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Using CFD the computational investigation of two axisymmetric and axiradial cones (diffusers) typical for the exhaust branch pipes of steam turbines was carried out in the wide range of the pressure variation at the entrance of the cones with available tip-clearance leakage. As the pressure decreased the Mach number at the cone entrance increased. It has been established that in subsonic modes the cone with continuous external by-pass without the circular blade had the flow without separation and considerably lower values of loss coefficients in comparison with the diffuser that has a projection of the internal by-pass and the circular blade. At trans-sonic rates the coefficients of total losses of both studied cones are increased reaching the values that exceed 1 and have similar values; however the coefficients of internal losses and exit velocity losses differ significantly. The obtained results allow us to extend somewhat the database for the designers that are involved in the solution of problems related to the design of outlet branch pipes for flow rates approaching the sound velocity.

Comparative design investigations of axiannular divergent cones were carried out using the CFD software. Turbulence models built-in into the CFD–software were used for the computations, in particular Spalart-Allmaras (S-A), k–w Shear Stress Transport (k–w SST), V2F. The aperture angle of the external by-pass of cones varied in the range of a = 11°–36°, while the cone expansion degree varied as n = 2–4. The data of CFD computations were compared with the known experimental curve obtained by Klein and confirmed by Zariankin, which divides the plane (a–n) into two regions: unseparated flow and separated flow regions. The region above this curve represents the separated flow in the cone at any combination of parameters (a–n), and the region below this curve represents the unseparated flow. The computations done using the CFD-software for axiannular divergent cones in the studied range of the relationships of geometric parameters (a–n) showed that the use of k–ω SST and V2F models results in the flow separation in the cones situated in the unseparated flow region, which runs counter to the experimental data, and the use of the turbulence model S-A provided the unseparable flow in the region below the experimental Klein curve and resulted in the flow separation in the region below this curve. Design investigations showed that the turbulence model (Spalart-Allmaras (S-A)) provides an opportunity for the satisfactory simulation of unseparable and separable flows in the cones of this type.

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.