Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

228 Fluid Mechanics, Thermodynamics of Turbomachinery
ideal head Hi defined by eqn. (7.3b) by the amount of the internal losses. The hydraulic
efficiency of the pump is defined as
From the velocity triangles of Figure 7.2
(7.20)
Therefore (7.20a)
where f 2 = cr2/U2 and b 2 is the actual averaged relative flow angle at impeller outlet.
With the definition of slip factor, s = cq2/c¢q2, H can, more usefully, be directly related
to the impeller vane outlet angle as
(7.20b)
In general, centrifugal pump impellers have between five and twelve vanes inclined
backwards to the direction of rotation, as suggested in Figure 7.2, with a vane tip angle
b¢2 of between 50 and 70deg. A knowledge of blade number, b¢ 2 and f 2 (usually small
and of the order 0.1) generally enables s to be found using the Busemann formula. The
effect of slip, it should be noted, causes the relative flow angle b 2 to become larger
than the vane tip angle b¢2.
EXAMPLE 7.3. A centrifugal pump delivers 0.1m 3 /s of water at a rotational speed of
1200rev/min. The impeller has seven vanes which lean backwards to the direction of
rotation such that the vane tip angle b¢2 is 50deg. The impeller has an external diameter
of 0.4m, an internal diameter of 0.2m and an axial width of 31.7mm. Assuming
that the diffuser efficiency is 51.5%, that the impeller head losses are 10% of the ideal
head rise and that the diffuser exit is 0.15m in diameter, estimate the slip factor, the
manometric head and the hydraulic efficiency.
Solution. Equation (7.16) is used for estimating the slip factor. Since exp(2pcosb¢2/Z)
= exp(2p ¥ 0.643/7) = 1.78 is less than r2/r1 = 2, then B = 1 and A 0.77, obtained by
replotting the values of A given in Figure 7.11 for b¢2 = 50deg and interpolating.
Hence the Busemann slip factor is
Hydraulic losses occur in the impeller and in the diffuser. The kinetic energy leaving
the diffuser is not normally recovered and must contribute to the total loss, H L. From
inspection of eqn. (2.45b), the loss in head in the diffuser is (1 - hD)(c 2 2 - c 2 3/(2g). The
head loss in the impeller is 0.1 ¥ U2cq2/g and the exit head loss is c 2 3/(2g). Summing the
losses,