This paper reviews modelling and interpretation advances of industrial fan stall phenomena, related stall detection methods, and control technologies. Competing theories have helped engineers refine fan stability and control technology. With the development of these theories, three major issues have emerged. In this paper, we first consider the interplay between aerodynamic perturbations and instability inception. An understanding of the key physical phenomena that occurs with stall inception is critical to alleviate stall by design or through active or passive control methods. We then review the use of passive and active control strategies to improve fan stability. Whilst historically compressor design engineers have used passive control techniques, recent technologies have prompted them to install high-response stall detection and control systems that provide industrial fan designers with new insight into how they may detect and control stall. Finally, the paper reviews the methods and prospects for early stall detection to complement control systems with a warning capability. Engineers may use an effective real-time stall warning system to extend a fan's operating range by allowing it to operate safely at a reduced stall margin. This may also enable the fan to operate in service at a more efficient point on its characteristic.

In this paper, the use of DIY transducers is proposed to detect the pressure instabilities in a low-speed industrial axial fan. The authors aim is to detect rotating stall, a well-studied aerodynamic instability with a typical frequency that can be even lower than 10 Hz in low-speed industrial fans. Pressure transducers and piezoelectric sensors, such as microphones, in turbomachinery are used respectively in the near and far field as standard methods to perform time-resolved pressure measurements. Other classes of sensors, such as electret microphones, may be not suited for pressure measurements, especially in the ultrasound region because their cutoff frequency is about 20 Hz. In this study, the authors use a lowcost DIY technology, as alternative technology to stall detection, in comparison with a high precision piezoelectric sensor. The authors performed the pressure measurements using a dynamic transducer, a piezoresistive transducer, and a piezoelectric high sensitivity sensor that provides the measurement baseline. They implemented and setup a measurement chain to identify the typical rotating stall pattern in low-speed axial fans. The results have been validated with respect to the state-of-the-art acoustic control techniques described in literature. The signals acquired using the two technologies are discussed using a combination of spectral and time-domain space reconstruction. The acoustic patterns obtained through the phase space reconstruction show that the DIY dynamic sensor is a good candidate solution for the rotating stall acoustic analysis.

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Stall-induced vibration places fundamental limitations on industrial fan performance and remains a persistent problem in the development of industrial fans and compressors. In this paper, we present an experimental programme that utilised two strategies to study rotating stall recovery patterns in an industrial fan. The experimental facility incorporated a fan with both variable pitch in motion blades and a variable speed drive. The facility enabled us first, to study the evolution of unsteady pressure signals on the fan casing whilst varying fan blade pitch and then to examine varying fan rotational speed. Consequently, we were able to recover a fan from stall either by varying blade pitch at constant speed, or by varying speed at constant blade pitch. We also studied the physical flow phenomena associated with stall recovery by cross-correlating signals from circumferentially offset high frequency response pressure transducers and then analysed the cross-spectra. This enabled us to gain insight into the transient fluid flow behaviour associated with stall recovery in the studied industrial fan class. The transient fluid flow behaviour associated with stall recovery by varying blade pitch was indicative of both a mild and progressive transition. In contrast, stall recovery by varying fan speed occurred more suddenly. In comparison with the variable pitch transition, the variable speed transition was more indicative of recovery from mild surge. From this we may conclude that stall recovery via variable speed resulted in significantly higher unsteady mechanical stress induced in fan rotating components than recovery via variable pitch. Higher bending stresses in the fan blades are more likely to result in mechanical failure, and therefore we may conclude that the studied fan is more ‘stall tolerant’ when one operates it as a variable pitch fan rather than as a variable speed fan

This work investigates the use of unconventional sensors to measure pressure modulation interpreted as pseudo sound in the near field on the casing of a fan in a view to detect rotating stall. Rotating stall is an aerodynamic issue with a frequency signature usually half the rotor frequency. In low speed turbomachines, such as industrial fans, this turns in very low frequencies, even lower than 10 Hz.Traditional methods use piezoelectric sensors, e.g., pressure transducers or microphones, respectively in the near and farfield, to detect instability from the signal patterns with broad frequency ranges. Recently electret microphones have been proposed, but with a cut-off frequency of 20 Hz as such not suitable for signal in near infrasound region.The sensor used in this work, have a narrower frequency range than more advanced technologies. The authors developed and set-up a measurement system able to acquire low frequency pressure signals using dynamic microphones.In this paper the authors developed a measurement chain based on dynamic microphone and pressure transducer in order to create a stall warning system. They tested the system on a low speed axial fan and they validated the work against state of the art acoustic control techniques. For this reason those devices represent candidate solutions for the detection of the patterns typical of rotating stall in turbomachines.