Convection Heat Transfer Characteristics Of Microencapsulated Phase Change Material Suspension In Microchannel Heat Exchanger

With increasing of the heat flux density of electronic chips,traditional air-cooled heat dissipation methods does not meet the needs of its development,and a new type of high efficient heat sink is urgently needed.The microchannel heat exchanger has the advantages of small volume and high heat exchange efficiency.Furthermore,the microencapsulated phase change material suspension(MPCMS)is a novel heat transfer medium,which has the advantages of large specific heat and high energy saving density.Therefore,the flow and heat transfer characteristics of MPCMS in microchannel heat exchangers are investigated in this work.The results has certain engineering value and can provide data support for the practical application of microchannel heat exchangers and MPCMS.Firstly,based on literature review,three types of heat exchanger models were designed,the effect of different inlet and outlet positions and microchannel arrangement on the flow distribution were analyzed and an ideal heat exchanger was obtained.The influence of flow rate,heat flux and particle concentration on convective heat transfer characteristics were studied in this heat exchanger.Finally,an experimental system was designed and established.The flow and heat transfer characteristics of MPCMS in heat exchanger were tested and compared with pure water.The results show that:(1)For 1#heat exchanger,the flow rate in the middle channel is large,and on two sides gradually decreases.For 2#and 3#heat exchangers are S-type distributions with high flow rate on two sides channel and middle channel decreases gradually.The flow distribution of the suspension in 3#heat exchanger is more uniform and its unevenness is the smallest(S=0.066).Compared with other heat exchangers,3#heat exchanger has the lowest differential pressure at inlet and outlet(Δp=1846Pa),which is 9.2%lower than that of 2#heat exchanger.For 3#heat exchanger,the local high temperature area on the bottom surface is relatively small,which is most conducive to the reliable operation of electronic devices.(2)The temperature difference between the inlet and outlet of the fluid is decreases gradually with the increasing of flow rate.The temperature difference of MPCMS between the inlet and outlet is lower than that of the pure water for same flow rate,When the inlet flow rate is 188.4 mL/min,the inlet and outlet temperature difference can be reduced by a maximum(19.6%)compared with the pure water.The pressure drop between the inlet and outlet of the fluid is increase gradually with the increasing of flow rate.The pressure drop of MPCMS between the inlet and outlet is higher than that of the pure water for same flow rate.With the increasing flow rate,the average temperature of the bottom surface of the heat exchanger is decrease gradually,and in the range of 188.4m L/min to 942m L/min flow rate,it tends to be stable.The heat surface temperature is decrease with the increasing of particle concentration.(3)The heat transfer coefficient K of the heat exchanger increases with the increase of the inlet flow rate,when the flow rate further increases,the heat transfer coefficient tends to be stable.At the same inlet flow rate,the surface heat transfer coefficient of the heat exchanger is increase with increasing of MPCMS concentration.In addition,the results also show that there is a certain difference in the_x~*of each channel in the heat exchanger due to the uneven flow distribution.(4)Experimental results show that the pressure drop of the fluid increases with the flow rate.When the flow rate is 800 mL/min,the pressure drop between the inlet and outlet of the suspension is 1.5 times that of pure water.For MPCMS as medium,the pressure drop of the heat exchanger decreases with the increasing of the heat flux.When the heat flux is 300 W,the pressure drop is reduced by 16.3%when compared with 120 W.(5)The outlet temperature of pure water and MPCMS all decreases with the increasing of inlet flow rate.At the same flow rate,the MPCMS temperature is lower than that of pure water,and can be reduced by 6.2%as compared with the pure water.The average temperature of the bottom surface decreases as the flow rate increases.When the MPCMS particle concentration is 2.5%,the average temperature of the bottom surface is lower than that of pure water,and the highest comparable temperature is 12.9%lower than that of pure water.(6)Under two kinds of heat transfer medium,the heat transfer coefficient of the heat exchanger all increase with the flow rate.When the flow rate reach 600 m L/min,the rising trend of the heat transfer coefficient is decrease gradually with the increase of the flow rate.Under the same flow conditions,the heat transfer coefficient of MPCMS is higher than that of pure water.