Unsteady Forces of Rotor Blades in Full and Partial Admission Turbines

A Numerical and experimental study of partial admission in a low reaction two-stage axial air test turbine is performed in this paper. In order to model one part load configuration, corresponding to zero flow in one of the admission arcs, the inlet was blocked at one segmental arc, at the leading edge of the first stage guide vanes. Because of the unsymmetrical geometry, the full annulus of the turbine was modeled numerically. The computational domain contained the shroud and disc cavities. The full admission turbine configuration was also modeled for reference comparisons. Computed unsteady forces of the first stage rotor blades showed cyclic change both in magnitude and direction while moving around the circumference. Unsteady forces of first stage rotor blades were plotted in frequency domain using Fourier analysis. The largest amplitudes caused by partial admission were at first and second multiples of rotational frequency due to the existence of single blockage and change in the force direction. Unsteady forces of rotating blades in a partial admission turbine could cause unexpected failures in operation; therefore knowledge about the frequency content of the unsteady force vector and the related amplitudes is vital in the design process of partial admission turbine blades. Pressure plots showed that the non-uniformity in the static pressure field decrease considerably downstream the second stage stator row, while the non-uniformity in the dynamic pressure field is still large. Numerical results between the first stage stator and rotor rows showed that the leakage flow leave the blade path down to the disc cavity in the admitted channel and re-enter into the main flow in the blocked channel. This process compensate the sudden pressure drop downstream the blockage but reduce the momentum of the main flow.