Liquid crystal images of surface temperature during incipient pool boiling

Abstract

Liquid crystal thermography was used to observe time-dependent temperature patterns on an electrically heated surface during the incipience of nucleate pool boiling. The images revealed spatial and temporal differences in the process by which boiling initiated and expanded over the surface in the presence of two different single-phase convection fields. The experiments were conducted in a pool of saturated refrigerant-11 (R-11). A 76-μm-thick, nickel-alloy foil formed the base of the pool. Thermochromic liquid crystals applied to the underside of the foil produced real-time images of the temperature patterns on the heated surface. Spectra of surface temperature were measured prior to and during boiling using a single thin-film resistance temperature gage. The transition from natural convection to pool boiling occurred through a transient process that began at a random point on the surface and expanded rapidly across the foil. The transition from a recirculating forced-convection flow to pool boiling did not occur by a transient process. Instead, convection and boiling regions coexisted on the surface with a sharp temperature gradient forming a thin front between the two regions. The front remained stationary for a fixed heat flux. An increase in heat flux caused sections of the foil adjacent to the front to burst into boiling. The front immediately reformed at the edge of the newly annexed area. At the conclusion of both transitions, the temperature images showed a uniform mottling from the presence of nucleation sites covering the entire surface.