PRE-WHIRL

In order to avoid acceleration to sonic or supersonic velocities, the Mach number of flow entering the impeller eye should be kept below the value of 0.9. When the absolute velocity of approach is high enough or the static absolute temperature of the entering air is low enough to make the Mach number more than 0.9, the method of providing a pre-whirl and thereby reducing the relative velocity entering the impeller eye is quite helpful.

Figure 14.10 Effect of pre-whirl on inlet velocity triangle: (a) With pre-whirl (v_w ≠ 0), (b) Without pre-whirl (v_w = 0)

Pre-whirl is provided by intake guide vane as shown in Fig. 14.10, which shows the velocity triangles for blades without pre-whirl and with pre-whirl. It may be observed that the magnitude of the relative velocity with pre-whirl vane gets reduced at the entrance, but the flow enters with a large relative angle. Thus, the flow inlet Mach number is lowered by pre-whirl vane. However, since v_w1 ≠ 0 with pre-whirl vane, the work done on the air must be greater for the same compressor inlet and outlet conditions.