A study for numerical estimation about the unsteady blade force in a partial arc admission stage of an axial turbine is presented in this paper.
Firstly, in order to investigate the influence of modeling domains in CFD (Computational Fluid Dynamics) analysis upon unsteady blade forces and turbine performance in partial admission stages, CFD analyses of three models were carried out. The analysis models consist of “2D” model; only on a mean diameter, “Simple-3D” model; 3-Dimensional passages without a tip clearance and a disc cavity, and “Full-3D” model; 3-Dimensional passages with a tip clearance and a disc cavity.
It results in clear difference in unsteady blade forces among these models. And it was revealed that numerical estimations of unsteady blade forces in partial admission stages should be carried out with analysis models that include disc cavities and tip clearances. In a partial admission stage there is extremely large circumferential pressure distribution and it causes the typical leakage flow through the gaps around a blade row. The leakage flow affects the circumferential pressure distribution that is a dominant factor in the unsteady blade forces. Air turbine experiments were conducted to validate the accuracy of the CFD analyses. Turbine efficiency of the Full-3D model with the disc cavity and the tip clearance is the closest to that of the experiment among the three models. In addition, comparison of pressure histories on a blade surface between an experiment and a calculation of the Full-3D model shows remarkably good agreements.
Next, CFD analyses with two different cascade setups (Cascade A and Cascade B) of partial admission stages were conducted in order to reveal the relationship between unsteady blade forces and cascade geometries such as a pitch of blades and a pitch of nozzles. These results show that there are clear differences in unsteady blade forces in an admission arc and those at leaving an admission arc. And some of trade-off relationships among the unsteady blade forces due to the geometries in partial admission stages are pointed out.