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• Pumped two-phase loop (PTL) integrated with thermal storage (LHS) is proposed. • Thermal response behaviors of PTL-LHS under step heat load are investigated. • Fast tracking and delay response is sequentially observed as heat load increases. • Thermal response behavior regime diagram is quantitatively recognized. • Variable flow rate optimization strategy is validated by comparative experiments. Pumped two-phase loop integrated with thermal storage is an emerging solution for the short-term high-heat-flux cooling. The transient microchannel boiling heat transfer of pumped two-phase loop (PTL) integrated with latent heat storage (LHS) under step heat load are experimentally investigated, with special focuses on dynamic thermal response of two-phase loop and latent heat storage unit. The transient thermal behaviors of PTL-LHS are determined based on the wide variations of heat load and flow rate. A thermal behavior regime diagram is proposed to quantitatively recognize the transient thermal response of PTL-LHS, depending on the heat load and flow rate. The results indicate that the fast tracking mode and the delay response mode are observed and sequentially experienced for PTL-LHS as the step heat load increases. With respect to delay response mode, thermal storage efficiency is high with short thermal storage duration but cooling performance is insufficient, while the transient thermal performances are contrary under the fast tracking mode. The increase of heat load accelerates the transformation of the heat transfer mechanism in LHS from sensitive-heat-dominated to latent-heat-dominated, while increasing flow rate is conducive for the enhancements of thermal capacity and thermal stability of PTL-LHS. Increasing heat load brings about a larger critical flow rate of the transient thermal behavior regime transition, and hence the range of flow rate tends to be larger for the delay response mode. Particularly, the proposed variable flow rate optimization strategy is validated by comparative experiments, showing better cooling performance (the average evaporator wall temperature decreases by 10 °C) and more remaining load of phase change material. [Display omitted] [ABSTRACT FROM AUTHOR]

• Evaporation regimes in an enclosed narrow space is studied by MRT LB simulation. • Evaporation regimes are evaluated in a regime diagram depending on Bo and Ja. • Five typical evaporation regimes are identified by changing local heat flux. • Occurrence of liquid-bridge evaporation arises from the chamber confinement. • Transition boiling and film boiling are more likely to occur under a smaller Bo. A hybrid multiphase lattice Boltzmann method is utilized to investigate the vapor-liquid thermos-hydrodynamic behaviors in a flat two-phase thermosiphon. The evaporation regimes in the enclosed narrow space are quantitatively evaluated in a regime diagram with the Bond number and Jakob number, which are directly related to the heat flux, chamber height and filling ratio. The results indicate that the surface evaporation, nucleate boiling, transition boiling and film boiling are sequentially experienced in a flat two-phase thermosiphon as the local heat flux increases (i.e. Jakob number increases). The surface evaporation regime can be subdivided into the pool-surface evaporation and the liquid-bridge evaporation. Liquid-bridge evaporation is a unique evaporation regime due to the confinement of a narrow space under the combined effects of evaporation and condensation. In addition, for a smaller Bond number (i.e. the enclosed narrow space is more confined), the transition boiling and film boiling are more likely to occur, and the interval of Jakob number is smaller for the nucleate boiling in which the thermal performance is superior. [ABSTRACT FROM AUTHOR]

A model of vapor condensation on a solid surface is developed and numerically analyzed using the free-energy lattice Boltzmann method. Based on the model, the condensation phase change on hydrophobic, hydrophilic and gradient surfaces are simulated with a particular focus on the condensation on a gradient surface. The droplet nucleation, growth, deformation, coalescence and motion during the condensation on a gradient surface are investigated. The present simulation reproduces the self-propelled dropwise condensation on a gradient surface, the film condensation on a hydrophilic surface and the conventional dropwise condensation on a hydrophobic surface. The results indicate that the condensed droplets on a gradient surface can be swept in time to provide a favorable condition for the subsequent condensation. On a smooth gradient surface, owing to the unbalanced wetting force, the vapor condenses into a thin film firstly and then fractures into droplet nucleation as the condensation process goes on. The larger wettability gradient results in a larger amplitude oscillation of condensation rate and a slighter variation of surface coverage. [ABSTRACT FROM AUTHOR]

The condensation flow patterns of ethanol–water mixture in a wide range of ethanol mass concentration inside a hydrophobic microchannel is experimentally studied via a high speed imaging system. The effects of channel surface wettability and ethanol concentration on flow condensation are compared and discussed. The experimental results indicate that the surface hydrophobic modification and ethanol concentration play a significant role in the flow condensation of mixture in a microchannel. The droplet condensation appears almost the whole two-phase flow region when the water steam is the main component, in which the droplet flow, droplet-streak flow, droplet-annular flow, droplet-injection flow and droplet-slug/bubble flow occur sequentially in a hydrophobic microchannel. With increasing ethanol concentration, the droplet-slug/bubble flow after droplet-injection flow disappears and is replaced by the pure slug/bubble flow. When the ethanol vapor is the main component, the droplet condensation almost disappears, and the annular-streak flow, annular flow, injection flow and slug/bubble flow appear sequentially along the flow direction. Both an increase in vapor Reynolds number and a decrease in ethanol concentration cause the injection location move toward the channel outlet. In addition, the surface hydrophobic modification introduces the droplet condensation, which is beneficial for the flow condensation heat transfer. [ABSTRACT FROM AUTHOR]

• LHS is proposed to enhance thermal performance of AGHP under intermittent heating. • Thermal performance of AGHPs with three LHS modes are experimentally investigated. • Introduction of LHS unit into AGHP reduces the temperature of heat source by 27.4%. • The presence of LHS unit greatly enhances the thermostat performance of an AGHP. • Temperature homogenization of AGHP & LHS units becomes worse as heat load increases. Heat pipe coupled with phase change material (PCM) based latent heat storage (LHS) is a significant thermal management system that can efficiently control the intermittent high heat loads of electronic devices. Herein, an axially grooved heat pipe-LHS (AGHP&LHS) system has been proposed. The influence of LHS units and their location on dynamic temperature response of AGHP has been investigated both experimentally and theoretically. The effect of filling rates of phase change materials (PCMs) as well as cooling temperature on thermal response of the AGHP has been compared and analyzed. Results indicate that the LHS effectively stabilizes the heat source temperature under the intermittent heating process, enhances the thermostat performance and brings the temperature uniformity in AGHP. In addition, LHS units arranged at the evaporator section are optimal for AGHP under intermittent heat load conditions, reducing the heat source temperature by 27.4% and prolonging the melting time by 81.5%. As the heat load increases, the melting time of AGHP&LHS system becomes shorter, and the overall temperature uniformity is worsened. In particular, the melting time is prolonged by 34.8% for AGHP&LHS system with PCM filling rate of 89% than that of 44% filling rate. [ABSTRACT FROM AUTHOR]

The fractal Cantor structure is introduced to characterize the surface topography of solid wall. By this fractal characterization, a molecular dynamics simulation of heat conduction in rough nanochannels is conducted to investigate how surface topography affects the thermal conductance at liquid–solid interfaces. The role of surface roughness on temperature profiles, liquid atom trajectories, interfacial interaction energy and interfacial conductance are all examined and analyzed. The results indicate that, the temperature jump at liquid–solid interfaces is observed irrespective of surface condition, and the existence of surface roughness reduces the interfacial temperature jump. When compared with smooth surface, the presence of surface roughness diminishes the motion capability of liquid atoms in the wall-neighboring region and these atoms could maintain contact with the solid surface for a long time, which intensifies the energy transfer between the liquids and solid surface and hence contribute to large thermal conductance at a liquid–solid interface. Interestingly, it is found that the thermal conductance at a rough liquid–solid interface is affected by more than just statistical roughness height, but also fractal dimension (topographical irregularity). In addition, increases in liquid–solid interaction strength, roughness height and fractal dimension are all beneficial to enhance thermal conduction at liquid–solid interfaces. In particular, a longer trapping time of liquid atoms inside the valley of the fractal surfaces is observed for a larger fractal dimension. [ABSTRACT FROM AUTHOR]

Abstract: A lattice Boltzmann simulation of heat transfer for gas flowing in microchannels incorporating surface roughness as characterized by fractal Cantor structure is conducted to investigate the temperature jump at rough gas–solid interfaces in the slip flow regime. The gas temperature jump at rough interface as quantified by the temperature jump length is evaluated and compared with smooth interface. It is indicated that the temperature jump at a rough gas–solid interface is mainly dependent on Knudsen number, Prandtl number, and surface roughness. The presence of roughness is beneficial to the energy exchange at the gas–solid interface and introduces a smaller temperature jump when compared with a smooth surface. The local gas temperatures in the valley of rough surface are approximately equal to the corresponding surface temperatures while an obvious interfacial temperature jump is detected over the peaks of the rough surface. In addition, increase in Prandtl number, roughness height, surface fractal dimension as well as the decrease in Knudsen number can lead to the reduction of interface temperature jump for gas convection heat transfer in microchannels. Interestingly, in microchannels, the roughness height play a considerable role in the temperature jump at the gas–solid interface, however, the effect of fractal dimension on interfacial temperature jump is not so significant. [Copyright &y& Elsevier]

Abstract: The self-diffusion model of Lennard-Jones fluid confined in a nanoscale space is developed by using equilibrium molecular dynamics simulation. The radial distribution function in free and confined space is evaluated and compared to analyze the fluid microstructure. The effects of the confined scale, strength of the fluid–solid coupling and commensurability of wall and fluid density on the fluid self-diffusion are investigated and discussed. The results indicate that the confined scale, fluid–solid coupling strength and commensurability of wall and fluid density play considerable role in the self-diffusion coefficient for the fluid confined in nanoscale space. Decrease in the confined scale lead to large portion of fluid molecules ‘feel’ the wall interaction, which results in the reduction of self-diffusion coefficient. In addition, the reduction of fluid self-diffusion coefficient in confined space is also demonstrated when the wall and fluid densities are incommensurate and wall–fluid coupling strength is large. [Copyright &y& Elsevier]

Abstract: Flow boiling in constructal tree-shaped minichannel network with an inlet diameter of 4mm is numerically investigated using a one-dimensional model, taking into consideration the minor losses at junctions. The pumping power requirement, pressure drop, temperature uniformity and coefficient of performance of the constructal tree-shaped minichannel network are all evaluated and compared with those of the corresponding traditional serpentine channel, and the fluid stream undergoes a phase change from saturated liquid to saturated vapor. The effects of the length dimension and top view area (i.e. the path length) on saturated gas–liquid two-phase flow boiling heat transfer in tree-shaped minichannel networks are all analyzed and discussed. The results indicated that, the tree-shaped network configured with length dimension of two is able to maximum flow access; the path length plays a significant role in the determination of flow boiling in tree-shaped minichannel networks. In particular, compared to the traditional serpentine channel, flow boiling in constructal tree-shaped minichannnel network possesses less pressure drop, lower pumping power requirement, better temperature uniformity and higher coefficient of performance (COP). [ABSTRACT FROM AUTHOR]

Abstract: A theoretical model of fluid flow and heat transfer in a heat pipe with axial “Ω”-shaped grooves has been conducted to study the maximum heat transport capability of these types of heat pipes. The influence of variations in the capillary radius, liquid–vapor interfacial shear stress and the contact angle are all considered and analyzed. The effect of vapor core and wick structure on the fluid flow characteristics and the effect of the heat load on the capillary radius at the evaporator end cap, as well as the effect of the wick structure on the heat transfer performance are all analyzed numerically and discussed. The axial distribution of the capillary radius, fluid pressure and mean velocity are obtained. In addition, the calculated maximum heat transport capability of the heat pipe at different working temperatures is compared with that obtained from a traditional capillary pressure balance model, in which the interfacial shear stress is neglected. The accuracy of the present model is verified by experimental data obtained in this paper. [Copyright &y& Elsevier]

• Ice storage process with natural convection is numerically studied and verified. • Natural convection is negatively related to the ice storage performance. • Buoyancy reversal induces the intensification of natural convection. • Optimization of shell-tube ice storage units is performed by Taguchi method. • Fin height is the dominated factor to the overall ice storage performance. The present paper reports on a numerical study of the ice storage process in finned shell-tube ice storage (STIS) units, with a focus on the special solidification behavior using water as the phase-change material (PCM). The proposed model is experimentally verified using an energy-discharging process in an ice storage unit. The effects of natural convection and buoyancy reversal on the solidification behavior are examined and investigated. Moreover, the Taguchi method is utilized to optimize the fin geometry of STIS units. The results indicate that the natural convection and buoyancy reversal are negatively correlated with the ice storage performance. An increase of the superheat factor leads to the enhancement of the buoyancy reversal intensity, which is not conducive to the acceleration of the solidification rate. In addition, the increases in fin height, fin width, and fin number are positively correlated with ice storage performance. It is demonstrated that the fin height is the dominant factor affecting the overall ice storage performance, and it is independent of the superheat factor. From the perspective of trade-off between ice storage rate and ice storage capacity, the optimal fin parameters for the STIS unit are fin height H = 40 mm, fin number N = 10, fin width Δ = 3 mm for engineering applications. [ABSTRACT FROM AUTHOR]

The fractal Brownian motion is introduced to describe the pore distribution of porous metal foam. By the fractal description, a model of melting heat transfer for the phase change material (PCM) is developed and applied to investigate the melting behaviors in porous metal foam with a particular focus on the role of pore distribution. The dynamic response of temperature and the evolution of melting front are presented. The effects of porosity and fractal dimension on the melting heat transfer are all examined and investigated. In addition, an experiment of melting heat transfer is performed to verify the present model. The results indicate that the porous metal foam is of significance for the enhancement of melting heat transfer. When compared with PCM alone, the melting process in porous metal foam possesses the larger melting rate, the faster evolution of melting front and the higher liquid fraction. Unlike the PCM alone, the melting front is no longer continuous and many independent solid-liquid interfaces are formed inside pores owing to the interstitial heat transfer. Interestingly, the melting phase change is also affected by the fractal dimension even though the porosity is identical. A porous metal foam with smaller fractal dimension is beneficial for the melting heat transfer. [ABSTRACT FROM AUTHOR]

In this paper, the thermo-hydrodynamic behaviors in miniaturized two-phase thermosyphons are experimentally investigated by a visualization system. The effects of heat load and channel dimension on the thermo-hydrodynamic behaviors are examined and further quantitatively recognized by a state diagram depending on the Weber number and Bond number. In addition, the vapor–liquid two-phase flow, heat transfer regimes and thermal performances are analyzed and discussed. The results indicate that, differing from the conventional two-phase thermosyphon, the state of working fluid in a miniaturized two-phase thermosyphon is no longer only in a fashion of pool and annular-flow states, but also in a fashion of oscillation state. The oscillation state is the characteristic two-phase flow regime of a miniaturized two-phase thermosyphon and possesses the feature of random formation and oscillatory motion of liquid plug in minichannels. The pool state occurs at small Weber number, the annular-flow state is observed at large Weber number, and the oscillation state takes place at medium Weber number when Bond number is approximately smaller than 0.7. The thermal performance of oscillation state is better than that of pool state but is less powerful than that of annular-flow state. The heat transfer regimes are the film evaporation and condensation combined with thermal conduction in the pool state, the forced evaporation and condensation in the oscillation state, and the film evaporation and condensation in the annular-flow state, respectively. [ABSTRACT FROM AUTHOR]

• The boiling characteristics of plate heat exchanger under sloshing is explored. • Boiling characteristics are vulnerable under self-driven mode with low mass flux. • Heat transfer coefficient varies 44 % during ±20° angle under inclined state. • Heat transfer performances are apparently enhanced by increasing swing amplitude. • High mass flux is conducive to eliminate the sloshing effect on flow boiling. Aiming to explore the boiling heat transfer characteristics in plate heat exchanger under ocean sloshing scenarios, a two-phase flow boiling experimental system integrated with visualized plate heat exchanger and three degree-of-freedom (DOF) motion platform is designed and built. With special focuses on two-phase flow pattern, temperature and pressure responses, the sloshing effect on flow behaviors and boiling characteristics of plate heat exchanger under different driven modes (pump-driven and self-driven), liquid filled charge ratio, mass flux, sloshing postures (inclined state and swing state) and sloshing orientation (frontal and lateral) are investigated. The results indicate that when plate heat exchanger is fully filled with working fluid under pump-driven mode, the heat transfer efficiency is insensitive to sloshing effect regardless of inclined state or swing state. When plate heat exchanger is not fully filled with working fluid under self-driven mode, the inclined angle poses a great influence on the heat transfer performance by changing the liquid-vapor distribution, with heat transfer coefficient variations of 44 % and 15 % at mass flux of 10 kg/(m2·s) during frontal inclined angle of ±20° and lateral inclined angle of ±20°, respectively. Moreover, there are heat flux variations of 24 % and 177 % at mass flux of 10 kg/(m2·s) during ±20° frontal incline and lateral incline, respectively. For swing state, heat transfer performances are apparently enhanced with the effect of inertial force, characterized by the increases of heat transfer coefficient, heat flux and pressure fluctuation with increasing swing amplitude. Under ocean sloshing scenarios, the operation of plate heat exchanger under high liquid filled charge ratio and mass flux is conducive to eliminate the sloshing effect on flow boiling. [ABSTRACT FROM AUTHOR]

• An innovative LIBCS based on the flow guide with fish-shaped holes is proposed. • An electrochemical-thermal model for battery module is established. • Common flow guides provide good heat dissipation but generate higher pumping costs. • Employing flow guides with fish-shaped holes has the best comprehensive performance. A battery thermal management system (BTMS) is crucial for the safety and performance of lithium-ion batteries (LIBs) in electric vehicles. To improve the BTMS in terms of cooling performance and pumping cost, an innovative liquid immersion battery cooling system (LIBCS) using flow guides with fish-shaped holes is proposed. The design of this flow guide is grounded in conformal mapping technology and bionics principles. An electrochemical-thermal model for the battery module is established by the Multi-Scale Multi-Domain approach with the Newman, Tiedemann, Gu, and Kim model. The simulation and experimental results show good agreement, with mean absolute errors of 0.0174 V, 0.126 °C, and 0.042 Pa for LIB voltage, maximum temperature, and average pressure drop, respectively. Furthermore, the effects of four different battery pack structures on the performance of LIBCSs under different operating conditions are investigated. For LIBs at a 3C discharge rate, the LIBCS using common flow guides decreases the maximum LIB temperature by 5.3 % at a mass flow rate of 0.00273 kg/s compared to the without flow guide case, but it increases the pump power consumption by 81.4 %. However, using flow guides with circular holes or fish-shaped holes reduces the maximum LIB temperature by 9.2 % and 12.2 %, respectively, compared to LIBCS using common flow guides, while maintaining maximum temperature differences within 5 °C. Interestingly, the LIBCS using flow guides with fish-shaped holes reduces the pump power consumption by 42.1 % and 11.8 %, respectively, compared to the common flow guide and flow guide with circular holes cases under the same operating conditions. Additionally, the comprehensive performance factor of LIBCS using flow guides with fish-shaped holes improves by 24 %, 39.3 % and 7.3 % at a mass flow rate of 0.00273 kg/s, respectively, compared to cases using no flow guide, common flow guides and flow guides with circular holes, indicating greater value for application in practical engineering. [ABSTRACT FROM AUTHOR]

• Thermal performance evaluation of electronic fluorinated liquids (EFLs) is studied. • A figure of merit is proposed to guide the selection and development of EFLs in immersion cooling. • Importance of EFL thermal properties for its immersion cooling is quantitatively analyzed. • Temperature increase results in a higher weightage of dynamic viscosity on EFL performance. The regulation and prediction of the cooling performance by thermal properties of electronic fluorinated liquids (EFLs) are critical challenges in EFL selection and development for single-phase immersion liquid cooling (SPILC) systems in data centers. To address this, this paper proposes a figure of merit (FOM) to guide the selection and development of EFLs for SPILC systems. An integrated experimental platform is constructed to measure EFL thermophysical properties over a wide temperature range. The influence mechanism of thermophysical properties of different EFLs on the flow and heat transfer performance of SPILC systems is numerically investigated. The results indicate that FOM is effective and reliable in evaluating EFL performance. According to the importance of FOM, the ranking order of thermophysical properties for EFLs is dynamic viscosity > thermal conductivity > density > specific heat capacity. With increasing temperature, only the weight factor of dynamic viscosity increases, while weight factors of other thermophysical properties decrease. Moreover, the average Nusselt number and pressure drop of a SPILC cabinet using the EFL with the lowest dynamic viscosity increases by 57.3% and decreases by 59%, respectively. Therefore, when selecting and designing EFLs in data centers, efforts should be made to minimize their dynamic viscosity. [ABSTRACT FROM AUTHOR]

• Buoyancy-driven melting/solidification performance with different PCMs is studied. • Thermal expansion and contraction properties of PCMs affect the natural convection trend. • Downward and upward flow modes have opposite effects on PCM phase change performance. • Suitable non-uniform fin layout enhances melting but weakens solidification performance. The thermal expansion and contraction property of phase change materials (PCMs) is a significant factor in affecting the buoyancy-driven melting/solidification heat transfer performance in latent heat storage (LHS) units. To reveal the intrinsic mechanisms, a mathematical model of buoyancy-driven melting/solidification processes in finned LHS units is constructed. The phase-change behaviors, thermal transport properties of different PCMs, and roles of heat transfer fluid (HTF) direction and fin layout in melting/solidification performance are explored. The results indicated that when compared to conduction-dominated cases, the buoyancy-driven melting duration of gallium and n-octadecane increases by 1.8% and decreases by 32.6%, respectively, while the corresponding solidification times decrease by 8.4% and increase by 15.2%, respectively. The downward HTF enhances the melting rate of gallium with thermal-contraction properties and the solidification rate of n-octadecane with thermal-expansion properties. Conversely, the upward HTF enhances the melting rate of n-octadecane with thermal-expansion properties and the solidification rate of gallium with thermal-contraction properties. For finned LHS units with upward HTF, the ladder-type and reverse ladder-type fins facilitate the melting performance of n-octadecane and gallium, respectively. Moreover, the ladder fins with non-uniform layouts weaken the solidification performance of PCMs with different thermal characteristics. [ABSTRACT FROM AUTHOR]

• Pump-driven/buoyancy-driven single-phase immersion cooling systems are studied. • Pump-driven SPIC systems reduce average temperature and PUE by 55.5 and 11.6%. • Natural convection enhances heat transfer while inducing temperature stratification. • Low inlet temperature and appropriate flow rate are suggested for applications. Improving power usage effectiveness (PUE) in data centers becomes a key challenge for carbon neutrality and carbon peaking strategies. In this context, this paper investigates the single-phase immersion cooling (SPIC) system that promises to improve PUE. Three-dimensional mathematical models of pump-driven and buoyancy-driven immersion cooling tanks are established and numerically solved. The thermal characteristics, flow resistance, and energy consumption levels of immersion cooling tanks in three operating modes are investigated. Moreover, the roles of volume flow rate, inlet temperature, and device power in immersion cooling characteristics are analyzed. The results indicate that the heat dissipation of buoyancy-driven SPIC systems is less effective than pump-driven ones. Compared to buoyancy-driven SPIC systems, the average temperature of immersion coolants and PUE in pump-driven SPIC systems reduces by 55.5 and 11.6%, respectively. Interestingly, although natural convection in mixed convection operation enhances convective disturbance, it induces temperature stratification that weakens temperature homogeneity. Increases in inlet temperature and device power both worsen the immersion cooling performance. However, increasing inlet temperature improves temperature uniformity and reduces flow resistance losses, while increasing device power has the opposite effect. Besides, the increased flow rate enhances the convective heat transport and temperature uniformity but increases the flow resistance loss. [ABSTRACT FROM AUTHOR]

• A functional surface with periodical layout of multi-layer wettability gradient is developed to improve the condensation heat transfer. • The presence of a larger wettability gradient for the periodic gradient surface induces the efficient droplet removal and the preferable inter-layer and intra-layer actions. • The heat flux on the periodic gradient surface would increase about 20% to the hydrophobic surface when the surface subcooling ∆ T ≤ 12 °C. The functional surface is a crucial approach to enhance the condensation heat transfer. This paper designs and fabricates a functional surface with wettability gradient by the use of etching and chemical vapor deposition methods. The dynamic phase change behaviors and heat transfer characteristics of humid air condensation on vertical surfaces with wettability gradient have been experimentally investigated and compared with the corresponding hydrophilic and hydrophobic surfaces by a visualization system. The results indicate that the heat transfer performance of humid air condensation on traditional gradient surface is superior to hydrophilic case but inferior to hydrophobic case in large-scale application due to small wettability gradient. Considering this, the periodic gradient surface with periodical layout of multi-layer wettability gradient is introduced to improve the condensation heat transfer. The presence of a larger wettability gradient for the periodic gradient surface induces the efficient droplet removal and the preferable inter-layer and intra-layer actions, leading to an improvement of condensation heat transfer. [ABSTRACT FROM AUTHOR]

• Startup process of minichannel evaporator is investigated by a visual experiment. • Gradual and overshoot startup is sequentially experienced as heat load increases. • Temperature overshoot is essentially induced by the flow boiling incipience. • Startup regimes are quantitatively recognized by Jacob number and Weber number. • An increase in We leads to a larger critical Ja of startup regime transition. The startup process of minichannel evaporator in a two-phase mechanically pumped fluid loop is investigated by a visualized experimental system, with a particular focus on the dynamic temperature response under step heat load. The effects of heat load, flow rate and supercooling degree on the startup behaviors of the minichannel evaporator are examined and analyzed. A startup regime diagram is provided to quantitatively recognize the startup behavior, depending on Jacob number and Weber number. The results indicate that the gradual startup, once-rise overshoot startup and twice-rise overshoot startup are observed and sequentially experienced for a minichannel evaporator as the step heat load increases. The temperature overshoot is essentially induced by the boiling incipience. The dominated heat transfer mechanism is always the single-phase convection during the gradual startup process, and it changes from the single-phase convection to nucleate boiling during the once-rise overshoot startup process. In addition, it shifts from the single-phase convection to nucleate boiling, and then is the convective evaporation during the twice-rise overshoot startup process. The increase in Weber number leads to a larger critical Jacob number of the startup regime transition, and hence the range of Jacob number tends to be larger for the once-rise overshoot startup mode. Particularly, a higher supercooling degree also induces a larger range of Jacob number for the once-rise overshoot startup mode. [ABSTRACT FROM AUTHOR]

• Gradient fins are introduced to enhance melting performance of LHS units. • Geometric structure of gradient fin are optimized by response surface method. • Coordination between natural convection and thermal conduction is achieved. • Optimal gradients of fin thickness and central angle are 1.846 mm and 14.96°. • Complete melting time of optimal latent heat storage unit is reduced by 30.5%. Latent heat storage (LHS) is critical for the large-scale utilization of sustainable energy. The present paper designs the gradient fins (i.e., fins with gradient thickness and gradient central-angle) to enhance the melting performance of a LHS unit. A two-dimensional model of the melting heat transfer process for phase change material with Paraffin wax RT58 in a finned LHS unit is developed and numerically solved to investigate the melting behaviors. The melting front evolution and the dynamic temperature distribution are analyzed, and the effects of natural convection, fin layout, and Stefan number are investigated as well. Moreover, the fin configuration is optimized by the response surface method (RSM) with the objective function of the complete melting time. The results indicate that the natural convection seriously affects the melting behaviors in LHS units, and the melting rate is improved by 180% as compared with the case without natural convection. The LHS unit with gradient fins has a faster melting rate and a more uniform temperature distribution due to the enhancement in heat transfer synergy between natural convection and thermal conduction. Moreover, the increase of Stefan number leads to a faster melting rate and a shorter complete melting time. The RSM optimization indicates that the optimal gradients of fin thickness and central angle in this paper are 2.846 mm and 14.96°, respectively, and the complete melting time is reduced by 30.5% for the optimized LHS unit as compared with the corresponding uniform fin. Therefore, the gradient fins are recommended to be configured in a LHS unit in the engineering applications, where the fins are distributed with gradient angle, and they are thinner at the upper part and thicker at the lower part. [ABSTRACT FROM AUTHOR]