Numerical Simulation of a Gas Flow in a Stage-Diffuser System in a Wide Range of Modes with Experimental Verification

Abstract

Usually at the design stage of a gas turbine unit (GTU) the nominal operating mode is considered. This mode is defined by the axial exit of the flow from the turbine, in order to reduce losses with the exhaust velocity. Axial entry into the diffuser is optimal, due to the absence of incidence angle on the support struts. Struts are the integral part of the diffuser, where the rear bearing support of the turbine rotor is located. However, the GTU operates for a long time both in the nominal and in variable modes. Partial modes are characterized by significant incidence angles on the struts, which leads to flow separation and a notable increase in pressure losses in the diffuser. Maintaining a satisfactory flow in the diffuser in a wide range of modes is an important issue. In this paper, the stage-diffuser system is studied by experimental and numerical methods in a wide range of modes. This study shows that at the last stage when the flow angle is less than 50°, the flow in the diffuser acquires an asymmetric character with the developed vortices. This means that numerical simulation in an engineering approach cannot repeat the nature of the flow at low load modes. Nevertheless, a comparison of the total parameters of the experiment and simulation shows close agreement (a difference of about 5%) even for modes with large incidence angles on the diffuser struts.