Experimental investigation on heat transfer of supercritical water flowing in a horizontal tube with axially non-uniform heating

Abstract

For a heat exchanger, the heat flux is usually non-uniform in the axial direction, and the knowledge about heat transfer of supercritical water with axially non-uniform heating is scarce. To fill the gap, experiments were conducted in a horizontal tube to study the heat transfer characteristics of supercritical water with axially sinusoidal heating. The pressures varied from 23.5 to 26.5 MPa, mass fluxes 200 to 400 kg·m−2·s−1 and average heat fluxes 50 to 150 kW·m−2. Compared with uniform heat flux, the temperature difference between the top and bottom wall is larger than that of the axially sinusoidal heat flux. The inner-wall heat flux is non-uniform along the circumference due to the temperature difference, being higher at the side wall and lower at the top wall. A relative heat flux parameter is defined to describe the relative magnitude of the local heat flux. The relative heat flux parameter of the top generatrix, which is inversely proportional to the average heat flux, decreases linearly with the increase of the temperature difference between the top and side generatrix. The non-uniform degree of heat flux along the circumference is positively correlated with the average heat flux. Additionally, the local heat transfer characteristics should be considered in horizontal tubes. On the one hand, the heat transfer deterioration may occur when the fluid temperature near the top wall is higher than the pseudo-critical temperature and it can be suppressed with increasing Reynolds number. On the other hand, the local heat transfer can be enhanced if the side and bottom wall temperatures exceed the pseudo-critical temperature.