Experimental investigation of critical heat flux and post-critical heat flux in a carbon dioxide-cooled tube at high subcritical pressures

Supercritical fluids such as water or CO2 are an attractive choice for future nuclear reactor systems. The exact knowledge of the heat transfer in a trans-critical pressure is important for the design of those systems. In subcritical pressure, knowing how to avoid the reduction of heat removal when exceeding the critical heat flux (CHF) is of critical importance. A series of experiments is carried out with the SCARLETT (Supercritical CARbon dioxide Loop at IKE StuTTgart) test facility at the Institute of Nuclear Technology and Energy Systems of the University of Stuttgart, focusing on investigations of CHF and post-CHF heat transfer with CO2 as a working fluid. Two test sections have been used – each with a different internal diameter - to investigate CHF and post-CHF heat transfer. Both test sections with 10 mm and 6 mm internal diameters and heated lengths of 2000 mm are equipped with glass fiber in a stainless-steel capillary for semi-continuous wall temperature measurement and thermocouples for the reference temperature measurement. The test sections are instrumented accordingly for pressure, differential pressure, mass flow, inlet, and outlet temperature measurements. The paper presents CHF, upstream CHF, and boiling heat transfer measurements in selected experiments in the 10mm tube at subcritical pressures of pr=0.7 and pr=0.98, mass fluxes of 1000 kg/m2s, and heat fluxes from 80 to 200 kW/m2.