Boiling of Dilute Emulsions on a Heated Strip

Abstract

Measurements of heat transfer coefficients in 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 × 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 and FC-72 in water. Single phase and boiling heat transfer coefficients are measured for emulsions with a volume fraction of the dispersed component of 0.1 and 0.5 percent in an isothermal pool at approximately 25 degrees Centigrade. 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 slightly decreases the overall heat transfer coefficient, but when boiling commences, an enhancement of the heat transfer coefficient 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. Heat transfer coefficients are compared to recently published measurements of boiling in similar emulsions on a small diameter horizontal wire. Quantitative comparison of the boiling curves for the wire and plate geometries is made and discussed. The magnitude of the increase in heat transfer coefficient is smaller for emulsion boiling on the surface of the heated strip than is reported for boiling on the wire. The shape of the boiling curve is nearly the same for both geometries.