Flow and performance characteristics of an Allison 250 gas turbine S-shaped diffuser: Effects of geometry variations

The S-shaped diffuser which connects the exit of the compressor to the inlet of the combustion chamber of the Allison 250 gas turbine has been investigated using the Shear-Stress Transport turbulence model (SST) and the commercial code ANSYS-CFX. The diffuser geometry includes an initial conical diffuser which smoothly transitions into a constant cross-section S-duct. The numerical model and setup were validated using both in-house processed experimental data and experimental data from the literature on a similar geometry. The stream-wise velocity profile was observed to flatten in the initial divergent section, and then the region of the flow with the highest velocity is pushed toward the outer surface of the first bend, with a secondary-flow in the plane of the cross-section. This distortion of the stream-wise velocity intensified when the inlet turbulence intensity was decreased or when the Reynolds number was increased. An increase of the Reynolds number also translated into higher static pressure recovery potential and lower wall friction coefficients. Six variations of the diffuser geometry were considered, all having the same total cross-sectional area ratio and centreline offset. The qualitative results were the same as those of the Allison 250 diffuser, but unlike the base geometry, all the considered variants showed separated-flow regions (and reversed-flow regions in some cases) of different sizes and at different locations. The performance indicators for the Allison 250 S-shaped diffuser were the highest overall. Most interestingly, the current duct geometry outperformed its variant with a cross-sectional area expansion extending over its entire length, which is the most common inlet duct configuration.