Handbook of Turbomachinery Second Edition Revised - Ventech!

of the flow’s behavior can be had by employing the assumption that angular
momentum remains constant in the vaneless space, that is,
Where
rCy ¼ const ð18Þ
r ¼ distance from impeller rotational axis.
Cy ¼ tangential component of fluid velocity.
The constant in Eq. (18) can be determined by computing the angular
momentum at the impeller exit. The diffusion in the vaneless space comes
about with the reduction of Cy as the pumped fluid moves outboard to
increasing radii.
The prime difficulty with the vaneless diffuser comes about as a result
of the direction of the impeller outlet velocity in a highly loaded pump stage.
It often occurs that the very tight spiral angle (often less than 108 from the
tangential) causes flow in the vaneless space to undergo large frictional
losses on its way to the discharge volute. For this reason, vaneless diffusers
are generally less favored than vaned ones in rocket propellant pumps. This
is particularly true if peak pump efficiency must be maintained, as is often
the case with closed-cycle engine systems.
The incorporation of vaned diffusers of various kinds into propellant
pumps began in earnest in the early 1960s. This was primarily in response to
head requirements that were sufficiently high that we could not afford to
lose the impeller’s exit kinetic energy. Vaned diffusers generally fall into two
categories—the so-called vane island, or ‘‘wedge’’ diffuser, and the
‘‘cambered vane diffuser.’’ Which type is employed generally depends on
the level of sophistication required. Referring to Fig. 18, we can see that the
vane island diffuser consists of vanes (or wedges) that form straight,
diverging flow passages. Before the advent of computer-controlled machining
processes, the vane island diffuser was much easier to fabricate
accurately than a vane row that incorporated camber. In addition, the level
of diffusion and the general performance of the vane island diffuser are
often more amenable to analysis than its cambered counterpart, if for no
other reason than that a throat and exit area are more easily identified. It is
capable of generally the same level of performance as the cambered vane
row. In order to minimize the radial extent of the diffuser, a maximum rate
of diffusion might be sought. Past test data for vane island diffusers indicate
successful performance with diffuser cone angles of between 7 and 108, with
88 representing a good design compromise (the cone angle represents the
angle subtended by the two sides of the diffuser flow passage).
Copyright © 2003 Marcel Dekker, Inc.