The effect of outlet blade angle at the mean root diameter on the mixed inflow turbine

In turbomachinery, turbine blades are subjected to the most severe loads. High exhaust gas temperatures, significant temperature gradients during startup, substantial stresses from centrifugal forces, and complex three-dimensional geometries all contribute to blade damage. The geometry of the blades has a significant impact on turbine efficiency. Therefore, the three-dimensional shape of the blade warrants careful attention and thorough investigation. This study investigates the outlet blade angle effect on the mixed inflow turbine performances at the mean diameter under maximum load conditions. The other geometric parameters were kept constant to maintain the same rotor housing. The analysis also evaluates energy losses at different exit angles. Angles below -30° were considered to preserve the continuity of the camber line and the leading-edge geometry. A numerical model based on Bezier polynomials was developed to accurately define the blade profile and the distribution of exit angles. The flow within the inter-blade channels was simulated using three-dimensional Computational Fluid Dynamics (CFD), solving the Navier-Stokes equations for a viscous flow. It was found that turbine efficiency and blade geometry changed significantly with variations in the outlet blade angle. The optimal turbine performance occurred at -65°, resulting in an overall increase in static isentropic efficiency of 2.83%, operating efficiency of 2.85%, and a mass flow reduction of 4.77%. However, the blade size increased by 5.67%. It was also shown that for angles greater than -50°, a diffuser is required to recover the remaining kinetic energy.