Handbook of Turbomachinery Second Edition Revised - Ventech!

and pump flow derived from the engine balance, the pump specific speed can
be forced to take on a range of ‘‘reasonable’’ values by solving Eq. (1) for
the rotor speed, N. This ‘‘reasonable’’ range of specific speed might vary
from 700 to 2,000, in rpm-gpm-ft units.
In a typical application using a dense propellant (i.e., a propellant other
than liquid hydrogen), the low end of the examined Ns range will bring about
relatively low efficiency, relatively low operating speed, and relatively relaxed
suction-performance requirements, as denoted by a relatively low value of
suction specific speed [computable using Eq. (12)]. Due to the low operating
speed, the impeller tip diameter will be relatively large (to generate a fixed
amount of total head), and the impeller exit axial width (the so-called ‘‘b2’’
dimension) will be relatively small to maintain a fixed impeller outlet flow area.
Although challenges regarding suction performance, rotor dynamics,
seal durability, etc. can be alleviated by operating at reduced values of Ns,
pumpweightisadverselyaffected.ReferringtoFig.15,wecannotethatasthe
pump-stage specific speed is decreased, the increasing impeller outside
diameter causes the discharge volute to occupy ever-increasing radii (with
respect to the pump’s rotational axis). This results in the volute’s
circumference increasing in proportion to its radius. Since the discharge
volute must contain the pump’s total discharge pressure, the volute’s wall
thickness (and thus its weight per linear in.) tends to be the greatest of the
pump’s components. The end result of the decreasing specific speed trend is a
propellant pump whose weight quickly exceeds the allowable. In addition, for
closed-cycle engines (i.e., expander and staged combustion cycles), especially
‘‘small’’ ones whose propellant flow rates are relatively low, pump specific
speed values below approximately 500–700 Ns units may perform at isentropic
efficiencies that are below values that may be required by the engine cycles.
Examination of an increasing value of pump specific speed will show
the following trends—if we refer to Fig. 15(d) we see that increasing pump
Ns will yield an essentially monotonic (though not linear) trend of increasing
efficiency. This is due primarily to the progressively more favorable flow
passage geometry that increasing N s provides.
The increasing values of pump Ns also result in decreased impeller
diameter due to the higher rotating speeds involved. In all but the largest
pumps, this trend tends to bring about decreased impeller weight. In
addition, the decreased impeller diameter causes the discharge volute to be
‘‘wrapped’’ around a smaller outer diameter. This decreases volute
circumference and can (especially in high-pressure pumps) bring about
fairly dramatic decreases in total pump weight.
As we examine the effects of increasing pump N s, the pump’s suction
specific speed value (Nss) must be tracked as well. Constraints regarding
suction performance are most often the first fundamental constraint that
Copyright © 2003 Marcel Dekker, Inc.