Azimuthal Film-Boiling Heat Transfer from a Downward-Facing Hemisphere

Quenching experiments were performed to examine the effect of inclination angles and curvature on film-boiling heat transfer. The experiments employed a 294-mm-diam, 30-mm-thick stainless steel downward-facing hemisphere to substantiate the local film-boiling mechanism along the angular surface. Forty-six thermocouples were installed from 0 deg (the bottom) to 85 deg (near the equator) at three intervals: 10 deg (0 to 10 deg), 5 deg (10 to 55 deg), and 2.5 deg (55 to 85 deg) near the outer (1.5 mm) and inner (5 mm) surfaces of the test section. The angular film-boiling heat fluxes and heat transfer coefficients were obtained from the two-dimensional transient temperature profiles by solving a transient heat conduction equation in spherical coordinates. The test results were compared with those of the laminar and interfacial wavy film-boiling analysis. Undulating heat transfer coefficients were observed from the experimental data as the angle increases. These phenomena intensified near the equator, which has higher inclination angles than near the bottom. It was shown that the Helmholtz instability limited the vapor film thickness. In addition, the boiling mechanism on the downward-facing hemisphere was visualized utilizing a digital camera.