Handbook of Turbomachinery Second Edition Revised - Ventech!

from internal cooling passages, through orifices in the blade, to provide a
protective film of lower-temperature air along the airfoil surface. Cooling air
can also be injected along the end walls. As a result, the primary flow will be
influenced by this cooling flow. Additionally, the behavior of the hot gases
from the combustor will be altered as the flow passes through each
successive blade row of the turbine.
The performance of axial turbines can be affected by many complex
flow phenomena. Therefore, an understanding of the structure and heattransfer
characteristics of the flow is of paramount importance to a
successful design. While it is impossible to examine all these characteristics
here, a representative selection of typical flow phenomena will be discussed
briefly, to provide a general impression of the complex nature of axial
turbine flow fields.
Significant contributors to loss in axial turbines are the end-wall
boundary layer and the associated secondary flow. These flow structures
have been examined by a number of investigators over the years. Langston
et al. [22] performed a detailed study of three-dimensional flow within a
subsonic plane turbine cascade, which concentrated on end-wall flow.
Sieverding [23] compiled a comprehensive review of secondary flow
phenomena in straight turbine blade passages. He described secondary
vortex flow structures and their effect on end-wall boundary layers and
losses. Sharma and Butler [24] provided additional insight based upon
further experimentation with cascades. More recently, Detemple-Laake [25]
examined end-wall flows for a transonic plane turbine cascade. Based on
their descriptions, a summary of end-wall and secondary flow characteristics
may be compiled.
In the region of the leading edge, the end-wall boundary layer
separates and rolls up into a horseshoe vortex. This vortex has two legs,
which wrap around the leading edge onto both the pressure and the suction
sides of the blade. The two legs of the horseshoe vortices entering any
passage of the cascade are counterrotating.
As the pressure-side leg of the horseshoe vortex enters the passage, it is
convected toward the suction side of the adjacent blade, due to the blade-toblade
pressure gradient. Strong end-wall flows across the passage, from the
pressure to the suction side, feed the pressure-side leg of the vortex and it
grows into the passage vortex, which is the dominant feature of the flow
field.
The suction-side leg of the horseshoe vortex is swept onto the suction
surface behind the leading edge, due to the transverse pressure gradient in
the passage. The suction-side vortex follows the contour of the airfoil
suction surface, remaining near the end wall, until the passage vortex
interacts with it, and the suction-side leg of the vortex is forced away from
Copyright © 2003 Marcel Dekker, Inc.