Partial admission using Multi-channel Casing (MC) is a technique under investigation for controlling the operation of radial inflow turbines. The MC has different admission configurations according to the number of open channels which control the relation between the mass flow and expansion ratio at different operating speeds in order to provide the advantage of operational controllability. These different casing configurations will excite the structure in different manners and may cause high cycle fatigue on the rotor. Consequently, the source of excitation for a multi-channel casing is not only the casing asymmetries but also the different admission configurations.
The current study investigates the effect of different MC admission configurations on the aerodynamic excitation mechanisms of the turbine rotor blades. The Generalized Pressure (GP) and Generalized Force (GF) approach is used to compare the aerodynamic forcing field acting on the rotor blade for different admission arrangements. First, a 3D unsteady numerical simulation of a radial turbine featuring four channel casing is performed for different admission configurations to provide the raw unsteady pressure. In conjunction with this, a finite element modal analysis is applied to the turbine rotor to calculate the natural frequency and the mode shapes. Finally, the results from the unsteady simulation and the modal analysis are superimposed to calculate and compare the GP and GF.
It is found that using different MC admission configurations has a significant effect on the aerodynamic forces acting on the radial turbine featuring MC. One of the applied configurations increases the generalized force by up to 2.6 times compared to the full admission configuration, MC 4, at engine order 8. While another configuration reduces it by a factor of 0.7 compared with the same base case.