Experimental and Numerical Study of a Gas Turbine Exhaust Diffuser Applying Different Hub Extension Geometries

Exhaust diffusers are utilized to recover the residual kinetic energy of the last turbine stage. The investigated diffuser consists of an annular duct and an attached Carnot diffuser. The baseline diffuser of the investigated gas turbine features a blunt end at hub, which leads to an extensive separated flow region in the Carnot diffuser. In order to reduce the separation size and the implicated losses, four different hub extension geometries and the baseline are examined numerically and experimentally for two operating conditions. The wind tunnel tests are compared to the numerical results, which are obtained by using the CFD code TRACE. Three competing performance parameters are used for analysis: pressure recovery coefficient, total pressure loss and flow uniformity factor. The investigated hub extensions lead to a reduction of the separation zones downstream of the hubs end. This results in a better diffuser performance in terms of pressure recovery coefficient and total pressure loss of all extensions compared to the baseline. As expected, the ellipsoidal extension gives the highest pressure recovery coefficient for an infinite Carnot diffuser length. At the given length of the Carnot in this case, the Hollow Cylinder lead to the best compromise for all performance parameters. In sum, the choice of the geometry depends on the available length of the Carnot diffuser and its related costs.