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Slippery liquid-infused porous surfaces are vulnerable to lubricant depletion caused by three major factors encompassing, the evaporation, cloaking, and shearing. Herein, lubricant-infused iron palmitate surfaces (LI-IPSs) are presented addressing the challenges of (1) evaporation-driven lubricant depletion enabled by the immensely clustered micro-/nano-structures of iron palmitate surface (IPS), (2) completely suppressing the wetting ridge even with miscible water-oil combinations enabled by only the handful uptake of oil by IPS. The apparent contact angle θa and sliding angle α of water on LI-IPSs are 98° ± 2° and 3° ± 1°, respectively. Under evaporation-driven lubricant depletion analysis, the shelf-durability of LI-IPSs has been found to last for 10 days in partially open environment at the ambient temperature (25 °C). The heat fluxes and condensate fluxes have been determined. The highest condensate flux on LI-IPSs (14 kgh−1 m−2) is ascribed to the dropwise condensation mode with efficient droplet dynamics. However, the service durability of LI-IPSs has lasted only for 50 ± 5 min under rigorous condensation, which is further addressed by developing a LI-IPS prototype. Based on the excellent wicking capability of IPS, LI-IPS prototype is capable of maintaining the dropwise condensation unless the oil is available in the oil tub. The LI-IPS prototype has been subjected to open environment at ambient temperature for 168 h, and even after that, the stable dropwise condensation has been achieved with the same range of droplet speed as had been observed in the first hour. Meantime, the high condensation heat fluxes at various subcooling temperatures are obtained, justifying the promising ability of LI-IPS. [ABSTRACT FROM AUTHOR]

Low-grade thermal energy conversion and utilization are important for sustainable energy strategies and environmental conservation. Technologies such as ocean thermal convertors, thermoelectric generators, organic Rankine cycles, and heat pumps are being used to convert low-grade thermal energy into useful forms like electricity or heating. Advancements in materials science, engineering, and renewable energy technologies are improving the efficiency of converting low-grade heat into electricity and integrating renewable and waste heat sources into the grid. Ocean thermal energy conversion (OTEC) is particularly valuable for harnessing renewable energy from the oceans, while thermoelectric power generation converts waste heat into electricity. Organic Rankine cycles (ORCs) and loop heat pipes (LHPs) also play important roles in harnessing low-grade heat sources and thermal management systems. These technologies contribute to energy efficiency, reduced environmental impact, and enhanced resource utilization. [Extracted from the article]

The venation-finned porous network has been demonstrated as a promising method to maximize the thermal transport access. To improve the heat storage efficiency, an innovative venation-finned porous network is employed here to enhance the melting performance of phase change materials (PCMs). The venation-finned porous network is quantitatively described by Murray's law and Voronoi method, and a modified thermal lattice Boltzmann model of PCM melting processes in a square cavity is developed and numerically analyzed to optimize the venation-finned porous network. The melting performance of composite PCMs with three configurations (venation fin, porous network and venation-finned porous network) are compared and analyzed. Moreover, the effects of branching angle and Murray coefficient on melting performance are comprehensively studied. It indicates that venation-finned porous network is favorable to melting performance improvement due to venation's inherent efficient heat transfer paths. Compared to venation fins and porous networks, the melting duration time of venation-finned porous networks is reduced by 78.4% and 21.4%, respectively. Furthermore, the branching angle of 45 ∘ and Murray coefficient of 3 are suggested for maximizing the melting efficiency. Importantly, the melting mechanism is conduction and convective conjugated heat transfer in composite PCMs with venation fins, however, it is dominated by heat conduction for those with porous networks or venation-finned porous networks. [ABSTRACT FROM AUTHOR]

Summary: The performance enhancement of latent heat storage (LHS) units is of great consequence for the development of sustainable energy. In this article, the transient models of phase‐change heat transfer processes in LHS units with consideration of natural convection are developed and numerically analyzed, in an effort to explore the role of the fractal tree‐shaped fin in the energy charging performance. The solid‐liquid interface evolution and dynamic temperature response in the tree‐shaped finned LHS unit are presented and compared with the wheel‐shaped one. Moreover, the influence of the fin number is investigated for maximizing the energy charging performance. The results indicate that the tree‐shaped fin has merits of effective layout optimization for the point‐to‐area heat transfer and the time‐coordination of energy charging and discharging process. Compared with the wheel‐shaped fin, the melting duration time is decreased only a little for the presence of tree‐shaped fin, however, the solidification process is decreased significantly. The presence of tree‐shaped fin leads to a lower energy charging rate during the early and middle stages of the charging process due to the natural convection suppression, however, the energy charging rate is faster during the later stage due to the thermal conduction dominated interior heat transfer. For maximizing the melting performance, the appropriate fin number in practical applications is 16. [ABSTRACT FROM AUTHOR]

Based on the multiphase lattice Boltzmann method, a transient model of vapor–liquid phase change in an enclosed narrow space during startup operation is developed and numerically analyzed to investigate the two-phase dynamics and heat transfer behaviors in the confined flat two-phase thermosiphon. The transient temperature distribution, vapor–liquid interface evolution and thermal performance are investigated over a wide range of heat load and liquid filling ratio. The results indicate that, as heat load increases, the intermittent nucleation boiling, fully developed nucleation boiling and film boiling sequentially takes place at the evaporator section of the confined flat two-phase thermosiphon. Correspondingly, the transient temperature evolution shows the periodic large-amplitude fluctuation, the random medium-amplitude fluctuation, and gentle small-amplitude fluctuation. The fully developed boiling is desirable for the heat transfer enhancement on the evaporator surface due to smaller temperature fluctuation and lower thermal resistance. The moderate liquid filling ratio ensures the desired interaction between the evaporator and condenser, resulting in superior thermal performance. To optimize the thermal performance, the optimal liquid filling ratio is about 40%. [ABSTRACT FROM AUTHOR]

Inspired by the snowflake structure, an innovative Koch-fractal fin is proposed to optimize the fin geometry of the latent heat thermal energy storage (LHTES) units. A model of unsteady heat transfer accompanied with phase change is developed and numerically analyzed to investigate the effect of fin structure on the discharging process of a LHTES unit. The dynamic response of heat release, the dynamic temperature response and solidification front evolution of a LHTES unit with Koch-fractal fins are discussed and compared with the corresponding radial fins. Furthermore, a comprehensive evaluation of thermal performance of LHTES units is conducted in terms of the TES capacity, TES rate and solidification time. The results indicate that the heat release rate of a LHTES unit with Koch-fractal fins is faster than that with radial fins. Moreover, because the Koch-fractal fins have advantages of higher specific surface area, faster heat flow path from point to surface and smaller thermal resistance arising from the reasonable spatial layout, the evolution of solidification front is faster and the temperature distribution is more uniform. The results of quantitative evaluation show that although the TES capacity is identical, the TES rate of a LHTES unit with Koch-fractal fins is six times that with radial fins. [ABSTRACT FROM AUTHOR]

The fractal geometry provides a new insight for the description for actual pore structure of metal foams. In this paper, the fractal grids are introduced to describe the pore structure of metal foams. A two-dimensional unsteady melting model of phase change material (PCM) in the cross-fractal metal foam is developed and numerically simulated. The transient temperature variation and melting front evolution during the charging process in the cross-fractal metal foam are analyzed. The effects of initial temperature difference, porosity and fractal dimension on the melting heat transfer in fractal metal foam are examined and analyzed. The results indicate that the temperature distribution is more uniform and the melting rate is faster in the cross-fractal metal foam compared with that in the corresponding cavity structure. Interestingly, the fractal metal foam with smaller fractal dimension provides a faster heat flow path and hence enhances the melting performance though the porosity is identical. The melting performance in fractal metal foam can be enhanced when the metal foams have a lower porosity, a smaller fractal dimension and a larger initial temperature difference. [ABSTRACT FROM AUTHOR]

The gravity–capillary evaporation phase change in microgrooves inside a heat pipe is experimentally investigated by a visualization system. The evaporation regime, startup temperature response, and evaporation thermal resistance for gravity–capillary evaporation are evaluated and compared with those for capillary evaporation. The results identify oscillatory motion of vapor–liquid interface for gravity–capillary evaporation in rectangular microgrooves arising from the competition between gravity, the capillary force, and the shear force. In addition, the presence of gravity introduces the transition from fin–film evaporation regime to corner–film evaporation regime in rectangular microgrooves, and hence results in an overshoot startup and temperature oscillation during the process of gravity–capillary evaporation. Interestingly, the corner–film evaporation improves the evaporation phase change performance in the microgrooves. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1119–1125, 2019 [ABSTRACT FROM AUTHOR]

The Sierpinski fractal is introduced to construct the porous metal foam. Based on this fractal description, an unsteady heat transfer model accompanied with solidification phase change in fractal porous metal foam embedded with phase change material (PCM) is developed and numerically analyzed. The heat transfer processes associated with solidification of PCM embedded in fractal structure is investigated and compared with that in single-pore structure. The results indicate that, for the solidification of phase change material in fractal porous metal foam, the PCM is dispersedly distributed in metal foam and the existence of porous metal matrix provides a fast heat flow channel both horizontally and vertically, which induces the enhancement of interstitial heat transfer between the solid matrix and PCM. The solidification performance of the PCM, which is represented by liquid fraction and solidification time, in fractal structure is superior to that in single-pore structure. [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: 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 model of laminar flow and heat transfer in rough microchannels is developed and analyzed numerically to compare the effect of roughness elements on the thermal and hydrodynamic characteristics. In this model, the rough surfaces are configured with triangular, rectangular and semicircular roughness elements, respectively. Here, the effects of the Reynolds number, roughness height, and roughness element spacing on pressure drop and heat transfer in rough microchannels are all investigated and discussed. The results indicate that the global heat transfer performance is improved by the roughness elements at the expense of pressure head when compared to the smooth channel. Differing from the smooth microchannels, both the Poiseuille number and average Nusselt number of rough microchannels are no longer constant with Reynolds numbers and are larger than the classical value. Especially, the difference from the effects of three types of roughness elements is identified. With the increasing roughness height, the flow over surfaces with semicircular and triangular roughness elements induces stronger recirculation and flow separation. This contributes to heat transfer enhancement but also increases the pressure drop. However, the influence of the rectangular roughness element height is weaker than that of semicircular and triangular elements. In addition, the effects of the spacing of roughness elements on the Poiseuille number and average Nusselt number are in decreasing order for semicircular, triangular and rectangular roughness elements, respectively. [Copyright &y& Elsevier]

Abstract: A three-dimensional model of heat transfer and fluid flow in noncircular microchannel heat sinks is developed and analyzed numerically. It is found that Nusselt number has a much higher value at the inlet region, but quickly approaches the constant fully developed value. The temperature in both solid and fluid increases along the flow direction. In addition, the comparison of thermal efficiencies is conducted among triangular, rectangular and trapezoidal microchannels. The result indicates that the triangular microchannel has the highest thermal efficiency. [Copyright &y& Elsevier]

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]

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

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]

Highly dense and integrated data centers face key challenges of realizing efficient cooling and improved energy efficiency. To overcome these challenges, this study experimentally investigated the flow and heat transfer characteristics of single-phase immersion liquid cooling (SPILC) systems. The influence of two opposite coolant flow directions on the SPILC performance was examined. Furthermore, a correlation mechanism between coolant thermal properties and SPILC performance was established, along with a control chart for regulating the working conditions of SPILC systems. The results indicate that compared to a pro-gravity flow, an anti-gravity flow scheme reduces the chip case temperature and thermal resistance by 33.8% and 55.6%, respectively, while decreasing the power usage effectiveness (PUE) by 1.4%. Using coolant with the lowest viscosity reduces the chip case temperature and thermal resistance by 9.3% and 10.5%, respectively, while decreasing the PUE by 0.4%. Moreover, the cooling water temperature has a greater impact on the performance of SPILC systems than the volume flow rate of coolants. Additionally, this paper provides control charts for the cooling water temperature and coolant flow rate to improve the PUE while ensuring the safe operation of SPILC systems, with the highest chip temperature and total electricity consumption as indicators. • Immersion cooling performance of system-level data center servers is experimentally studied. • PUE of the anti-gravity flow decreases by 1.4% compared to the pro-gravity flow. • Correlation between coolant properties and immersion cooling performance is established. • Energy-efficient control charts for cooling water and immersion coolant are provided. [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]

Abstract: The topography of rough surfaces is characterized by using a Cantor set structure. Based on this fractal characterization, a model of laminar heat transfer in rough microchannels is developed and analyzed numerically. The effects of the Reynolds number, relative roughness, and fractal dimension on laminar heat transfer are all investigated and discussed. The results indicate that the local Nusselt numbers after the entrance region are no longer constant but tend to experience fluctuation along the rough microchannels. Differing from the smooth microchannels, the average Nusselt number increases nearly linearly with the Reynolds number and is larger than the classical value. For higher values of relative roughness, the flow-over roughness induces flow separation, which plays an enhancement on laminar convective heat transfer. Moreover, the laminar heat transfer in microchannels is also enhanced by roughness with a larger fractal dimension yielding more frequent variation in the surface profile even though at the same relative roughness. [Copyright &y& Elsevier]

• 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]

The use of the bi-conductive surface provides a promising prospect for the enhancement of the boiling phase change heat transfer. In this study, a theoretical model of two-phase flow and boiling heat transfer on bi-conductive surfaces is developed. With the consideration of thermal conductivity contrast and conjugate heat transfer, the boiling behavior is numerically investigated based on a modified lattice Boltzmann method. The bubble dynamics during the boiling process on the bi-conductive surface are reproduced and the effect of the structural size of these insets on the boiling performance is explored. The results indicate that boiling performance is enhanced on the bi-conductive surface when compared to homogeneous ones. In-plane temperature variation on the surface contributes to the pinning effect on bubble contact lines, and thus promotes boiling heat transfer. Boiling enhancement gets optimized once the embedding pitch space equal to the capillary length of the boiling fluid. Increasing the width of the insets also improves the boiling heat transfer coefficient on the bi-conductive surface under low superheat conditions, but also leads to the advance of CHF. However, changing the depth of the insets shows almost no effect on the boiling heat transfer performance. [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]