The traditional turbomachinery design systems are always based on the assumption of steady or quasi-steady flows. However, unsteady flows such as wake flow, separated flow and shedding vortices are the main factors inducing the excitation force on turbine blade which leads to high cycle fatigue failure of blade. In this paper, the three-dimensional, time dependent, Reynolds-Averaged Navier-Stokes (RANS) equations were resolved using a commercial program CFX based on finite volume method. The unsteady flow fields of three mass flow cases (design case, 110% design mass flow and 85% design mass flow) in a one-and-a-half stage axial turbine (stator/rotor/stator) were investigated in detail and then the unsteady aerodynamic force on the rotational blade was obtained. Frequencies of unsteady disturbances and excitation force factors were obtained by spectrum analysis. It can be seen clearly that the excitation factors at 110% mass flow case are larger than that at the design case. On the other side, the unsteady aerodynamic force on the rotational blade at 85% mass flow case is quite different from the design case. There are two peaks during a stator passing period and the dominate frequency of the tangential blade force is 6000Hz due to large amount of negative incidence angle. The 6000Hz component tangential aerodynamic force amplitude is 6.533N, which is 5.93 times of that at design case and 2.92 times of that at 110% mass flow case. Because of the large amplitude, the unsteady aerodynamic force at small mass flow case is necessary to be taken into account in the forced vibration analysis of blade.

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