The Impact of Low Speed Fan Design on Noise: An Exploratory Study
The steady evolution since the 1950s toward higher bypass ratio engines has enhanced the acoustic role of the fan compared to the jet. This paper addresses the following question: Does a further decrease in fan pressure ratio (FPR) and rotor tip speed provide a significant reduction of fan broadband and tonal noise? The paper presents two conceptual parametric studies conducted with a fast analytical aerodynamic and acoustic prediction tool. The tool includes an aerodynamic fan design model which provides the quantities necessary to assess the tradeoff between efficiency and noise at given thrust conditions. The fan acoustic model has a theoretical formulation for broadband and tonal noise sources which is not based on empirical correlations; it is applied on conventional and contrarotating fan concepts. The first study proposes a variation of the design FPR and evaluates for each concept its impact on noise at three acoustic off design points. The results obtained, which are in line with a past NASA study, indicate that the optimum pressure ratio in terms of fan noise is well below the fuel burn optimum. Significant noise reductions of the broadband and tonal interaction components can be achieved with fans operating in a fully subsonic domain. Alternatively, designing at higher speed and pressure ratio near the fuel burn optimum may invite to consider the contrarotating fan as a candidate: it performs very well in terms of buzz saw and broadband noise compared to the conventional fan. The second study addresses the variation of design rotor tip speed at constant FPR. Although reduced tip speed may suppress buzz saw noise, the increased loading related to it implies large blade solidities and wakes which causes a significant increase in broadband noise. Thus, there is an optimum loading that will depend on the severity of fan inflow distortion and on the onset of buzz saw noise. Here again these conclusions confirm some experimental work performed by NASA on two different fans, and by Rolls Royce on a third one.