Railway tunnel and metropolitan metro system ventilation fans are subjected to positive and negative pressure pulses. As a train travels along a tunnel it drives air down the tunnel. This creates a 'piston effect' that results in positive and negative pressure pulses on the ventilation fans. A pressure pulse effect transiently drives a ventilation fan to a higher pressure operating point. If the operating point is beyond the fan's pressure developing capability, there is a risk it may stall. Tunnel ventilation fan designers classically utilise a stabilisation ring to stabilise the fan's characteristic and thus mitigate the mechanical consequences of driving a fan into stall. A stabilisation ring consists of an annular chamber that is incorporated into the fan casing over the fan blade's leading edge. As a tip-limited axial fan approaches stall, boundary layer fluid centrifuges up the blade. The fan stalls at the point when flow inside the annulus reverses direction in the blade tip region. The stabilisation ring provides an annular chamber into which this fluid may flow. It incorporates a set of vanes that redirect the reverse flow into an axial direction, and then reintroduce it into the main-stream flow up-stream of the fan blade leading edge. Although effective in stabilising the fan's characteristic, stabilisation rings typically reduce fan efficiency by three per cent, and consequently are becoming progressively less acceptable as required minimum fan efficiencies increase. The reported research combines experimental measurements of overall fan performance with and without a fitted stabilisation ring and a numerical analysis of the flow-field within the stabilisation ring. Visualisation of the flow-field within the stabilisation ring provides an insight into the physical flow mechanisms that enable the stabilisation ring to stabilise the fan's characteristic. A conclusion of the research is that at the fan's peak efficiency operating point, flow through the stabilisation ring separated from the stabilisation ring vanes. Therefore, redesigning the vanes within the stabilisation ring to avoid separated flow offers the potential to eliminate this aerodynamic loss mechanism, thus reducing the efficiency loss classically associated with applying a stabilisation ring.
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