Study On Two-phase Heat Transfer And Critical Working Condition Characteristics Of Porous Wick Micro-channel Evaporator

Currently,with the enhancement of electronic equipment integration level and performance,thermal management of electronic components with high heat flux turns out to be a research hotspot.Loop Heat Pipe(LHP),as a novel two-phase thermal control technology,gained its extensive application in the cooling of both electronic devices in spacecraft and communication facilities on ground with its unique advantage in high heat transfer capacity,flexible location layout,and long-distance transmission of heat,ect.In whole system,porous wick micro-channel evaporator is a core component in heat transfer and exerts critical impact on system performance.Numerical simulation method is adopted in this context to investigate two-phase heat transfer and critical working condition characteristics of porous wick micro-channel evaporator.First of all,the volume averaging method is taken in commercial software Fluent to investigate heat transfer process in porous media,and it will cause CSF(Continuum Surface Force)model fail to characterize capillary force.Thus,on the basis of two-phase hybrid model and characteristics of capillary force near vapor-liquid interface,capillary force was characterized,and a three-dimensional axisymmetric model of the calculation element for porous wick micro-channel evaporator was established.The accuracy of the model was verified by the measured data of the existing literature.And the results showed that simulation results are in agreement with experiment ones in trend.Errors of simulation results are about 10%.Then,the driven condition of the loop heat pipe system without pump assistance taken into consideration,the independent effects of porosity,permeability,effective diameter,heat flux load and external loop pressure drop on heat transfer characteristics of the evaporator are investigated through controlling variable method.The research results show that with the increasement of porosity,effective diameter or external loop pressure drop,inlet volume flow rate of the calculation element declines and maximum temperature,outlet vapor volume fraction,external loop pressure drop as well as phase change heat transfer percentage enjoy different degrees of growth.A larger permeability will be beneficial to the increase of inlet volume flow rate,but contribute to a smaller maximum temperature,outlet vapor volume fraction and phase change heat transfer percentage on the contrary.Compared with permeability,although the increase of heat flux load leads to the increase of inlet volume flow rate,too,it exerts opposite impact on maximum temperature,outlet vapor volume fraction and phase change heat transfer percentage.Finally,a pump-assisted loop heat pipe system taken into consideration,with heat flux load on device interface and inlet liquid supply flow rate as variable operating parameters,four kinds of typical operation intervals were obtained.Additionally,the ultimate flow and heat transfer characteristics by the device miniaturization were predicted.The results show that,under the same heat flux condition,with the decrease of miniaturization ratio,three critical flow lines have the trend of convergence,and the flow range in two-phase safety region is shrinking.The relative height of vapor-liquid interface corresponding to the limit control temperature decreases significantly and the interface approaches the lower border of the capillary wick.Under the same liquid supply flow condition,with the decrease of miniaturization ratio,the heat flux range in two-phase safety region is almost unchanged,and the relative height of vapor-liquid interface corresponding to the limit control temperature is maintained at 2/3 height of the capillary wick.The conclusion of the study will provide theoretical reference on the miniaturization design and selection of optimum operating parameters.