Design of swirl recovery vanes in a slipstream of limited extension

Abstract

The angular momentum or swirl in the propeller slipstream is an energy loss. An effective method to recover the swirl and increase the propulsive efficiency is to use Swirl Recovery Vanes (SRVs). Former research showed an over-prediction in SRV thrust by a lifting line theory (LLT) model compared to wind tunnel experiments. The LLT model assumes an unbounded flow field from minus infinity to plus infinity, while in reality the SRV flow field is bounded due to a small axial spacing between the SRVs and the propeller plane. Using a 2D correction method, based on the application of the Kutta–Joukowski theorem in an axially confined domain combined with the thin airfoil approximation, a correction in the angle of attack can be computed for each blade element of the SRV. The LLT model including the correction method could match the SRV thrust of the experiments. Additionally, the original LLT thrust prediction was met with a corrected pitch angle of the SRVS. An SRV in off-design conditions would result in different correction angles. For a smaller thrust setting of the propeller, the thrust of the SRV will also be lower, hence the correction angle will be smaller. To make the correction method also applicable in off-design conditions, a fixed correction angle would be more practical since the whole vane can be rotated with a variable pitch to match the appropriate correction angle for the corresponding thrust setting. The fixed correction angle was determined by taking an average of all correction angles along the blade radius and the thrust prediction showed good agreement, with a negligible thrust loss of the SRV compared to correcting the pitch angle for each blade element separately. Finally, in the airfoil profile optimization, it was found that the profile drag of the profile is of less influence on the SRV thrust, hence more design freedom can be used to select the appropriate airfoil profile for the SRV.