# A Study on the Stall Detection of an Axial Compressor through Pressure Analysis
## 3 Experiment Analysis
### Pressure Analysis Results  
In order to research the developing laws of compressor pressure under different compressor conditions, two patterns of compressor working states are experienced. The compressor working patterns are: normal working conditions (where the compressor surge margin is 14.2%), and stall working conditions (where the compressor surge margin is 1.5%). Under both of these compressor working conditions, the compressor is analyzed through the FFT method [19; 20]. In this way, the pressure signal obtained in the time domain can be transferred into the frequency domain. The analysis of the compressor pressure signal in the frequency domain is to study the influence of different inlet conditions on the characteristics of the pressure signal. Because the rotational speed doesn't affect the purpose of this experiment, this paper takes the compressor experiment at 600 RPM randomly within examples to analyze the pressures. The experienced compressor contains 19 blades, so the compressor blade frequency with the compressor rotor at 600 RPM is 190 Hz. Figure 10 shows the variation of compressor inlet pressure with no inlet distortion at a surge margin of 14.2%, and Figure 11 shows the variation of compressor inlet pressure with sole inlet distortion. As is shown in Figure 10, the pressure periodicity is hard to maintain, but the spectrogram indicates that the compressor inlet pressure still holds the pressure frequency characteristic. However, with the impact of compressor inlet distortion, the compressor inlet pressure periodicity is further decreased, and the pressure frequency characteristic is obviously destroyed. In comparison to one another, it can be concluded that the compressor inlet pressure is not the best choice to describe the compressor's working characteristics. Figure 10: Compressor inlet pressure with no inlet distortion at a surge margin of 14.2%. Figures 12 and 13 show the variation of compressor outlet pressure with no inlet distortion and sole inlet distortion at a surge margin of 14.2%, respectively. It is not hard to find out that the pressure periodicity at the compressor outlet is not well held whether the compressor experiences inlet distortion or not. With the amplitude of the compressor pressure coefficient suddenly increased, the later pressure-signal processing difficulty in compressor stall detection is raised. However, the compressor outlet pressure frequency characteristic is well kept. Figure 11: Compressor inlet pressure with sole inlet distortion at a surge margin of 14.2%. Figure 12: Compressor outlet pressure with no inlet distortion at a surge margin of 14.2%. Figure 13: Compressor outlet pressure with sole inlet distortion at surge margin of 14.2%. Figures 14 and 15 show the variation of compressor blade tip region pressure with no inlet distortion and sole inlet distortion at surge margin of 14.2%. It is not hard to find out that both the pressure periodicity and frequency characteristics are well kept. The compressor blade tip region pressure signal integrates the advantages in compressor inlet and outlet pressure. Undoubtedly, the compressor blade tip pressure signal is the most appropriate pressure signal that could be used to analyze the compressor's working characteristics. Similar conclusions also could be deduced from the experimental compressor pressure under stall working conditions, as shown in Figures 16-21. Whether the compressor is working with inlet distortion or not, massive low-frequency pressure signals exist in the collected pressure signals at compressor inlet and outlet. The collected pressure signals at the compressor blade show the better frequency characteristic. Figure 14: Compressor blade tip pressure with no inlet distortion at a surge margin of 14.2%. Figure 15: Compressor blade tip pressure with sole inlet distortion at a surge margin of 14.2%. Figure 16: Compressor inlet pressure with no inlet distortion at a surge margin of 1.5%. Figure 17: Compressor inlet pressure with sole inlet distortion at surge margin of 1.5%.