OCR Text |
Show 7 Effect of percolation flow on flame stability As shown in Fig. 5, the perforated porous burner shows high flame stability compared with the perforated non-porous burner. The reason for this difference is the percolation flow from the porous burner. It should be noted that the percolation flow has two types of effect on flame stability. One is the effect of reducing the port flow velocity because a high proportion of the total flow passes through the porous plate. The other is the stabilization of the port flames by the percolation flames which are formed on the porous parts of the burner. (1) Effect of percolation flow on port flow velocity In the case of D1-P5, the percolation flow rate accounts for 70% of the total flow, which means that the port flow rate is only 30% of the total. This implies that the port flow velocity is substantially low compared with that of the perforated non-porous burner. This port flow velocity reduction makes the port flames more stable. On the other hand, in the perforated non-porous burner, all of the flow passes through the ports and the port flow velocity is much higher than that of the perforated porous one. Thus, the percolation from the porous surface has the effect of reducing the port flow velocity and the stability of the port flames increases. For example, the combustion states of both burners at a firing rate of 10 Mw/m2 are shown in Figs. 8 and 9. In these cases, the burners were combusted in the atmosphere but not in a water-cooled furnace. For the non-porous plate, its periphery is lifted up near the bumer edge and the flame length is longer than from the porous burner. The flames are clearly much more stable in the perforated porous burner. (2) Effect of percolation flames on POrt flame stability To identify whether the percolation flames playa role in stabilizing the port flames, the combustion states of the two burners were compared when both burners have the same port flow velocity. These two states are shown in Figs. 8 and 10. Figure 8 shows the flames of the perforated porous burner operating at the firing rate of 10 Mw/m2. Since the port flow rate is 30% of the total flow in the porous bumer, operating the perforated nonporous burner at a firing rate of 3 Mw/m2 produces the same port velocity as that in Fig. 8. In the case of a non-porous plate, its periphery is lifted up near the burner edge compared with the porous burner. In a perforated porous burner, however, combustion is more stable even though the port flow velocity is the same. These results demonstrate the percolation flames help to stabilize the port flames. |