Assessment of Prediction Capabilities of COCOSYS and CFX Code for Simplified Containment

The acceptable accuracy for simulation of severe accident scenarios in containments of nuclear power plants is required to investigate the consequences of severe accidents and effectiveness of potential counter measures. For this purpose, the actual capability of CFX tool and COCOSYS code is assessed in prototypical geometries for simplified physical process-plume (due to a heat source) under adiabatic and convection boundary condition, respectively. Results of the comparison under adiabatic boundary condition show that good agreement is obtained among the analytical solution, COCOSYS prediction, and CFX prediction for zone temperature. The general trend of the temperature distribution along the vertical direction predicted by COCOSYS agrees with the CFX prediction except in dome, and this phenomenon is predicted well by CFX and failed to be reproduced by COCOSYS. Both COCOSYS and CFX indicate that there is no temperature stratification inside dome. CFX prediction shows that temperature stratification area occurs beneath the dome and away from the heat source. Temperature stratification area under adiabatic boundary condition is bigger than that under convection boundary condition. The results indicate that the average temperature inside containment predicted with COCOSYS model is overestimated under adiabatic boundary condition, while it is underestimated under convection boundary condition compared to CFX prediction.

Study on Forced Convection Boundary Condition for Subcooled Water in the Simulation of Line-Heating Process

In the simulation of line-heating process, the convection boundary condition, especially the subcooled water forced convection, has great influence on the result. The calculation of the convection coefficient is a difficult problem in the simulation. This paper uses the theory of forced convection boiling to study subcooled water forced convection during the line-heating process. By solving the wall temperatures of originating nucleate boiling and critical heat flux for subcooled water and comparing the plate temperature with these two wall temperatures, the status of water can be determined, and then the corresponding convection coefficient is calculated. The simulation results show that the precision of the forced convection boiling boundary condition presented in this paper is much better than that of a boundary condition based on the pool boiling curve.

Investigation of the Natural Convection Boundary Condition in Microfabricated Structures

Heat loss through surrounding air has important thermal effect on microfabricated structures. This effect is generally modeled as a natural convection boundary condition. However, how to determine the convective coefficient (h) at microscales is a debate. In this paper, a micro heater is fabricated on a suspended thin film membrane. The natural convection is investigated using the 3-omega measurements and complex analytical modeling. It is found that h seems larger than that at larger scales; however, it is also proved that the increased h is actually contributed by heat conduction instead of heat convection. A method of determining the phenomenal h that can be used for microfabricated structures is proposed by using the heat conduction shape factor.