Comparative Assessment between Variable Area Fan Nozzle Concepts at Inner and Outer By-Pass Duct Surfaces

摘要

The next generation of civil turbofan engines target the by-pass ratios of up to 20:1, which are far beyond the current levels of 10:1 to 12:1. These jet engines require a significant step forward in low-pressure system technologies. The European Union supports within the 7th Framework the research activities for these new low-pressure systems through the program ENOVAL [1]. Such a new low-pressure technology can be achieved through an innovative fan design with a low fan pressure ratio and a radically increased fan diameter compared to the current conventional turbofan engines. The aerodynamic stability of such large fans is quite likely very sensitive against backpressure variations in the by-pass duct. For this reason, the back-pressure regulation of such a large fan, especially during the take-off operation, can be achieved through the introduction of a Variable Area Fan Nozzle (VAFN). On the given geometry, two potential design spaces were studied and the conceptual designs of two VAFN concepts were created. This paper presents a comparative study between both the concepts following the requirements of different attributes. These involve the performance assessment, structural design study, and a preliminary assessment of the design features for integration into the nacelle or in the core fairing structure. For the comparison, the clean nozzle, without any modulation of the BPD aero lines, was taken as the reference case. The VAFN assessments were carried out on two VAFN positions, over- and under-area, considering the maximum take-off and maximum climb flight conditions respectively. For the selected concepts, CFD trade studies were executed in order to compare the nozzle performance, in which three main governing design parameters i.e. discharge-, thrust performance coefficient, and drag, were examined. This research has received funding from the European Union's 7th Framework Program under grant agreement number 604999.