Impact of Using Chevrons Nozzle on the Acoustics and Performances of a Micro Turbojet Engine

This paper presents a study regarding the noise reduction of the turbojet engine, in particular the jet noise of a micro turbojet engine. The results of the measurement campaign are presented followed by a performances analysis which is based on the measured data by the test bench. Within the tests, beside the baseline nozzle other two nozzles with chevrons were tested and evaluated. First type of nozzle is foreseen with eight triangular chevrons, the length of the chevrons being L = 10 percentages from the equivalent diameter and an immersion angle of I = 0 deg. For the second nozzle the length and the immersion angle were maintained, only the chevrons number were increased at 16. The micro turbojet engine has been tested at four different regimes of speed. The engine performances were monitored by measuring the fuel flow, the temperature in front of the turbine, the intake air flow, the compression ratio, the propulsion force and the temperature before the compressor. In addition, during the testing, the vibrations were measured on axial and radial direction which indicate a normal functioning of the engine during the chevron nozzles testing. Regarding the noise, it was concluded that at low regimes the noise doesn’t presents any reduction when using the chevron nozzles, while at high regimes an overall noise reduction of 2–3 dB(A) was achieved. Regarding the engine performances, a decrease in the temperature in front of the turbine, compression ratio and the intake air and fuel flow was achieved and also a drop of few percent of the propulsion force.

Study of the Position of Sound Sources in a Turbulent Jet Using Nozzles of Different Configurations

The aim of this work is to study the position of dominant sound sources in a small-scale turbulent jet using the beamforming method. Two nozzles of equivalent diameter and different geometric configurations (conical and chevron) were used to create different initial conditions for the outflow. Based on the analysis of the results obtained, it can be concluded that they are in good agreement with the well-known concepts of the physics of noise generation processes by turbulent jets: higher-frequency sources are generated by smaller-scale turbulent structures located closer to the nozzle edge, which is confirmed by the localization of high-frequency noise sources also closer to the nozzle edge. The chevron nozzles loosen the initial section of the jet, making it less short and thereby facilitating a faster displacement of noise sources to the nozzle edge, as seen when comparing localization maps for conical and chevron nozzles at the same frequencies. The results of localization were compared with the data obtained by other researchers. The results obtained were found to provide confidence in the use of acoustic beamforming to measure the location of the jet noise source with accuracy similar to other methods that have been used in the past.