Profiles for Reversible Axial Fans
3.4 Fan Design for Improved Efficiency and Extended Operating Range
Most fans have a distinct flow direction, nevertheless a reversible flow direction for fans is desirable in certain circumstances in ventilation and drying applications. In order for the fan performance to be similar in both directions, the blade profiles of these axial fans have to conform to distinct symmetry conditions. As data for this kind of profiles is very scarce, the flow around such reversible profiles is investigated. Thus it is possible to get further insight into the aerodynamics in order to improve the efficiency of the profile design.
Most common are elliptical airfoils and S-shaped profiles. These are mostly derived from existing profiles using a twice cambered (i.e. S-shaped) camber-line.
The flow over elliptical airfoils and S-shaped profiles is investigated numerically solving the two dimensional incompressible Reynolds averaged Navier-Stokes (RANS) equations. The influence of different turbulence models is shown. The results are validated and compared with experimental data from the literature. A comparison with data obtained using other computational methods is performed.
The flow around various profiles at various incident angles and for various low Reynolds numbers, as appropriate for fan profile applications, is computed. The flow patterns of the flow around the profiles are investigated and polar diagrams are derived from the computations. The performance of the S-shaped profiles is compared to the elliptical airfoils and the normal (once cambered) airfoils.
With an increase in camber and profile thickness lift and drag also increases. The most important parameter is the camber. The thickness distribution has a smaller influence. Due to the S-shape the flow is asymmetric and complex: There is always flow separation as well as reattachment and changes in the direction of the pressure gradient due to the curvature changes. For strong cambered profiles, these effects are predominant and the advantages using a twice cambered profile might diminish. Based on the analysis, design recommendations for an improved design of reversible profiles are derived.