Research On Heat Transfer Of A Closed-loop Thermosyphon Using Microencapsulated Phase Change Material Suspensions

The development trend of electronic equipment determines its miniaturization and multifunctionalization,which increases its power consumption and heat capacity per unit volume.The use of phase change materials as the cooling medium is an effective method to develop large-scale and high-performance equipment and to meet the high heat flux density of the data center.Microencapsulated phase change material suspension(MPCMS)as a solid-liquid cooling medium has good fluidity and high energy density.It can effectively solve large volume change and other problems of gas-liquid phase-change materials.The existing experimental investigations of MPCMS mainly focused on testing the heat transfer characteristics of suspensions in tube sections by pump.There were a few experimental studies about a closed-loop thermosyphon system with MPCMS.Therefore,MPCMS as a cooling medium can provide new ideas for the study of heat transfer performance in a closed-loop thermosyphon system.This paper first analyzed the effect of preparation time and base fluid ratio on the stability of MPCMS,and determined that the mixed base fluid was conducted with 46 wt.% anhydrous ethanol and 54 wt.% deionized water.Stirring the mixture of microencapsulated phase change material particles and the base fluid with a magnetic stirrer for 0.5 h and dispersing particles into the base fluid homogeneously with ultrasonic cleaner for 0.5 h was adopted to prepare the MPCMS.MPCMS with mass concentration not exceeding 15.0 wt.% can be regarded as a Newtonian fluid.The heat transfer experiment of a closed-loop thermosyphon system operating with MPCMS was compared and analyzed in two different heating sections.The first heating section was formed that nichrome heating wire was tightly wound around the outer surface of a copper straight tube.The second heating section contained a cross-connected microchannel heat sink and a ceramic heating plate.It indicated that the closed-loop thermosyphon system in two different heating sections with various MPCMS can start up successfully and presented the similar temperature change under different working conditions.The Nusselt number at the heating section inlet not only increased with the increase of the heating power,but also tended to increase as the temperature of the cold source increased.It was revealed that the reasonable match between the cold source temperature and the heating power had a significant effect on the operation of the system with MPCMS.The heat transfer performance of the system having a cross-connected microchannel heat sink in the heating section with 5.0 wt.% MPCMS was better than that with the base fluid and 2.0 wt.% MPCMS at 17.5 W / 26 °C.The heat transfer performance of the system in which the heating section was a straight tube with 0.5 wt.% MPCMS is better,and the convective heat transfer effect at the heat source position of the loop is obvious at 20~30 W/22~30°C.It indicated that the heating plate temperature and the loop system thermal resistance of the system having a cross-connected microchannel heat sink in the heating section were lower than the heating wire temperature and the loop system thermal resistance of the system in which the heating section was a straight tube,and the Nusselt number at the first heating section inlet was higher than the second.The set of a cross-connected microchannel heat sink improved heat flux input value and increased the contact area of MPCMS and the high-temperature wall,thereby promoting the circulation of MPCMS and the cooling effect of MPCMS on the heating plate.The closed-loop thermosyphon system having a cross-connected microchannel heat sink at the heating section can be further applied to chip cooling in the data center with high heat flux density.The relative error between the experimental and the analytical value of the temperature at the second heating section outlet was within 11%,so that the correctness of the numerical simulation results was verified.