Influence of the Above-Shroud Leakage on the Aerodynamics of Exit Cones of the High Pressure Cylinders of Steam Turbines

The leakage above the rotor wheel in the last turbine stage can positively influence the characteristics of the diffuser acting by analogy to the special inflation of the flow into the boundary layer in the external bypass of the diffuser. Contemporary structures of high pressure cylinders are equipped with axial-radial diffuser or diffuser installed in the exhaust pipe branch; therefore the problem of influence of the above-shroud leakage in the last stage on the aerodynamics of the exit diffuser of high pressure cylinders is rather vital. Computing investigations were carried out using CFD of the axisymmetric axial-radial diffuser peculiar for the exhaust pipe branches of high pressure cylinders of steam turbines in a wide range of pressure variations that define a value of the above-shroud leakage in the rotor wheel of the last stage. The flow behavior in the diffuser, the coefficients of losses and the pressure distribution on the diffuser cowling have been studied. It has been established that at the minimum values of leakage, which is peculiar for the seals of a cellular structure the flow in the diffuser is detachable and a coefficient of full losses has maximum values and at an increased above-shroud leakage that occurs in the traditional seals of a labyrinth type the point of flow separation from the cowling shifts towards the exit section lowering considerably the coefficient of full losses.