Numerical Investigations of Secondary Flow around Blades with Adjacent Endwalls by Variations of the Inlet Boundary Conditions

Abstract

The objective of this thesis was to numerically investigate conditions which provoke secondary flow and related phenomena at the hub of compressor blades with adjacent endwalls. Theoretical studies have been carried out with idealizations of the actual flow conditions in order to evaluate their impact in comparison to the actual condi- tions. The idealizations were designed regarding the influence of the viscous endwall, of the clearance flow and the clearance vortex as well as the generally inhomogeneous inlet boundary conditions at the hub of the rotating blade. As a result of these investigations, a waved contour was implemented on the upstream cantilevered stator in order to influence the inlet boundary conditions. The last rotor row of a four-stage research compressor, the Rig250, of the German Aerospace Center (DLR) was the subject of the investigations. The Rig250 is a state-of- the-art compressor and has been the subject of several research projects which allows to assess the theoretical potentials presented in this thesis. The operating point was chosen from a numerical speed line calculation and is in between the aerodynamic design point and the surge limit of the rotor at design speed of the Rig250. The numerical simulationswere performedwith theDLR in-house flowsolver TRACE. For the investigations multi-block structured meshes were used and only those passages relevant were simulated, the inlet and outlet boundary conditions are extracted from the simulation of the full rig. All simulations performed were steady state calculations using mixing-planes which set the radially averaged outlet conditions as inlet boundary conditions to the downstream row. Finally the results of the numerical simulations were analyzed. The idealizations included a simulation with an inviscid endwall, a simulation without a hub clearance gap of the upstream stator as well as variations of the inlet boundary conditions total pressure, total temperature, flow angles and turbulence to the rotor. The results show that the losses related to secondary flow can be suppressed by the idealizations and almost only profile losses remain. In addition it is shown that the different idealizations can add up as their beneficial effects can be related to the suppression or reduction of specific secondary flow phenomena. It was found that the skew of the inflow angle and the reduced total pressure as inlet boundary conditions are particularly important. By introducing a wave-like contour at the hub of the upstream cantilevered stator it was attempted to influence those parameters. Although the flow analysis indicated a positive impact on the formation of the clearance vortex the expected results on the outflow conditions could not be achieved. It is assumed that the reason for this is the small clearance size of the stator.