Influence of Turbulent Flow Characteristics and Coherent Vortices on the Pressure Recovery of Annular Diffusers: Part A — Experimental Results

Abstract

Exhaust diffusers downstream of turbines are used to transform the kinetic energy of the flow into static pressure. The static pressure at the turbine outlet is thus decreased by the diffuser, which in turn increases the technical work as well as the efficiency of the turbine significantly. Consequently, diffuser designs aim to achieve high pressure recovery at a wide range of operating points. Current diffuser design is based on conservative design charts, developed for laminar, uniform, axial flow. However, several previous investigations have shown that the aerodynamic loading and the pressure recovery of diffusers can be increased significantly if the turbine outflow is taken into consideration. Although it is known that the turbine outflow can reduce boundary layer separations in the diffuser, less information is available regarding the physical mechanisms that are responsible for the stabilization of the diffuser flow. An analysis using the Lumley invariance charts shows that high pressure recovery is only achieved for those operating points in which the near-shroud turbulence structure is axi-symmetric with a major radial turbulent transport component. This turbulent transport originates mainly from the wake and the tip vortices of the upstream rotor. These structures energize the boundary layer and thus suppress separation. A logarithmic function is shown that correlates empirically the pressure recovery vs. the relevant Reynolds stresses. The present results suggest that an improved prediction of diffuser performance requires modeling approaches that account for the anisotropy of turbulence.

On the Numerical Prediction of the Influence of Tip Flow on Diffuser Stability

“…By resolving the turbulent flow scales, the anisotropic character of the flow is also resolved. It has been shown that the calculated Reynolds shear stresses and the resulting pressure recovery of the annular diffuser follow the correlation presented by [8].…”

“…Furthermore, they came to the conclusion that the Reynolds stresses of the rotor outflow at the blade tip are anisotropic. Based on these findings, Kuschel et al [8] presented a correlation between the Reynolds shear stresses at the blade tip region of the rotor outflow and the pressure recovery in the two examined annular diffusers (see Fig. 1).…”

“…1). A detailed description of this rig can be found in [8] and [11]. For the experimental investigations the half-cone opening angle of the annular part was chosen to be 15 • , and a NACA0020-profiled rotor with 30 blades was used as a wake generator.…”

“…They stated that the design of efficient diffusers requires taking the rotor outflow of the upstream located turbine stage into account. STRESSES OF THE ROTOR OUTFLOW AND THE PRESSURE RECOVERY OF THE ANNULAR DIF-FUSER [8] Drechsel et al [9] performed scale-resolving simulations for a annular diffuser with a half cone opening angle of 15 • , that is susceptible to flow separation. A rotor with symmetrical shaped blade is placed at the diffuser inlet.…”

Correlation between total pressure losses of highly loaded annular diffusers and integral stage design parameters

“…Kuschel and Seume (2011) conducted additional experiments using a NACA profile instead of cylindrical spokes. A detailed analysis of these experimental results can be found in Kuschel et al (2015) and Drechsel et al (2015). They conclude that the increase in pressure recovery is caused by the interaction of tip vortices of the rotor blades with the diffuser boundary layer.…”