Experimental and Numerical Analysis of Heat Transfer on the Annular Test Section

A water-heated annular test section for studies of convective boiling was recently installed in the Thermal-Hydraulics Experimental Laboratory for Multiphase Applications (THELMA). The test section consists of a heat exchanger between the primary and secondary fluid, which is designed to investigate flow behaviour in the primary fluid under various heat transfer regimes in reactor-like conditions. As the total exchanged heat flux between primary and secondary fluid is calculated from the temperature difference between inlet and outlet thermocouples, accurate determination of heat losses on the test section is required. Our previous experimental studies in single-phase flow have shown that surface heat losses are not negligible and cannot be estimated solely from the measurements of inlet and outlet temperatures. For prediction of heat losses, a detailed three-dimensional computational model of the test section including its inlet/outlet heads was prepared in ANSYS Fluent. Two limiting cases with low and high temperature differences between the water and the working liquid (refrigerant R245fa) have been simulated. The measured axial temperature profile of the heating water and the measured temperatures at the outlet head were used to validate the simulation results. After that, the simulated velocity and temperature fields were used to determine and quantify different heat loss contributions in the test section.