In the framework of Research and Development in the field of turbomachinery, in order to meet the forthcoming regulations in civilian air traffic related to environmental constraints on noise and emissions, new concepts of engine integration are currently under study. These evolutions and constraints could lead, for instance, to installations over the airframe to reduce noise. Due to the evolutions of size, a close coupled installation, with engines partly buried inside the airframe could be an optimum solution. On the aerodynamic point of view, this solution presents drawbacks due to significant aerodynamic engine/airframe interference effects or the fact that the boundary layer on the airframe can be swallowed by the engine intake, with worse engine operating conditions. Indeed, from the turbo-machinery point of view, semi-buried installations lead to high levels of distortion of the aerodynamic field upstream the engine. The distortion impacts the engine performance but also reduces the operating range because of loss in surge margin.
Possible future engine installation architecture, with semi-buried air intake
(Source: Godard, Int. Congress of Aeronautical Sc.)
In the present study, part of the RAPRO (RAdical PROpulsion) program, the effect of a total pressure distortion on the surge margin of a fan has been numerically investigated. The distortion map has been delivered by Safran Snecma and applied as injection condition. The computational domain has been meshed by Altran and the simulations performed by Andheo with the use of Onera's elsA software.
Firstly, only radial distortion has been considered, enabling steady simulations. The performances of the fan have been compared with the case without distortion. The loss in surge margin, although visible, is limited to 5 percent. Special attention has been paid to the flow in the tip clearance, since the tip vortex is responsible for surge inception.
Secondly, circumferential distortion has been considered, leading to 3D time-accurate computations. The effect on the surge margin is much larger, since the loss reaches 12 percent. The conditions of maximum efficiency are never reached. To understand the physical phenomena responsible for this loss, and to help the turboreactor manufacturer to identify solutions, the effects of the distortion on the upstream flow conditions have been analyzed and correlated to the mass flow time history, aerodynamic load on the blade, local flow regimes and topologies.
The computation resources required for such time-accurate simulations are very large. In the present study, the calculations have involved more than 1500 hours / 62 days of CPU (Computational Power Unit) on a vectorial supercomputer, provided by Onera.
The study has been presented during the 46th AAAF International Symposium on Applied Aerodynamics, held in Orléans (France) in 2011, dedicated to aerodynamics of rotating bodies.