Intake Analysis

Moderator: Mike Everman

hinote
Posts: 1241
Joined: Sat Oct 04, 2003 1:54 am
Antipspambot question: 0
Location: Central California

Intake Analysis

Post by hinote » Sat Aug 21, 2004 3:19 pm

Hello all:

I was going to post this on Larry's thread about his Uflow analysis, but I decided to start a new one instead.

Attached is a Uflow "snapshot" of the intake analysis of my current engine project. The location is just behind the intake flare.

It's amazing how the engine can force the intake to do its bidding, when the design is working.

Note the sharp pressure spikes (top left graph) as the intake tube resonates during the outflow; it's possible to estimate the frequency (by going to the numerical file and finding the peak pressure events and then looking at the time differential). Note also the damping of the pressure oscillations that takes place as cold air is drawn in, during the last half of the cycle.

On the mass flow graph (lower right) the outflow (below the horizontal line) is also notched in appearance, from the pressure fluctuations--but it's all outflow; then, the intake switches over into solid inbreathing (above the line) during the 2nd half of the cycle.

Also note how rapidly the intake flow goes up to about Mach .7 or so, (top right graph), and stays there until very near the end of the cycle when the building pressure inside the comb. chamber starts to overpower the flow somewhat--but its's still pretty high at the end of the cycle.

This intake analysis would roughly apply to Lockwoods, Ecrevisse--but not the Snecma Escopette.

I hope everyone finds this interesting.

Bill H.
Acoustic Propulsion Concepts
Attachments
M1E intake analysis.JPG
M1E intake analysis.JPG (36.66 KiB) Viewed 7114 times

NanoSoft
Posts: 261
Joined: Fri Feb 20, 2004 5:08 am
Antipspambot question: 125

Re: Intake Analysis

Post by NanoSoft » Sat Aug 21, 2004 4:09 pm

Very interesting indeed. I have been playing around with Uflow but due to the lack of nowing temps and how to analyis the graphs, its not going to well.

Nanosoft

skyfrog
Posts: 539
Joined: Fri Jul 23, 2004 11:39 am
Antipspambot question: 0
Location: Kaohsiung, Taiwan
Contact:

Re: Intake Analysis

Post by skyfrog » Sat Aug 21, 2004 4:20 pm

Hi Bill,

Very interesting indeed.

I'd like to do this analysis to my little creature too, how do I start ?

Any help will be much appreciated.
Long live jet engine !
Horace
Jetbeetle

Graham C. Williams
Posts: 571
Joined: Fri Oct 03, 2003 11:33 pm
Antipspambot question: 0
Location: England
Contact:

Re: Intake Analysis

Post by Graham C. Williams » Sat Aug 21, 2004 10:16 pm

Dear Bill.
I don't want to hijack your thread just make a few things clear to others.

1st look at the Mass flow chart. If the Graph is above the central line Flow is going to the Right and when the Graph is below the central line flow is going to the Left. The first thing to notice is that at each small time division the mass flow rate at this one location in the induction pipe is changing.
Next look at the density chart. It is roughly in three parts. The high-density gas at both ends and the patch of low density gas about 1/3 way along. This low-density gas will be the combustion (hot) gas and the high-density gas is most probably cold air.
So, just looking at these two graphs we can see the movement of first cold gas then hot gas then cold again past Bill's chosen point in the induction pipe. The Mass flow chart tells us the direction that the partials of gas are taking at a given time and the density chart can tells us about the Hot/cold interfaces. The density chart tells a lot about the motion of the various Hot/Cold interfaces in the motor. The Hot /Cold interface is that almost wall like transition between the densities of the gas. It should be no surprise that the highest mass flow rates can come from the movement of high-density (cold) gas.

Let's step through time with just these two graphs. I must remember that we are looking at one point in space and stepping through time.
Things start with cold gas moving to the Right and it is not until some time later that a pressure wave changes the density at this point that we can have any knowledge of events further down the pipe. During the major pressure waves the mass of gas at this point is brought to rest then changes direction. Cold air is now going to the Left.
A short time after the major pressure changes the Cold/Hot interface passes us, again moving to the Left. Around this time the gas flow is brought first to rest then changes direction again. We see the Hot/Cold interface then the cold (high density) air moving to the Right. This condition continues till the end of the analysis.

Graham.

NanoSoft
Posts: 261
Joined: Fri Feb 20, 2004 5:08 am
Antipspambot question: 125

Re: Intake Analysis

Post by NanoSoft » Sun Aug 22, 2004 1:03 am

Thank you Graham for that complete explanation. I put my gas lockwood into Uflow and this is the result i get. Analysis point is half way in to the intake stack.

Looks very interesting to me. A lot more waving then hinote's.

Nanosoft
Attachments
LH Intake Graph.jpg
Intake Analysis of lockwood
LH Intake Graph.jpg (55.84 KiB) Viewed 7061 times

mk
Posts: 1053
Joined: Mon Jan 19, 2004 8:38 pm
Antipspambot question: 0
Location: FRG

Re: Intake Analysis

Post by mk » Sun Aug 22, 2004 11:16 am

To NanoSoft:

You're watching the effects of ONE single combustion (Kadenacy oscillation if you will)!

Look at Bill's graph limit (0.0062) and your graph limit (0.1). Try to equal the scale first.
mk

NanoSoft
Posts: 261
Joined: Fri Feb 20, 2004 5:08 am
Antipspambot question: 125

Re: Intake Analysis

Post by NanoSoft » Sun Aug 22, 2004 3:09 pm

Woops my bad. I thought it looked a little funny. Shows how much i know.

Nanosoft

Mike Everman
Posts: 4930
Joined: Fri Oct 31, 2003 7:25 am
Antipspambot question: 0
Location: santa barbara, CA
Contact:

Re: Intake Analysis

Post by Mike Everman » Sun Aug 22, 2004 3:21 pm

Everyone take the time to decipher what Graham is saying. Uflow does not model real life transitions in temperature, you end up with sharply defined walls of temperature change, which as he says are very reflective acoustically.
Sometimes it's good to make all of the temps the same, so that you can see if these local modes you are looking at are not just "dinging back and forth between cold fronts". Real life temperature gradients are much softer due to turbulence and heat transfer.

Back to basics a bit, but look at the plots that Bill and Nano have posted. If you want to know the frequency, count the peaks and divide by the time at the bottom of the graph. You'll notice that the thing Bill is focusing on is a very high frequency thing, about 1,400 Hz, many times the operational cycle; whereas Nano's plot is of an event that "rings down" at about 115 Hz.
Mike
__________________________
Follow my technical science blog at: http://mikeeverman.com/
Get alerts for the above on twitter at: http://twitter.com/mikeeverman

hinote
Posts: 1241
Joined: Sat Oct 04, 2003 1:54 am
Antipspambot question: 0
Location: Central California

Re: Intake Analysis

Post by hinote » Sun Aug 22, 2004 4:08 pm

hinote wrote:

Note the sharp pressure spikes (top left graph) as the intake tube resonates during the outflow; it's possible to estimate the frequency (by going to the numerical file and finding the peak pressure events and then looking at the time differential). Note also the damping of the pressure oscillations that takes place as cold air is drawn in, during the last half of the cycle.
The frequency of the interval between the 2 sharp spikes is 1739 Hz (divide the time difference between the 2 spikes into unity); this corresponds to an acoustic temp of 711C or 1312 F, for the acoustic length of the intake tube I'm using.

This seems like a reasonable number, don't you think?

Bill H.
Acoustic Propulsion Concepts

Mike Everman
Posts: 4930
Joined: Fri Oct 31, 2003 7:25 am
Antipspambot question: 0
Location: santa barbara, CA
Contact:

Re: Intake Analysis

Post by Mike Everman » Sun Aug 22, 2004 4:49 pm

Sure, sounds right. Isn't the question going to be: Should this frequency match or avoid one of the higher harmonics of the entire duct? or: Can we hope to get this higher frequency in synch, and is it worth it if we have?
Mike
__________________________
Follow my technical science blog at: http://mikeeverman.com/
Get alerts for the above on twitter at: http://twitter.com/mikeeverman

hinote
Posts: 1241
Joined: Sat Oct 04, 2003 1:54 am
Antipspambot question: 0
Location: Central California

Re: Intake Analysis

Post by hinote » Sun Aug 22, 2004 6:01 pm

Mike Everman wrote:Sure, sounds right. Isn't the question going to be: Should this frequency match or avoid one of the higher harmonics of the entire duct? or: Can we hope to get this higher frequency in synch, and is it worth it if we have?
Well, that was an interesting question, when I looked at the answer.

I'll tell you one part, and keep the other to myself.

Mike, try using the temporal analysis on one of your engines. Enter a point not too far inside the front of the intake, and a point just after the front frustrum (the front-most part of the comb. chamber).

My results show that the 2 points are almost directly opposite each other (i.e., pressure is high on one while low on the other), during the first part of the cycle. I would have thought the opposite.

Given the typical proportions in an L-H, this also means the front-end wave is augmenting the outflow in the exhaust direction, at the other end of the combustion chamber; it's also aiding the impedance at the front-end.

This would make it clear why the intake length is so critical--and why it must match the rest of the engine's acoustic properties, phase-wise.

Any comments? Graham?

Bill H.
Acoustic Propulsion Concepts
Last edited by hinote on Mon Aug 23, 2004 4:18 am, edited 1 time in total.

hinote
Posts: 1241
Joined: Sat Oct 04, 2003 1:54 am
Antipspambot question: 0
Location: Central California

Re: Intake Analysis

Post by hinote » Sun Aug 22, 2004 6:15 pm

hinote wrote:

My results show that the 2 points are almost directly opposite each other (i.e., pressure is high on one while low on the other), during the first part of the cycle. I would have thought the opposite.
Attached is the pressure trace for the 2 points described. The red one is the same at the top of this thread, the yellow is at the very front of the combustion chamber.

Note the wave action moving in opposite directions in the 2 big pulses during the outflow event.

Bill H.
Acoustic Propulsion Concepts
Attachments
intakecc phasing M1E.JPG
intakecc phasing M1E.JPG (22.13 KiB) Viewed 6994 times

Graham C. Williams
Posts: 571
Joined: Fri Oct 03, 2003 11:33 pm
Antipspambot question: 0
Location: England
Contact:

Re: Intake Analysis

Post by Graham C. Williams » Sun Aug 22, 2004 8:02 pm

Dear Bill.
I can only see events propagating down the pipe; to the left. This graph shows a compression event (or two) moving down the intake pipe (to the left). It starts at the combustion chamber data point and moves towards the data point further down the induction pipe. The rate of propagation is given by the time taken and the distance travelled. Only further investigation would tell me where the second compression event comes from; it could be a reflection from the Back cone of the combustion chamber?

Graham

hinote
Posts: 1241
Joined: Sat Oct 04, 2003 1:54 am
Antipspambot question: 0
Location: Central California

Re: Intake Analysis

Post by hinote » Mon Aug 23, 2004 3:30 am

Graham C. Williams wrote:Dear Bill.
I can only see events propagating down the pipe; to the left. This graph shows a compression event (or two) moving down the intake pipe (to the left). It starts at the combustion chamber data point and moves towards the data point further down the induction pipe. The rate of propagation is given by the time taken and the distance travelled. Only further investigation would tell me where the second compression event comes from; it could be a reflection from the Back cone of the combustion chamber?

Graham
OK Graham--I'm game for the game here, so here goes:

Attached is a pressure plot--the same 2 traces as the last one, and the back of the combustion chamber added on top.

Now, the red trace (as before ) is near the front of the intake; the yellow (as before) is at the front of the combustion chamber; the white trace is at the rear of the combustion chamber.

Luckily, this is conforming to my current conjecture. The white trace is again diametrically opposite to the yellow one. This is increasing evidence (IMHO) about my previous conjecture--the combustion wave is being opposed by the intake wave, while the back of the combustion chamber is reflecting a wave that augments the acoustic event at the other end of the comb chamber.

BTW this engine is based on the Ecrevisse architecture; I think we're seeing the reason why there's a brief frustrum at the back end of the combustion chamber--it augments the wave action returning to the front.

All comments appreciated. I'm learning as I go here.

Bill H.
Acoustic Propulsion Concepts
Attachments
M1E 3 traces.JPG
M1E 3 traces.JPG (23.58 KiB) Viewed 6964 times

Graham C. Williams
Posts: 571
Joined: Fri Oct 03, 2003 11:33 pm
Antipspambot question: 0
Location: England
Contact:

Re: Intake Analysis

Post by Graham C. Williams » Mon Aug 23, 2004 9:32 am

Dear Bill.

Simp:- will have to think a little before coming back.

Best Regards
Graham.

Post Reply