Stall Inception, Evolution and Control in a Low Speed Axial Fan With Variable Pitch in Motion

Abstract

Obtaining the right pitch in turbomachinery blading is crucial to efficient and successful operations. Engineers adjust the rotor's pitch angle to control the production or absorption of power. Even for low speed fans this is a promising tool. This paper focuses on the inception and the evolution of the flow instabilities in the tip region which drive the stall onset in low speed axial fans. The authors conducted an experimental study to investigate the inception patterns of rotating stall evolution at different rotor blade stagger-angle settings with the aim of speculating on stable operating margin. The authors drove the fan to stall at the design stagger-angle setting and then operated the variable pitch mechanism in order to recover the unstable operation. They measured pressure fluctuations in the tip region of the low-speed axial-flow fan fitted with a variable pitch in motion mechanism, with flush mounted probes. The authors studied the flow mechanisms for spike and modal stall inceptions in this ¡Qw-speed axial-flow fan which showed relatively small tip clearance. The authors cross-correlated the pressure fluctuations and analyzed the crossspectra in order to clarify blade pitch, end wall flow, and tip-leakage flow influences on stall inception during the transient at the rotor blades' different stagger-angle settings. The authors observed a rotating instability near the maximum pressure-rise point at both design and low stagger-angle settings. The stall inception patterns were a spike type at the design stagger-angle setting as a result of the interaction between the incoming flow, tip-leakage flow and end wall backflow.

A Critical Review of Stall Control Techniques in Industrial Fans

“…The researchers studying short wavelength disturbances refer to them as "spikes" or "pips" that are responsible for localised part-span stall cells [32][33][34][35]. The stall cell's spikelike inception in a single stage is clearly evident in data that researchers obtained from a model fan [36]. We can see the spike-like inception at 28.5 seconds, Figure 2, with the flow then returning to its steady state condition for half a second before becoming unsteady at 29 seconds.…”

“…It is the movement from the unstable to stable region that allows a reduction in blade angle to constitute a method by which a stall control system may recover a fan from stall. Bianchi et al [36] studied experimental data from a variable pitch in motion fan as pitch angle reduced, observing that the pressure stabilised after 89 seconds, Figure 6, with reducing pitch angle. Consequently, a fan that was stalling with a 70 ∘ pitch angle no longer stalls when the pitch angle reduces to 50 ∘ .…”

“…We can observe a spike-like feature at 28.5 seconds that is characteristic of stall inception. From 29 to 36 seconds the variation in pressure is associated with the fan's stalled operation [36].…”

“…The symmetrised dot pattern technique differentiates between critical and noncritical stall conditions and provides a form of visualisation that enables one to identify stall precursors. Bianchi et al [36] first reconstructed the symmetrised dot patterns from the unsteady pressure which they measured on an industrial fan's casing and more recently Figure 12: Unsteady pressure data from a 2.24 meter diameter tunnel ventilation fan at 100 per cent, 50 per cent, and 25 per cent speed in stable operation, when stall is incipient and during stalled operation. The authors processed the data using the symmetrised dot pattern (SDP) technique to produce a set of patterns that are distinctly different at each speed and operating condition [41].…”

“…At 50 ∘ the fan is operating in the stable part of its characteristic, to the right of the characteristics peak pressure [36].…”