Matching the flow between the impeller and the diffuser is complex because the flow path changes from a rotating system into a stationary one.This complex， unsteady flow is strongly affected by the jet-wake ofthe flow leaving the impeller， as seen in Figure 6-29. The three-dimensionalboundary layers， the secondary flows in the vanelessregion， and the flow separation at the blades also affects the overall flow in the diffuser.
The flow in the diffuser is usually assumed to be of a steady nature to obtain the overall geometric configuration of the diffuser. In a channel-type diffuser the viscous shearing forces create a boundary layer with reducedkinetic energy. If the kinetic energy is reduced below a certain limit， the flow in this layer becomes stagnant and then reverses. This flow reversal causes
Figure 6-2.. Jet-wake flow distribution from an impeller.
separation in a diffuser passage， which results in eddylosses， mixinglosses， and changed-flow angles. Separation should be avoided or delayed to improve compressor performance.
The high-pressure-ratio centrifugal compressor has a narrow yet stable operating range. This operating range is due to the close proximity of the surge and choke flow limits. The word ..surge'' is widely used to express unstable operation of a compressor. Surge is the flow breakdown period during unstable operation. The unsteady flow phenomena during the onset of surge in a high-pressure-ratio centrifugal compressor causes the mass flow throughout the compressor to oscillate during supposedly ..stable'' operations. The throat pressure in the diffuser increases during the precursor period up to collector pressure .col at the beginning of surge. All pressure traces (except plenum pressure) suddenly drop at the surge point. The sudden change of pressure can be explained by the measured occurrence of backflow from the collector through the impeller during the period between the two sudden changes.
Scroll or Volute
The purpose of the volute is to collect the fluid leaving the impeller ordiffuser， and deliver it to the compressor outlet pipe. The volute has an important effect on the overall efficiency of the compressor. Volute designembraces two schools of thought. First， the angular momentum of the flow in the volute isconstant， neglecting any friction effects. The tangential velocity V58 is the velocity at any radius in the volute. The following equa-tion shows the relationship if the angular momentum is held constant
V58r二 constant二 K (6-12)
Assuming no leakage past the tongue and a constant pressure around theimpeller periphery， the relationship of flow at any section Q8 to the overall flow in the impeller Q is given by
Q8二 28王 Q (6-13)
Thus， the area distribution at any section 8 can be given by the following relationship:
二 Qr x 8 x L (6-14)
A82王 K
where:
r二 radius to the center of gravity
L二 volume width
Second， design the volute by assuming that the pressure and velocity are independent of 8. The area distribution in the volute is given by
A8二 K VQ 58 28王 (6-15)
To define the volute section at a given 8， the shape and area of the section must be decided. Flow patterns in various types of volute are shown in Figure 6-30. The flow in the asymmetrical volute has a single-vortex instead of the double-vortex in the symmetrical volute. Where the impeller is dis-charging directly into the volute， it is better to have the volute width larger than the impeller width. This enlargement results in the flow from the
Figure 6-... Flow patterns in volute.
impeller being bounded by the vortex generated from the gap between the impeller and the casing.
At flows different from design conditions， there exists a circumferential pressure gradient at the impeller tip and in the volute at a given radius.
At low flows， the pressure rises with the peripheral distance from the volutetongue. At high flows， the pressure falls with distance from the tongue. This condition results because near the tongue the flow is guided by the outer wall of the passage. The circumferential pressure gradients reduce efficiency away from the design point. Nonuniform pressure at the impeller discharge resultsin unsteady flows in the impeller passage， causing flow reversal and separ-ation in the impeller.
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