Single-phase subchannel mixing in a simulated nuclear fuel assembly

Abstract Single-phase subchannel mixing data were obtained from a 25 rod square array by measuring precisely subchannel exit temperatures over a range of test conditions. A least-squares type statistic operating on exit enthalpy differences was developed to ascertain, in conjunction with the COBRA-II subchannel computer analysis, an optimum value for the coefficient of Rowe and Angles's single-phase mixing correlation β. When the same analytical procedures were applied to data taken under conditions of subcooled nucleate boiling, an approximately linear correlation of β with average exit quality was found. The values obtained were for conditions of natural, or unaided mixing, and for design purposes should be considered as a dependable lower limit. Commercial rod-spacer designs intended to increase mixing will, of course, show substantially higher values of β.