Experimental investigation on heat transfer deterioration of supercritical pressure water in vertically-upward internally-ribbed tubes

Abstract

This paper presented an experimental investigation into heat transfer deterioration (HTD) characteristics of supercritical pressure water (SCW) flowing vertically upward in eight internally-ribbed tubes (IRT). The experiment was carried out at the following operating conditions: pressures from 25 to 28 MPa, mass velocities from 400 to 800 kg m−2 s−1, and inner wall heat fluxes from 400 to 1000 kW m−2. The results show that HTD of SCW might occur in the large specific heat region (LSHR) with increasing of q/G, but the increase of pressure could suppress the severity of HTD in the test IRTs. Based on the experimental data, the differences of heat transfer behavior among a series of IRTs, and the differences between IRTs and smooth tubes were compared separately. To explain the mechanism of unusual heat transfer, the buoyancy effect and flow acceleration were further analyzed based on the previous criteria. It was found that the dimensionless number Grb‾/Reb2.7 in all IRTs was much larger than that in smooth tubes under HTD occurrence, while dimensionless number Kvt for flow acceleration in all test IRTs was much less than the threshold value when HTD came out in smooth tubes for SCW. Therefore, a new improved heat transfer criterion by adding the rib geometry parameters of IRTs was proposed for predicating HTD of SCW according to the experimental data. Also, after evaluating the existing heat transfer coefficient (HTC) correlations of SCW for IRTs, it was found that all previous correlations cannot accurately predict HTC and wall temperature profiles of IRTs at HTD conditions. Thus, an improved HTC predicating correlation for HTD was proposed regarding to the effect of rib geometry and buoyancy that can be to precisely predict wall temperatures and HTCs in various IRTs. Meanwhile, the new correlation was also verified to be more effective than the previous correlations.