Ventilation fans operating in underground metropolitan tunnels are subjected to abrupt changes in operations due to the pressure wavefronts generated by the passage of the trains, and the magnitude of these pressure waves is increasing due to increasing speed of passing trains in modern mass transport systems. To avoid fans being driven into stall designers can fit fans with a stabilisation ring, i.e. a casing treatment that was found to mitigate the mechanical consequences of being inadvertedly driven into stall due to pressure pulses.
A stabilisation ring is a circumferential cavity in the casing of the fan, placed upstream of the rotor in order to allow the fluid to recirculate in stalled operations. A series of fins inside this cavity is used in order to drive the recirculating fluid back into the blade vane with a proper alignment with the leading edge of the rotor.
Following a previous RANS investigation that lead to the conclusion that the drive mechanism of the stabilisation ring onto the fan is based on azimuthal pressure unbalance we present here a U-RANS investigation aiming at understanding the dynamics of the interaction of the anti-stall ring with the fan and to provide insight on possible development of the geometry of the casing treatment.
The fan selected for this study is a real fan for tunnel and metro applications (9 rotor blades, 1490 rpm) with a real-geometry stabilisation ring (27 fins). Computations account for different operating points (peak efficiency, design point, peak pressure and stalled operations) and rely on the low-Reynolds cubic k-ε model of Lien et al. All the simulations were carried out with the open-source OpenFOAM code. Results were validated against available experimental data and then analysed to understand the unsteady interaction between the rotor of the fan and the cavity of the stabilisation ring.