Figure 6-.2. Secondary flow at the back of an impeller.
Clearance loss. When a fluid particle has a translatory motion relativeto a noninertial rotating coordinate system， it experiences the Coriolis force. A pressure difference exists between the driving and trailing faces of an impeller blade caused by Coriolis acceleration. The shortest and least resis-tant path for the fluid to flow and neutralize this pressure differential is provided by the clearance between the rotating impeller and the stationarycasing. With shrouded impellers， such a leakage from the pressure side to thesuction side of an impeller blade is not possible. Instead， the existence of a pressure gradient in the clearance between the casing and the impellershrouds， predominant along the direction shown in Figure6-33， accounts for the clearance loss. Tip seals at the impeller eye can reduce this loss considerably.
This loss may be quite substantial. The leaking flow undergoes a large expansion and contraction caused by temperature variation across the clear-ance gap that affects both the leaking flow and the stream into which it discharges.
Skin friction loss. Skin friction loss is the loss from the shear forces on the impeller wall caused by turbulent friction. This loss is determined by considering the flow as an equivalent circular cross section with a hydraulic diameter. The loss is then computed based on well-known pipe flow pressure loss equations.
Figure 6-... Leakage affecting clearance loss.
Stator Losses
.ecirculating loss. This loss occurs because of backflow into the impel-ler exit of a compressor and is a direct function of the air exit angle. As theflow through the compressor decreases， there is an increase in the absolute flow angle at the exit of the impeller as seen in Figure 6-34. Part of the fluid isrecirculated from the diffuser to the impeller， and its energy is returned to the impeller.
Wake-mixing loss.This loss is from the impellerblades， and it causesa wake in the vaneless space behind the rotor. It is minimized in a diffuser， which is symmetric around the axis of rotation.
Figure 6-.4. Recirculating loss.
Vaneless diffuser loss. This loss is experienced in the vaneless diffuser and results from friction and the absolute flow angle.
Vaned diffuser loss. Vaned diffuser losses are based on conical diffuser test results. They are a function of the impeller blade loading and the vaneless space radius ratio. They also take into account the blade incidence angle and skin friction from the vanes.
Exit loss. The exit loss assumes that one-half of the kinetic energy leav-ing the vaned diffuser is lost.
Losses are complex phenomena and as discussed here are a function ofmany factors， including inlet conditions， pressure ratios， blade angles， and flow. Figure 6-35 shows the losses distributed in a typical centrifugal stage of pressure ratio below 2:1 with backward-curved blades. This figure is only a guideline.
Compressor Surge
A plot showing the variation of total pressure ratio across a compressor as a function of the mass flow rate through it at various speeds is known as a performance map. Figure 6-36 shows such a plot. a...
The actual mass flow rates and speeds are corrected by factors ( 8/8)
...
a
and (1/8)， respectively， to account for variation in the inlet conditions of temperature and pressure. The surge line joins the different speed lines where
Figure 6-.5. Losses in a centrifugal compressor.
the compressor's operation becomes unstable. A compressor is in ..surge'' when the main flow through the compressor reverses its direction and flowsfrom the exit to the inlet for short time intervals. If allowed to persist， this unsteady process may result in irreparable damage to the machine. Lines of constant adiabatic efficiency (sometimes called efficiency islands) are also plotted on the compressor map. A condition known as ..choke'' or ..stone walling''is indicated on themap， showing the maximum mass flow rate possible through the compressor at that operating speed.
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