DOWNFLOW POST-CRITICAL HEAT-FLUX HEAT-TRANSFER TO LOW-PRESSURE WATER

During the worst loss-of-coolant accidents for nuclear reactors, the flow through the cores is calculated to be down much of the time. Very little downflow heat transfer data exist, however, so there is some question as to how to calculate the heat transfer in this transient. Post-critical heat flux heat transfer data for water in downflow have been obtained for the following conditions: mass velocity, 48. 8 to 147 kg/s multiplied by (times) m**2; wall temperature, 538 to 760 degree C; pressure, 1. 3 to 2. 6 bars; quality, 4. 1 to plus 5. 8%; tube diameter, 1. 25 cm; and tube length 66 cm. A low mass velocity, a frozen equilibrium model predicts the data well. At high mass velocity, droplet-vapor heat transfer is good enough so that a homogeneous equilibrium model predicts the data. Under no circumstances is droplet-wall heat transfer significant. When the vapor is in laminar flow, the heat transfer is particularly poor and the radiant heat transfer becomes a significant fraction of the total.