Heat transfer to an in-containment heat exchanger in natural convection flow: Validation of the AEA Technology computational fluid dynamics code CFDS-FLOW3D

Validation is presented of an appropriate computer code for modelling heat transfer from the containment atmosphere to an in-containment heat exchanger using new data from ENEL. This work has been carried out in collaboration with ENEL, CISE and ANSALDO. The study helps to identify conditions under which natural circulation induced by the heat exchanger does initiate. The Computational Fluid Dynamics (CFD) code CFDS-FLOW3D, developed by AEA Technology, has been used, initially in a 2-dimensional mode, to simulate the natural convection flow generated within a test vessel by an internal heat exchanger operating in a steam-air gas mixture. The model incorporates a calculations of the heat exchanger condensation rate based on local conditions. Calculational parameters have identified which allow the transient timesteps to converge sufficiently but without using excessive CPU time. Results of pre-test calculations performed for 2 different geometrical configurations are presented. These calculations suggest that the heat exchanger will operate as intended and, at the design values of pressure and temperature, would exceed the planned test power by up to 28%. Post-test simulation results are presented for the first test performed. Good general agreement with major measured parameters is found and a possible explanation for the high upward velocity measured outside the heat exchanger exit is offered. The simulation underestimated the total condenser power by about 16%; this is believed to be due to unpredicting the steady state steam fraction in the vessel. CFDS-FLOW3D is found to be a suitable tool for simulating the details of complex buoyancy driven flows, including non-condensibles, in passive containment cooling applications. The code is sufficiently flexible to be able to represent correctly heat exchanger condensation effects and to be able to simulate the resultant natural convection flows in either 2-D or, if required, in 3-D. (author). 3 refs, 9 figs