Page 63 - Fluid Mechanics and Thermodynamics of Turbomachinery

44 Fluid Mechanics, Thermodynamics of Turbomachinery

L L

q q
R 1
W 1 Flow
Flow
1 q
1
2
2
N N
2
2N
A 2 A 2 1 + N
A R = = 1 + tan q A R = = tan q
A 1 W 1 A 1 R 1
(a) (b)

Lo

Dr 1
q i q o
r t1

N

(c)
FIG. 2.13. Some subsonic diffuser geometries and their parameters: (a) two-dimensional;
(b) conical; (c) annular.

difficult than might be anticipated, and some aspects of the flow processes are still
not fully understood. There is now a vast literature about the flow in diffusers and
their performance. Only a few of the more prominent investigations are referenced
here. A noteworthy and recommended reference, however, which reviews many
diverse and recondite aspects of diffuser design and flow phenomena is that of
Kline and Johnson (1986).
The primary fluid mechanical problem of the diffusion process is caused by the
tendency of the boundary layers to separate from the diffuser walls if the rate
of diffusion is too rapid. The result of too rapid diffusion is always large losses
in stagnation pressure. On the other hand, if the rate of diffusion is too low, the
fluid is exposed to an excessive length of wall and fluid friction losses become
predominant. Clearly, there must be an optimum rate of diffusion between these
two extremes for which the losses are minimised. Test results from many sources
indicate that an included angle of about 2 D 7 degrees gives the optimum recovery
for both two-dimensional and conical diffusers.