Boiling Heat Transfer to Dilute Emulsions From a Vertical Heated Strip

Abstract

Heat transfer measurements for nucleate pool boiling of a dilute emulsion on a short vertical surface are reported. The vertical surface is a thin steel ribbon of 1.35 mm height أ— 101 mm length. Direct current resistance heating produces boiling either on the surface or in the free convection boundary layer of dilute emulsions of pentane in water and FC72 in water. Single phase and boiling heat transfer is measured for emulsions with a volume fraction of the dispersed component of 0.1% and 0.5% in a pool at approximately 25 آ°C. The dispersed component is created by a simple atomization process and no surfactants are employed to maintain the droplets of the dispersed phase in suspension. In free convection, the presence of the dispersed component somewhat impedes heat transfer, but when boiling commences enhancement of heat transfer is observed. Boiling is observed in the emulsions at lower surface temperatures than for water alone, and significantly more superheat is required to initiate boiling of the dispersed component than would be needed for a pool of the dispersed component alone. Consequently, a temperature over shoot is observed prior to initiation of boiling, and such an over shoot has been observed in several prior studies. Boiling heat fluxes are compared to recently published measurements of boiling in similar emulsions on a small diameter horizontal wire. Boiling generally occurs at a slightly higher degree of superheat of the dispersed component on the heated strip as compared to thin wires.