OCR Text |
Show the data were acquired by the computer system at a certain firing rate, while excess air was controlled at different levels. The experimental results are presented as follows. Effect of Excess Air Because air/fuel stoichiometric ratio is the key parameter to govern combustion stability in the porous matrix, all tests were conducted at a fixed firing rate, but with variable excess air. Although direct monitoring for flame stabilization was not performed, flame stability can be evaluated based on heat-transfer characteristics. To maximize the overall thermal efficiency of the combustor-heater, the flame is desired to be stabilized between the grate and the first row of tubes. Heat transfer to each row of tubes as well as gas temperature measured within the bed can indicate a tendency for flame movement. The effect of excess air on the heat-transfer rate to the grate and each row of tubes at a 4S-kW firing rate is shown in Figure 3. The highest heat-transfer rate occurs at the first row of tubes over most of the range of excess air, while the lowest heat-transfer rate occurs at the grate. However, as excess air increases, the heat-transfer rates to the grate and the first row of tubes are decreased, while the rates are increased for the second row of tubes. That indicates move up of the flame with an increase in excess air. At higher excess air, for instance, 40% in this typical condition, heat extraction by the first row of tubes is dramatically decreased because the flame is almost moved away from that row of tubes. Therefore, the range of operating parameters for stable flame that is desired below the first row of tubes can be defined, depending upon the configuration of the combustor-heater and the bed material. ~O~------------------------------------~ N E JOO i-. ,- o 0:: 200 -..~,, c f ~ c; 100 C) :I: 0... ... o GRATE • ROW No.1 • ROW No.2 • OVERAU.. ...... ......... ............ .0.. ... ...... f1RJNG RATE: 45 kW ...... ... cr ......... _ .... _-....... - ...... 4 O~--__ --__ --~--~--~--~--__ --~--~~ o 50 Excess Ak. % Figure 3. EFFECT OF EXCESS AIR ON HEAT-TRANSFER RATES The data shown in Figure 3 also show that a heat-transfer rate as high as 300 kW/m2 was achieved for the first row of tubes. The overall average heat-transfer rate for this configuration of the water heater is 215 kW/m2 at low excess air operation. 6 |