Experiments and Analyses on Heat Transfer Characteristics from a Solid Wall to a Strip-ShapedWick Structure

Centered or striped wick structures have been used to develop ultrathin heat pipes.Differing from traditional heat pipes,the centered or striped wick structures leave noncontact container surfaces with the wick structure.In this study,experiments andnumerical analyseswere conductedtoinvestigate the influenceof thesenoncontact surfaces.In the experiments,a strip-shaped wick structure was placed vertically,the top was sandwiched between wider rods and the bottom was immersed in a working fluid.The rod width was greater than the wick width;thus,noncontact surfaces were left between the rod and the wick structure.The heat was applied from the rod to the wick structure,and the evaporation heat transfer characteristics of the working fluid from the wick structure were evaluated.Water was used as the working fluid.The experiments were conducted by varying the rod and wick widths.The experimental results were obtained when the wick structures were placed separately.In the numerical analyses,the temperature and heat flux distributions in the rod were obtained.From the experimental and numerical results,it was confirmed that the noncontact surfaces caused the heat flux in the rod near both surfaces of the wick structure to concentrate,which increased the evaporation thermal resistance of the wick structure.A reduction in the noncontact surface area by increasing the wick width was found to be effective in decreasing the evaporation thermal resistance and increasing themaximumheat transfer rate of the wick structure.The separation of the wick structure increased the evaporation surface area.However,its effectiveness was limited when the heat transfer rate was small.

Evaporation Heat Transfer Characteristics from a Sintered Powder Wick Structure Sandwiched between Two Solid Walls

An ultra-thin flattened heat pipe has been developed with a centered wick structure.This structure is essential to make the heat pipe thinner.However,the centered wick structure reduces the evaporation and condensation surface areas of the wick structure because it is sandwiched between heat pipe walls.In this study,because detailed discussion has not been made,heat transfer experiments were conducted for the wick structure sandwiched between two solid walls.This study focused on the evaporation heat transfer characteristics from the sandwiched wick structure.The experiments were conducted with three wick structures,that is,strip-shaped sintered copper powders with thicknesses of 0.5,1.0,and 1.5 mm.Water was used as working fluid.The capillary pumping performance,that is,the liquid lifting velocities of the three wick structures were the same.The experimental results of the three wick structures were compared regarding the relation between the evaporation heat transfer rate and the superheat of the working fluid.The heat transfer experiments were also conducted when one of the solid walls was removed from the wick structure.It was confirmed that even if the wick structure was sandwiched between the solid walls,sufficient evaporation of the working fluid occurred from the thin sides of the wick structure.

Effect of the Filling Liquid Ratio on the Thermal Performance of a Novel Thermal Diode with Wick

The application of thermal diodes,which allow heat to flow more readily in one direction than the other,is an important way to reduce energy consumption in buildings and enhance the battery heat dissipation of electric vehicles.Depending on various factors including the specific design,materials used,and operating conditions,the convective thermal diode can exhibit the best thermal rectification effect in intended applications compared to the other thermal diodes.In this study,a novel convective thermal diode with a wick was proposed based on the phase change heat transfer mechanism.This design takes advantage of both capillary forces provided by the wick and gravity to achieve enhanced unidirectional heat transfer performance for the designed convective thermal diode.The effect of the filling liquid ratio on the thermal performance of the thermal diode was experimentally investigated,which was in good agreement with the theoretical analysis.The research findings showed that with an optimal liquid filling ratio of 140%,the thermal diode with a wick can achieve a better thermal rectification ratio when subjected to a lower heating power,and the maximum thermal rectification ratio of 21.76 was experimentally achieved when the heating power of the thermal diode was 40 W.

Biomimetic Capillary Inspired Heat Pipe Wicks

The possibility of mimicking desirable properties from nature accelerates material improvement and generates commercial interests. For heat pipe development, many attempts have been made in heat pipe wicks to enhance its capillary performance by mimicking biology. Constructing biporous, composite, or nanopillar wicks with aim of achieving hierarchical structure has been found in many studies. Mimicking beetle shell surface to obtain hybrid wettability shows biomimetic potential in heat pipe wicks. This paper firstly reviews some fundamental studies in biomimetics, establishing a general idea of surface wetting and capillary effect. MR/scanning of two live plants （Musa X Paradisiaca and Salix Flamingo） provides the possibility of visualising internal structures in vivo and obtaining rates of water transport in xylem vessels. In addition, by investigating the work inspired directly or indirectly from biomimetics, the role that biomimetics plays in modern heat pipe technology is revealed. Our innovation which syntheses a low level of hierarchical structure and integrates integral wicks for different heat pipe sections including evaporator, adiabatic, and condenser is introduced. Mathematical modelling in terms of capillary pressure and capillary rise rate to characterise such new structure is provided.

Heat Transfer with Flow and Phase Change in an Evaporator of Miniature Flat Plate Capillary Pumped Loop

An overall two-dimensional numerical model of the miniature flat plate capillary pumped loop （CPL） evaporator is developed to describe the liquid and vapor flow, heat transfer and phase change in the porous wick structure, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The entire evaporator is solved with SIMPLE algorithm as a conjugate problem. The effect of heat conduction of metallic side wall on the performance of miniature flat plate CPL evaporator is analyzed, and side wall effect heat transfer limit is introduced to estimate the performance of evaporator. The shape and location of vapor-liquid interface inside the wick are calculated and the influences of applied heat flux, liquid subcooling, wick material and metallic wall material on the evaporator performance are investigated in detail. The numerical results obtained are useful for the miniature flat plate evaporator performance optimization and design of CPL.