Air inlets or intakes are used in flight vehicles to convey ambient air to the propulsion system. Inlets have received particular attention from both flight vehicle and aero-engine manufacturers because of their performance effects on the vehicle drag and on the safe and efficient operation of the engine itself. For a fuselage-embedded engine, a curved diffuser called an S-duct, as shown in Fig. 1, is used to route air from the side or underside of the flight vehicle to the interior and is therefore an important part of the inlet. The design and aerodynamic performance of the inlet and consequently that of the S-duct diffuser vary from one flight vehicle to another depending upon the engine location, airframe configuration, and flight environment [1,2]; therefore, it is prudent to document the performance of each and every type of inlet in various flight conditions.
The flow in the S-duct diffuser is complex due to two or more curves in the duct and to changes, sometime abrupt, in the cross-section area and shape. The important geometric characteristics of an S-duct diffuser, as shown in Fig. 2, are the centerline curvature, the wall curvature profile, the area ratio between the exit and entrance cross-section, axial length and offset. The ratio of the entrance and exit area of an inlet depends on the flight Mach number of the vehicle and the required Mach number at the engine face, which is generally between 0.4 and 0.6, the highest level being for aero-engines in supersonic applications [3]. The overall goal of the present study on an inlet of a propulsion system of a high-subsonic flight vehicle was to conduct a parametric investigation of generic (or representative) geometry (area ratio, offset, length, and entrance aspect ratio) and flight condition (e.g. Mach number, incidence and sideslip angle of the flight-vehicle).
This paper focuses on the effect of the entrance aspect ratio on the internal-aerodynamic performance of the S-duct diffuser. The generic baseline was a rectangular-entrance, transitioning S-duct diffuser in high subsonic (Mach number 0.8 to 0.90) flow. The test section, shown in Fig. 3, was manufactured using rapid prototyping to facilitate parametric investigation of geometry. Since the S-duct diffuser performance is characterized by static pressure recovery, total pressure loss and engine face steady-state distortion [2], streamwise static pressure and exit-plane total pressure were measured in a test-rig, as shown in Fig. 4, and was simulated through computational fluid dynamics. The investigation indicated the presence of streamwise and circumferential pressure gradients leading to three dimensional flow in the S-duct diffuser and distortion at the exit plane. Total pressure losses and circumferential and radial distortions at the exit plane were higher than that of the podded nacelle type of inlet. The results for two aspect ratios, while keeping all the other geometric parameters constant, will be presented in the final paper. The work represents the beginning of the development of a parametric database for the performance of a general S-duct inlet. This database will be useful for predicting the performance of aero-engines and air vehicles in high subsonic flight.
References
1. Goldsmith, E.L. and Seddon, J., 1993, Practical Intake Aerodynamic Design, AIAA Education Series, 2nd Edition
2. Seddon, J. and Goldsmith, E.L., 1999, Intake Aerodynamics, AIAA Education Series.
3. Walsh, P.P. and Fletcher, P., 2004, Gas Turbine Performance, 2nd Edition, Blackwell Publishing, ASME Press
Figures:
Fig. 1. Example of an Inlet with an S-Duct Diffuser.
Fig. 2. Main Geometric Parameter of an S-Duct Diffuser
Fig. 3. S-Duct Diffuser Test Section without a Bellmouth
Fig. 4. S-Duct Diffuser Test Rig at RMC
Topics: Integration of propulsion systems into vehicles , Topics: Experience: operational, qualification, certification
