Aerodynamic load of multistage vaneless counterrotating turbine under wake/shock rotor/rotor interactions

To investigate the impact of wake/shock on the aerodynamic load distribution of downstream turbine blades in a multi-stage vaneless counter-rotating turbine, unsteady numerical simulation was utilized to analyze the influence of complex inter-stage flow on pressure fluctuation on the downstream blade surfaces. The study reveals that in the multi-stage vaneless counter-rotating turbine, all guide vanes are eliminated, resulting in suction side of the first-stage moving blade with an extended uncovered section. The extended uncovered suction side forms a converging-diverging wake flow passage resembling a Laval nozzle in interaction with the wake from adjacent blades. Under the unsteady interaction between the stages, the exit Mach number of the first-stage moving blade decreases, leading to the formation of a new compression wave (wake flow passage shock) near the trailing edge of the suction side of the first-stage moving blade superimposed on the existing suction side trailing edge shock. At specific moments within one cycle, both the reflected shock and the wake flow passage shock act on the 28.8% axial position on the suction side of the second-stage moving blade leading to a significant increase in pressure fluctuation peak value at that location. The peak value reaches 81.2% of the peak value induced by the suction side trailing edge shock sweeping, indicating that the suction side trailing edge shock is the primary factor causing pressure load variations on the blade surface of the second-stage moving blade. The main perturbation region for pressure fluctuation induced by the suction side trailing edge shock wave is the leading edge of the second-stage moving blade, with the maximum peak value of pressure fluctuation in this region within one cycle reaching 47.7% of the mean pressure. Due to the dissipation effects of the reflected shock of pressure side trailing edge shock and wake flow passage shock, the wake strength of the first-stage moving blade significantly decreases, resulting in a minor impact on the aerodynamic load distribution on the blade surface of the second-stage moving blade. Frequency analysis results indicate that due to the combined effects of the wake flow passage shock and the reflected shock of pressure side trailing edge shock, the main frequency of pressure fluctuation on the blade surface of the second-stage moving blade is twice the sweeping frequency of the suction side trailing edge shock.