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The current technologies to treat hypersaline produced water (PW), such as thermal evaporation, are usually energy-intensive and cost-prohibitive. This study developed a low-cost, robust, solar-driven carbon black and airlaid paper-based evaporator (CAPER) for desalination of PW in the Permian Basin, United States. The study aims to better understand the removal of aromatic organic compounds and heavy metals during solar distillation, water output, and heat transfer. Outdoor experiments using CAPER assisted with polystyrene foam in a single slope, single basin solar still achieved an enhanced average evaporation rate of 2.23 L per m2 per day, 165% higher than that of a conventional solar still. Analysis of heat transfer models demonstrated that CAPER solar evaporation achieved an evaporative heat transfer coefficient of ∼28.9 W m−2·K−1, 27.9% higher than without CAPER. The maximum fractional energy of evaporation and convection heat transfer inside the solar still with and without CAPER was ∼81.4% and ∼78.2%, respectively. For the PW with a total dissolved solids concentration of 134 g L−1, solar distillation removed 99.97% salts and over 98% heavy metals. The high removal efficiency of 99.99% was achieved for Ca, Na, Mg, Mn, Ni, Se, Sr, and V. Organic characterization revealed that solar distillation removed over 83% aromatic compounds. Solar desalination using CAPER provides a low-cost and high-performance process to treat PW with high salinity and complex water chemistry for potential fit-for-purpose beneficial uses. Image 1 • A carbon black and airlaid paper-based solar evaporator (CAPER) was developed. • Outdoor distillation of produced water (PW) achieved evaporation of 2.23 L m−2·d−1. • The evaporation rate of PW with CAPER is 165% higher than without CAPER. • Heat transfer coefficient with CAPER is 27.9% higher than without CAPER. • 99.97% salts, 98% metals, >83% aromatic compounds removed from 134 g L−1 TDS PW. [ABSTRACT FROM AUTHOR]

Mixing of surface microstructure and wettability is one of the future development trends. Their combination always affects the bubble motion and heat transfer coefficient under pool boiling/flow boiling, but it is difficult to distinguish their respective contributions. This work studies the combination of new microstructure and mixed wettability to improve the flow boiling heat transfer in the channel. The nucleation, growth, coalescence and departure behaviors of R134a boiling bubbles were numerically analyzed. Effects of different heights, gaps and mixed wettability of microstructures and heat fluxes on bubble emission frequency and boiling heat transfer coefficient were discussed. Results showed that this new microstructure provides the bubble with the momentum of lateral motion, which helps the bubble to merge and leave faster. The surface of the hydrophobic groove coupled with the hydrophilicity as the substrate can effectively increase the bubble emission frequency, up to 134.37%. At the same time, the heat transfer coefficient decreases with the increase of the height of the microstructure, and it also increases with the increase of the microstructure gap. Finally, the contribution of each variable was determined by analysis of variance. The research results in this paper provide a reference for heat pipe and vapor chamber. • A creative microchannels with mixed wettability was proposed and optimized. • Synergistic effect of groove microstructure and mixed wettability was studied. • Surface mixed wettability has more influence on boiling heat transfer. • Bubble emission frequency of surface-A is 134.37% higher than surface-B. [ABSTRACT FROM AUTHOR]

• Subcooled flow boiling heat transfer of water in the porous metal foam tubes was investigated experimentally. • Filling a tube with metal foam could improve the heat transfer and also pressure drop. • The thermal efficiency of metal foam tubes was investigated. • Based on experimental data, a correlation was presented to predict heat transfer in the metal foam pipes. This paper has investigated the effect of different porosities (0.80–0.90) of the metal foam pipes on heat transfer coefficient and pressure drop of subcooled flow boiling of water experimentally. The metal foam pipe with 0.80 porosity has increased the Nusselt number and pressure drop by 41 % and 21 % compared to the 0.90 metal foam pipe, respectively. The heat transfer of water phase change in a porous medium has received less attention due to the complexities of the problem. One of the challenges in this field is the absence of a correlation for predicting heat transfer in metal foams. In this paper, a systematic method based on dimensionless numbers considering the essential parameters, like mass flux, subcooling, and heat flux, was used to provide a correlation for calculating the Nu-number based on 106 experimental points. The mean absolute deviation (MAD) of the results predicted by the new correlation is 8.9 %, and it predicts 95 % of the database with ±20 %. [ABSTRACT FROM AUTHOR]

• Synergistic enhancement spray cooling use micropillars and capillary-driven wicking hybrid structures. • The coolant can self-supplied reach the target surface under the capillary force. • The circulation heat transfer enhancement process of wicking-rewetting-vapor evacuation. • HTC and maximum heat flux were improved 79.6% and 80.0%, respectively on the hybrid structure surface. • A maximum reduction of approximately 30.8 °C was obtained for ONB. Spray cooling, which has a fast thermal response and low flow rate consumption, is a common and effective thermal management approach for high power electronic systems. Typically, to improve the heat transfer coefficient (HTC) for spray cooling, the target surface wettability is decreased, promoting nucleation boiling by reducing the surface energy barrier for bubble nucleation. However, the merging of bubbles and the formation of a vapor film have limitations in further increasing the critical heat flux (CHF). Hence, maximizing both the HTC and CHF of spray cooling can be mutually exclusive on the same heated surface. This paper presented a novel design for a vapor-liquid separation structure surface. It consisted of lower micropillars that serve as the vapor separation channel. On top of that, there was an upper multi-layer copper micromesh that wicks and spreads the coolant. Additionally, nano-grass attached to the micromesh to enhance capillary force and increase the nucleation sites simultaneously. Subsequently, we calculated the forward velocity and pressure drop of the droplets passing through the copper mesh to study the effects of droplet velocity and capillary force on heat transfer performance. The results show that the forward velocity of the droplets decreases rapidly as the increase of copper mesh layers and it drops to zero when passing through the third mesh. Therefore, the capillary force generated by the copper mesh is another important factor for the atomized droplets to pass through the copper mesh and reach the target surface. We have proven that all the vapor-liquid separation structure surfaces satisfy the condition ΔP cap > ΔP pore + ΔP local. Hence, the synergistic effects of the micropillar, copper micromesh, and nano-grass enable a circulation process of wicking-rewetting-vapor evacuation, which improves the onset of nucleate boiling (ONB), enhances the HTC, and increases the maximum heat flux dissipated by the system. Enhanced HTC (7.94 W/cm2/K) and maximum heat flux (817 W/cm2) were achieved at 1.50 ml/s on the vapor-liquid separation structure surfaces (#200 2-layer), representing an enhancement of 79.6% and 80.0% compared with that for the flat surface and the temperature of the heated surface at the ONB maximum decreased by approximately 30.8 °C. [ABSTRACT FROM AUTHOR]

An extensive development of two-phase thermal control systems based on two- and multi-phase devices (standard heat pipes, loop heat pipes, heat switches, etc.) offers a potential to assume an increasingly important position in the instrumental thermal control for a number of space and earth applications. However, experimental data regarding long-life testing for such systems is insufficient. The first ammonia loop heat pipe (LHP) was developed in 1976 in the Ural Polytechnic Institute under supervision of Prof. Gerasimov, Iu. F. (1921–2001) (Kiseev, 1977; Gerasimov et al., 1981; Maydanik, 2005) [1–3]. This paper features its long-life test, covering 41 years with varying operation times. The life test consists of two parts: permanent and periodic LHP operation (approx. 5000 h) and LHP stand-by storage (approx. 152,600 h). Experimental investigations were performed using the developed loop heat pipe based on composite nickel wick with effective pumping pores of 0.5–10 µm in radius and 1 × 10 −14 m 2 to 4 × 10 −12 m 2 in permeability. We have provided experimental data demonstrating the impact of orientation, varying heat load and temperature on the LHP operation. In addition, a qualitative and quantitative chemical element analysis of the distilled water in a loop heat pipe without capillary structure (loop thermosyphon) was performed before and after its long-term operation (one year) to identify any possible interaction between the working fluid and the pipe materials. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, heat transfer performance of a 40cm-length circular heat pipe with screen mesh wick is experimentally investigated. This heat pipe is made of copper with two diameters; larger in the evaporator and smaller in the adiabatic and condenser. Three different liquids including water, methanol, and ethanol are separately filled within the heat pipe. Low heat fluxes are applied (up to 2500W/m2) in the evaporator and constant temperature water bath is used at three levels including 15, 25, and 35°C in the condenser. Results demonstrate that higher heat transfer coefficients are obtained for water and ethanol in comparison with methanol. Furthermore, increasing heat flux increases the evaporator heat transfer coefficient. For the case of methanol, some degradation in heat transfer coefficient is occurred at high heat fluxes which can be due to the surface dryout effect. Increasing the inclination angle decreases the heat pipe thermal resistance. [Copyright &y& Elsevier]

• Nanomaterial deposition on heating surface are used to attain reduction in wall superheat, enhancing nucleate boiling HTC and CHF. • Different working fluid is used such as de-ionized water, fluorocarbon fluid, hydofluoroethers and refrigeration-based fluid for enhancing CHF. • The surface modification enhanced the CHF due to the reduction in wall superheat temperature, increased nucleation site, porosity rise, enhanced surface wettability, small nano-pores/cavities generation, rise in bubble release frequency. • Nanoparticle size, nanofluid preparation techniques and concentration of nanofluid mainly influenced the CHF. Boiling is the most significant phase change phenomena for various applications such as cooling of electronic devices, heat pipes, cooling in nuclear reactor, different industrial application and plenty more. Due to its wide scope of application, any enhancement in this domain leads to remarkable economic and energy efficient consequences. Augmentations in boiling heat transfer performance are very crucial which establishes the various industrial applications further energy efficient. To reduce the energy dissipation in heat transfer equipment, boiling heat transfer augmentation is one of the significant techniques used by researchers. This paper demonstrates a thorough review and investigation of published articles describing the various surface modification method (nanomaterial coating, nano/micro porous coating, nano/micro structured surface) for pool boiling and flow boiling heat transfer enhancement as well as the effect of surface modification on wettability, porosity, surface roughness, nucleation sites are studied. Various augmentation methods are utilized to attain three apparent heat transfer desire: reducing wall superheat temperature, enhancing pool boiling heat transfer coefficient (HTC) and improving critical heat flux (CHF). [ABSTRACT FROM AUTHOR]

Visual and instrumental investigations of the processes of condensation and redistribution of a working fluid in a loop heat pipe have been carried out. This paper presents the results of an experimental investigation of the heat transfer and hydrodynamics during the condensation of water vapor in a flat gap condenser measuring 80×40×1mm. Investigations have been conducted at a condenser cooling temperature of 20, 40 and 60°С. During all operating modes a stratified two-phase flow and film condensation have been observed. The temperature field in the condenser has been measured, and the heat-transfer coefficients and the thermal resistances have been determined. [ABSTRACT FROM AUTHOR]

In the framework of cooling technology applied to electronics, multiple setups and coolants have been used to achieve effective methods to remove high heat density fluxes. In this paper, an innovative device based on the combination of two fluids in a closed loop two-phase thermosiphon is proposed as an effective passive system to cool electronic components. The cooling device is an external flow boiling system that uses NOVEC 7000 as a circulating boiling fluid to transport heat from an evaporator capsule to a condenser. The bubble lifting effect in the closed-loop thermosiphon accelerates the fluid circulation and creates a natural jet impingement effect in the evaporator. In the condenser, the NOVEC bubbles ascend through a water pool, achieving a highly effective heat exchange between the fluids. The advantage of this two-fluid condenser system is highly efficient condensation since the vapour of NOVEC is condensed in a water pool, which produces a high heat transfer between the fluids and is not sensitive to the presence of non-condensable gases. In addition, the movement of the NOVEC bubbles produces high agitation in the water that enhances the heat transfer to the condenser walls. The system was tested under different working conditions with several power generation rates in the heat source and operating at atmospheric pressure and in a vacuum. The system was tested to assess the effect of water in the condenser, and heat transfer coefficients were calculated for the setup operating with and without water. In addition, a numerical model was developed to predict the different temperatures in the system and extrapolate the results for different operating conditions. Several tests were performed, and the results showed robust working of the system and good control of temperatures in the element to be cooled. The system was able to maintain a temperature lower than 70 °C in the aluminium block working with a heat flux of 220 kW/m2 without any extra energy consumption. The effect of water in the condenser was shown to be effective in improving heat transfer and coolant condensation. The heat transfer coefficients in the condenser can be improved by 22 to 133%. The model for temperature prediction showed reasonably good results for most of the experiments regarding both transient behaviour and stabilization temperatures. • A 2-fluid CLTPT is designed to operate as a passive cooling system. • A flow boiling evaporator produces a highly stirred flow and effective heat transfer. • The condenser contains NOVEC 7000 and water combined to improve the condensation efficiency. • The biphasic condenser prevents the negative effects of non-condensable gases. • A numerical model is proposed to predict the system behaviour under different operating conditions. [ABSTRACT FROM AUTHOR]

• Study the effect of condenser temperature on the performance of a wickless heat pipe. • Performance in microgravity declined as the condenser temperature decreased. • Temperature data was used to extract a heat transfer coefficient in the evaporator. • Nusselt number was found to scale with Marangoni number to the -1/3 power. • A single correlation using the Marangoni number collapsed all the experimental data. The Constrained Vapor Bubble (CVB), a simple, wickless, heat pipe design that depends on interfacial forces to drive corner flow in a square cuvette, was studied in the microgravity environment aboard the International Space Station (ISS). In this paper, we consider the effects of different condenser temperatures on the heat transfer and fluid flow behavior using pentane as the working fluid. As the condenser temperature was decreased, the performance of the system decreased. This performance decrease using the pure working fluid was opposite to the behavior observed when using a mixture of 94 vol% pentane and 6 vol% isohexane. The mechanism for the decline in performance as the condenser temperature was lowered was a stronger than expected increase in the apparent strength of Marangoni flows at the heater end of the system. A simple mathematical model was fit to the experimental data and used to extract an evaporator heat transfer coefficient for experiments where we held the condenser temperature constant while increasing the heater power and where we held the heater power constant while decreasing the condenser temperature. All the results could be collapsed onto a single Nusselt number vs. Marangoni number curve. In this formulation, the Nusselt number was found to decrease with increasing Marangoni number to the 1/3 power. [ABSTRACT FROM AUTHOR]

• Critical review of morphological and thermal-hydraulic performance of the conventionally and additively manufactured lattices is conducted. • Conventionally manufactured wire-meshes can provide heat transfer enhancements similar to stochastic metal foams and relatively easy and cheap to manufacture. • Additively manufactured lattices demonstrate higher interfacial heat transfer coefficient values than stochastic metal foams, but the role of inherent surface roughness needs more investigation. The advantages offered by the stochastic metal foams when used in high-performance compact heat exchangers is evident in the open literature. Recently, there has been a surge in investigations on regular metal lattices which can be manufactured additively, because of their "potential" to provide multifunctional and tunable characteristics to heat exchangers. These lattices are also reported to lend superior mechanical properties as well. The literature, however, lacks in the critical assessment of these additively manufactured regular lattices in terms of the quality of printed parts (defects, surface roughness, etc.), their compatibility in compact heat exchangers, flow and thermal transport characteristics, and whether or not they are viable alternatives to the commercial metal foams conventionally manufactured. This paper reports different manufacturing routes for the fabrication of conventional stochastic and regular lattices and additively manufactured regular lattices, and how these technologies have evolved over the years. The additively manufactured lattices different from the commercial TKD may result in significantly higher interstitial heat transfer coefficient. Further, the design freedom allows different porosities through which higher effective thermal conductivity can be achieved. The usage of regular lattices in tube-heat exchangers, as wicks in heat pipes, heat transfer enhancement features in boiling heat transfer and transpiration cooling mechanisms of turbine blades as an alternative to conventional technologies is promising. The role of inherent surface roughness in dictating the thermal and flow transport in additively manufactured lattices, especially for multi-phase flow applications, should be studied in more detail. [ABSTRACT FROM AUTHOR]

• Experimental analysis on hysteresis phenomenon during flow boiling heat transfer • Dedicated experimental procedure with variation of the imposed heat flux over time • Detection and isolation of parameters affecting the hysteresis phenomenon. • Hysteresis effect related to activation of surface cavities with higher heat fluxes This paper presents an experimental analysis on the hysteresis phenomenon occurring during flow boiling heat transfer in a horizontal stainless-steel tube having an internal diameter of 6.0 mm. Pure R134a is the working fluid employed, working at a saturation temperature and a vapor quality fixed to 40°C and 0.30, respectively, whereas two different mass velocities of 400 and 800 kg/m2s are studied. A dedicated experimental procedure has been carried-out to highlight the variation of the two-phase heat transfer performance due to the hysteresis effect. Specifically, tests performed at the same operating conditions but having a different thermal history (a higher imposed heat flux in the transient phase) were seen to provide lower wall superheat values and therefore higher heat transfer performances. The nucleate boiling contribution is isolated from the overall heat transfer and the parameters affecting the boiling curve behavior are highlighted and discussed, pointing out the differences when increasing and decreasing the imposed heat flux once a peak value is reached. The experimental trends are finally upheld by the boiling theory, considering the hysteresis effect as the activation of the surface cavities undergone a higher given thermal boundary condition. [ABSTRACT FROM AUTHOR]

• Condensation of R134a/R245fa in a plate heat exchanger is experimentally tested. • Heat transfer and pressure drop characteristics of mixtures are analyzed. • Prediction methods of heat transfer and pressure drop are developed. Zeotropic mixtures have shown great potential to improve the system efficiency compared with the use of pure fluids as the working fluids. However, there is a lack of analysis of the heat transfer during the phase change of zeotropic mixtures in plate heat exchangers, which is a common heat transfer device used as the evaporator and condenser in these systems. This paper presents an experimental analysis of the flow condensation heat transfer and pressure drop characteristics of zeotropic mixtures in a plate heat exchanger. The objectives are to obtain prediction methods for the heat transfer coefficient and frictional pressure drop and to quantify the degradation of the heat transfer coefficient by using mixtures. Zeotropic mixtures of R134a/R245fa with five different quantitative compositions were experimentally tested with the bubble point temperatures of 30°C to 50°C and various mass fluxes. The experimental results indicate that condensation of the mixtures is governed by a shear-controlled process, similar to what happens with pure fluids. Moreover, the heat transfer degradation of the zeotropic mixture caused by the mixture effects increases with decreasing condensation temperature and mass flux. There is a 48% maximum decrease in the heat transfer coefficient for the mixture of R134a/R245fa with 0.431/0.569 in mass fraction. A modified Silver-Bell-Ghaly method provides a good prediction for the heat transfer data with a mean absolute percentage deviation of 12.2%. A pure-fluid correlation developed in a previous work by the authors enables a prediction with an 8.6% mean absolute percentage deviation for the pressure drop data. [ABSTRACT FROM AUTHOR]

The research on working fluids of refrigerant/absorbent mixture is important for the development of the absorption refrigeration area. R290/POE-oil mixture is a new potential working fluid, in which R290 (propane) is the refrigerant and POE oil is the absorbent. This refrigerant/oil working fluid has a wide operating range, low generation temperature, and no corrosion to metals. Flow boiling heat transfer performance of the new working fluid in smooth horizontal tubes has been experimentally investigated in this paper. The R290 concentration for the experimental working fluid was selected as 11 wt%. The test sections were horizontal smooth stainless steel annular tubes with outer diameters of 6, 8 and 10 mm. The boiling saturation pressures in the test sections coincided with the actual operation of the absorption refrigeration cycle, which were set as 1.2 MPa, 1.3 MPa, 1.4 MPa, respectively, corresponding to boiling temperatures 55.3 °C, 58.7 °C, 62.5 °C. The mass fluxes were 166 kg m−2s−1, 221 kg m−2s−1, 276 kg m−2s−1, 332 kg m−2s−1 and 387 kg m−2s−1. The effects of heat flux, vapor quality, mass flux, boiling temperature and tube diameter on the flow boiling heat transfer coefficient were displayed and analyzed. Experimental results show that heat flux, mass flux and vapor quality have obvious effects on the coefficients; the nucleate boiling mechanism plays an important role, and the convective boiling mechanism plays a significant role in the boiling process. The present study is of good significance to the application of R290/POE-oil working fluid and the design and optimization of the finned-tube generators in absorption refrigeration systems. • R290/POE-oil working pair has obvious advantages in absorption refrigeration. • The flow boiling heat transfer performance of R290/POE-oil mixture are studied. • The influences of different operating conditions on the coefficient are analyzed. • Both nuclear boiling and convective boiling mechanisms play an important role. [ABSTRACT FROM AUTHOR]

• Effect of tilt and length on porous miniature heat pipes thermal performance is investigated experimentally. • Thermal resistance network of 1D of miniature heat pipes with fin is detailed for thermal modelling. • Prediction of evaporation and condensation heat transfer coefficients using experimental results. • Effect of adiabatic length on capillary pressure of the two studied heat pipes using the developed hydraulic model. • Effect of porous media parameters on thermal resistance and heat pipes thermal performance and evaluation of optimal porosity. Miniature heat pipes are considered as an innovative solution able to dissipate high heat fluxes with low working fluid charge, temperature control, and operating with minimum energy consumption and less noise. In this paper, results show that tilt ranging [10°:60°] has not an impact on heat transfer meanwhile the system thermal resistance is the lowest at horizontal position. A theoretical analysis on heat pipe thermal performance has been carried out based on experimental data by predicting heat pipe evaporation and condensation heat transfer coefficient. The measured heat transfer coefficients are close to the predicted results with a mean deviation less than 10%. A hydraulic mathematical model is developed and shows a good agreement with Kaya's study using water as working fluid. This work comprises a parametric study that investigates the effect of particles size, porosity, permeability, heat pipe length on heat transfer and capillary pressure. The obtained results show that the heat pipe operating temperature rises when particles size increases from 1 μ m to 90 μ m and the optimum porosity range is between [10%–40%]. [ABSTRACT FROM AUTHOR]

• A numerical model of R290 condensation in minichannels was proposed and verified. • The effect of diameter on heat transfer and frictional pressure drop was analyzed. • The refrigerant charge of R290 in different pipe diameters was analyzed. • Reducing the diameter can reduce the refrigerant charge of R290 by 94.8% • Wall effects have a remarkable influence on HTC in small diameter and mass flux. In order to enhance the heat transfer performance and decrease the refrigerant charge of R290 in condensers, this paper numerically studied the condensation and refrigerant charge of R290 in minichannels (not the entire system) with different diameters to analyze the effect of diameter on them. The VOF multiphase flow model and SST k − ω turbulence model were used for 3D transient calculation in the simulation. The simulation results show that the diameter has a remarkable effect on the flow pattern, heat transfer coefficient, friction pressure drop and refrigerant charge. When the mass flux is small, a remarkable eddy current appears between vapor and liquid on the circular section, which can sharply enhance the heat transfer. At high vapor quality (x >0.8), the heat transfer coefficient and frictional pressure drop do not almost increase and even decrease with the increase of vapor quality under some working conditions, which is due to the influence of wall effect. Under the same heat transfer rate and heat transfer temperature difference, with the decrease of diameter, the maximum reduction of refrigerant charge reaches 94.8%. But when the diameter is reduced to a certain value, continuing to reduce the diameter will have little effect on the refrigerant charge. [ABSTRACT FROM AUTHOR]

• HTC and pressure drop rises with mass velocity and quality in plain and TTI tubes. • Gravity-dominated regimes (stratified-wavy) are more sensitive to vapor quality. • Shear-dominated regimes (intermittent/annular) are more sensitive to mass velocity; • Tapes increase HTC and pressure drop, the less twist ratio, the more increment. • Twist ratio of 9 has the highest thermal performance, usually less than one. This paper deals with condensation flow inside plain and twisted tape inserted (TTI) tubes with diameter and length of 8.7 mm and 700 mm, respectively. R1234yf, identified as a viable alternative for R134a with low global warning potential (GWP), is examined as the working fluid. The heat transfer coefficient and the pressure drop of the flow are measured in different mass velocities and vapor qualities ranging 160–310 kg m−2 s−1 and 0.12–0.84, respectively. A well-equipped test rig consisting of a test condenser and other necessary equipment is employed to precisely adjust the test conditions. Experimental results for the plain tube are compared with some well-known correlations. As a result, Shah and Souza et al. equations are the most reliable equations to predict the flow heat transfer coefficient and its pressure drop, respectively. Different twisted tape inserts with three twist ratios of 6, 9, and 12 are exploited to study the effect of twisted tape geometry. The results indicate that up to 42% and 235% increment in the heat transfer coefficient and the pressure drop, respectively, is associated with implementing twisted tape inserts. In addition, it was shown that, among some correlations proposed previously in the literature, the experimental heat transfer coefficient and pressure drop in TTI tubes can be predicted by Akhavan Behabadi et al. and Hejazi et al. correlations with reasonable accuracy. According to the results, the twist ratio of 6 gives the highest increase in the heat transfer coefficient, while the ratio of 9 is associated with the highest overall enhancement ratio. [ABSTRACT FROM AUTHOR]

• Experimental study of evaporation from asymmetric microdroplets. • Increasing levels of droplet asymmetry significantly enhance evaporative transport. • Asymmetric droplets have higher heat transfer performance than hemispherical droplets. • Asymmetric droplets yield smaller thicknesses near regions with high curvature. Droplet evaporation is ubiquitous both in everyday life and numerous engineering, biomedical, and environmental applications. For example, the large amount of latent heat associated with the liquid-vapor phase change process makes droplet evaporation an ideal solution for cooling high-powered electronic devices. Over the past 100 years, many new theories and models of droplet evaporation have been proposed. Still much of the fundamental transport physics remains elusive, which makes it challenging to describe diverse evaporation behavior by a universal mechanism. In particular, evaporation from asymmetric droplets remains grossly unexplored due to their rarity in nature. However, recent advances in micro- and nanoengineering technology have made it possible to tune and maintain droplets in non-spherical geometries, where the interfacial transport rate becomes highly anisotropic along the circumferential direction. Such a characteristic represents a distinctive feature from evaporation of capped-spherical droplets and has not been comprehensively explained by existing theories. This paper exams the evaporation rate and heat transfer performance of continuously–fed water microdroplets confined on heated silicon micropillar structures with circular, square, and triangular cross sections. The evaporation experiments are performed at substrate temperatures ranging from 60 °C to 98 °C. The droplets exhibit a capped spherical shape on the circular micropillar but an asymmetric geometry on non-circular micropillars. Our experimental results show that increasing levels of droplet asymmetry significantly enhance evaporative transport. Specifically, for a 60 °C substrate, a triangular micropillar has 45% greater heat transfer coefficient than a circular one; at 98 °C, the increase reaches 71%. This enhancement is validated by multiphase numerical simulations which show agreement with experimental values. The enhanced heat transfer coefficient from asymmetric droplets is attributed to a reduced conduction resistance inside the droplet and smaller diffusion resistance at the droplet interface, both of which result from the non-uniform geometric features of the droplet along the circumferential direction. Finally, a thermal resistance network is developed to demonstrate the variation in the local droplet thickness and interfacial curvature along the circumferential direction for droplets confined on circular, square, and triangular micropillars. Asymmetric droplets yield smaller thicknesses near regions with high curvature, which results in a smaller conduction resistance and higher fraction of heat transport near the contact line region compared to a capped spherical droplet. Furthermore, with increasing droplet temperature, the thermal resistance associated with vapor diffusion is reduced significantly, leading to greater enhancement of evaporative heat transport. These findings not only provide insights in the evaporation behavior of asymmetric droplets, but also provide important guidance for relevant applications, such as the design of evaporative cooling systems. [ABSTRACT FROM AUTHOR]