LIQUID-CRYSTAL IMAGES OF THE TRANSITION FROM JET IMPINGEMENT CONVECTION TO NUCLEATE BOILING .2. NONMONOTONIC DISTRIBUTION OF THE CONVECTIONCOEFFICIENT

This paper reports the second phase of a two-part study of jet impinge ment boiling. Liquid crystal thermography was used to study nucleate b oiling incipience on a thin uniform-heat-generation surface that was c ooled by a submerged impinging jet of refrigerant R-11. Thermographic images of the temperature distribution on the test surface were acquir ed synchronously with single-point temperature measurements from three locations on the surface. The nozzle-to-plate spacing was such that t he single-phase flow produced a nonmonotonic radial distribution of th e convective heat transfer coefficient. The convection coefficient dis played a secondary peak at approximately 3.5 jet radii from the stagna tion point and decreased smoothly for radii beyond that value. Single- phase and boiling regions were found to coexist stably when the interf ace between the two regions, the boiling front, was positioned at a ra dius greater than the radius of the secondary peak. The front was obse rved to suddenly collapse across the part of the surface associated wi th the nomonotonic behavior of the convection coefficient. The collaps e was attributed to the surface temperature at more than one radius re aching the incipience level simultaneously. A simple model for correla ting the typical radius of the boiling front to the convection coeffic ient and to a specific surface temperature at incipience was successfu lly applied at radii beyond the location of the secondary peak.