Figure 6-1.. Inducer centrifugal compressor.
(a) (b)
Figure 6-2.. Impeller channel flow.
Because of choking conditions in theinducer， many compressors incor-porate a splitter-blade design. The flow pattern in such an inducer section is shown in Figure 6-20a. This flow pattern indicates a separation on the suction side of the splitter blade. Other designs include tandem inducers. In tandem inducers the inducer section is slightly rotated as shown in Figure6-20b. This modification gives additional kinetic energy to the boundary， which is otherwise likely to separate.
Centrifugal Section of an 1mpeller
The flow in this section of the impeller enters from the inducer section and leaves the impeller in the radial direction. The flow in this section is not com-pletely guided by theblades， and hence the effective fluid outlet angle does not equal the blade outlet angle.
To account for flow deviation (which is similar to the effect accounted forby the deviation angle in axial-flow machines)， the slip factor is used:
μ二 V82 (6-8)
V82∞
where V82 is the tangential component of the absolute exit velocity with a finitenumber ofblades， and V82∞ is the tangential component of theabsolute exit velocity， if the impeller were to have an infinite number of blades (no slipping back of the relative velocity at outlet).
With radial blades at theexit，
μ二 V82 (6-9)
U2
Flow in a rotating impeller channel (blade passage) will be a vector sum of flow with the impeller stationary and the flow due to rotation of the impeller as seen in Figure 6-21.
In a stationary impeller， the flow is expected to follow the blade shape and exit tangentially to it. A high adverse pressure gradient along the blade passage and subsequent flow separation are not considered to be general possibilities.
Inertia and centrifugal forces cause the fluid elements to move closer to and along the leading surface of the blade toward the exit. Once out of theblade passage， where there is no positive impelling action present， these fluid elements slow down.
Causes of Slip in an 1mpeller
The definite cause of the slip phenomenon that occurs within an impelleris not known. However， some general reasons can be used to explain why the flow is changed.
Coriolis circulation. Because of the pressure gradient between the wallsof two adjacent blades， the Coriolisforces， the centrifugalforces， and the fluid follow the Helmholtz vorticity law. The combined gradient that results causes a fluid movement from one wall to the other and vice versa. This movement sets up circulation within the passage as seen in Figure 6-22.Because of this circulation， a velocity gradient results at the impeller exit with a net change in the exit angle.
Boundar'-la'er development. The boundary layer that develops within an impeller passage causes the flowing fluid to experience a smaller exit area as shown in Figure 6-23. This smaller exit is due to small flow(if any) within the boundary layer. For the fluid to exit this smallerarea， its velocity must increase. This increase gives a higher relative exit velocity.
Figure 6-2.. Boundary-layer development.
Since the meridional velocity remainsconstant， the increase in relative velocity must be accompanied with a decrease in absolute velocity.
Although it is not a new approach， boundary-layer control is being used more than ever before. It has been used with success on airfoil designs whenit has delayed separation， thus giving a larger usable angle of attack. Control of the flow over an airfoil has been accomplished in two ways: by using slots through the airfoil and by injecting a stream of fast-moving air.
Separation regions are also encountered in the centrifugal impeller as shown previously. Applying the same concept (separation causes a loss in efficiency and power) reduces and delays their formation. Diverting the slow-moving fluid away lets the separation regions be occupied by a faster streamof fluid， which reduces boundary-layer build-up and thus decreases separation.
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