Study of droplet sprays prior to impact on a heated horizontal surface

Spray cooling is one of the most promising techniques under consideration for the removal of high heat fluxes. Potential and current applications of spray cooling can be found in the microelectronics, nuclear, ceramic and metallurgical industries as well as cooling of medical devices, among others. The thermal behavior of spray cooling can be divided into two coupled sub problems: pre-impact and post-impact thermal behavior of the spray. This paper concerns a quantitative assessment of the heat and mass transfer behavior of spray droplets downward oriented prior to their impact on a heated horizontal surface. An experimental and theoretical investigation of the coupling effects between a downward oriented spray and a rising saturated buoyant jet that results from evaporation on a heated surface has been successfully completed. A model describing the thermal and hydrodynamic behavior of both the spray and the saturated buoyant jet has been developed. The theoretical and experimental results indicate that: (1) the temperature of the saturated buoyant jet is highly affected by the presence of a subcooled spray; (2) small droplet sprays vertically projected experience high condensation rates as they pass through the saturated buoyant jet reaching the saturation temperature before impacting on the heated surface thus inhibiting any sub-cooling effect; (3) these droplets also can experience acceleration as a consequence of an increase in mass due to initial condensation; (4) the presence of the saturated buoyant jet increases the drag forces on the spray resulting in a considerable reduction in the droplet velocity, and (5) droplets close to the edge of the jet free boundary experience a trapping effect that forces them toward the centerline due to a radial velocity component resulting from entrainment of air from the dry ambient