CHEVRON NOZZLES TITLE CHEVRON NOZZLES Students name zeinab

CHEVRON NOZZLES

TITLE : CHEVRON NOZZLES Student’s name : zeinab tahmasi jahed Date of preparation & adjust : fall / 1397 (helical hegira)

1. INTRODUCTION Environmental concern and strict noise regulations around major airports have made jet noise a crucial problem in present-day aeroacoustics research. Jet noise continues to be the dominant noise component during take-off, in which the exhaust conditions may become underexpanded. Current jet noise research is directed towards three main areas: improvement in noise prediction tools; better understanding of the noise generation mechanisms; and investigation of noise reduction devices. While designing noise reduction strategies, the associated penalty such as increasedweight, thrust and performance losses of the engine, and so on have to be

What is “Chevron Nozzle & its roles“?

Turbofan Engine & Its Noisy Problem : Let me get a little basic first. A nozzle is any hollow cylinder which is larger in circumference at the inlet side and narrower at the outlet. Hence a turbofan engine, ones used on modern airliners like the Boeing 737 and the Airbus A 320 are essentially, nozzles. A modern airliner engine is a TURBOFAN engine. This is a modified jet engine. a TURBOFAN engine derives 70% of its thrust from this huge fan at the front.

The air the fan throws back is not all sent into the compressor in the center, most of it bypasses the jet engine and goes out the sides of the engine casing right to the back and gets shot out at the SAME place where your jet engine is shooting out its hot air steam. The air the TURBOFAN is throwing back directly is COLD and the air coming out the jet engine turbines is very HOT. the mixture of this air, owing to the temperature difference is very NOISY.

… and Chevron Nozzle is used to counter the problem of NOISE.

Geometrical Difference Between Chevron & Baseline Nozzles: A Chevron Nozzle is a relatively new concept. The Boeing 737 engines apply this concept. It simply cuts the rear circumferential end (trailing edge) of the engine, not straight but in a repeated sawtooth pattern. You can name nozzles without chevrons , baseline nozzle.

How Chevron Nozzles Counter Generating Huge Noise? A. Low-pressure spool B. High-pressure spool C. Stationary components 1. Nacelle 2. Fan 3. Low-pressure compressor 4. High-pressure compressor 5. Combustion chamber 6. High-pressure turbine 7. Low-pressure turbine

The air coming out of the fan nozzle(9) is subsonic, whereas the hot jet coming of the core section of the engine is relatively hot due to combustion energy and high in speed, due to CD (convergent and divergent) duct made by the core nozzle (8) and its shroud. this imparts supersonic profile to the core air. Now we will discuss the mixing of these two air viz, fan nozzle air (9)─ subsonic relatively higher speed, and core nozzle air(8)─ supersonic, and the ambient air (10)─ subsonic, relatively low speed; all mixing at the aft-most section of the engine.

The separate exhausts from the core nozzle and the fan nozzle are high velocity jets typically having maximum velocity during take-off operation of the aircraft with the engine operated under relatively high power. The high velocity jets interact with each other as well as with the ambient air and produce substantial noise along the take-off path of the aircraft.

Furthermore, the core jet is hot and produces an infrared signature which may be detected from afar. The solutions typically rely on vigorously mixing the hot core jet with the fan jet, or ambient air, or both. for reducing the velocities thereof and reducing the temperature thereof, by allowing the air mix in 3 Dimensions, where, in addition to the existing axial component of the air mixing, a transversal(radial) component of the air is allowed to escape thru the "valleys“ into the jet stream or bypass stream based on the direction of local differential pressure.

Therefore, chevron nozzles constitute a 3 D mixing profile where the air interaction path is increased by the virtue of zig- zag profile of the chevron nozzle. . .

Some Geometrically Types Of Chevron Nozzles: Now let spend a few time on some geometrically types of chevron nozzles. The chevron lobe profile has a significant effect on performance. Most popular of these lobes are: baseline chevron nozzles conventional chevron nozzles sinusoidal chevron nozzle asymmetric chevron nozzle

Two new chevron nozzle concepts brought here ; One concept explores the possibility of using a sinusoidal chevron profile whereas the other looks at the effect of asymmetry in the chevron configuration. Now we want to make comparison between them at geometry and some Acoustic characteristics such as OASPL(=overall sound pressure level) , spectra, directivity, acoustic power, and broadband noise.

A Few Comparison Between Geometrically Types Of Chevron Nozzles: Geometrically , For sinusoidal profile chevrons (Chev 6 -Sine), the edges of the sinusoidal protrusions are made smooth to eliminate sharp corners. The asymmetric chevron nozzle (Chev 6 -Asym) is fabricated by cutting equilateral and isosceles triangular lobes at the trailing edge of the nozzle. The isosceles triangular chevron lobes have larger depth when compared with equilateral chevron lobes. The triangular lobes are Placed such that the apexes of all the chevrons lie on a plane perpendicular to the nozzle axis.

Some Experiments were conducted over a range of NPRs(= nozzle pressure ratios) from 1. 5 to 5. 0. The results indicate that both chevron asymmetry and chevron lobe profile have considerable effect on noise reduction. There is a reduction in OASPL of ∼ 2. 5 d. B for both of the symmetric chevron nozzles (Chevron 6 -0 and Chev 6 Sine) tested. However, at higher values of NPR, the sinusoidal chevron nozzle (Chev 6 -Sine) shows better noise reduction benefits for all emission angles due to the influence of the smooth chevron lobe profile.

A marginal reduction in OASPL of∼ 1 d. B is observed for Chev 6 -Asym asymmetric nozzle along the longer isosceles triangular side, and a considerable reduction of 4. 5 d. B is observed along the smaller equilateral triangular side. This asymmetry may be usefully exploited in practical implementations. It is also observed that the forward angles are noisier compared to aft angles for all chevron configurations , at all NPRs. The quadruple sources ( = OASPL, spectra, directivity, acoustic power ) convected downstream by the mean flow at a relatively high velocity tend to radiate

Now I want spend more on the Acoustic power and subsequently the Acoustic efficiency as the last part of my presentation. The acoustic power in a given slice area termed, as slice power, is the product of the intensity at the corresponding slice area. The total acoustic power is obtained by summing up the slice power over the entire spherical measurement surface as shown in Fig. 18.

The baseline nozzle has a peak value for total power when it screeches, and as the NPR increases the total power decreases due to the cessation of screech. The same trend can also be visualized in the variation of acoustic efficiency. Since screech is not present in chevron nozzles , an increase in total acoustic power is due to the influence of broad band shock associated noise. The ratio of the total acoustic power over the spherical measurement surface for a given chevron nozzle to the total acoustic power of the baseline nozzle is defined as the acoustic power index(API)

it is seen that the minimum value of API occurs at a Mach number close to 1. 5, which falls in the screeching zone, where the total power developed for the baseline nozzle is maximum. API values higher than unity are not desirable since the nozzles corresponding to these values are noisier than the baseline nozzle. The sinusoidal chevron nozzle is marginally better than Chevron 6 -0 even though both have same trends of API variation. Chev 6 -Asym has low values for API that lie entirely in the acoustic benefit region, and hence this nozzle gives the maximum effectiveness in noise reduction. The total power of the asymmetric chevron nozzle is calculated by averaging the sound

Conclusion: Acoustic efficiency is calculated by taking the ratio of the total acoustic output power to the input power represented by the kinetic energy of the jet. The kinetic energy of the jet can be obtained by the simple relation connecting choked mass flowrate and the square of the exit velocity of jet from the nozzle. the acoustic efficiency of baseline nozzle is higher at all Mach numbers since the kinetic energy of the jet is converted to acoustic energy in a better way. Since this acoustic energy is significantly small , the values of acoustic efficiencies are less than 1 per cent. The efficiency of the chevron nozzles are less when compared with the baseline round nozzle, which is obviously due to increased mixing in the chevron lobe region. The least acoustically efficient nozzle gives the maximum noise reduction benefit since the conversion of kinetic energy of jet to the acoustic power is comparatively less. Chevron

References: 1. Samarth Singh (Sam), Commercial Pilot - King Air B 200 / 350, “Jet Engines: What is the role of "Chevron Nozzle" in Boeing B 787 Dreamliner Aircraft? ”. 2. P S Tide 1, 2 and K Srinivasan 1∗, ”Novel chevron nozzle concepts for jet noise reduction”.

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