# A Study on the Stall Detection of an Axial Compressor through Pressure Analysis
## 3 Experiment Analysis  
Compressor experiments are always designed under the restrictions of compressor characteristics. Figures 4 and 5 show a compressor characteristic map without inlet distortion. With given compressor revolutions per minute, the compressor pressure ratio will increase while the compressor air mass flow is decreasing. However, excessive reduction of compressor air mass flow would lessen compressor compressing ability, as shown in Figure 4. Meanwhile, the efficiency of the compressor is gradually lessening with the reduction of compressor air mass flow, as the compressor experiment facility duct would block mass flow in the experiment, as is shown in Figure 5. Figures 6 and 7 show the compressor characteristic map with sole inlet distortion, and Figures 8 and 9 show the compressor characteristic map with symmetrical inlet distortion. When the compressor works in the sole distortion situation, the inlet is influenced by stall flow, and the surge margin of the compressor ought to move towards the bottom right, theoretically. Although the measuring point of compressor inlet pressure is set behind the distortion-imitating device, there is little difference in the surge margin the between inlet with distortion, and the inlet without distortion. However, when compressor operation is close to its surge margin, the stall flow of compressor inlet distortion is obviously different from the situation without \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Blade Parameters**} & \multicolumn{4}{c}{**Blade Position**} \\ \cline{2-5}  & **First-Stage Rotor** & **First-Stage Stator** & **Second-Stage Rotor** & **Second-Stage Stator** \\ \hline Type & NACA-65-010 & NACA-65-010 & NACA-65-010 & NACA-65-010 \\ Chord length/mm & 130 & 106 & 130 & 117 \\ Number & 19 & 22 & 18 & 20 \\ Radial clearance/mm & 1.3 & 1 & 1 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relevant design parameters of the compressor blade. Figure 3: Symmetrical inlet distortion-imitating device. distortion. As the compressor flow decreases, the pressure ratio increases and the operating efficiency gets higher; when the flow reduces to a certain degree, the pressure ratio and efficiency both go down. Figure 4: Compressor characteristic of pressure ratio versus mass flow rate with no inlet distortion. Figure 5: Compressor characteristic of efficiency versus mass flow rate with no inlet distortion. Figure 6: Compressor characteristic of pressure ratio versus mass flow rate with sole inlet distortion. The compressor experiment design fully utilizes the compressor characteristics. The control valve adjusts the compressor air mass flow in order to let the compressor get into stall or surge state. In this paper, the collected pressure signals are pre-processed by the following function,  
\[Cp=(P-P_{0})/P_{0} \tag{1}\]  
where \(P\) is the monitored pressure, which is filtered by a low-pass filter with any convenient cut-off frequency, and \(P_{0}\) is the compressor inlet pressure. Therefore, \(Cp\) is a dimensionless variable, which is called the pressure coefficient in this paper. Figure 8: Compressor characteristic of pressure versus mass flow rate with symmetrical inlet distortion. Figure 7: Compressor characteristic of efficiency versus mass flow rate with sole inlet distortion. Figure 9: Compressor characteristic of efficiency versus mass flow rate with symmetrical inlet distortion. The tested pressures are always disturbed by unsteady environmental factors and other unavoidable measurement disturbances. Therefore, in order to research the laws by which compressors work, the pressure characteristics should be pre-analyzed. In this paper, the pressures are mainly calculated by fast Fourier transform (FFT). With the FFT method, the experiment's results would be analyzed through the comparison of the sensor location at the compressor inlet, blade tip region, and compressor outlet. In this experiment, the main useful signal frequency is no more than 500 Hz.