STUDY ON BOILING HEAT TRANSFER AND CRITICAL HEAT FLUX IN MIST COOLING (EFFECT OF DROPLET SIZE ON HEAT TRANSFER CHARACTERISTICS)

摘要

Boiling heat transfer and critical heat flux-CHF - in mist cooling were investigated experimentally and analytically. Especially, the heat transfer in the mist cooling was examined focusing on the effects of droplet size and droplet velocity on the heat transfer characteristics. Steady state experiments of heat transfer were conducted using a pure copper cylinder and mist flow of water-air at room temperature. Liquid flow rate was 0.3, 0.9, 1.8, 4 and 8 l/hr, respectively; each air flow rate on normal condition was 0, 40, 75 and 120 l_N/min. Furthermore, liquid mass flux on the heater surface for each experimental condition was measured by using a cylinder with a scale and the same diameter as the heater. Distribution of air velocity, average velocity of droplets and average diameter of droplets were measured by using a fine Pilot tube, laser doppler anemometry and immersion method, respectively. Three correlations of the mist cooling rate for non-boiling, evaporation of droplets and evaporation of the liquid film were developed by using the measured liquid mass flux, characteristic droplet velocity and wall superheat. A CHF model was presented by focusing on maximum evaporation rate of the liquid mass flux on a heater. A droplet evaporation model was proposed by using the transient heat conduction in a sphere. Finally, three dimensionless correlations for the mist cooling were presented.