An Experimental Study of Heat Transfer Deterioration at Supercritical Pressures

Abstract

Convective heat transfer to CO2 flowing upward in electrically heated vertical tubes at supercritical pressures was studied for wall heat fluxes q within ranges that included values corresponding to the onset of heat transfer deterioration (HTD). The inlet pressure was P = 8.35 MPa, the mass flux was in the range 200 kg/m2s ≤ G ≤ 1500 kg/m2s, and the inlet temperature was in the range 0 ◦C ≤ Tin ≤ 35 ◦C. Wall temperature measurements were collected in three tubular test sections, having inner diameters of D = 4.6, 8, and 22 mm. The abilities of three different HTD identification methods to separate the entire data set into deteriorated and normal heat transfer modes were tested. Two types of buoyancy parameters were tested as HTD detection methods, and correction factors for changes in mass flux were devised. The minimum heat flux at HTD onset was found to follow a power law of mass flux with the same exponent for all three sections and the same proportionality coefficient for the two smaller sections but a smaller one for the larger test section. 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. The heat transfer coefficient for normal heat transfer was expressed as an exponential function of the diameter.