79
2.4 Diffusers
2.4 Diffusers
2.4.1 Dump Diffusers
Figure 2.20 sketches the flow in a sudden expansion of a channel. Flow is decelerated. A sudden expansion thus functions as a diffuser. The figure also illustrates the
loss mechanism by mixing. The loss is the kinetic energy related to the velocity
difference (see Exercise 2.5.4):
(2.31)
Diffusion by sudden expansion or dump diffusion is efficient with a rather small
area ratio. For instance, A 2 /A 1 = 1.25 gives x = 0.04. A higher area ratio is useful as
well. For instance, A 2 /A 1 = 2 gives x = 0.25. A loss coefficient with value 0.25 is not
disadvantageous for a diffuser, as will be discussed below. That is why dump diffusion is applied in turbomachines, which is rather surprising at a first glance. The
main advantage of dump diffusion is its realisation in a short distance.
2.4.2 Inlet Flow Distortion
Diffusers are channels where flow is decelerated and dynamic pressure is converted
into static pressure. This process is called pressure recovery. The attainable pressure recovery or diffusion strongly depends on the uniformity of the incoming flow.
Figure 2.21 compares the diffusion with uniform and non-uniform incoming flow of
an ideal fluid. In both cases, 50 % dynamic pressure recovery based on the average
inlet velocity is intended. The figure shows the position where the 50 % recovery
is obtained. With the uniform flow, the corresponding velocity is 1 0.5 0.71
−
≈
times the inflow velocity and the corresponding section area is 1.41 times the inflow
section area. With a non-uniform inlet, a larger area ratio is required, so a larger
covered length with a given opening angle, due to the much faster velocity decrease
of the slower part of the flow. At inflow, the velocities are 1.25 and 0.75 times the
average velocity. The necessary reduced velocities are
2
( 1.25 ) 0.5 1.03
−
≈
and
2
2
2
2
2
2
1
2
2
1
1
1
1
irr
1
2
( v v )
v
v
A
v
v
q
( 1
)
( 1
)
.
2
v
2
A
2
2
x
−
=
= −
= −
=
Fig. 2.21 Diffusion with
uniform and non-uniform
inlet flow
2.4 Diffusers
2.4 Diffusers
2.4.1 Dump Diffusers
Figure 2.20 sketches the flow in a sudden expansion of a channel. Flow is decelerated. A sudden expansion thus functions as a diffuser. The figure also illustrates the
loss mechanism by mixing. The loss is the kinetic energy related to the velocity
difference (see Exercise 2.5.4):
(2.31)
Diffusion by sudden expansion or dump diffusion is efficient with a rather small
area ratio. For instance, A 2 /A 1 = 1.25 gives x = 0.04. A higher area ratio is useful as
well. For instance, A 2 /A 1 = 2 gives x = 0.25. A loss coefficient with value 0.25 is not
disadvantageous for a diffuser, as will be discussed below. That is why dump diffusion is applied in turbomachines, which is rather surprising at a first glance. The
main advantage of dump diffusion is its realisation in a short distance.
2.4.2 Inlet Flow Distortion
Diffusers are channels where flow is decelerated and dynamic pressure is converted
into static pressure. This process is called pressure recovery. The attainable pressure recovery or diffusion strongly depends on the uniformity of the incoming flow.
Figure 2.21 compares the diffusion with uniform and non-uniform incoming flow of
an ideal fluid. In both cases, 50 % dynamic pressure recovery based on the average
inlet velocity is intended. The figure shows the position where the 50 % recovery
is obtained. With the uniform flow, the corresponding velocity is 1 0.5 0.71
−
≈
times the inflow velocity and the corresponding section area is 1.41 times the inflow
section area. With a non-uniform inlet, a larger area ratio is required, so a larger
covered length with a given opening angle, due to the much faster velocity decrease
of the slower part of the flow. At inflow, the velocities are 1.25 and 0.75 times the
average velocity. The necessary reduced velocities are
2
( 1.25 ) 0.5 1.03
−
≈
and
2
2
2
2
2
2
1
2
2
1
1
1
1
irr
1
2
( v v )
v
v
A
v
v
q
( 1
)
( 1
)
.
2
v
2
A
2
2
x
−
=
= −
= −
=
Fig. 2.21 Diffusion with
uniform and non-uniform
inlet flow