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Heat pipe is a two-phase heat transfer device with high effective thermal conductivity and transfer huge amount of heat with minimum temperature gradient in between evaporator and condenser section. This paper objective is to predict the thermal performance in terms of thermal resistance, R, and heat transfer coefficient, h, of screen mesh wick heat pipe with deionized water-TiO2 as working fluid. The input process parameters of heat pipe such as heat load, Q, tilt angle, θ, and concentration of nanofluid, ϕ, were modeled and optimized by utilizing response surface methodology with MINITAB-17 software to attain minimum thermal resistance and maximum heat transfer coefficient. The minimum thermal resistance of 0.1764 °C per W and maximum heat transfer coefficient of 1411.52 W/m² °C was obtained under the optimized conditions of 200 W heat load, 57.2° tilt angle, and 0.159 vol.% concentration of nanofluid. [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]

• 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]

• 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]