Very High Temperature Reactor (VHTR) is one of the six reactor concepts selected for further consideration by Generation IV International Forum (GIF) members. Adequate heat removal under steady-state and transient condition is important for the safe operation of VHTR. In one of the proposed designs for reactor cavity cooling system, under off normal conditions, coolant comes down through a downcomer and is then distributed into riser channels. The purpose of this design is to enhance heat transfer rates. The heat transfer characteristics in such a cavity are modeled by natural convection in a square cavity with plate. The plate divides the cavity into channels. The cavity is open at the top and the left side wall is isothermal. The plate is parallel to the side walls. The plate is isothermal and is maintained at a lower temperature than the left wall. The plate is shorter than the height of the cavity so that flow down in one channel turns up in the next channel. The right wall has a temperature lower than the plate, and bottom of the cavity is adiabatic. The flow in the cavity is simulated by recently developed Modified Nodal Integral Method (MNIM). Simulations are done for two different heights of the plate. The cavity without plate is also studied for comparison purposes. The numerical investigation is done for Rayleigh numbers of 103, 104 and 105. Nusselt numbers at hot wall and volumetric flow rate into the cavity are reported for all nine cases. Results show that there is significant enhancement in heat transfer in the presence of a plate for higher Rayleigh numbers.

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