Influence of Pressure on Film Boiling Heat Transfer

Experiments on film boiling of carbon dioxide were performed covering the range of pressures from the triple point to the critical point. Measurements were also made at supercritical pressures. Three different heating wire sizes were employed with diameters of 0.0508, 0.1, and 0.4 mm. The boiling curves, plotted in terms of heat flux and temperature difference, were found to be pressure dependent, with a more marked dependence for smaller diameter heating wires. The role of pressure level was exhibited in greater detail in a presentation in which the heat transfer coefficient is plotted against pressure at fixed values of heat flux. The most rapid variations of the heat transfer coefficient with pressure occur in the neighborhoods of the triple and critical points, with relatively gradual variations in evidence in the intermediate range of pressures. The curves of heat transfer coefficient versus pressure take on minimum values at the triple and critical points. Nusselt numbers evaluated from the experimental data agree satisfactorily with available predictive equations. Photographs of the vapor separation patterns revealed that with increasing pressure, the bubble columns which break away from the vapor film successively evolve into vapor columns and vapor sheets. As the critical pressure is approached, the height of the sheet diminishes.