &y& Elsevier]

Abstract: An experimental investigation was performed for single-phase flow and condensation characteristics inside five micro-fin tubes with the same outer diameter 5mm and helix angle 18°. Data are for mass fluxes ranging from about 200 to 650kg/m2 s. The nominal saturation temperature is 320K, with inlet and outlet qualities of 0.8 and 0.1, respectively. The results suggest that Tube 4 has the highest condensation heat transfer coefficient and also the highest condensation pressure drop penalty, while Tube 5 has the highest enhancement ratio due to its lowest pressure drop penalty and intermediate heat transfer coefficient. Condensation heat transfer coefficient flattens out gradually as G decreases when G <400kg/(m2 s) for Tube 2 and Tube 4. This nonlinear mass-flux effect may be explained by the complex interactions between micro-fins and fluid, including liquid drainage by surface tension and interfacial turbulence. In addition, the experimental data was analyzed using seven existing pressure-drop correlations and four heat-transfer models to verify their respective accuracies. [Copyright &y& Elsevier]

Abstract: Existing database in literature on the adiabatic two-phase frictional pressure drop in evaporative micro/mini-channels were reviewed. The collected database contains 769 data points, covering 12 fluids, for a wide range of operational conditions and channel dimensions. The whole database was analyzed using five existing correlations to verify their respective accuracies. The importance of the Bond number, which relates the nominal bubble dimension or capillary parameter with the channel size, was revealed. A particular trend was observed with the Bond number that distinguished the entire database into three ranges. Using the Bond number, improved correlations of adiabatic two-phase pressure drop were established for small Bond number regions. The newly proposed correlations can predict the database well for the region where ⩽200. [Copyright &y& Elsevier]

Abstract: This paper verified the macro-to-mini-scale criterion because data points where and show very different trends for the entire database (1672 data points). Boiling number at critical heat flux (Blchf ) decreases greatly with heated length-to-diameter ratio (Lh /dhe ) when Lh /dhe is small while a relatively smooth trend occurs with large Lh /dhe values. The paper proposed a threshold value of Lh /dhe =150, beyond which Lh /dhe presents negligible effect on saturated critical heat flux. The combined dimensionless number was introduced to analyze saturated critical heat flux, which represents the significance of inertia, surface tension, and viscous force, without considering gravitational force for the region where Lh /dhe >150 and . In addition, a new predictive tool for saturated critical heat flux in micro/mini-channels was obtained, predicting almost 95.5% of the non-aqueous data and 93.5% of the water data within a ±30% error band. [Copyright &y& Elsevier]

Abstract: Experimental results of the saturated-flow boiling critical heat flux (CHF) in microchannels for both multi- and single-channel configurations were obtained from the literature. The collected database contains 629 data points, covering five halogenated refrigerants, nitrogen, and water, for a wide range of operational conditions, and different microchannel dimensions. The whole database was analyzed by using five empirical correlations to verify their respective accuracies. However, none of the existing correlations could predict the entire database precisely. A saturated CHF correlation was proposed by using boiling number, length-to-diameter ratio, and exit quality. Combining with the energy balance equation, the new correlation can predict the overall microchannel database accurately on the whole. It predicts almost 97.0% of the non-aqueous data (except R12 data points located in the macro-scale region) and 94.0% of the water data within the ±30% error band. [ABSTRACT FROM AUTHOR]

Abstract: Experimental results of adiabatic two-phase pressure drop in micro/mini-channels for both multi- and single-channel configurations were obtained from the literature. The collected database contains 769 data points, covering 12 fluids, for a wide range of operational conditions and channel dimensions. The whole database was analyzed using eleven existing correlations to verify their respective accuracies. In addition, the Bond number and the Reynolds number were introduced to modify the Chisholm parameter of two-phase multipliers to develop new generalized correlations. A particular trend was observed with the Bond number that distinguished the data in three ranges, indicating the relative importance of surface tension. [Copyright &y& Elsevier]

Abstract: This paper presents a general criterion to classify a channel as micro-channel or macro-channel. Experimental results of saturated-flow boiling heat transfer in micro/mini-channels for both multi- and single-channel configurations were obtained from the literature. The collected database contains 4228 data points, covering a wide range of working fluids, operational conditions, and different micro-channel dimensions. Seven existing correlations were evaluated against the database to verify their respective accuracies. A combined non-dimensional number was introduced as the new conventional-to-micro/mini-channel criterion. In addition, a generalized prediction method was proposed based on the new criterion, with accurate predictive capability. [Copyright &y& Elsevier]

The manifold microchannel heat sink shows great potential for cooling high-heat-flux electronic equipment owing to its exceptional thermal and flow performance. A new manifold ring-shaped channel design is introduced to improve the temperature uniformity for circular heat sources. A comparative analysis of the manifold straight channel, radial expansion channel and manifold ring-shaped channel designs is conducted based on the same heating area under equal mass flow conditions by numerical investigation using single-phase de-ionized water. The findings indicate that the newly proposed manifold ring-shaped channel design exhibits a lower maximum temperature, a lower average temperature, a better temperature uniformity and a smaller pressure drop in comparison to the straight channel and radial channel designs. Specifically, at a mass flow rate of 2.5 g/s, the ring-shaped channel design demonstrates a 71.4 % and 45.4 % reduction in the temperature difference between the maximum temperature and the minimum temperature of the heating surface, and the coefficient of performance is enhanced by 458.2 % and 169.7 % when compared to the straight channel and radial channel designs, respectively. Additionally, three improved designs based on the original manifold ring-shaped channel design are proposed to further enhance fluid flow distribution in the ring-shaped channel design. The results reveal that all three manifold ring-shaped channel optimizations further lower the maximum temperature, the average temperature and the temperature difference. Notably, the improved case3 exhibits the best thermal and hydraulic performances among all structures. When compared to straight channel and radial channel designs, the improved case3 demonstrates an 86.9 % and 75.0 % reduction in the temperature difference, and an increase of 201.2 % and 523.4 % in coefficient of performance under the equal mass flow comparison basis. A heat flux of 1200 W/cm2 can be dissipated at 6 g/s when the maximum temperature rise is kept below 60 K, with a pressure drop of 38 kPa. • Propose a new manifold ring-shaped channel (MRC) design for electronics cooling of circular heat source. • The MRC design improves temperature uniformity by 71.4% and decreases pressure drop by 71.7%. • The improved MRC design can further improve temperature uniformity by 86.9%, and increase COP by 500%. • 1200 W/cm2 is dissipated for single-phase water cooling with a maximum temperature rise less than 60 K. [ABSTRACT FROM AUTHOR]

As a marine engine fuel of great concern, ammonia needs to be mixed with another high reactive fuel to improve its combustion performance. In this work, the combustion performance of NH 3 /NH 4 NO 2 and NH 3 /H 2 was compared under different boundary conditions (excess air coefficient, initial temperature, pressure and mixing ratio). The numerical simulation of compression combustion is carried out under different power loads. The addition of ammonium nitrite decreases the ignition requirement of ammonia and shortens the ignition delay time of the mixture fuel. The boundary conditions of compression ignition can be reduced by mixing hydrogen and mixing ammonium nitrite, but it is not enough to achieve compression ignition under NH 3 /H 2 mode. The addition of 30% ammonium nitrite can reduce the intake temperature to 300–360 K, which makes the compression ignition of the mixed fuel feasible. Meanwhile, in order to reduce the high in-cylinder combustion pressure and improve the combustion performance of the mixed fuel, the fuel injection strategy was proposed to achieve constant combustion pressure of 30 MPa under the premise of less power loss, which is a potential solution for the combustion of ammonia fuel. • The combustion law of NH 3 /NH 4 NO 2 is studied. • Temperature affects ammonia combustion performance more than other conditions. • The higher mixing ratio within the study, the better the combustion performance. • NH 4 NO 2 reduces the compression ignition boundary more than H 2. • A reliable compression ignition improvement scheme is proposed. [ABSTRACT FROM AUTHOR]

• Wedge-patterned biphilic surface is used for droplet transport; • Apex angle of the wedged patterns can help droplet transport on biphilic surface between 36.9° and 67.4°. • A force balance model is promoted on droplet transport on biphilic surface with wedge patterns and agrees well with experimental results. Droplet impact on biphilic surfaces with a wettability contrast has been intensively studied in recent years. In this work the effects of tilting and apex angles on droplet transport dynamics after impacting on a wedge-patterned biphilic surface at low Weber numbers were investigated experimentally. The biphilic surface was fabricated by applying a hydrophobic polymer coating on a bare silicon surface. According to the experimental results, a larger apex angle below 67.4° can accelerate the droplet effectively at first. Then the friction force controls the droplet movement and reduces the speed. The tilting angle along the hydrophilic direction activates the droplet. If the gravity component is opposite to the hydrophilic direction and the tilting angle is over 15°, the droplet can hardly move toward the hydrophilic area. By modeling the hydrodynamics of the droplet movement after impact on a biphilic surface with assumptions of no evaporation, no Marangoni effect, negligible dynamic contact angle variation and negligible rotation effect, the surface tension values versus the position at different apex angles are derived. The predicted position versus time trends agree well with the experimental data. This study aims to provide a better understanding of the mechanisms of droplet hydrodynamics on wedge-patterned biphilic surfaces at low Weber numbers. [ABSTRACT FROM AUTHOR]

Modulated nanoparticle-coating surfaces were fabricated by an improved electrophoretic deposition technique in this study. Pool boiling experiments were studied for HFE-7200 on the modulated nanoparticle-coating surfaces, with a smooth surface and uniform coating surfaces as comparison. It was found that the present modulated coating surfaces can enhance the heat transfer coefficient and the critical heat flux by 60% and 20%–40%, respectively, in comparison to the smooth surface, while the uniform coating surface can improve heat transfer coefficients by maximum 100%, but cannot enhance critical heat fluxes. Heat transfer on the modulated nanoparticle-coating surfaces was theoretically analyzed by a mechanistic model which considered free convection, transient conduction and microlayer evaporation. The heat transfer can be predicted by the model, especially at low-to-moderate heat fluxes. Additionally, referring to the bubble visualization at critical heat fluxes, possible mechanisms to trigger critical heat fluxes were discussed. Afterwards, a critical heat flux model originating from the Zuber hydrodynamic instability model, was employed to predict the experimental results, showing a good prediction ability. [ABSTRACT FROM AUTHOR]

• A single deep learning model can regress ignition delay time. • Data-driven approach can predict ignition and variation trend under high pressure. • SEGA performance excellent in mechanism optimization of multiple parameters. • The ammonia reaction mechanism adapted to high pressure was obtained. • Suggest mechanism optimization method under needed calculation requirements. Ammonia is a potential zero-carbon fuel, but the difficulties of experimenting at high pressure limit its development of application in power engineering. In this work, deep learning and genetic algorithm are combined to optimize the chemical reaction kinetics mechanism of ammonia under high pressure. The result shows that deep learning model is able to regress experimental data of ignition delay time in the range of less than 0.2% |E log | (logarithmic absolute error). At the same time, the trained model can be applied to the trend prediction of ignition delay time at high pressure, enriching combustion data of ammonia fuel that is not available from experiments. Optimization of PLOG reactions with a large population size was performed by strengthen elitist genetic algorithm, and the prediction accuracy of ammonia chemical reaction kinetics mechanism was improved by about 5% finally. The application value of artificial intelligence in combustion science is demonstrated. [ABSTRACT FROM AUTHOR]

Hydrodynamics and mass transfer in liquid-liquid slug flow inside a square microchannel (MC-1, aspect ratio 1) and a rectangular microchannel (MC-2, aspect ratio 2) with identical hydraulic diameter (400 µm) were investigated experimentally. The results showed that the overall volumetric mass transfer coefficient ( K L a ) in MC-2 was 16.2% lower than that in MC-1 due to enlargement of the dispersed phase slugs in MC-2, leading to development of stagnant zones within plugs and reduction of wall film renewal. Three different junction configurations (JC) were studied. K L a was enhanced by up to 34.8% using JC-1, compared to JC-2 or JC-3 due to greater movement of the biphasic interface, resulting in higher mixing within slugs using JC-1. The junction arrangement influenced not only the mass transfer during the slug-formation stage, but also the slug-flowing stage due to effects on the vortex patterns along the microchannel. [ABSTRACT FROM AUTHOR]

To realize the transition of our society to a low-carbon future with innovative subsurface energy solutions, understanding the dynamic behavior of gas invading multi-fluid systems in underground pore space is critical. In this work, a joint approach of flow imaging and digital image processing is employed to investigate the fingering dynamics of gas invading multi-fluids in porous media. We examined various gas (G) invasion scenarios of a high-viscosity defending liquid (HL), low-viscosity defending liquid (LL), and their co-existing multi-fluid system, focusing on the viscosity effect. Quantification of phase saturation shows that the displacement efficiency follows the order of G → (L → L) > L → L > G → L, regardless of the varieties in injection flow rate in the viscous-dominated flow regime. In other words, the enhancement in displacement efficiency and potential energy savings are achieved by solely introducing a third phase without the cost of the higher pumping power. When gas invades the HL and LL multi-liquid system, the fingering pattern in G → (HL → LL) and G → (LL → HL) significantly differs and highly depends on the sequential occupation of HL and LL in the pore spaces. The previously unobserved yarn-liked gas pattern in G → (LL → HL) is suspected as the main reason for the fast gas displacement. Through Local dynamics analysis, we identified that the preferential invasion into interconnected LL channels and the inhibitory effect of scattered HL on bypass invasion are the primary mechanisms behind the formation of yarn-liked fingers. We classified two distinct categories of ganglia mobilization and connection in G → (LL → HL), i.e. "catch up to connect" and "expand to connect". Finally, the topological connectivity of the gas finger in G → (LL → HL) is evaluated using Euler number. Euler number shows an ascending trajectory before breakthrough, followed by a rapid descent and stabilization at steady state. This signifies that disconnected ganglia emerge before breakthrough and subsequently expand and reconnect. Our new findings are of great importance for subsurface extraction/storage strategy innovation through enriching multi-fluids injection scenarios. • Investigate various viscous-dependent gas invasion scenarios of a high-viscosity and low-viscosity liquid as well as their co-existing multi-fluid system. • Demonstrate the highest displacement efficiency in three-phase displacement followed by the liquid–liquid and gas–liquid two-phase displacements, regardless of the varieties in injection flow rate. • Discover a novel yarn-liked gas pattern in three phase displacement and reveal the formation mechanism by local invading analysis. • Identify two types of ganglia mobilization and analyze time evolution of the topological ganglia connectivity. [ABSTRACT FROM AUTHOR]

Regenerative cooling system at supercritical conditions can accommodate high heat fluxes effectively in aerospace applications. The potential of nanofluids as regenerative coolants at supercritical pressures was evaluated in this work. Experiments were carried out to study the heat transfer characteristics of Al 2 O 3 -kerosene nanofluids flowing upward in a vertical minitube at supercritical pressures. Parametric effects of mass flow rate, heat flux, pressure and particle content on the heat transfer performance are presented. Results show that increasing the mass flow rate or pressure enhances heat transfer, while higher heat fluxes lead to poorer heat transfer performance. Nanofluids tend to deteriorate heat transfer at supercritical pressures because deposition of the nanoparticles smoothens the wall roughness and presents an additional thermal resistance. As the particle content increases, the heat transfer performance becomes worse. Based on the experimental data, a heat transfer correlation was established for Al 2 O 3 -kerosene nanofluids at supercritical pressures and the correlation shows good predictive ability. [ABSTRACT FROM AUTHOR]

• Phase-transition-associated vapor transport is investigated for geothermal extraction. • Interfacial instability of liquid bridge rupture occurs in throat during vaporization. • Capillary-condensation surprisingly enhances vapor extraction under certain conditions. Vapor transport in porous media, often associated with liquid-vapor phase change, is an fundamental process in many emerging underground energy storage and extraction processes (i.e., seasonal solar thermal aquifer storage, geothermal extraction, extraterrestrial in-situ water extraction). By jointly using experimental imaging and numerical modeling at the micro-scale, we conduct mechanistic pore scale investigation of capillarity-dominated phase change dynamics and its influence on vapor transport in partially saturated porous rock micromodel. Strongly linked to surface roughness and wettability condition, the capillarity hinders water vaporization from rock surface micro/nano-structures as observed under the environmental scanning electron microscope. By varying the contact angle of 0°, 60°, and 120°, the lattice Boltzmann simulation shows the wettability-dependent vaporization process of capillary-hold water, where pores with hydrophilic surfaces contains significantly more liquid water than that of the hydrophobic ones under the same temperature. When saturated vapor flows through rock porous patterns, capillarity further induces water condensation on the strongly water-wet surfaces. Water condensation, yet forming water bridges/islands and causing the blockage of vapor diffusion, enhances the vapor diffusion ability counterintuitively. The reduction of diffusion path is revealed as the main reason by assessing the local vapor pressure distribution before and after the pore filling by condensate. The findings support the debatable enhancement mechanisms postulated by Philip and de Vries. This work offers the insightful interfacial hydrodynamics of vapor transport in porous media and potentially provides operational guidance for geothermal applications and beyond. [Display omitted] [ABSTRACT FROM AUTHOR]

• Reveal the cooling performance of Z-type manifold microchannel heat sink. • Trapezoidal manifold presents better thermal and hydraulic characteristics. • Dissipate an ultra-high heat flux of 1842 W/cm2 with an ultra-low pumping power. Ultra-high heat flux (up to 1000 W/cm2) thermal management has been an urgent need for advanced power electronic devices to maintain their superior electrical performance and reliability. Embedded microchannel cooling is a promising and challenging thermal management method for high-heat-flux chips. This study presents an embedded Z-type manifold microchannel (Z-MMC) cooling design for a 5 × 5 mm2 heating zone, which can dissipate an ultra-high heat flux up to 1842 W/cm2 by using water single-phase flow. The microchannel array is etched on silicon wafer, while the manifold structure is fabricated by PDMS molding. The test chips are assembled using O 2 plasma bonding technique, which provides an observation window for visualizing the flowing state of the coolant during the heat transfer process. We experimentally test and compare four test chips with different microchannel geometries and manifold designs. The average heater temperature, temperature distribution, thermal resistance, heat transfer coefficient and pressure drop are reported for mass flow rates of 2–6 g/s. The proposed Z-MMC cooler is capable of dissipating up to 1800 W/cm2 with a total thermal resistance of only 0.075 cm2·K/W (only 0.039 cm2·K/W after subtracting the conduction thermal resistance), at a flow rate of 6 g/s and a pressure drop of 54 kPa. Compared to the rectangular manifold arrangement, the trapezoidal manifold demonstrates a lower temperature rise, a more uniform temperature distribution and a smaller pressure drop at the same heat flux and flow rate, mainly due to a more uniform fluid distribution in the microchannel. Furthermore, we compare our proposed design's thermo-hydraulic performance with those of representative manifold microchannel cooling data in the literature, and the proposed Z-MMC design demonstrates relatively low thermal resistance and pressure drop values. [ABSTRACT FROM AUTHOR]

• Novel MMC with single- and double-layer countercurrent regions are proposed. • Better heat transfer performance of lower pressure drops are obtained. • Temperature difference is about 25% of the original Z-type MMC. • 1100 W/cm2 can be dissipated with pressure drop of 22 kPa for single phase flows. Manifold microchannel (MMC) heat sink is a potential method to dissipate high heat fluxes of electronic devices. Traditional Z-type MMC is a simple configuration but usually encounters high temperature difference and pressure drop. In this work, we introduced countercurrent flows to alleviate these problems by revising the design of manifold configurations. Six cases with different manifold arrangements are numerically investigated using single-phase deionized water at mass flow rates ranging from 0.04 - 0.12 g/s. The results show that the maximum temperature, average temperature, and temperature difference of the revised arrangements are all lower than these of the traditional Z-type manifold arrangement. The cases with countercurrent regions can reduce the maximum temperature around 5 K compared with the original cases with parallel regions. The temperature difference of the ZU-type MMC with double-layer countercurrent flow configuration is lower than 4 K, which is about 25% of the original Z-type MMC. For the revised manifold configuration with countercurrent flows, the pressure drops also decrease due to the split of inlets and outlets of the manifold. The pressure drops of single-layer and double-layer countercurrent manifolds are respectively 27.13% and 33.36% lower than that of the case with traditional Z-type manifold. This leads to a higher comprehensive performance of revised cases with countercurrent flows. Besides the better temperature uniformity, the fluid flow becomes more uniform in the microchannels for the revised case. The improved ZU-type MMC with double-layer countercurrent flow can dissipate 1100 W/cm2 for single-phase flow with a temperature increase of 80 K and a pressure drop of 22 kPa at mass flow rate of 1.2 g/s. [ABSTRACT FROM AUTHOR]

In the present study, two types of micro-pin–fin configurations were fabricated on silicon surfaces by a dry etching method, i.e., staggered pin fins (#1) and aligned pin fins with empty areas (#2). The micro-pin–fin surfaces were then further modified by depositing FeMn oxide nanoparticles (∼35 nm) electrostatically for 8 h and 16 h, respectively, namely #1-8h, #1-16h, #2-8h and #2-16h. Subcooled pool boiling heat transfer was experimentally studied on these surfaces at atmospheric pressure, using FC-72 as the working fluid. The results showed that in comparison to the smooth surface, pool boiling heat transfer was significantly enhanced by the micro-pin-fin surfaces and the maximum superheat was considerably decreased. Additionally, critical heat fluxes were also greatly improved, e.g., the critical heat flux on #1 was almost twice of that on the smooth surface. Generally, the nanoparticle deposition could further enhance pool boiling heat transfer, including the heat transfer coefficient and critical heat flux (CHF). High speed visualizations were taken to explore the mechanisms behind the heat transfer performance. The bubble behavior on the micro-pin–fin surfaces with and without nanoparticles was compared at low, moderate and high heat fluxes, respectively. The wickability of FC-72 on the test surfaces was measured, based on which, a modified CHF model was proposed to predict the experimental CHFs. Accordingly, a possible mechanism of CHF enhancement was described. [ABSTRACT FROM AUTHOR]

Heat transfer performance of submerged jet impingement boiling over staggered micro-pin-finned surfaces was investigated using air-dissolved FC-72. The dimension of the silicon chips is 10 × 10 × 0.5 mm 3 (length × width × thickness) on staggered micro-pin-fins with four dimensions of 30 × 30 × 60 μm 3 , 50 × 50 × 60 μm 3 , 30 × 30 × 120 μm 3 and 50 × 50 × 120 μm 3 (width × thickness × height, named S-PF30-60, S-PF50-60, S-PF30-120, and S-PF50-120) were fabricated by using the dry etching technique. The effects of micro-pin-fins, jet-to-target distance ( H  = 3, 6, and 9 mm), and jet Reynolds number ( Re  = 2853, 5707, and 8560) on jet impingement boiling heat transfer performance were explored. For comparison, experiments with jet impinging on a smooth surface were also conducted. The results showed that all micro-pin-finned surfaces show better heat transfer performance than that of a smooth surface. The largest Nusselt number is 1367, corresponding to a heat transfer coefficient of 26387 W·m −2 ·K −1 with S-PF30-120 at Re  = 8560, H / d  = 2, and q  = 151 W·cm −2 , which is approximately twice the largest Nusselt number of Chip S. In the single-phase heat-transfer-dominant region, the Nusselt number ( Nu ) is mainly influenced by several dimensionless numbers, including Reynolds number ( Re ), boiling number ( Bo ), the ratio of jet-to-target distance to jet diameter ( H / d ), the ratio of micro-pin-finned surface area to smooth surface area A / A S , and a dimensionless number corresponding to flow resistance D h / L h . Correlations to predict Nu in both single-phase heat-transfer-dominant region and two-phase heat-transfer-dominant region for smooth and micro-pin-finned surfaces were proposed. The results show that most data (96%) in the single-phase heat-transfer-dominant region and most data (96%) in the two-phase heat-transfer-dominant region were predicted within ±13% and ±15%, respectively. In addition, CHF correlations for smooth and micro-pin-finned surfaces were also proposed, and most data (95%) are predicted within ±20% for a smooth surface and all the data within ±5% for the micro-pin-finned surfaces. [ABSTRACT FROM AUTHOR]

• Novel manifolds with two inlets and countercurrent regions are proposed. • Better uniformity in terms of flow and temperature distributions are obtained. • Higher comprehensive performance are obtained with novel manifolds. • New performance evaluation criterion based on thermal resistance is developed. Manifold microchannel heat sink is promising to dissipate high heat fluxes for electronic devices due to its high surface to volume ratio. Traditional Z-type manifold encounters fluid flow maldistribution and a high temperature difference. In this work, two inlets and countercurrent flows are introduced into the manifold arrangement to alleviate these problems in the Z-type manifold microchannels. Six cases with different manifold arrangements are numerically investigated using single-phase HFE-7100 at mass flow rates ranging from 56.8 to 117.78 mg/s. The results show that the maximum temperature, average temperature, and temperature difference of the revised arrangements are all lower than these of the traditional Z-type manifold arrangement. The minimum temperature difference for an ideal multi inlet manifold arrangement can be less than 0.2 K, while the value is about 5.3 K for the traditional Z-type manifold arrangement. The revised arrangement can also improve the uniformity of mass flux distributions. The ratio of the maximum to the minimum mass flux within microchannels is 12.83 for the traditional Z-type manifold, while this value is about 5.82 for cases with two inlets and countercurrent zones. A longer flow path and a higher local pressure drop in the revised arrangements results in higher pressure drops than the traditional Z-type manifold microchannels. Compared with the traditional Z-type arrangement, heat transfer coefficients and performance evaluation criterion are higher for the improved cases. The presence of a countercurrent zone can improve the comprehensive performance of the manifold microchannel. New criterion based on the thermal resistance calculated at the maximum temperature is proposed to evaluate the cooling performance of electronics. [ABSTRACT FROM AUTHOR]

• Mixture model without interfacial reconstruction is adopted and validated. • Computational time of VOF model is about 6.6 to 8.4 times of Mixture model. • A complex case of MMC with a hotspot heat flux of 800 W/cm2 was studied. • Maldistributions of mass flux, wall temperature and vapor void fraction are observed. The Manifold Microchannel (MMC) heat sink is an emerging micro-scale electronic cooling technology with high heat dissipation potential and application prospects. This paper numerically studied the subcooled flow boiling in MMC heat sinks. The feasibility of using the Mixture multiphase flow model to simulate the subcooled flow boiling in the MMC heat sink is validated. As the vapor-liquid interfaces are not reconstructed in this model, less computational resources are needed compared with other multiphase models. The results show that at constant channel width, the maximum and average temperatures of the MMC decrease with the increase of the microchannel aspect ratio. For cases with an inlet velocity of 0.35 m/s, the average wall temperature decreases from 388.53 K to 374.26 K when the aspect ratio increases from 6.67 to 16.67. The difference between the maximum and the minimum temperature on the heated surface increases with the increase of channel aspect ratio, resulting in an uneven temperature distribution. The thickness of temperature boundary layer near the divider is thicker than that near the heated base at low aspect ratio. Low pressure drops are obtained for cases with high aspect ratios due to the increase in cross-sectional area. The pressure drop of MMC with aspect ratio of 6.67 is nearly twice of the case with aspect ratio of 16.67. Although the Mixture model could not characterize the bubble shapes, the numerical results are similar to those obtained by the VOF model and the experimental visual results. A complex case of subcooled flow boiling in Z-type MMC with a hotspot heat flux of 800 W/cm2 is also conducted using the whole computational domain. Mal-distributions of mass flux, wall temperature and vapor void fraction are observed for this complicated problem. The difference between the maximum and the minimum temperatures is about 12 K. The Mixture model is more suitable to obtain reasonable numerical results of flow boiling in complex MMC with much less computational resource. [ABSTRACT FROM AUTHOR]