OCR Text |
Show A water-cooled probe was used to sample the combustion products. The probe at the cold end was equipped with a section of externally heated stainless steel tubing to maintain a sample temperature of about 1900 F so that moisture would not condense. The warm sample was passed through a millipore filter to remove any solid particles and a Permapure dryer to remove moisture. The dried sample was supplied to oxygen (02), carbon monoxide (CO), and nitric oxide (NO and NOx ) analyzers through Teflon tubing. A suction pyrometer was installed in the exhaust plenum to measure flue gas temperatures for the overall thermal balancing. A surface temperature probe was employed to measure temperatures on the unit's outer surface for estimating heat loss from the system. In addition to the regular measurements described above, six fine thermocouple probes were embedded in different depths of the porous bed to diagnose combustion within the porous media. Combustion stabilization can be monitored by continuously recording the temperature distribution measured by these thermocouples. A moveable thermocouple .probe installed at a rig was employed to measure vertical temperature distribution in the exhaust gas flow above the radiating surface, as shown in Figure 2. By extrapolating temperature distribution measured along the flow stream above and below the burner surface, the temperature at the radiating surface can be determined. Radiant heat flux from the surface to an open environment can be wellestimated based on its temperature and emissivity. EXPERIMENTAL RESULTS START-UP Fuel-lean gas/air mixture is ignited in the gas-phase above the burner surface. Blue flames can be held on the porous radiating surface. By reducing excess air, the flame is gradually regressed toward the surface. As a result, the surface temperature is continuously increased , while the flue gas temperature is decreased due to an increase in radiant heat loss from the surface. However, the surface temperature is always below the flue gas temperature in this surface-combustion mode because the surface is heated by heat conduction and radiation from the downstream flame. Further reducing excess air could draw the flame back in the porous bed. The surface-combustion mode is thereafter changed to the porouscombustion mode. Certain evidence of change in the combustion mode is that the temperature on the surface is higher than that in the gas phase above the surface because the surface is heated by heat convection from the upstream combustion gases and heat conduction from the lower part of the porous bed, whereas, the flue gas leaving the surface is cooled by the radiant heat loss from the surface. Establishment of the porous-combustion mode is dependent upon the operating conditions and structure of the radiating surface. |