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

本文研究了包覆薄层绝缘（低导热）材料的热金属对空气-水混合物雾流的瞬态沸腾传热。本实验选用的试样为银盘和不锈钢盘，其传热表面涂有耐火漆。加热后的圆盘垂直投入雾流中，在大气压下冷却到水的饱和温度。为了测量金属盘的温度，将两个镀铬铝mel护套热电偶与焊料一起埋入与流体接触表面分别在2.0 mm和8.0 mm范围内的孔中。电炉用于将测试盘加热到所需的温度约750ºC。加热后的圆盘垂直投入雾流中，在大气压下冷却到水的饱和温度。热电偶的输出信号通过a /D转换器由个人计算机记录。在运行期间，雾的过冷度在大气压力下保持在82K。实验条件为：涂层厚度：3- 38mm水滴平均直径：27- 66mm水质量速度：0.04-0.78kg/m^2s空气线速度：3-7m/s。涂层在传热方面有很大的增强，特别是在过渡区和膜沸腾区。这种增强随着涂层厚度的增加而增强。传热随着水的质量速度的增加而增强，尽管它对空气的线速度有较弱的依赖性。

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.