The Benefits of Variable Area Fan Nozzles on Turbofan Engines

Abstract

A variable area nozzle (VAN) on the bypass stream of a turbofan engine may prove to be necessary for a sufficient surge margin on an ultra-high bypass ratio fan. It is shown that such a nozzle also has benefits with respect to fuel burn and noise. The fuel burn will reduce typically about 10 % during departure and approach and likely 2 % in cruise. The noise reduction is in the order of 2 dB for sideline and flyover. Jet mixing noise in cruise is reduced by typically 1.0 dB, the reduction of broadband shock noise being larger. A VAN complements the fan rotational speed n with an additional parameter for controlling the working point on the fan performance map. The smallest nozzle area is needed only for a single flight condition, TOC, while for all other conditions, the nozzle exit area can be enlarged, yielding lower fan pressure ratios, higher mass flows and propulsive efficiencies, lower noise emissions and higher flutter margins for the fan. The benefits are studied for a UHBR fan with a pressure ratio of 1.5 at TOC. The optimal nozzle enlargement for this fan varies from small values in the range of a few percent during cruise to values larger than 15 % for take-off and approach, depending on the actual fan performance map.