Studies of flow redistribution of external parallel coolant or moderator flow through tube bundles

Abstract

The problem of transverse flow redistribution for geometries of interest to nuclear reactor technology are investigated from a macroscopic point of view by studying methods of solving the pertinent equations, their finite difference representation, convergence criteria, the choice of boundary conditions and the selection of the "control volume" within the matrix of fuel rods. The arrays selected consist of a fixed number of fuel rods surrounded by the coolant or moderator inside a square, rectangular, or cylindrical structural member. The driving function is the set of initial mass velocities which are assumed to be constant and known at the flow channel entrances. Because the number of degrees of freedom for the flow redistribution in these geometries exceeds the number of flow equations, special techniques have been used to achieve convergence of not only the axial pressure drop, but which will also yield a symmetrical flow redistribution when symmetrical problems are solved. A Fortran program has been devised which will solve the continuity equation for the outlet velocity and transverse velocities for given input values. The momentum equation is then solved to obtain the pressure drop in each control volume which results from dividing the flow channel length in equal axial increments. An average pressure drop is estimated toward which all channel pressure drops are corrected by correcting the transverse mass velocities. Thus, having corrected the transverse mass velocities, the above steps are repeated until the pressure dropper control volume is the same in the axial interval selected, within a convergence criteria. The outlet enthalpy are then evaluated assuming the inlet enthalpy at each control volume and the heat source per fuel rod. The results of this axial calculation are used as input to the next set of calculations for the next axial interval and the procedure is repeated until the whole length of the flow channel is transversed. Results of sample calculations for a square (3x3 rods) and a cylindrical (19 rod bundle) array are presented. In the former system, results for both cases, symmetrical and unsymmetrical, are reported. The computer programs used in the calculations are also included.