Influence of the vortex design method in the performance characteristics of reversible axial rotors

Abstract

This paper presents a methodology for the aerodynamic design of reversible axial-flow rotors of jet fans, usually used for ventilation of road tunnels. Due to the low hub-to-tip ratio, typical of this kind of rotors, a particular type of non-free vortex is adopted to solve the radial equilibrium equation, in order to reduce the twist in the blades when compared to the free-vortex case. Values of the deviation and incidence angles, lift and profile drag coefficients, were interpolated from the literature data, for elliptical profiles with maximum aspect ratio of 8%. In order to reliably use these data in the computational routine, written in FORTRAN, for obtaining the main geometric and operational characteristics, a technique of scattered data interpolation was utilized. Radial basis functions (RBF) of type thin plate spline (TPS) were thus used for this purpose. This function showed to be efficient for the iterative procedure in the aerodynamic design of the rotor blades. Two reversible axial-flow rotors, formed by elliptical profiles, were finally obtained with the proposed methodology. The first profile resulted from the free-vortex condition and the second one from the non-free vortex condition. Both rotors have the same dimensions and the same number of blades, but differ in the stagger angle of each profile forming the blades. Through the use of the commercial code FLUENT, the main aerodynamic performance characteristics of these two rotors were determined, for a specific rotation, within a wide range of operation conditions. The non-free vortex rotor showed a higher maximum efficiency when compared to the free-vortex one, which was located in the region of stability of the rotor. On the contrary, the free-vortex rotor maximum efficiency was located in the region of instability of the rotor.