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Show 3 Discussion 3.1 Transient furnace temperature changes Figures 8 and 9 show change in the transient temperature rise properties of the atmosphere and work for four speeds of rotation of the circulating fan. It can be seen from this figure that as the rotating speed of the fan increases, the temperature rise time of the atmosphere and the work was increased. To specify delay times for each fan speed, temperature rise times of the atmosphere and the work up to the target temperature (773/0 are tabulated in table 4. Although these values measured were in the same furnace, difference in the fan rotating speed caused differences in the temperature rise time, and maximum delay times were 4.68 min for the atmosphere and 5.97 min for the work. As already noted, the fuel gas supply rate was fixed. Therefore, temperature rise time delay causes an increase in the consumption of fuel gas and a decrease in furnace efficiency. In the indirectly heating furnace, effects of change of the emissivity of the radiant tube surface and the specific heat value and thermal conductivity value of the thermal insulating material with temperature rise of the furnace werw examined. However, it was difficult to set these values to targetted values due to experimental limitations, so we estimated these effects using the C F D . Three emmisivity values of e - 0.5, 0.7, 0.9 were intially testesd. These values are commonly used in industrial furnaces with indirectly heating by radiant tubes. Figure 10 shows work temperature rise properties with the three tube emissivities and experimental results. It can be seen from this figure that nearly the same temperature rise properties were predicted for each emissivity. In addition, each property agreed well with the experimental result. These finding suggest that change in emissivity does not practically affect temperature rise properties. Effects of change of the specific heat value and heat conductivity value of the thermal insulating wall of the indirectly heating furnace were also predicted using the C F D. The thermal properties of the thermal insulating wall used here are tabulated in table 5. Notably, since the temperature dependency of the specific heat value of the insulating wall was rather small, we ignore change in temperature. O n the other hand, because the thermal conductivity value may change within the range 0.05 ~ 0.4 W/m2K in these experimental conditions, we approximate it as a polynomial function of temperature. Figure 11 shows work temperature rise properties predicted with three different values of specific heat and thermal conductivities of the thermal insulating wall. A n experimental result is also shown in this figure. Comparison of the experimental result and the prediction shows that the predicted values always exceeded the experimental value. These behaviors appeard to be caused by thermal storage into insulating wall retainers installed in the thermal insulating walls. Therefore, we included these effects in the C F D code and attempted to improve the prediction. The dashed line in figure 11 shows the result, there |