Simulation and Analysis of a Novel Open Rotor Propeller - the Boxprop

Licentiate thesis, 2016

Economic factors and environmental awareness is driving the evolution of aircraft engines towards increasingly higher efficiencies, reaching for lower fuel consumption and lower emissions. The Counter-Rotating Open Rotor (CROR) is actively being researched around the world, promising a significantly increased propulsion efficiency relative to existing turbofans by employing two, counter-rotating propeller blade rows, thereby increasing the bypass ratio of the engine. Historically, these engines have been plagued by very high noise levels, mainly due to the impingement of the front rotor tip vortices on the rear rotor. In modern designs, the noise levels have been significantly decreased by clipping the rear, counter-rotating propeller. Unfortunately, this comes at a cost of decreased efficiency. An alternative, potential solution lies with the Boxprop, which was invented by Richard Avellán and Anders Lundbladh. The Boxprop consist of blade pairs joined at the tip, and are conceptually similar to box wings. It is hypothesized that the Boxprop can eliminate the tip vortex found in conventional blades, consequently increasing the efficiency of the blades, and reducing their acoustic signature. The present work highlights advances done in the research surrounding the Boxprop. A validation of the deployed CFD methodology is presented, in which numerical and experimental results compare favourably. Performance results for a Boxprop (GP-X-701) designed for cruise conditions are presented and compared with a generic conventional propeller (GP-S-609). It is shown that the present Boxprop cruise design can reach the required thrust for replacing the front rotor of a modern CROR, but with increased swirl relative to the analyzed conventional propeller. This is mainly due to the effect of the blade passage unloading one of the Boxprop blade halves near the tip, forcing the blade to be more highly loaded closer to the hub. The swirl generated by the Boxprop could be partially recovered if it is used together with a rear counter-rotating propeller. A Wake Analysis Method (WAM) is presented in this work and is used to quantify the power flows inherent to the flow features of the propeller wake. The power flows can be characterized as propulsively beneficial, recoverable, or pure losses. It has the ability to distinguish the kinetic energies of the tip vortices, wakes, and other disturbances from the flow field. The Wake Analysis Method was applied on the two propellers mentioned earlier, and confirmed that the Boxprop produces 50\% more swirl than the conventional propeller. Additionally, the method very clearly shows the lack of tip vortex on the Boxprop, and the presence of it in the flow field of the conventional propeller.