# Physics of Prestall Propagating Disturbances in Axial Compressors and Their Potential as a Stall Warning Indicator

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## Abstract

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## 1. Introduction

- Rotating Instability occurs when the compressor is operating at off-design conditions. If the mass flow is reduced at a constant rotational speed, the nominally uniform still unstalled flow becomes unstable and transitions into a “prestall condition” which is significantly non-uniform. Meanwhile, the compressor is nominally unstalled.
- The RI phenomenon can be located in the vicinity of the rotor tips, where the occurrence of RI is more likely with larger tip gaps [9].
- The inception of RI can be identified by a broad chain of peaks in the frequency spectrum taken by time-resolved pressure measurements near the rotor blade tips.

## 2. Experimental Setup

#### Instrumentation

## 3. Computational Setup

## 4. Model Validation

## 5. Performance Characteristics

## 6. Prestall Disturbances

#### 6.1. Nature of Prestall Disturbances

#### 6.2. Prestall Propagating Disturbances

#### 6.3. Prestall Propagating Disturbances; Numerical Results

## 7. Discussion

## 8. Conclusions

- Discrete propagating prestall disturbances can be measured over the tips of the rotor blades. They represent a kind of flow non-uniformity and appear in a broad operating range before the compressor reaches its stalling limit.
- The disturbances can be clearly identified and quantified by analysing the blade passing signature irregularity. As the mass flow is reduced, the intensity of flow disturbances increases. Furthermore, they become amplified disproportionately with increasing rotor speed.
- Based on a spectral analysis, it could be proven that these particular disturbances can be assigned to the subject of Rotating Instabilities. The number of disturbances coincides with the mode order in the spectral signature of RI.
- It could be demonstrated that RI and rotating stall cells can indeed coexist in an operating range where the flow coefficient is below the stalling flow coefficient.
- A new stall indicator has been found which indicates the last stable operating point before stall. For this purpose, the third statistical momentum was applied to the unsteady pressure data measured upstream of the leading edge plane.
- Numerical simulations revealed that the flow disturbances appear as small vortex tubes where the casing ends of the tubes induce low pressure spots while propagating around the circumference. The highly deflected vortices behave like a characteristic feature of a Kelvin–Helmholtz-Instability which is triggered when the entire tip region of the rotor is affected by blockage. It is assumed that the prestall vortices arise if the critical rotor incidence is not exceeded in this flow regime.

## Supplementary Materials

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## Abbreviations

BPF | Blade passing frequency |

KHI | Kelvin–Helmholtz-Instability |

LE | Leading edge |

LLSC | Long length-scale stall cell |

PSD | Power spectral density |

RI | Rotating Instability |

RMS | Root mean square |

RS | Rotating stall |

SST-SAS | Shear-stress transport scale-adaptive simulation |

TCV | Tip clearance vortex |

TE | Trailing edge |

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**Figure 4.**Filtered pressure fluctuations at B18; Difference between mean blade passing (black line in Figure 3b) and the first revolution (one exemplary red line).

**Figure 6.**Stall indicator; distribution for each pressure transducer in red—average distribution marked with circles (Experiment).

**Figure 7.**Analysis of the Power Spectral Density (PSD); Development of prestall instability at various operating points for different speeds (BPF: blade passing frequency) (Experiment).

(a) Design specifications of rotor | |||||

$r/{r}_{mid}$ | ${\beta}_{1}$ [${}^{\circ}$] | ${\beta}_{2}$ [${}^{\circ}$] | solidity | stagger [${}^{\circ}$] | ${C}_{A}$ |

0.39 | 25 | 10.8 | 0.55 | 17.9 | 3.17 |

1 | 53.9 | 45.7 | 0.89 | 49.8 | 1.84 |

1.61 | 65.2 | 59.9 | 1.19 | 62.4 | 1.22 |

(b) Datum parameters | |||||

Design speed n | 22,000 rpm | ||||

Flow coefficient, $\phi =\frac{C}{{U}_{mid}}$ | 0.7 | ||||

Pressure rise, $\mathsf{\Psi}=\frac{{p}_{exit}-{p}_{t,in}}{\frac{1}{2}\rho {U}_{mid}^{2}}$ | 0.53 | ||||

tip clearance | $2.4\%$ blade height |

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**MDPI and ACS Style**

Eck, M.; Geist, S.; Peitsch, D. Physics of Prestall Propagating Disturbances in Axial Compressors and Their Potential as a Stall Warning Indicator. *Appl. Sci.* **2017**, *7*, 285.
https://doi.org/10.3390/app7030285

**AMA Style**

Eck M, Geist S, Peitsch D. Physics of Prestall Propagating Disturbances in Axial Compressors and Their Potential as a Stall Warning Indicator. *Applied Sciences*. 2017; 7(3):285.
https://doi.org/10.3390/app7030285

**Chicago/Turabian Style**

Eck, Mario, Silvio Geist, and Dieter Peitsch. 2017. "Physics of Prestall Propagating Disturbances in Axial Compressors and Their Potential as a Stall Warning Indicator" *Applied Sciences* 7, no. 3: 285.
https://doi.org/10.3390/app7030285