Handbook of Turbomachinery Second Edition Revised - Ventech!

approximately 35% each. Although the turbine efficiency is quoted at
approximately 78%, there is evidence from various cold-flow component
tests that the actual turbine aerodynamic efficiency is in fact in the mid-80s
percent range. The observed efficiency penalty is primarily due to leakage
flow through various seals and turbine coolant paths. These are accepted for
the sake of improved packaging. Also, the lack of a turbine outlet pressure
measurement in the flight hardware configuration makes determination of
actual turbine performance very difficult and subject to certain inferences.
In the preceding discussions, it has been assumed that the turbine
working fluid enters the rotating blade row around its entire circumference.
With the bulk of rocket engine turbines, this is certainly the case. However,
some low-power systems may have unique requirements. If the turbine
power required is low enough, and the turbine working fluid specific energy
is high enough in comparison, then the working fluid flow rate required
becomes ‘‘small.’’ When this happens, the rotor annulus area becomes small,
as determined by the equation of continuity. As a result, turbine blade
heights can become quite small. When this occurs, the wetted areas of the
hub and tip end walls can increase to the extent that boundary-layer losses
on these end walls become a large fraction of total blade row losses. Also,
fabrication limitations make it impossible to reduce blade tip-to-housing
clearances below a certain absolute value. When blade tip clearances become
a significant fraction of the blade height, excessive blade tip leakage can
result. These two phenomena can bring about large decreases in turbine
performance. In order to keep blade heights reasonable, the turbine inlet
manifold can be arranged so as to limit the entry of working fluid to only a
portion of the rotor circumference. This portion of the rotor into which
drive fluid is entering is known as the arc of admission. A turbine operating
with an arc of admission of less than 3608 is known as a partial-admission
turbine. It is the task of the turbine designer to adjust both the arc of
admission and the rotor diameter such that the best compromise of all
design factors is obtained, resulting in minimum losses and maximum
efficiency. The design factors we refer to here include normal loss
mechanisms, as well as some losses unique to the partial-admission scheme.
We attempt to touch on the highlights of these partial-admission unique
losses.
It is this author’s impression that the losses unique to partialadmission
turbines are not quantified particularly thoroughly in the
literature, undoubtedly due to the complexity of the mechanisms involved.
However, for the purposes of this discussion, these losses can be
approximated as falling into two main categories. The first of these is
what I refer to as the fill and drain loss. When two adjacent turbine blades
are moving through an inactive portion of the nozzle arc, the intended
Copyright © 2003 Marcel Dekker, Inc.