Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

Flow
R1
r t1
1
Basic Thermodynamics, Fluid Mechanics: Definitions of Efficiency 45
L
N
q
AR = A2 2N
= 1 + tan q
A1 W1 Dr 1
(a)
Lo
N
(c)
2
ficult 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
W1
1
Flow
L
N
A R = A 2 =
A 1
q
q
1 + N
tan q
FIG. 2.13. Some subsonic diffuser geometries and their parameters: (a) twodimensional;
(b) conical; (c) annular.
q i
q o
(b)
R 1
2
2