The Enhancement of Mist Cooling of Hot Metals by a Coating Material

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This paper deals with transient boiling heat transfer to mist flow of air-water mixture from hot metals coated with a thin layr of insulating (low thermal conductivity) material. The test specimens selected for the present experiment are silver and stainless steel disks whose heat transfer surface is coated with a refractory paint. The heated disk is plunged vartically into the mist flow and is cooled down to the saturation temperature of water under atmospheric pressure.In order to measure the temperature of the metal disk, two Chromel-Alumel sheathed thermocouples are buried with solder into holes, which are within 2.0 mm and 8.0 mm, respectively, of the surface in contact with flow. An electric furnace is used to heat the test disk to a desired temperature of approximately 750ﾟC.The heated disk is plunged vertically into mist flow and is cooled down to the saturation temperature of water under atmospheric pressure. The output signals from the thermocouples are recorded by a personal computer through an A/D converter. During the runs the subcooling of mist is maintained at 82K under atmospheric pressure.The experimental conditions are as follows :Coating thickness : 3-38mumAverage diameter of droplet : 27-66mumMass velocity of water : 0.04-0.78kg/m^2sLinear velocity of air : 3-7m/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. Heat transfer is more enhanced as mass velocity of water increases although it has a weak dependency on linear velocity of air.