| Cryogenic engineering is closely related to space technology,and the temperature requirement for space cryogenic refrigeration is also increasing.In the current development of space technology,space cryogenic cold shield technology comes into being,the essence is to achieve suppression characteristics of target radia tion,on the realization way of the technology is mainly composed of the following three,reducing radiation intensity and radiation characteristics and changing regulation of infrared radiation.The most widely applied technology is to reduce the radiatio n intensity of the target surface.Based on the detection principle of infrared imaging instrument,as long as reducing surface emissivity or reducing surface temperature can realize this technology,the former technology uses the material with low emission coefficient to reduce surface emission coefficient;the latter needs target surface covered by a layer of insulating material or using certain refrigerants and phase change materials to reduce temperature.At present,cryogenic fluid transformation using refrigeration principle has been most widely used,with the help of the evaporation of refrigerant,it can reduce the temperature of the aircraft surface,thereby reducing the surface infrared radiation intensity to realize the isolation of heat.Liquid nitrogen cold shield system using liquid nitrogen as refrigerant and capillary material to storage liquid in layers,applying the principle of the triple point of refrigeration and the diffuser flow to reduce the surface temperature of aircraft.The temperature is getting lower and the cooling capacity is bigger and bigger.Cold shield system is an effective refrigeration equipment in space,its internal phase change refrigeration and throttling refrigeration are two important processes,which involve complex physical processes such the change of gas-liquid two-phase flow interface,evaporation and condensation,two-phase flow pattern and so on.Because of the lack of basic theory support for cold shield system in high vacuum,microgravity,cold background on triple point refrigeration,the related research on flow field and heat transfer is very important significance,meanwhile it can provide a theoretical basis for the optimization of the final structure of the cold shield system.According to the characteristics of the model and applicable conditions of various models in the numerical simulation,this paper adopts VOF multiphase model,RN G k-ε turbulence model,the CSF model and the evaporation and condensation model to simulate the flow field of liquid-nitrogen interlayer element model and the throttle valve pressure expanding cavity model.The VOF model of multiphase flow is a function of vo lume of fluid method,surface tracking in fixed Eulerian grid,each grid in the flow flied,the function is defined as the ratio of the target fluid volume and grid volume,as long as knowing the function in each grid value,it can realize the moving inter face tracking.The VOF model also contains the surface tension effect on the interface among the multiphase flows,which is reflected by the contact angle between the fluid and the wall.FLUENT provides a CSF model for the surface tension.The RNG k-epsilon turbulence model is stronger than the standard k-ε model.The model can calculate the low Reynolds number effect after proper treatment of the near wall area.Evaporation condensation model is based on the multiphase flow and surface heat transfer model to express the mass transfer of evaporation model and condensation model.The Lee model is widely used.The model is based on the physical basis of the mechanical model.By changing the size of contact angle and Bo number and according to the characteristics of thermocapillary convection in microgravity environment,we can observe the change of gas-liquid interface in the interlayer unit when liquid nitrogen is at triple point,and obtain the flow field distribution in different conditions at the same time.Through the calculation and comparing the Hewitt-Roberts vertical pipe flow map,verify the numerical simulation of the flow pattern in throttle valve pressure expanding cavity is reasonable and flow field distribution of the corresponding flow patterns are analyzed.By changing the superficial gas velocity and superficial liquid velocity,the distribution of flow patterns,gas volume fraction and differential pressure are obtained.The results show that the contact angle and Bo number is smaller,liquid nitrogen interlayer element is easy to form the large internal closed air bubbles surrounded by liquid phase,and internal pressure difference,temperature difference,maximum speed accordingly decreases.When the contact angle is 90 degrees,zero gravity and the contact angle is 10 degrees,gravity is 1g,gas-liquid interface has no significant changes.When the contact angle is more than 90 degrees and the wall climbing ability of liquid phase is very weak,the gas-liquid interface appears concave phenomenon.The simulation results combined with the actual situation,the small contact angle can make the refrigeration process is relatively stable,but prone to flooding phenomenon.So it should appopriatly increase the contact angle to suppress the flooding pheno menon.There are four kinds of flow type in throttle valve pressure expanding cavity model,including two kinds of bubble flow,slug flow and chum flow.With the increase of height,gas volume fraction of different flow type is increasing,and volune fraction of slug flow and chun flow is high;When the superficial liquid velocity and superficial gas velocity both change,the simulated flow type distribution results are consistent with theoretical calculation results.when the superficial liquid velocity is constant,with the increase of gas superficial velocity,gas-liquid phase force increases,gas volume fraction increases and pressure differentials decreases.Combined with the numerical results,using the refrigerant calculation software REFPROP to draw the phase-change refrigeration process and throttling refrigeration process,reading the required physical quantity to calculate the cooling capacity of two refrigeration processes.Comparing the cooling capacity under different throttle pressure ratio shows that the throttle pressure ratio is larger,the cooling capacity is greater. |
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