Quantitative characterization of the pseudo-boiling contribution to supercritical heat transfer

Abstract

This paper explores the supercritical heat transfer mechanism by characterizing the boiling contribution ratio qb/q, where qb is the boiling heat flux and q is the applied heat flux. Experiments are performed using nickel–chromium wire in 15 °C liquid carbon dioxide at 5.2, 7.6, 9.0, and 11.0 MPa. The evaporation heat flux qe is the amount of heat used for vapor generation, while qb is the heat transfer in the bulk liquid due to the disturbance of the flow/temperature field by vapor–liquid interface motion. A data processing procedure is developed to measure qb/q from the captured images. Similar trends appear for both supercritical pseudo-boiling and subcritical boiling. The evaporation-like regime at supercritical pressures reaches qb/q = 0.21–0.43, while the film boiling (evaporation) regime achieves qb/q = 0.08. In the supercritical-boiling-like regime, qb/q increases sharply from 0.19 to 0.65, whereas in the subcritical-nucleate-boiling regime, qb/q maintains a value of 0.30 followed by a rapid rise to 0.68 under a vigorous bubble merging and departing mechanism. At both subcritical and supercritical pressures, the heat transfer deteriorates in the evaporation regime, but is significantly enhanced by phase-change-induced flow/temperature field perturbations. The boiling curves differ in the two pressure domains. At supercritical pressures, natural convection transitions smoothly to the evaporation-like regime, then to the boiling-like regime. At subcritical pressures, a steep transition from natural convection to nucleate boiling occurs, and then, film boiling is induced through the action of surface tension. The above findings complete the inverse boiling curves in the two pressure domains.