Numerical Investigations on Fluid Flow and Heat Transfer Characteristics of an Ultra-Thin Heat Pipe with Separated Wick Structures

Thermal and fluid-flow characteristics were numerically analyzed for ultra-thin heat pipes.Many studies have been conducted for ultra-thin heat pipes with a centered wick structure,but this study focused on separated wick structures to increase the evaporation/condensation surface areas within the heat pipe and to reduce the concentration of heat flux within the wick structure.A mathematical heat-pipe model was made in the threedimensional coordinate system,and the model consisted of three regions:a vapor channel,liquid-wick,and container wall regions.The conservation equations for mass,momentum,and energy were solved numerically with boundary conditions by using a code developed by one of the authors.The numerical results with the separated wick structures were compared with those with the centered,which confirmed the effectiveness of the separation of the wick structure.However,the effectiveness of the separation was affected by the position of the separated wick structure.A simple equation was presented to determine the optimum position of the separated wick structures.Numerical analyses were also conducted when the width of the heat pipe was increased with the cooled section,which clarified that the increase in the cooled-section width with the addition of wick structures wasmore effective than the increase in the cooled-section length.A 44%reduction in the total temperature difference of the heat pipe was obtained under the present numerical conditions.Furthermore,a comparison wasmade between experimental results and numerical results.

Experimental Investigation on the Vapor Chambers with Sintered Copper Powder Wick

High power electronics units generate high-density heat flux,which poses a significant threat to the reliability of these devices.The vapor chamber(VC)has a very high heat transfer rate and has a wide range of applications in the heat dissipation of electronic products.VCs with sintered copper powder wicks sized as irregular shape copper powder 50μm(I-50μm),I-75μm,I-110μm,I-150μm were studied in this paper.The effect of liquid filling ratio was discussed.The results indicated that the thermal resistance of all VCs decreased with the increase of heating power.The capillary performance of wick was the dominant factor for the heat transfer performance of VCs with different kinds of wick.A capillary performance factor was proposed to evaluate the capillary performance of VCs.The capillary performance factor of VC with the wick of I-75μm was much higher than that of the other types of VC in this study.The heat source surface temperature of VCs could be affected by filling ratio and wick structure.But,it should be insensitive to the filling ratio when a better capillary performance factor was obtained.With the same heating area and similar structure,the maximum heat flux density of the VC would decrease as the height of vapor cavity decreased.

Effects of water vapor in high vacuum chamber on the properties of HfO_2 films

The influence of water vapor content in high vacuum chamber during the coating process on physical properties of HfO2 films was investigated. Coatings were deposited on BK7 substrates by electron beam evaporation and photoelectric maximum control method. An in situ residual gas analyzer （RGA） was used to monitor the residual gas composition in the vacuum chamber. The optical properties, microstructure, absorption and laser-induced damage threshold （LIDT） of the samples were characterized by Lambda 900 spectrophotometer, X-ray diffraction （XRD）, surface thermal lensing （STL） technique and 1064-nm Qswitched pulsed laser at a pulse duration of 12 ns respectively. It was found that a cold trap is an effective equipment to suppress water vapor in the vacuum chamber during the pumping process, and the coatings deposited in the vacuum atmosphere with relatively low water vapor composition show higher refractive index and smaller grain size. Meanwhile, the higher LIDT value is corresponding to lower absorbance.