EFFECTS OF THERMAL BOUNDARY CONDITIONS ON HEAT TRANSFER CHARACTERISTICS OF SUPERCRITICAL WATER

The design of some heat exchangers faces some challenges, among which a prominent one is the effect of thermal boundary conditions. This study aims at heat transfer characteristics of supercritical water at three thermal boundary conditions, i.e., convective boundary condition, constant heat flux boundary condition, and constant wall temperature boundary condition, in a horizontal tube. The convective boundary condition is realized via a counterflow double-pipe heat exchanger at the pressure of 25 MPa, and inlet temperatures and mass flux varying from 623 K to 733 K and 250 kg.m(-2).s(-1) to 1500 kg.m(-2).s(-1), respectively. The inner diameter of the tube is 0.015 m and the inner and outer diameters of the shell side are 0.02 m and 0.035 m, respectively. To realize the latter two boundary conditions, the shell side of the double-pipe is removed, and the heat flux or wall temperatures obtained from results under the convective boundary condition are applied to the outer tube wall. The results indicate that the maximum heat transfer coefficient deviation is approximately 8.8% between constant heat flux and convective boundary conditions. Under the convective boundary condition, the coupling buoyancy effect of the tube and shell side results in approximately uniform temperature distribution on the outer tube wall, just like a constant wall temperature boundary condition. In addition, local heat transfer behaviors of the inner tube exhibit a large discrepancy for a different combination of the inlet mass flux and temperature of the shell side, although average parameters of the external wall of the inner tube are the same.