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Exhaust System
Published in Ahmed F. El-Sayed, Aircraft Propulsion and Gas Turbine Engines, 2017
Turbofan engines are quieter than turbojet engines as the exhaust velocities of both streams of turbofans are less than those for turbojets. Mixing of the flows in turbofan engines for the purpose of noise reduction was examined as early as the 1970s [23]. Extensive research was carried out by air forces in the 1980s as they started looking for ways to reduce fighters’ infrared signature. It was noticed that those same nozzles had an effect on noise emissions as well. Several nozzle designs were tested to reduce the fully expanded jet velocity by mixing (1) core flow with fan flow only, (2) fan flow with ambient flow only, or (3) both flows simultaneously. Depending on the type of mixing attempted, these designs fell into two broad categories: tabs and chevrons. Tabs are severe protrusions into the flow at the nozzle exit plane. Chevrons are also protrusions, but of much less severity than tabs. The aggressive mixing produced by the tabs greatly reduced low-frequency noise, but with the penalty of tab-induced high-frequency noise. Chevrons, which provided a more balanced approach to mixing, reduced low-frequency noise without significant chevron-induced high-frequency noise. Other nozzle designs attempted to shield the core flow by using a scarf-fan nozzle and an offset-fan nozzle for mixed-flow turbofan engines; mixing of both hot and cold streams greatly reduces the noise.
Experimental investigation on effect of exit shape of nozzle on the flow Mach number and noise
Published in International Journal of Ambient Energy, 2021
R. Sarweswaran, P. Manivannan, P. Booma Devi
Myung and Rokaf (2013) explained about Technologies of jet Noise Abatement for the Republic of Korea Air Force and they introduced the various nozzle designs from the Advanced Subsonic Transport (AST) program such as Doublets, scar fed, tabbed, chevrons, tongue, lobed nozzles. They completely showed the Schematic diagram of a localized arcing plasma actuator system. Shriwas, Prashant Singh, and Kaurase (2015) completely explained about the decline of jet noise in exhaust nozzle owing to the high turbulence structure created in the jet shear. They showed different types of nozzles images with specifications such as cone, bell, spike, E-D, R-F, H-F nozzles. They also mentioned various probable techniques to decrease jet noise such as chevron nozzles, nano-technology, aircraft engine configuration, acoustic liner, configuration of nozzle and increasing the bypass ratio in turbo fans.