| OCR Text |
Show 9 temperature becomes lower than that of unoxidized iron. Note that the same mass of aluminum and iron is assumed in this calculations; therefore, the thicknesses of the materials are different. The effect of wall material on the performance of the furnace was also examined by comparing with the results in the absence of the steel casing in the wall construction. It was found that the calculated surface temperature difference of the load is negligible (within 0.5 % ) . Furnace Efficiency. Furnace efficiencies for four different materials are presented in Fig. 9 as a function of time. The efficiency is defined as the ratio of the heat absorbed by the load to the total heat emitted from the panel heaters. It is found that higher heat conductivity can more effectively enhance the furnace efficiency than higher emissivity. This is owing to the fact that more heat can be transferred to the interior of the load with less steep temperature gradient near the surface (see Fig. 8). Temperature Deviations in the Load. The temperature distribution in the load is of major concern during heat treatment and materials processing. The effect of emissivity on the surface temperature deviation is shown in Fig. 10. The ordinate in the figure indicates the maximum temperature difference on the load surface. The temperature deviation gradually increases with the decrease in the load emissivity. This becomes evident by comparing the results in Figs. 4 and 10. In the case of oxidized iron, which has a different trend compared to the other materials, the temperature deviation rapidly increases to its peak value, then gradually decreases. The explanation for this trend is that the load with higher emissivity can absorb a greater amount of direct radiation from the panel heaters. Therefore, at early times into the heating process the temperature distribution is reflected by the panel placement and the temperature deviation increases to the peak. Then, as the wall temperature gradually approaches the panel temperature, the temperature distribution on the load surface also becomes more uniform. Effect of Panel Size and Arrangement The effect of panel size and arrangement has been examined and the results are shown in Fig. 11. The maximum temperature deviation for each panel placement is shown in Fig. 12. In these three cases the total area of the panels is the same. The results reveal that smaller panels can improve the temperature uniformity of the load. Unfortunately, the cost of the panels would also increase. Effect of Well Stirred Gas The effect of gas stirring on the furnace performance has been examined and the results are shown in Fig. 13. Gas in the furnace enclosure is assumed to be perfectly mixed (Le., using a fan), and the convective heat transfer coefficient distribution is assumed to be uniform. As a consequence, a fraction of the heat is indirectly transferred from the panel heaters and the walls to the gas, and the gas heats or cools the load. Convection contributes to the uniformity of the temperature of the load by 1 5 % (about 0.5 K ) , compared with the case in the absence of convection. Figure 14 illustrates the timewise variation of the radiative and convective heat fluxes to the load. At the beginning of heating, convective heat flux is higher than the radiative heat flux. But, as the temperature of panel heaters increases the heat flux by radiation rises rapidly. Then, finally radiation becomes the dominant mode of heat transfer in the furnace. |
