Flow Distortion Into the Core Engine for an Installed Variable Pitch Fan in Reverse Thrust Mode

Abstract

The flow distortion at core engine entry for a Variable Pitch Fan (VPF) in reverse thrust mode is described from a realistic flow field obtained using an integrated airframe-engine model. The model includes the VPF, core entry splitter, complete bypass nozzle flow path wrapped in a nacelle and installed to an airframe in landing configuration through a pylon. A moving ground plane to mimic the rolling runway is included. 3D RANS solutions are generated at two combinations of VPF stagger angle and rotational speed settings for the entire aircraft landing run from 140 to 20 knots. The internal reverse thrust flow field is characterized by bypass nozzle lip separation, pylon wake and recirculation of flow turned back from the VPF. A portion of the reverse stream flow turns 180° with separation at the splitter leading edge to feed the core engine. The core engine feed flow exhibits circumferential and radial non-uniformities that depend on the reverse flow development at different landing speeds. The temporal dependence of the distorted flow features is also explored by an URANS analysis. Total pressure and swirl angle distortion descriptors, as defined by the Society of Automotive Engineers (SAE) S-16 committee, and, total pressure loss into the core engine are described for the core feed flow at different operating conditions and landing speeds. It is observed that the radial intensity of total pressure distortion is critical to core engine operation, while the circumferential intensity is within acceptable limits. Therefore, the baseline sharp splitter edge is replaced by two larger rounded splitter edges of radii, ∼0.1x and ∼0.2x times the core duct height. This was found to reduce the radial intensity of total pressure distortion to acceptable levels. The description of the installed core feed flow distortion, as described in this study, is necessary to ascertain stable core engine operation, which powers the VPF in reverse thrust mode.

Estimation of Resultant Airframe Forces for a Variable Pitch Fan Operating in Reverse Thrust Mode

“…The computational time in the 3D URANS runs for one timestep is 24 minutes, and typically 2500 to 3000 timesteps are required for flow development and meeting the convergence criteria, requiring nearly 650 to 750 hours of computational time for each aircraft landing speed. Details of the model development, computational domain discretization and computational solution methods are discussed in further detail in other publications by the authors [11][12][13][14].…”

“…Therefore, to address this research gap, the authors developed an integrated airframe-engine-VPF model to computationally obtain the realistic installed dynamic VPF reverse thrust behavior during the aircraft landing run [11]. Several aspects of the VPF reverse thrust behavior like the installed dynamic fan flow field, distortion transferred on to the core engine inlet during reverse thrust operation and the use of an inflatable rubber lip to improve the reverse flow were discussed by the authors in separate publications [12][13][14].…”

On the Use of an Inflatable Rubber Lip to Improve the Reverse Thrust Flow Field in a Variable Pitch Fan

“…the realistic dynamic installed reverse thrust behavior using flow solutions obtained from an integrated airframe-engine research model [11][12][13]. The installed reverse thrust behavior was obtained by considering the VPF operating in a baseline representative unmodified aircraft-engine configuration without any flared nozzle 'exlets' as in the outdoor engine tests.…”