ABSTRAKT

Investigation of the melting behavior of solid particles such as iron ore, coal and so on immersed in a hot metal bath subject to gas injection is of practical importance for the inbath smelting reduction process [1]. This is also useful for predicting the melting of scraps in electrical resistance furnaces [2]. Fluid flows in the baths of these processes are turbulent under operating conditions and the melting behavior is controlled mainly by forced convection heat transfer between the particles and the ambient fluid accompanying bubbles. Since great amount of gas is injected to enhance the efficiency of the processes as much as possible, the local turbulence intensity in the bath, Tu, is usually very high (Tu≥ 40%) [3]. Therefore, the effect of turbulence intensity should be considered for the correlation of heat transfer data. Previously published data for forced convection heat transfer from a solid body were obtained for single-phase flows with turbulence intensity less than about 20% [4, 5]. Very few heat transfer data for gas-liquid two-phase flows are available at present.
Since it is very difficult to make heat transfer measurements under realistic high temperature conditions, so-called "cold model experiments" were made here. A vertical bubbling jet was generated in a cylindrical water bath by blowing air into the bath through a single-hole centric nozzle. A sphere made of ice was immersed into the bubbling jet and its melting behavior was observed with a high-speed video camera. The local heat transfer coefficient between the water and the ice sphere was calculated from an decrease in local radius. The decreasing rate was determined using image processing technique. The mean heat transfer coefficient was obtained by averaging the local heat transfer coefficient over the surface of the sphere. A correlation for the mean heat transfer coefficient was proposed as a function of Reynolds number, Prandtl number, and turbulence intensity.