Investigation Of Numerical Method On Thermal-hydraulic In Passive Containment Cooling System

The containment cooling system acted as the third safety net to prevent the radioactive product in the reactor leaking to the environment.With the natural circulation in and out of the containment,combined with vapor condensation and film evaporation,the advanced passive containment cooling system keeps the containment in integrity.According to the literature surveys,research of experiments and simulations on passive containment has been intensively studied.Also,the surveys show that CFD method used in passive containment analysis always focused on simulating the outer-containment or inner-containment separately.Nevertheless,these two fields were interacted with each other through their shared interface,so they should be coupled.Furthermore,the numerical method on conjugate heat transfer in fluids still needs to be investigated.Based on that,this thesis will theoretical analyze the numerical method on conjugate heat transfer,and then in the thermal-hydraulic in passive containment system.Based on ANSYS FLUENT commercial CFD codes,the numerical investigation was performed on conjugate convective heat transfer.An alternative computation method was used to investigate conjugate convective heat transfer problem,with the coupling of boundary conditions.Convergence characteristics,error propagation were theoretically analyzed and main influence factors upon the numerical iteration procedure were revealed.The results show that the ratio of convective heat transfer coefficient(RCHTC)has a great impact upon convergence characteristics.The computation could lead to converged result only under the case of the ratio smaller than 1 and the smaller the ratio,the faster the convergence.Meanwhile,the results show that the convergence characteristics of iteration depend on the type of boundary conditions to choose and the choice of initial calculating domain.In addition,an improved strategy was proposed to deal with the divergence situation with complex RCHTC distribution,and the results show good convergence behavior and better speed.The ability to control the iterative process is quite important when using partitioned solution method repeatedly,which is realized by combining the scheme program language and user defined function together in FLUENT.Considering there is difficulty in calculating heat transfer coefficient in inner-containment,three methods were proposed to solve this problem.They were relaxation factor method,reference temperature method and RCHTC method.Subsequently,the conjugate heat transfer process in passive containment cooling system was analyzed using partitioned solution method under the circumstance of single phase.The results were compared with those from monolithic solution,and perfect agreements were found,which indicates that the numerical method is right.In addition,the uniform temperature and uniform heat flux were calculated based on the partitioned solution results,which were used as boundary conditions in the single-field calculation.The results show that,compared to uniform heat flux,the uniform temperature calculation was less reasonable,and the heat amount of it was 6% less than the coupled way outside containment and 12%bigger inside containment.At last,the partitioned solution was used in the thermal coupling of evaporation and condensation in the presence of non-condensable gas.The accuracy tests of evaporation and condensation model were conducted respectively.Reasonable outcomes were obtained and analyzed.