Design Study to Axial Flow Fan Stator Blade Row Used in Reverse Ventilation
3.4 Fan Design for Improved Efficiency and Extended Operating Range
Fan flow directional change is frequently required in emergency situations in traffic tunnels, chemical plants and mine ventilation systems. Our paper focuses on the reversing method, namely changing direction of the fan rotation and turning of the stator vanes. The paper will discuss the aerodynamic design of stator blades of two reverse ventilation axial fan stages A and B with different stage parameters, specifically the flow coefficients of 0.40 and 0.35 and pressure coefficients of 0.30 and 0.42. Free vortex blading was used when designing both fan stages.
Companies which procure reverse axial fans request that the one-shot stator vanes will have the capability of rotating around their axis in emergency events to enable the flow reversal. Consequently, the blade cascades have the higher pitch/chord ratio than one along the full blade span. New stator vane shape for the reverse fan is characterised by the decrease in the chord length in the radial direction to the fan axis. Thus the cascade aerodynamic loading increases in this region in comparison with the original stage design.
This is only possible if the aerodynamic loading of blade cascades is moderate. The cascade aerodynamic loading is often expressed by the Lieblein’s definition of the diffusion factor D. If the diffusion factor is higher than 0.6, the flow is separated on the cascade profile suction surface. The paper also presents new derived relationships to determine the fan stage parameters area, the pressure and flow coefficients, where the stator vane cascades would not incur the flow separation.
Following the analysis of the stage A we conclude that using the stator vanes for the flow reversal is acceptable. The maximum diffusion factor at the stage hub is 0.56. However, in analysing the fan stage B we find that the stator diffusion factor is above the critical value of 0.6 in lower half of vane height near the fan hub. This condition will cause large flow separation area together with unacceptable decrease in the fan efficiency.
For that reason the authors designed the stator vanes consisting of two parts, straight which is fixed and curved which is moveable. At the basic flow conditions these parts are connected. During the flow reversal the moveable curved stator blade part is rotated to ensure the maximum flow rate.
Computed (CFD) and measured aerodynamic performance of the original stage B with profile blades was compared resulting in acceptable agreement for the basic flow direction. As a result it was possible to investigate the effect of reversing the one-shot stator vane design variants on the fan stage efficiency with the use of CFD data. The decrease in efficiency at the design conditions was 0,6 % to 1,9 % in both studied stages.