OCR Text |
Show heat transfer rate increases slightly. The fraction of the radiative heat transfer rate to the total heat transfer rate is about 1 3 % at maximum. Convective heat transfer is thus the dominant mode of heat transfer in this direct heating furnace. In previous studies estimating convective heat transfer rate in industrial furnaces, the temperature dependencies of this value have not been evaluated in detail. It thus must be determined whether the convective heat transfer coefficient on the work surface exhibits temperature dependency. Figure 18 shows changes in the average convective heat transfer coefficient on the work surface with atmospheric temperature. In this figure, an applicable work Reynolds number range is shown. Here the work Reynolds number is determined by the work length scale, kinematic viscosity and area mean velocity of the cross-section of the working space in the muffle. These convective heat transfer coefficients are calculated from revised heat transfer rate data obtained by subtracting the radiant heat transfer rate from the total heat transfer rate. It is evident from figure 18 that the convective heat transfer coefficient in the direct heating furnace doesn't change with atmospheric temperature, and that its value is A0W/m2K. Convective heat transfer rates in industrial furnaces hav been estimated using the naphthalene sublimation method in several cold models. For example, Sousa and Tucker[3] mentioned that using the turbulent heat transfer coefficient in fully developed pipe flow is insufficient for estimating the convective heat transfer rate in industrial furnaces, instead, they measured the coefficient in a 1:8.8 scale cold model of a direct heating bogie-hearth production furnace. The result of their experiment showed that the convective heat transfer coefficient of the work set on the hearth was 37.0 W/m2K. This value is almost the same as that estimated in our study. The value estimated in our study is thus valid sufficient for use in estimating convective heat transfer rates in industrial furnaces, and the effect of atmosphere circulation rate on this value is very weak within the range of our experiments. O n the other hand, the convective heat transfer rate in the indirect heating furnace was also estimated from predicted temperature rise properties of the work. Although the radiant heat transfer is the dominant mode of heat transfer in the indirect heating furnace, the convective heat transfer coefficient was of the same order, 3bW/m2K as that for the direct gas-fired furnace over the entire temperature range. Thus, even in furnaces in which flow is dominated by buoyancy, i.e. in which flow in the working space is very weak, the convective heat transfer coefficient in the direct heating furnace can be used for estimation in the same manner as in the furnaces in which flow is dominated by convection. 4 Conclusion Unsteady heat transfer phenomena were investigated during transient heating process using a directly gas-fired furnace with a circulating fan whose m a x i m u m temperature was 773K and heat input was 27'kW', a indirectrly gas-fired furnace with radiant tubes of |