OCR Text |
Show slightly more complicated, since a burner sleeve is n o w required to divert the combustion air away from the wall and through the vanes. The simulation results shown in Figure 3 represented this swirl vane position. The swirl vane angle and flow area were limited by pressure drop. The power required for running the fan and pump is parasitic and, therefore, must be limited. The fan power is proportional to pressure drop. It was experimentally observed that burner pressure differential around 5 inches W C was sufficient to generate good mixing in the current design. A pressure differential as high as 10 inches W C is acceptable, since there are commercially available fans that can produce this static pressure at the required operating flow with low power consumption. Partial Rapid Premix. Premixing refers to the mixing of fuel and air upstream of the ignition zone, and promotes a short compact flame with rapid heat release near the flame by eliminating mixing limitations. However, premixing must be achieved without premature ignition or heat release. In earlier iterations on the burner design, the fuel was injected into the air downstream of the swirl vanes. Partial premixing occurred in a short slot prior to ignition. The slot also protected the metallic vanes from direct exposure to the hot flame zone. In latter designs, the fuel was introduced into the air just upstream of the swirl vanes. In this case, the improved premixing yielded immediate ignition at the burner face. The high fluid velocity upstream of the burner prevented premature ignition. The visible flame length was about 1 inch (Figure 6) with the improved premixing. Figure 6: Fuel introduced into the air downstream of the swirl vanes (left), and upstream of the swirl vanes (right) |