Numerical Simulation On Desublimation And Heat Transfer Of UF₆ Gas In An 8L Cryogenic Container

Desublimation of UF₆ is used extensively in collecting UF₆ gas from a cascade system in a uranium enriching plant.Based on the fact that desublimation of UF₆ takes place in a cryogenic container,the method can be easily operated.Ways of accelerating the solid depositing rate in cryogenic containers have been improved,but the major one is through experimental approaches and analysis approaches for the steady-state in heat transfer.In this thesis,both the gas desublimation and the solid layer formation in an 8L cryogenic container are studied by numerical simulation,and the main content is summarized as follows:For the prediction of the desublimation and heat transfer,an appropriate mathematical model is developed,in which the mass transfer rate in desublimation is characterised in relation to gas density,temperature and saturation temperature,according to the principle that gas supercooling degree determines the rate of phase changing in nucleation theory.In condition,a criterion of desublimation in terms of the gas speed is defined in the model.The numerical simulation generates the variation in time of the average pressure,solid depositing rate and temperature field in the container,the distribution of solid layer as well as the heat flux through the wall of the container.To evaluate the effect of operating condition on the desublimation and heat transfer,four sets of numerical simulation have been performed under different conditions: initial pressure of 1000 Pa and 4000 Pa in the container,wall temperature of 103.15 K and153.15 K of the container.The calculation results have shown that in the early period of the gas collecting process,the solid layer,growing under the higher initial pressure,is thicker and denser as a result of the higher mass transfer rate in the desublimation.Hence,increasing the initial pressure can accelerate the solid depositing rate in the early period,but it also demands more cooling capacity from the refrigerator due to the larger heat flux through the wall of the container.Lowering the wall temperature of the container can also speed up the solid depositing at first;however,when the solid layer thickness reaches to a certain value,the local solid depositing rate in the position beneath the layer surface declines,even being lower than that in the control temperature condition.Meanwhile,the final total solid mass is slightly less than that obtained in thecontrol condition.Nevertheless,due to the lower internal pressure in the container,the condition of lower wall temperature is more likely to guarantee maximizing the mass flux in the gas collecting process.