| The air flow in a Rolls Royce/Allison 250 gas turbine diffuser tube was investigated experimentally and numerically. The diffuser tube consists of a conical diffuser section which transitions into an S-shaped constant diameter tube. Velocity profiles were measured using a Pitot-static tube at three measurement planes on the one hand, and the flow was also simulated using the Reynolds-Averaged Navier-Stokes approach and the Shear Stress Transport turbulence model in ANSYS CFX 12.1 on the other hand. A long, constant diameter tube was used upstream the diffuser tube to generate a fully-developed velocity profile at the inlet of the component. The numerical predictions agreed with the experimentally measured data excellently. The sensitivity of the flow and aerodynamic performances to the inlet turbulence level and Reynolds number was also investigated, showing that the pressure recovery consistently improved when either of the two parameters was increased. The sensitivity to the diffuser geometry was also investigated by considering several diffuser tubes with the same total length, total area ratio and offset, but presenting different area expansion locations along the component. Separated flow regions appeared in most cases, and a wide range of performance potentials was predicted.;The flow in the Rolls Royce/Allison 250 diffuser tube was also simulated by prescribing the velocity distribution actually measured at the exit of the compressor to the inlet plane of the diffuser. Separated flow regions were observed together with a pronounced non-uniformity of the velocity profile at the exit plane. However, the pressure recovery potential and the total pressure loss coefficient were somewhat similar to those predicted when using a fully developed flow condition at the inlet of the diffuser. |
