Detailed Measurements on a Modern Combustor Dump Diffuser System

An experimental investigation has been carried out to determine the flow characteristics and aerodynamic performance of a modern gas turbine combustor dump diffuser. The system comprised a straight walled prediffuser, of area ratio 1.35, which projected into a dump cavity where the flow divided to pass either into the flame tube or surrounding feed annuli. In addition, a limited amount of air was removed to simulate flow used for turbine cooling. The flame tube was relatively deep, having a radial depth 5.5 times that of the passage height at prediffuser inlet, and incorporated burner feed arms, cowl head porosity, cooling rings, and primary ports. Representative inlet conditions to the diffuser system were generated by a single-stage axial flow compressor. Results are presented for the datum configuration, and for a further three geometries in which the distance between prediffuser exit and the head of the flame tube (i.e., dump gap) was reduced. Relatively high values of stagnation pressure loss were indicated, with further significant increases occurring at smaller dump gaps. These high losses, which suggest a correlation with other published data, are due to the relatively deep flame tube and short diffuser length. Furthermore, the results also focus attention on how the presence of a small degree of diffuser inlet swirl, typical of that which may be found within a gas turbine engine, can result in large swirl angles being generated farther downstream around the flame tube. This is particularly true for flow passing to the inner annulus.