Handbook of Turbomachinery Second Edition Revised - Ventech!

influenced by the geometry of the scroll cross sections and the area variation
around the scroll circumference, in addition to the configuration of the
scroll-to-nozzle transition. The cross-sectional geometry impacted the
accumulation of low-energy flow regions, and hence contributed to the
loss distribution. In the earlier scroll sections, the low-energy flow
accumulation was controlled by the cross-flow velocity pattern. Through
later sections the flow discharge into the nozzle became the more dominant
effect, and its influence was reflected in the behavior of both through-flow
and cross-flow velocity.
Details of the flow in a radial-inflow turbine nozzle were studied by
Eroglu and Tabakoff [38].
Secondary flows in radial turbine nozzles were found to differ from
those in axial-flow turbine cascades, because of the presence of radial
pressure gradients and incidence effects. Although radial turbine nozzles
produced leading-edge vortices, either the passage vortices were too weak to
be detected or they did not form.
The upstream scroll geometry was found to strongly influence the flow
field through the nozzle blade passages, resulting in a lack of periodicity
between flow channels, and also an asymmetric distribution with respect to
the two end walls. Other effects observed included significant end-wall cross
flows just upstream of the trailing edge.
In addition, turbulence tended to increase as the flow proceeded
downstream, which contributed to rapid mixing at the exit of the nozzle
blade passages. This rapid mixing, along with high-turbulence fluctuations
downstream of the trailing edge, tended to distort or smear the wakes and
the free-stream regions, to the point that the wakes extended across more of
the flow-path cross section than the free-stream flow.
The vaneless region downstream of the nozzle was investigated by
Lakshminarasimha et al. [39]. Flow in the vaneless region was found to
directly influence nozzle losses and rotor performance, through rotor–stator
interaction.
The vaneless region consisted of a highly viscous, swirling flow field.
The passage pressure field strongly impacted the spanwise variation of both
the flow velocities and flow angles. The pitchwise variation in the velocities
was most pronounced near the midchannel, in contrast to the flow angles,
which were found to deviate more near the end walls.
Because of the difficulty of obtaining flow measurements within a
radial turbine rotor, Zangeneh-Kazemi et al. [40] combined instantaneous
measurements of shroud static pressure and exit flow distribution with
numerical predictions to infer the behavior of the flow within the rotor of a
low-speed radial-inflow turbine. The flow through the rotor was computed
using a fully three-dimensional viscous program, and the predictions of
Copyright © 2003 Marcel Dekker, Inc.