Numerical Studies on the Effect of Purge Flow on the Performance and Flow Field of a Gas Turbine with Disc Cavity

Abstract

An axial gas turbine stage with disc cavity was numerically investigated to assess the effect of purge flow on cooling of turbine disc and rotor blades, and on turbine overall performance. Steady state flow analysis was carried out using ANSYS FLUENT software without purge flow, and with three purge flow rates of 7, 6.5 and 5.75 % of the primary turbine design mass flow rate. The purge flow from disc cavity, driven by the pressure differential, combines with and affects the main annulus flow. At any purge flow rate, the turbine efficiency continuously drops with increasing pressure ratio owing to reduced cooling effect. The purge flow causes compression of the primary gas flow, resulting in higher static pressure and temperature above mid-span level of the rotor blade compared to the case without purge flow. However, the rotor blade temperature is reduced in the hub region where the stresses are large. The cooling is found to be more pronounced on the suction surface of the rotor blade than on the pressure surface. Flow through the disc cavity shows complex behaviour with dominant recirculation zones. It is concluded that the turbine efficiency as well as the blade surface temperature distribution is affected by purge flow and that the overall turbine performance is sensitive to both modeling assumptions and cooling flow rate.