VELOCITY DIAGRAMS

Figure 15.5 shows the velocity triangles for one stage of an axial flow compressor. All angles are measured from the axial direction. The blade velocity u is taken to be same at blade entry and exit as the air enters and leaves the blades at almost equal radii. Due to the diffusion process in the moving blades, v_r2 < v_r1 but v_a2 > v_a1. The air leaves the fixed blades with velocity v_a3 at an angle α₃ and is redirected to the next page. It is assumed that v_a1 = v_a3.

From the inlet velocity triangle, we have

Figure 15.5 Velocity diagrams for axial flow compressor

From the outlet velocity triangle, we have

Assuming 1 kg flow of air through the compressor stage, we have

Tangential force per kg of air = v_w2 − v_w1

Work required by the stage per kg of air,

Now v_f1 = v_f2 = v_f

Also, isentropic efficiency of the stage,