Liquid-solid contact phenomenon in film boiling of Leidenfrost drops

The purpose of this study was to determine the effect of surface macro-roughness elements on the film boiling of discrete stationary liquid drops. The possible enhancement of boiling heat transfer rates due to the presence of these roughness elements as well as the conditions under which such enhancement might be expected was also to be determined. Film boiling of stationary discrete drops was selected as the focus of this study rather than I flow boiling since flow boiling introduces additional experimental complexities normally associated with two-phase flow phenomena which might obscure the effect on heat transfer due to the macro-roughness elements alone.

Instantaneous heat transfer coefficients were obtained from photographic measurements of drop vaporization. Experiments were conducted at atmospheric pressure with four liquids on five heating surfaces at temperatures of up to 620°C. The drop sizes investigated ranged from 0.01 cc. to 10.0 cc. The liquids investigated were water, denatured ethanol, iso-propanol, and ethylene-chloride. The heating surfaces which were investigated consisted of one smooth surface (for baseline comparison data), two surfaces having concentric grooves, one surface having 492 embedded cylindrical pins arranged in an evenly spaced square matrix, and one having evenly spaced hexagonal pins which were fabricated by excavating diagonal slots in the heating surface. One of the cylindrical pins and one of the hexagonal pins in each of the surfaces so fitted was fabricated with a flush-mount micro-thermocouple at the protruding surface, having a measured in-place response rate of at least 12,000°C/sec.

Increases in heat transfer rates of up to 500% were measured on the macro-roughened surfaces (compared to that which was measured on the smooth surface with the same fluid and bulk surface temperature). Also, substantial increases (up to 450°C in the case of water) in the minimum bulk surface temperature required to maintain stable film boiling on the macro-roughened surfaces were measured (as compared to that required on the smooth surface).

Since the height of the macro-roughness elements was of the same order of magnitude as the thickness of the vapor layer which characteristically separates the heating surface from a liquid undergoing film boiling, it was postulated that the macro-roughness elements penetrating this vapor layer between the liquid and the heating surface intermittently come into direct contact with the liquid, thus providing a possible means of enhancing the heat transfer in film boiling. Transient surface temperature measurements obtained from the flush-mounted micro-thermocouples demonstrated that direct contact between the elements and the boiling liquid does in fact occur in film boiling and that at such times substantial heat flow through the elements takes place. Thermal gradients within the elements indicated that the heat which is transferred through the macro-roughness elements as a result of direct contact with the liquid is the primary mechanism responsible for the increase in heat transfer rates observed for the surfaces having the macro-roughness elements.

A model for intermittent liquid-solid contact in film boiling on a macro-roughened surface was developed as well as a two-dimensional finite difference computer program for cylindrical macro-roughness geometry. This model in conjunction with the computer program was used to calculate heat transfer coefficients from measured contact duration and period for two of the macro-roughened surfaces. These calculated heat transfer coefficients were in reasonable agreement with measured heat transfer coefficients.