The Effect of Bypass Nozzle Exit Area on Fan Aerodynamic Performance and Noise in a Model Turbofan Simulator
Authors: Hughes Christopher E. , Podboy Gary G. , Woodward Richard P. , Jeracki Robert J. 
Published: November 2008

The design of effective new technologies to reduce aircraft propulsion noise is dependent on identifying and understanding the noise sources and noise generation mechanisms in the modern turbofan engine, as well as determining their contribution to the overall aircraft noise signature. Therefore, a comprehensive aeroacoustic wind tunnel test program was conducted as part of the NASA Quiet Aircraft Technology program called the Fan Broadband Source Diagnostic Test. The test was performed in the anechoic NASA Glenn 9- by 15-Foot Low Speed Wind Tunnel using a 1/5 scale model turbofan simulator that representative of a current generation, medium pressure ratio high bypass turbofan engine. The investigation was focused on the simulated bypass section of the turbofan engine. The technical objectives of the test were not only to identify the noise sources within the model and determine their noise level, but also to investigate several component design technologies by evaluating their impact on the aerodynamic and acoustic performance as well as conducting detailed flow diagnostics within the research model to help in understanding the physics of the flowfield. This report will present details of the results obtained for one aspect of the test that investigated the effect of the bypass nozzle exit area on the bypass stage performance, specifically the fan and outlet guide vanes, or stators. The aerodynamic performance, farfield acoustics, and Laser Doppler Velocimeter measurements obtained for the fan and four different fixed-area bypass nozzles. The nozzles represented fixed engine operating lines encompassing the operating envelope of the turbofan engine from near stall to cruise, with a total change in area from the smallest to the largest nozzle of 12.9%. One nozzle exit area was selected as a baseline reference, and its area was 2% larger than the smallest nozzle and 10.9% smaller than the largest nozzle. The results will show that there are significant changes in aerodynamic performance and farfield acoustics as the nozzle area is increased. As the fan exit nozzle area was increased, the weight flow through the fan model increased between 7% and 9%, the fan and stage pressure dropped between 8% and 10%, and the adiabatic efficiencies increased between 2% and 3% — the magnitude of the change dependent on the fan speed. Results from force balance measurements made of fan and outlet guide vane thrust will show that as the nozzle exit area is increased the combined thrust of the fan and outlet guide vanes together also increases, between 2% and 3.5%. In terms of farfield acoustics, the overall sound power level produced by the fan model dropped between 1 and 3.5 dB as the nozzle exit area was increased, with the larger decrease in noise occurring near approach conditions and the smaller decrease near takeoff condition. Both fan tone and broadband levels are discussed. The amount of area the fan exit nozzle can be opened was limited, as the largest of the four nozzle designs encountered performance problems at full power takeoff conditions, at which point its performance was actually worse both in terms of lower aerodynamic performance and higher noise levels compared to the baseline nozzle. Finally, flow diagnostic results in the form of fan swirl angle survey data and Laser Doppler Velocimeter mean velocity and turbulence measurements obtained downstream of the fan within the wake will show that the noise of the fan module decreases as a result of lower swirl angles and lower turbulence levels within the wake as the fan exit nozzle area increases.