A fundamental study of spray evaporative cooling
Grissom, William M.
Wierum, Frederic A.
Master of Science
"Spray evaporative cooling" is defined as the mode of spray cooling heat transfer for which no liquid film would form on a heated surface of infinite extent. The heat flux during this mode is simply that required to vaporize all of the impinging spray. The lower surface temperature range limit for the existence of spray evaporative cooling is determined experimentally to be an essentially linear function of the impinging spray mass flux. This suggests a conduction-controlled droplet evaporation mechanism. An analytical model of this form gives fairly good agreement with the experimental measurements at atmospheric pressure. The effect of lowering the surrounding pressure appears to be a decreased "wettability" of the liquid (distilled water) upon the aluminum surface. This would account for the correspondingly lower droplet evaporation times observed. "Spray film cooling" is defined as the mode of spray cooling heat transfer for which a liquid film would exist upon the heated surface. An analysis of this mode is of importance in determining several characteristics of the spray evaporative cooling mode. At atmospheric pressure the mechanism governing spray film cooling appears to be quite similar to that of ordinary pool boiling with little or no dependence upon the liquid film thickness. At vacuum pressures spray film cooling appears to be governed by the simple mechanism of heat conduction through the liquid film, and very much dependent upon the liquid film thickness. The "Leidenfrost State" is defined as the mode in which impinging droplets rebound off of the surface. The initiation of the Leidenfrost state imposes: the upper range limit for the existence of spray evaporative cooling. The surface temperature at which this state is initiated is found to be very much a function of the surrounding pressure. Interestingly, this variation with pressure is such that it counteracts the variation of the lower range limit with pressure, resulting in essentially the same maximum possible heat flux during spray evaporative cooling for all surrounding pressures.