Noise from Turbojet's Exhaust

[Can-type]

The Turbofan engine had been conceptualized because of the Noise from the exhaust of a turbojet.

My question is : The noise created by the exhaust of a turbojet is due to the difference in velocity between ambient air and exhaust or due to difference in temperature of the exhaust and ambient?

[Intruder]

Velocity -- sheer stresses between the static air and high-velocity exhaust stream.

[Can-type]

Yeah.. thanks... I felt the same too... but the temperature factor came into the picture while discussing with my friend...

Thanks again!!!

[Landroger]

The Turbofan engine had been conceptualized because of the Noise from the exhaust of a turbojet.

Err..... point of order Mr. Chairman. :rolleyes: Whittle conceptualized the multi spool, high bypass fan jet a long, long time before it was possible, let alone fashionable. He knew that a very large volume of relatively slow moving air was much more efficient than a small volume of very fast moving air, but he had to put up with what was just possible in 1936. :=

The fact that a High Bypass fan jet is orders of magnitude quieter than a plain turbojet is by way of being a bonus, it was efficiency Mr. Whittle sought. :)

[BBK]

Can type

I'm sure temperature plays a part but the noise, IIRC, is proportional to the eighth power of the jet velocity. Believe this is called the Lighthill law. All the more reason to get the exhaust velocity down - which modern turbofans do of course.

Also from my dodgy memory, Lighthill was a director of RAE Farnborough??

[Mr Optimistic]

This was suggested as the subject for a PhD thesis at CalTech in the 70's (characterisation of noise sources for a jet engine). So no easy answer ! I know this because it was in my class.

Chaos and turbulence. Good luck.

[Can-type]

BBK and Mr. Optimistic Thank you!!

Thinking about the temperature factor, I guess it would play a part or else engine designers wouldn't have had a mixer unit in the exhaust assembly to allow fan air to mix with exhaust to reduce its temperature before being expelled from the engine...
(I guess i shouldn't think too much about this and focus on flying an aircraft and leave the designing part of it to the manufacturers):)

LandRoger

Sir Frank Whittle successfully flew the first Jet engined aircraft. He did not conceptualize the TurboFan
Manufacturers realised that in terms of Propulsive efficiency the turbojet and turboprop were at opposite ends of the graph and needed something intermediate.. Hence the Turbofan (Low Bypass and High Bypass)
If u can get access to an Oxford Powerplant book, pls refer Gas Turbine Engines Section, Chapter 1, Page 13.

[Landroger]

Sir Frank Whittle successfully flew the first Jet engined aircraft. He did not conceptualize the TurboFan
Manufacturers realised that in terms of Propulsive efficiency the turbojet and turboprop were at opposite ends of the graph and needed something intermediate.. Hence the Turbofan (Low Bypass and High Bypass)

Err ... no, the Germans flew the first jet engined aircraft - Heinkel He178? - using a axial compressor type engine developed from Whittle's paper on the subject. 1927 0r 1928 was that, without looking a reference material.

Whittle had patents on a wide variety of jet engine developments which had all expired by the time they were possible. Fan or at least high bypass jets, propjets, axial compressors, water injection and so on. I understand that Whittle's calculations were so sound, they are used today in the design of modern engines?

ROger.

[Can-type]

I stand corrected. Thank you for the information Sir!!

[AerialNinja]

Landroger,

I'm pretty sure that the He 178 was powered by a centrifugal-flow engine

[henry crun]

I agree with AerialNinja.

[Brian Abraham]

He 178 was powered by the HeS 3 engine which had a centrifugal compressor of 16 blades.

http://www.xs4all.nl/~jqmgrdyk/jetpower/HeS3a-Ohain.jpg

Dr. Hans-Joachim Pabst von Ohain standing beside the HeS 3A jet engine in the Deutsches Museum.

http://www.century-of-flight.net/Aviation%20history/jet%20age/25.jpg

[Landroger]

I too stand corrected guys, thanks. :) I knew Whittle's engine was a centrifugal compressor and I knew that the Me262 had Jumo engines that were axial compressor and I somehow assumed (! :eek:) that the Germans had all gone for that type. I believe the Me 262 was prone to and maybe plagued by tangentially departing compressor blades? :uhoh:

Whittle definitely had patents for axial flow compressors, but elected (was driven?) to go with the centrifugal compressor as being slightly less technically challenging, the rest of the engine being challenging enough. Especially given the enduring penury and politics that plagued Power Jets Ltd. :ugh:

Thanks for the images Brian, never seen by me before. :ok:

Roger.

[Jane-DoH]

Now here's a puzzling question. I understand that hotter exhaust gasses produce more noise; I understand that turbofans work by surrounding the hot gasses with a stream of cooler gasses, which additionally absorb heat from the core; what puzzles me is that there was at least one proposal for a nose reducing mechanism that had the exhaust velocity lower in the middle of the exhaust stream and higher around the edges?

Why does that work? That sounds like the worst thing you could do!