For the nuclear reactors, operating under supercritical conditions, the degradation of heat transfer is a similar phenomenon as the boiling crises in conventional light water reactors. The increase of cladding surface temperature is rather mild comparing to the boiling crises, but it could under some circumstances rise above the acceptable level. For the safe and reliable performance of a light water reactor operating at supercritical pressure, such a heat transfer regime must be avoided. Therefore the detailed knowledge of the deterioration phenomenon is necessary in order to be able to predict its onset. Different authors claims that the buoyancy, streamwise acceleration and the temperature variations of thermo-physical properties are the mechanisms, which cause the heat transfer deterioration (HTD)
In the current paper, the numerical investigation of the HTD phenomenon is performed for the flow of water in a uniformly heated pipe at supercritical pressure using the low-Re k − w turbulence model. Steady state Reynolds-averaged Navier-Stokes equations are solved together with equations for the transport of enthalpy and turbulence. Equations are solved for the supercritical water flow at different pressures, using water properties from the standard IAPWS-IF97 tables. All results are extensively validated against experimental data. The influence of buoyancy on the HTD is demonstrated for different mass flow rates in the heated pipes and the influence of thermo-physical properties variation is demonstrated at the coolant flow rates close to the designed operating conditions of supercritical water reactor.
Numerical results prove that the RANS low-Re turbulence modeling approach is fully capable to simulate the heat transfer in pipes with the water flow at supercritical pressures, even in the deteriorated region. A study of buoyancy influence shows that for the low mass flow rates of coolant, the influence of buoyancy forces on the heat transfer in heated pipes is significant. For the high flow rates, buoyancy influence could be neglected and there are clearly other mechanisms applying in this region.
The preceding study of the influence of temperature variations of thermo-physical properties on heat transfer shows that the variations in thermal conductivity creates the low-conducting layer close to the heated wall, which acts as a thermal barrier, causing the deterioration. The effect of coolant flow rate on the magnitude of deterioration is studied on the case, where the thermal conductivity variations cause the HTD.
NUTHOS-7. Seoul, Korea. October 5-9, 2008