WATER SPRAY COOLING IN A VACUUM (HEAT TRANSFER, IMPINGEMENT, NUCLEATE BOILING, EVAPORATION, NOZZLE)

Water spray cooling in a vacuum is a technique that appears to have excellent potential for achieving high heat transfer rates by using the high heat of vaporization of water and the low saturation temperatures at vacuum conditions. Important to this technique is a thorough understanding of the geometric and physical parameters that affect nozzle sprays and an understanding of the spray cooling heat transfer mechanisms. Investigations into these two areas were undertaken by building a test facility to provide a vacuum in a bell jar down to the triple point pressure of water while removing 12 lbm/hr of steam. Three different oil furnace type nozzles, rated from 0.75 to 2.50 GPH, were tested at vacuums from 6 to 14 mm Hg. Water was sprayed onto a one inch diameter heated copper surface; the surface temperature and heat transfer were obtained from the gradient of temperatures measured by thermistors. Heat was supplied by five cartridge type heaters.;A general theory of spray cooling is presented which places in perspective the various heat transfer mechanisms. A relation was developed for the liquid film evaporative cooling mechanism in terms of only nozzle orifice diameter, water properties, and operating conditions. This relation is valid for ambient pressures from 6 to 760 mm Hg, spray fluxes from 0 to 3660 lbm/hr-ft('2), and for spray droplet Sauter mean diameters from 20 to 270 microns. Heat transfer coefficients up to 330 Btu/hr-dt('2)-F were obtained and were found to increase with higher spray flows, smaller droplet diameters, and higher ambient pressures.;The nozzle spray characteristics of flow distribution depended not only upon the nozzle inlet pressure but also upon the vacuum and the water temperature. With higher vacuums and water temperatures, the spray changed from a full solid cone to a hollow-cone. Test observations corroborated data in the literature which showed that spray droplet diameters decrease with higher nozzle inlet pressures, higher water temperatures, and smaller nozzle orifice diameters.