OCR Text |
Show FLAME STABILITY WITHIN THE POROUS MEDIA Stable porous-phase combustion means combustion is completely carried out within the porous media in a certain range of operating conditions that would allow a regular fluctuation or disturbance of the operating parameters . Transient temperature distribution within the porous bed indicates stabilization and location of the flame. Tests conducted in the present study using different types of porous materials and different burner configurations have shown that flame stabilization is strongly dependent on operating conditions (combustion rate and air/fuel ratio), material and structure of the porous bed, and the radiating surface as well. Tests performed in a single-layer ceramic foam burner showed that the flame could be held within the foam only by carefully controlling operating parameters. Either lift off or flashback would happen eventually if control was released. Flame stabilization was studied in detail in the porous burner consisting of dual- or multiple-layer porous media with different materials and configurations. Temperature profiles obtained along the flow stream at different levels of excess air, as shown in Figure 3, demonstrate that the flame can be stabilized within the porous media in a relatively large range of excess air. "Flame location is closer to the radiating surface at higher excess air because of correspondingly higher flow velocity and lower flame speed. It is interesting to note that with an increase in excess air, combustion temperature is decreased, "as expected, but the temperature on the radiating surface is just slightly decreased and the flue gas temperature is almost unchanged. The reason is due to the different locations of the combustion zone in the bed as indicated in this figure. As a resul t, radiant heat flux from the burner does not significantly depend upon excess air. In other words, the heat radiation rate or thermal efficiency of the burner-heater unit is higher at lower excess air operation. The effect of the firing rate on flame stabilization is shown in Figure 4. Data obtained at the same level of excess air show flame location in the porous bed is not sensitive to the firing rate. This is important evidence that this porous burner has good capabili ty of high turndown operation. The surface temperature and flue gas temperature are all dependent on the overall firing rate as shown in this figure. RANGE OF FLAME STABILIZATION Based on transient temperature distribution measured in the porous bed, the limits of flame stabilization at different operating and configuration conditions can be define. Figure 5 presents the effect of firing rate and excess air on combustion mode of the tested porous burner unit . The range of the stabilized porous-combustion mode is defined between the two operating limits that correspond with the outer blue flame |