Towards optimum swirl recovery for propeller propulsion systems

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In a propeller propulsion system, due to the torque working on the propeller, a rotational motion of the fluid is generated. This rotational motion, expressed as a swirl component in the slipstream, does not result in any useful propulsive power, but causes a decrease in propeller efficiency. By recovering the momentum in the crosswise direction with other aerodynamic components located in the slipstream, either extra thrust can be produced or the overall drag of the aircraft can be reduced with the same power input from the propeller. This dissertation provides aerodynamic design and investigation of swirl recovery for both uninstalled and installed propeller propulsion systems. Swirl recovery vanes (SRVs) are a set of stationary vanes located behind a propeller, by which the angular momentum contained in the propeller slipstream can be recovered and thereby extra thrust can be generated. In this thesis, a design framework of SRVs is developed based on a lifting line model. The design method features a fast turnaround time, which makes it suitable for system level design and parameter studies. As a test example, a set of SRVs was designed for an uninstalled six-bladed propeller at a high propeller loading condition. A parametric study was performed of the SRV performance as a function of the blade count and radius. In order to validate the design routine, an experiment was performed with a propeller and the SRVs in a low-speed open-jet wind tunnel. The thrust generated by the SRVs was measured at different propeller loading conditions. The experimental results show that the SRVs provided thrust at all the measured propeller advance ratios. Since the SRVs did not require any extra power input, the propulsive efficiency of the system (propeller + SRVs) has improved accordingly for all the loading conditions considered. For an installed tractor-propeller propulsion system, both the downstream wing and the SRV have the ability of recovering the swirl of propeller slipstream. In the first case of swirl recovery from the trailing wing, reduction of wing induced drag can be achieved. In order to determine the optimum wing shape for maximum drag reduction, a multi-fidelity optimization procedure is developed, where the low-fidelity method corresponds to the potential flow-based method, and the high-fidelity method is based on an analysis by solving Euler equations. As a test case, the twist distribution of the wing is optimized at the cruise condition of a typical turboprop aircraft. Compared to the baseline wing (untwisted), the induced drag of the optimized wing has decreased by 1.4% of the propeller thrust. In the second case of swirl recovery from the SRV, extra thrust can be generated by the vanes. Four different cases of SRVs installation positions are investigated (with assumption of inviscid flow) with different axial and azimuthal positions relative to the wing. An optimum configuration is identified where SRVs are positioned on the blade-downgoing side downstream of the wing. For the identified optimum configuration, a set of SRVs was designed taking the effect Summary II of viscosity into account. The SRV design is subsequently validated by RANS simulation. Good agreement is observed in the lift, circulation, and thrust distributions of the SRV between the lifting line prediction and the RANS result. A thrust of 1.6% of propeller thrust from SRVs was validated by the RANS simulation. Comparing the two ways of swirl recovery, further investigation has shown that for the installed propeller propulsion system, due to the different aerodynamic consequences of the two (drag reduction of the wing compared with thrust enhancement from the SRV), they can be algebraically added up.