Improvement Of Vapor-liquid Phase Change Model In Condensation And Its Cryogenic Application

Cryocoolers are now capable of kilowatt level cooling power at liquid nitrogen temperature,which means the heat flux at the cold head has achieved the scale of 105W/m2.The cooling power at cryogenic temperature requires a large quantity of work,therefore the heat transfer loss is of great concern.Condensation is a regular scheme of high flux heat transfer.However,the condensation heat transfer coefficient is restricted in cryogens for the low thermal conductivity,low surface tension and low latent heat.This thesis extends the Patankar&Sparrow model to cryogenic temperature,the high fin efficiency is discovered for the high fins,which is quite different from the common sense from the conventional fluid.With the theoretical analysis on condensation model,a correlation between the interfacial temperature and the model parameters is proposed.Based on the correlation,both the convergence and accuracy is improved for the condensation model.Furthermore,the correlation can be extended to superheated vapor condensation.Taking the limit,the correlation for superheated vapor condensation can be reduced to the saturated correlation.The improved model is applied in the CFD model of high aspect ratio minichannel.Various flow rate of nitrogen condensation are computed,and the heat transfer,volume fraction,outlet backflow and the fin temperature distributions are all discussed.As a result,the correlation for condensation in such a minichannel is yielded.The mechanism of cryocooler condensation deterioration is discovered in the discussion on liquid backflow boundary.The model indicates that high fins are capable of high heat flux,reducing the temperature difference from 17.5 K to 1.5 K compared with the bare surface under natural condensation.Taking the improved model in superheated nitrogen vapor condensation,it is discovered that the dry area on the fins near the inlet can sustain high fin efficiency via large temperature difference.Additionally,the heat transfer curve is nonlinearity compared to the saturated condensation for that the superheat vapor condensation process is a combination of condensation,forced convection and natural convection.According to the numerical results,a fin-type cryocooler cold head is manufactured.The test bench is built and the cooling power extracted by nitrogen condensation is tested.The performance attained is in consistency with that of electrical heating test,which indicates that the finned cold head is of high heat transfer coefficient.