Effect of Buoyancy on the Mechanism of Heat Transfer Deterioration of Supercritical Water in Horizontal Tubes

Abstract

In order to get insights into the mechanisms governing the heat transfer deterioration (HTD) of supercritical water, systematical numerical simulations were carried out in the present study for the flow and heat transfer of supercritical pressure water in horizontal smooth tubes. The numerical results were found in very good agreement with the corresponding experimental data, validating the reliability and accuracy of the numerical model and the computational method. It was found that from these profiles along the top generatrix of the wall of the horizontal tube, there exists a thin fluid layer in which the thermophysical properties of the fluid, including the specific heat capacity, thermal conductivity, density and viscosity, all approach its minimum at a roughly identical axial position of the tube with the increasing of the bulk fluid enthalpy along the flow direction. The maximum wall temperature of the top generatrix, obviously show the occurrence of HTD. It was especially interesting that the axial position of the maximum top generatrix wall temperature (HTD position) just coincided with the axial positions of the minimum of the abovementioned thermophysical properties in the near top generatrix layer, which reveals the inherent connection between the HTD and the minimum value of the abovementioned thermophysical properties of the supercritical water. It was concluded that the HTD of supercritical water in horizontal tubes was evidently due to the vertical stratification and the accumulation of light supercritical pressure fluid (very high enthalpy but low density) in the near top generatrix region. Also, the HTD phenomena under supercritical condition was similar to that of the film boiling of the subcritical pressure water. This result clearly reveals why the axial position of the HTD occurred on the top wall of horizontal tubes (with bulk fluid enthalpy of roughly 1750 kJ/kg) is axially far ahead of the position corresponding to the critical point of the supercritical water (with bulk fluid enthalpy of roughly 2150 kJ/kg) in terms of the bulk fluid enthalpy.