Boiling Heat Transfer Rates for Small Precisely Placed Water Droplets on a Heated Horizontal Plate

Two small horizontal surfaces, heated to temperatures up to 220°C, were cooled by small (50–300 mm diameter) room-temperature droplets at 1 atmosphere pressure. One surface was a 10×10 mm thin-film nichrome heater that was used to measure heat fluxes below 100 W/cm2. The other surface, used for fluxes in excess of 100 W/cm2, was a solid copper heater with an 8×8 mm exposed surface. A continuous jet droplet generator coupled with two mutually perpendicular deflection plates was used to manipulate the path of constant diameter water droplets so that the impact of the drops could be precisely located on the heated surfaces. The droplet generator and the deflection plates were employed so that the effect of the impact frequency, droplet diameter, droplet velocity and spacing on the resulting heat transfer rates could be studied under controlled conditions. Optimal droplet spacing between 0.75 and 1.5 times the droplet diameter increased the critical heat flux approximately 30 percent above the value that was achieved when the drops were deposited in one location. For area-averaged mass flow rates less than about 0.08 g/(cm2s), there was no trend in the critical heat flux with the Weber number. However, for larger mass flux rates, the critical heat flux increased with an increasing Weber number. The measured critical heat flux values were roughly twice the heat flux of traditional pool boiling for identical superheat temperatures. Two droplet cooling dimensionless critical heat flux correlations are proposed as a function of Weber and Strouhal numbers; one for a single stream of drops and the other for drops that are spaced across the heated surface. The correlation for the spaced droplets is a function of a dimensionless droplet spacing on the heater.