Design And Heat Transfer Characteristics Of High-capacity Loop Heat Pipe With A Flat Evaporator

Loop heat pipe（LHP）is a passive,efficient two-phase heat transfer device with many advantages including high heat dissipation capacity,large heat transfer distance,zero power consumption,strong anti-gravity,compact structure and flexible layout,which is one of the effective measures to solve the cooling of server chips with high input power and high heat flux.At present,the flat evaporator LHP suitable for electronic cooling generally has the disadvantage of low heat transfer capacity,and it is difficult to meet the current requirement for power growth of server chips.The design and development of high-capacity LHP with a flat evaporator has become the focus of research.In this study,the preparation and related properties of capillary wick materials are studied.Then,the influence of evaporator and condenser design,composite wick design,and condensing line layout design on the heat transfer characteristics of the flat evaporator LHP is systematically studied.Finally,the steady-state mathematical model of LHP is established,and the heat and mass transfer performance of LHP is analyzed based on the simulation results.The main research contents of this study are as follows:（1）Porous copper materials with different particle size are prepared,and their micromorphology,porosity,thermal conductivity and capillary performance are evaluated.The results show that the sintered porous copper powder with particle size of 106-150μm has better performance and is more suitable for the preparation of wick of LHP.In addition,a novel low temperature pressureless sintering technology is proposed,which successfully realizes the low temperature sintering of porous mullite ceramic materials,so that it can be used as the secondary wick to reduce the heat leakage of evaporator.The properties of porous mullite ceramic materials with different mass ratios（Mullite powder:Clay powder:Na HCO₃powder）were studied.The results show that when the sintering mass ratio is 1:0.2:1.2,the porous mullite ceramic materials have the highest porosity,the lowest thermal conductivity and better capillary performance.（2）The evaporator,water/air-cooled condenser and the overall structure of LHP are designed and fabricated,and the heat transfer characteristics test platform of flat evaporator LHP is established.The heat transfer characteristics of water/air-cooled LHP with different filling ratios are studied.The results indicate that the start-up characteristics for the water/air-cooled LHP are better when the filling ratio is 33.1%.At higher input power,the junction temperature of heat source of the water/air-cooled LHP are lower when the filling ratio is 46.1%.Under the input power of 750 W（83.33 W/cm²）,the junction temperature of heat source of the water-cooled LHP is 87.27°C.The maximum input power of the air-cooled LHP is 550 W（61.11 W/cm²）,and the corresponding junction temperature of heat source is90.93°C.With the increase of input power,the heat dissipation advantage of water-cooled LHP is more and more obvious than that of air-cooled LHP.（3）A flat evaporator LHP with a composite wick is designed,and its heat transfer characteristics is studied and compared with that of LHP with a mono wick.The results display that the composite wick design can effectively reduce the heat leakage from the primary wick of sintered copper powder to the compensation chamber,decrease the startup power and startup temperature of LHP,and reduce the temperature fluctuation during the startup process of the LHP.In the low input power range,the junction temperature of the heat source and thermal resistance for the LHP with a composite wick is lower than that of the LHP with a mono wick.However,at the high input power,the heat dissipation advantage of the LHP with a composite wick disappears due to the effect of interface resistance of the primary wick and the secondary wick.（4）A water-cooled LHP with parallel condensing line is designed.The effects of the LHP orientations,cooling water temperatures and cooling water flow rates on the heat transfer characteristics of series/parallel type LHP are studied.The results show that the LHP orientations have significant influence on the heat transfer characteristics of series/parallel type LHP.Compared with the series type LHP,the influence of cooling water temperatures and flow rates on the junction temperature of heat source of the parallel type LHP are more significant.Compared with the LHP orientations and cooling water temperatures,the cooling water flow rates have less influence on the heat transfer characteristics of series/parallel type LHP.The influence difference of LHP orientations,cooling water temperatures and cooling water flow rates on the heat transfer characteristics of series/parallel type LHP are caused by the condensation status of the working fluid in the condensing line and the volume of liquid working fluid in the compensation chamber.（5）A steady-state mathematical model is established for the flat evaporator LHP,and the operation characteristics of the LHP system are simulated.The results indicate that the relative errors between the experimental and simulated values of saturated steam temperature and evaporator temperature are less than 10%in the whole input power range.In addition,the steady-state mathematical model is applied to study the distribution of two phase zones in the condensing line,the pressure drop of LHP components and the heat leakage of evaporator at different input powers.