Handbook of Turbomachinery Second Edition Revised - Ventech!

Further downstream, the wake was even more skewed to the radial
direction. Near the shroud, the wake was deformed by the shroud passage
vortex. Also present within the wake were other vortices associated with
both trailing-edge shed vorticity and vorticity remaining from the suctionside
leg of the horseshoe vortex. Both the mixing of the wake and the radialinward
flow of the low-energy fluid tended to reduce losses in the wake. By
approximately 3.3 chord lengths downstream, the discrete regions of high
loss had been mixed out, and the wake itself was no longer visible.
Shocks and shock–boundary-layer interaction also represent major
sources of loss in a transonic axial turbine flow field. Graham and Kost [28]
and Detemple-Laake [25] examined the interactions between shocks and
boundary-layer flows for high-turning transonic turbine cascades, over a
range of exit Mach numbers. A summary of their findings provides some
insight into the complexity of shock–boundary-layer interactions.
For subsonic exit Mach numbers, the passage flow accelerates rapidly
along the suction surface, expanding down to the throat, with local regions
of supersonic flow. These supersonic regions terminate downstream at a
normal shock. As the exit Mach number increases through sonic conditions,
the cascade becomes choked and the flow expands supersonically downstream
of the throat, producing a strong normal shock at the trailing edge.
With further increases in exit Mach number, this trailing-edge shock
structure becomes stronger and more oblique, eventually producing a
system of deflected and reflected shocks. The suction side of the trailing-edge
shock is deflected by the adjacent blade wake. The pressure-side branch of
the trailing-edge shock crosses the passage to impinge on the suction surface
of the adjacent blade. This impinging shock is then reflected away from the
suction surface, as a sequence of compression–expansion–compression
waves.
Depending on the strength of the trailing-edge shock, the pressure
increase across the branch of the shock impinging on the suction surface
may cause the laminar suction surface boundary layer to be lifted,
developing a separation bubble. As exit Mach numbers are increased into
the supersonic range, the trailing-edge shock structure becomes more
oblique, such that both the impingement point and the boundary-layer
separation move along the suction surface toward the trailing edge. At
reattachment, the boundary layer becomes turbulent.
Unsteady flow and blade-row interaction effects can strongly influence
the structure of the secondary flow field in axial turbines. Zeschky and
Gallus [29] examined the effect of upstream stator wakes on the flow
through a subsonic axial turbine rotor. They found that the rotor flow was
significantly influenced by the periodically unsteady inlet conditions caused
by the stator.
Copyright © 2003 Marcel Dekker, Inc.