Performance comparison of rectangular nozzle with circular nozzle of military aircraft engine

Abstract

Aircraft performance depends on its propulsion system, and the type of exhaust nozzle used in the aircraft plays a major role in jet propulsion. The exhaust nozzle of an aircraft is also the prominent source of infrared (IR) signature from the rear-view, especially in short-wave (1.9–2.9 μm) and mid-wave (3–5 μm) IR bands. Aircraft IR signatures (passive) are used for the detection and tracking by the IR-guided (heat-seeking) missiles. Alterations in engine exhaust geometry are important to reduce the IR emissions from the aircraft’s rear-view. Nozzle shape (e.g., two-dimensional exit, lobed nozzle, S-shaped nozzle) has an important influence on the aircraft and plume IR signature, especially from the rear-view. Rectangular nozzle exit with a high aspect ratio (AR > 5) has an advantage in terms of reduced IR signature, convenience in thrust vectoring, and ease in maneuverability. The wetted perimeter increases with an increase in AR of the rectangular nozzle exit, which results in an increase in the mixing of plume (hot exhaust gases) with the surrounding. This analytical study compares the aero-engine performance and visibility of a rectangular nozzle exit of different AR with that of the circular nozzle. The rectangular exit nozzles of different AR are designed using Solid Edge and their visibility is compared with a circular nozzle of same cross-sectional area, from the direct rear-view. This study shows that the visibility of hot engine parts decreases from the rear and side views with the increase in AR, which is important in reducing aircraft IR-signature. But results in increase in visibility from the bottom view, which is undesirable concerning for the surface to air missiles. The weights of rectangular exit nozzle of different AR are compared with that of the circular nozzle. It is found that the nozzle weight increases with an increase in AR, which increases the aircraft’s lift-induced drag. For performance analysis, the effect on specific thrust is obtained for different nozzle isentropic efficiency. For isentropic flow through nozzle, the results for specific thrust are compared with the values obtained from GasTurb software.