Onset of heat transfer deterioration in vertical pipe flows of CO2 at supercritical pressures

Convective heat transfer to CO2 flowing upward in electrically heated vertical tubes at supercritical pressures was studied for wall heat fluxes within ranges that included values corresponding to the onset of heat transfer deterioration (HTD). The inlet pressure was 8.35 or 7.76 MPa, the mass flux was in the range from 200 to 1500 kg/m2 s and the inlet temperature was in the range from 0 to 40 °C. Wall temperature measurements were collected in three tubular test sections, having inner diameters of 4.6, 8, and 22 mm. The abilities of different HTD identification methods to separate the entire data set into deteriorated and normal heat transfer modes were tested. Measurements show that the minimum heat flux at HTD onset followed power laws of mass flux. For heat flux values that were larger than this minimum, HTD was found to occur only within a limited range of Tin, whose width increased with increasing heat flux.