No abstract

Of late, the aerospace industry has taken increasing interest in Micro Air Vehicles (MAV) powered by electric motor driven micro propellers. The endurance of the MAV largely depends upon the propulsive efficiency of its propeller. This has created a need for improved design of propellers through an in-depth understanding of the relevant aerodynamics. Design of micro propellers operating at low Reynolds numbers with sufficiently high propulsive efficiency is a challenging task. This paper deals with the parametric studies on a micro propeller for MAV application through numerical simulations. A propeller of known geometry was selected from the published literature. Geometrical model of the baseline propeller was prepared using CATIA V5 software and CFD analysis was carried out using ANSYS FLUENT 12.0 software. The baseline geometry of the micro propeller was modified by varying the spanwise position of maximum blade chord, maximum chord length, and pitch to diameter ratio to generate new design variants. The performances of these design variants were analysed through CFD simulations and compared in terms of variation of efficiency, torque coefficient and thrust coefficient against advance ratio. No significant change was observed in performance by changing the location of maximum blade chord. However, the required thrust of 1 N was achieved by increasing the chord length by 1.2 times the base line design at an efficiency of 64.2%. The propeller efficiency was further increased to 70.8% at an increased pitch to diameter ratio of 1.2 and at an advance ratio of 1.033.

This paper deals with the numerical investigations on the effect of trailing edge crenulation on the performance of a transonic axial compressor rotor. Crenulation is broadly considered as a series of small notches or slots at the edge of a thin object, like a plate. Incorporating such notches at the trailing edge of a compressor cascade has shown beneficial effect in terms of reduction in total pressure loss due to enhanced mixing in the wake region. These notches act as vortex generators to produce counter rotating vortices, which increase intermixing between the free stream flow and the low momentum wake fluid. Considering the positive effects of crenulation in a cascade, it was hypothesized that the same technique would work in a rotating compressor to enhance its performance and stall margin. However, the present CFD simulations on a transonic compressor rotor have given mixed results. Whereas the peak total pressure ratio in the presence of trailing edge crenulation reduced, the stall margin improved by 2.97% compared to the rotor with straight edge blades. The vortex generation at the crenulated trailing edge was not as strong as reported in case of linear compressor cascade, but it was able to influence the flow field in the rotor tip region so as to energize the low momentum end-wall flow in the aft part of the blade passage. This beneficial effect delayed flow separation and allowed the mass flow rate to be reduced to still lower levels resulting in improved stall margin. The reduction in pressure ratio with crenulation was surprising and might be due to increased mixing losses downstream of the blade.