Development of Correlation for Heat Transfer Enhancement and Deterioration for Supercritical Fluid Using Freon 22 Experimental Results

The heat transfer behaviour of a supercritical fluid is an important input for the performance evaluation of SuperCritical Water-cooled Reactors (SCWR) of Generation IV nuclear power plants. SCWRs have very high overall thermal efficiency of about 45-50% because their operating pressures and temperatures are high. Accident analyses for licensing are carried out using system thermal hydraulics codes in which well established and validated correlations for Heat Transfer Coefficient (HTC) have to be built in. Experimental studies have shown that there is Heat Transfer Enhancement (HTE) for supercritical fluid near the pseudocritical temperature at relatively low heat flux to mass flux ratios. The peak HTC decreases as this ratio increases. At very high values of heat flux, a peak in wall temperature appears due to Heat Transfer Deterioration (HTD). This phenomenon is important as the nuclear fuel clad may fail at high temperatures induced by HTD and result in release of radioactive nuclides into the coolant streams. It is also important for the sizing of core. Several investigators have carried out experiments using water, CO2 and R22 etc. due to similarity in their thermophysical properties, and it was observed that the bulk fluid enthalpy at which peak wall temperature appears is different at different test specifications (heat flux, mass flux and different inlet temperature). Effects of inlet temperature are found to be significant for HTD but results reported in literature have not reported this effect in detail. Experimental wall temperatures available in open literature were measured by thermocouples positioned at regular intervals. However, since the HTD is shown to result in very steep temperature changes, highly local temperature measurements are desirable. HTC correlations are available which are able to predict HTE satisfactorily but HTD predictions from available correlations are poor and therefore, better correlations are required to predict HTD.