THE EFFECT OF SURFACE COATING LAYER ON AIR-WATER MIST COOLING OF HOT METALS

This paper deals with transient boiling heat transfer to mist flow of air-water mixture from hot metals coated with a thin layer of low ther mal conductivity material. The test specimens used for the present exp eriment are silver and stainless steel disks whose heat transfer surfa ce is coated with a refractory paint over the thickness range, 3-38 mu m. The heated disk of 750 degrees C is plunged vertically into the mi st flow and is cooled down to the temperature of air-water mixture und er atmospheric pressure. The mass velocity of water ranges between 0.0 4 and 0.78 kg/m(2) . s. The air velocity is varied from 3 to 7 m/s. Th e coating produces a great enhancement in heat transfer, especially in transition and film boiling regions. This enhancement becomes higher with increasing coating thickness since minimum film boiling temperatu re becomes higher and an earlier transition from film to transition bo iling occurs. Heat transfer is more enhanced as mass velocity of water increases although it has a weak dependence on air velocity.