**4. Effect of pure distortions on aerodynamic stability of fans**

## **4.1 Effect of the pure total pressure distortion on aerodynamic stability of fans**

The distortion screens of variable solidity in the inlet duct block a 180° sector of the inlet annulus to generate the total pressure distortion of the compressor inlet. **Figure 7** shows typical circumferential profiles resulting from averaging the pressures on each rake. The total pressure profile reasonably approximates a 180° extent square wave at the compressor inlet, whereas the static pressure profile appears sinusoidal.

**Figure 7.**

*Compressor profiles with the 180° circumferential pressure distortion at the 94% designed speed [18]. (a) Compressor inlet. (b) Compressor outlet.*

The compressor-outlet profiles show very little circumferential variation in the total/ static pressure; however, a total temperature distortion is evident. If the region of higher exit temperature is the result of the higher compression heat in the distorted sector, distorted flow rotates approximately 90° in the rotor-rotating direction passing from the low-pressure sector of the compressor inlet to the compressor outlet [18].

The effects of different distortion amplitudes (screen solidity) on the compressorstall line are shown on the undistorted compressor map (see **Figure 8**). Each stall line is defined as the locus of stall points generated. *P*<sup>d</sup> is the average of the pressure measurements on the rakes behind the distortion screen; *P*ud is that in the undistorted region; *P*av is the average of *P*<sup>d</sup> and *P*ud. The increased amplitude of the pressure distortion causes the stall line to move to lower pressure ratios. Also, the corrected rotor speed lines shift to the left or lower airflow for 93% of the corrected speed. The pressure distortion is not constant with airflow. The pressure distortion for screen "A" varies from 5% at 90% of the speed to 12% at the rated speed [18].

**Figure 9** shows the effects of different total pressure distortions on Δ(*PRS*). Parameters Δ(*PRS*)W and Δ(*PRS*)N, describing losses in the pressure ratio of stall at constant corrected airflow or a constant corrected speed, are used to evaluate the effects of flow distortions. Δ*P/P*, the ratio of Δ(*PRS*)N to the amplitude of the distortion, is approximately 0.60 and essentially independent of speeds with inlet pressure distortion alone. The ratio of Δ(*PRS*)W to Δ*P/P*, however, varies with speeds and amplitudes from 0.50 to 1.0, and the ratio is constant with the amplitude only at 93% of the corrected speed [18].
