Experiments on the Use of Symmetrized Dot Patterns for In-Service Stall Detection in Industrial Fans

C2 - Performance Measurements

Aerodynamic instability when throttling flow rate constrains the operational envelope of fans, blowers and compressors. To avoid instability, aerodynamicists must provide adequate stability (stall) margin to accommodate inlet distortions, degradation due to wear, throttle transients, and other factors that reduce fan, blower and compressor stability from the original design base-line.

Because aerodynamic stall, resulting in increased alternating aerodynamic loads, is a major potential cause of mechanical failure in axial fans stall-detection techniques have had wide application for many years.
The detection and analysis of the different forms of aerodynamic instability have been studied for several decades. Two main types of aerodynamic flow instability exist: (i) 'rotating stall' (in which regions of reversed flow occur locally); and (ii) 'surge' (which is characterised by periodic backflow over the entire annulus involving violent oscillations in the air flow which can result in mechanical failure such as fan blade breakage.

Prior methods have had drawbacks in their inability to enable a sufficiently rapid response to the onset of stall to avoid damage and their inability to sense the approach to stall. First, alert methods have been identified for individual test beds but reliable warnings of general validity require further research. Second, techniques for the detection of stall initiation based on experimental observation of pre-stall behaviours have sought to identify such behaviour as early as possible to enable an active control system to react and suppress the incipient stall. As a third factor, all the non-model based detection techniques rely on the use of on-board probes able to sense the unsteady pressure evolution in the vicinity of the blade rows, i.e. usually flush-mounted on the casing.

The paper describes a stall-detections criterion based on the use of symmetrized dot pattern (SDP) visual waveform analysis and the stall-warning methodology based on that analysis recently developed. The present programme of work is aimed at the verification of the SDP based early-stall warning technique in a test environment relevant of the in-service operation of a cooling fan unit.

The experimental study explores the capability of the SDP technique to detect the stall incipience and evolution in presence of low signal to noise ratios, i.e. noisy working environment. Moreover, the investigation presents a systematic analysis on the influence of the probe position with respect to the fan section. As such the SDP technique in combination with an acoustic measurement is able to create a visual pattern that can be used to detect stall in any locations, not just with the microphone over the blade itself.