Handbook of Turbomachinery Second Edition Revised - Ventech!

lade speed at the reduced shaft speed, it is necessary to increase the turbine
diameter, which causes the blade height to decrease. The short blades cause
an increase in secondary flow losses reducing turbine efficiency. With partial
admission, the blade height can be increased, reducing secondary flow
losses. In a low-flow-rate situation, maintaining a given hub-to-tip radius
ratio results in an increase in the design shaft speed and a decrease in the
overall size of the turbine. However, manufacturing limits restrict the radial
tip clearance and blade thickness. With a small blade height, tip clearance
losses are increased. With a limitation on how thin blades can be made, it is
necessary to reduce blade count in order to keep trailing-edge blockage to a
reasonable level. Fewer blades result in longer blade chord and reduced
aspect ratio, leading to higher secondary flow losses. The taller blades
associated with partial admission can increase turbine performance. For
high-head applications a high blade speed is necessary for peak efficiency.
With shaft speed restricted by bearing and manufacturing limitations, an
increase in turbine diameter is required, resulting in a situation similar to the
no-gearbox case discussed earlier. Here, too, partial admission can result in
improved turbine efficiency.
The penalty for partial admission is two additional losses not found in
full-admission turbines. These are the pumping loss and sector loss. The
pumping loss accounts for the drag of the rotor blades as they pass through
the inactive arc, the portion of the circumference not supplied with flow
from the stator. The sector loss arises from the decrease in momentum
caused by the mixing of the stator exit flow with the relatively stagnant fluid
occupying the blade passage just as it enters the active arc. Instead of being
converted into useful shaft work, the stator exit flow is used to accelerate
this stagnant fluid up to the rotor exit velocity. An additional loss occurs at
the other end of the active arc as the blade passages leave the active zone.
Just as a blade passage is at the edge of the last active stator vane passage,
the flow into the rotor blade passage is reduced. This reduced flow has the
entire blade passage to expand into. The sudden expansion causes a loss in
momentum resulting in decreased power output from the turbine. Loss
models for partial-admission effects are not as well developed as those for
conventional, full-admission turbines. As a historical basis, Glassman [1]
presents Stodola’s [20] pumping loss model and Stenning’s [21] sector loss
model in an understandable form and discusses their use. More recently,
Macchi and Lozza [22] have compiled a number of more modern loss
models and exercised them during the design of partial-admission turbines.
The reader is referred to those sources for detailed information regarding
the estimation of partial-admission losses.
Copyright © 2003 Marcel Dekker, Inc.