Determination of heat transfer coefficient of air mist sprays

Abstract

Continuously cast copper and nickel samples were cooled using combined air and water jet. Temperatures close to the ingot surface were measured continuously during the cooling process and the coefficient of heat transfer from the ingot to the cooling water as a function of surface temperature was calculated with the aid of a suitable mathematical model. The experiments were carried out at spray water densities from 3·4 × 10−4 to 2·72 × 10−3 m3 m−2 s−l with air pressures in the range 0·2–0·4 MPa. At lower flowrate or lower air pressure, no uniformity in droplet distribution within the spray cones could be achieved. It was found that the heat transfer coefficient when spraying an air and water mixture (air mist) is higher than that obtained for water spraying, densities being equal. As the water density is increased, the difference between the results obtained using the two methods is reduced. A formula was found by which the heat transfer coefficient can be calculated as a function of the surface temperature (above 250°C), the thermophysical properties of the material, and the water flux density (above 3·4 × 10−4 m3 m−2 s−1).MST/844