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Show produced from the part of the flow with the lower rate because of excessive local cooling. At the higher firing rate, however, the elevated flame location caused some incompleteness of combustion. The effect of combustion location on the performance of the surface combustorheater was also seen in the unit pressure drop change, as shown in Figure 8. In general, the pressure drop increases with firing rate in a parabolic relationship. In the surface combustorheater, however, because the combustion zone moved upward with an increase in firing rate that caused a reduction of the high temperature area within the porous matrix, an increase in pressure drop across the bed was somewhat restrained. As a result, pressure drop increased linearly with the firing rate within the testing range. The heat-transfer study also provided interesting results on the interaction of combustion and heat transfer. There were two types of heat-exchange surfaces installed in the porous matrix, the embedded tubes, and the grate. The heat-extraction rate in the combustorheater, defined as the rate of heat extraction by the heat load to the gross heat input, was calculated. Figure 9 shows the effect of excess air on the heat flux to the tubes and heatextraction rates by the tubes and the grate. It is seen that at very low level of excess air (below 10%), heat flux to the tubes slightly increased with excess air increase probably because of enhanced convective heat transfer from the hot gas. However, at relatively high levels of excess air (above 20%), heat flux decreased significantly because combustion moved upward. Because there was only one row of tubes installed in the preliminary tests, the moving upward of the combustion zone resulted in an increase in the exhaust gas temperature and, correspondingly, in a decrease in heat-extraction rate. Data presented in Figure 9 show that the maximum heat flux was 68 X 103 Btu/h-ft2 when the unit was operated at 65 X 103 Btu/h firing rate and 11 % excess air. The impact of the combustion location on heat transfer to the grate was even more significant. For example, the heat-extraction rate by the grate decreased from 11 % to 6% when the excess air was increased from 11 % to 21 %, as shown in Figure 9. Therefore, excess air is an important operating parameter controlling the interaction between combustion and heat transfer in the porous matrix. Other evidence indicating the interaction of combustion and heat transfer was found from analyzing the effect of firing rate on heat flux and heat-extraction rate, as shown in Figure 10. At the lower firing rate, heat flux to the tubes increased with the firing rate increase partially because convective heat transfer to the tubes was enhanced. The heat-extraction rate by the tubes remained almost constant. However, further increases in the firing rate probably caused elevation of the combustion zone, resulting in reduction of both heat flux and 17 INSTITUTE o F GAS TEe H N 0 LOG Y |