The Effect of Surface Coating Layer on Air-Water Mist Cooling of Hot Metals.

AbstractThis paper deals with transient boiling heat transfer to mist flow of air-water mixture from hot metals coated with a thin layer of low thermal conductivity material. The test specimens used for the present experiment are silver and stainless steel disks whose heat transfer surface is coated with a refractory paint over the thickness range, 3–38μm. The heated disk of 750°C is plunged vertically into the mist flow and is cooled down to the temperature of air-water mixture under atmospheric pressure. The mass velocity of water ranges between 0.04 and 0.78kg/m2.s. The air velocity is varied from 3 to 7m/s. The 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 temperature becomes higher and an earlier transition from film to transition boiling occurs. Heat transfer is more enhanced as mass velocity of water increases although it has a weak dependence on air velocity.KEYWORDS: boiling heat transfermist flowwater-air mixturehot metalcoating materialheat transfer enhancementmaximum heat fluxminimum heat flux