COMBUSTION CHAMBER DESIGN

COMBUSTION CHAMBER DESIGN:
The simplest combustor consists of a straight-walled duct connecting the compressor and turbine. This combustor is impractical due to the excessive pressure drop across it. The pressure drop from combustion is proportional to the square of the air velocity. Since the compressor air discharge velocity is around (170 m/s), the pressure drop will be around one-third of the pressure increase developed by the compressor. This pressure loss can be reduced to an acceptable value by installing a diffuser. Even with a diffuser, the air velocity is still high to permit stable combustion. A low-velocity region is required to anchor the flame. This is accomplished by installing a baffle (Fig. 1). An eddy region forms behind the baffle. It draws the gases in to be burned completely. This steady circulation of the flow stabilizes the flame and provides continuous ignition.



Fig.(1)Baffle added to straight-walled duct to create a flame stabilization zone


Other methods are used to stabilize the flame in the primary zone. Figures (2) and (3) illustrate two such designs. A strong vortex is created by swirl vanes around the fuel nozzle in the first design. The second design relies on formation of another flow pattern by admitting combustor air through rings of radial jets. The jet impingement at the combustor axis results in the formation of a torroidal recirculation zone that stabilizes the flame. The air velocity has a significant effect on the stabilization of the flame. Figure (4) is a general stability diagram. It illustrates the decrease in the range of burnable mixtures as velocity increases. The size of the baffle