Impact of installation on a civil large turbofan exhaust system at idle descent conditions

Abstract

Recent trends in civil aero-engine design aim at lowering specific thrust and improving propulsive efficiency by increasing the bypass ratio and therefore, usually also the fan diameter. The integration of these larger diameter engines with the airframe is critical to exhaust performance, and it is important to include these effects in engine performance analysis. The discharge coefficient of the bypass and core nozzles of a high-bypass ratio aero-engine at idle descent conditions is investigated numerically for an aero-engine in isolation and installed on an airframe. The discharge coefficients influence the engine operating conditions and turbomachinery re-matching at these off-design conditions. The maximum difference in the bypass nozzle discharge coefficient between the installed and isolated aero-engine across the descent phase is ≃1.6%. The differences in the core nozzle discharge coefficient between the installed and the reference isolated configuration are ≃43% and ≃−5.4% at the start and the end of the descent phase, respectively. The nozzle discharge coefficients depend on flight Mach number, incidence angle, and the nozzle pressure ratios of the fan and core nozzles. Multiple competing flow mechanisms govern the static pressure on the core nozzle base, which influences the core nozzle discharge coefficient. A novel reduced-order model is developed to estimate the core nozzle discharge coefficient for the installed configuration in idle descent conditions. This approach is based on the effective nozzle pressure ratio and can be implemented in engine performance simulations.