Intake Analysis

[Mike Everman]

Bill, how long is your inlet cone? Just trying to make temporal sense to the very top graph on this thread.

[Mike Everman]

Bill, I'm not sure I'm seeing what you're seeing. I look at the plot at the very top of this thread, and read Graham's comment on it, agree, but I'm forced to put it in my own words: the fluid flow makes for a travelling reference frame to the left, and I would conclude (not knowing anything else) that this snapshot in time is when fluid is being expelled by the intake. That's why I wanted to see how far between these two data points, I was so cleverly going to tell you what the velocity of the gas was in between ;-)

Take a look at the plots Larry has posted, if you squint your eyes, the signature of the trace is virtually identical at all points, just displaced in time. One of your points is close enough to the end to get a bit whacky, sence the open ends would be flat lines, but the yellow trace is clearly the same signature as the red, shifted to the left. I take your meaning that it is in fact high at one while low at the other, but the reference frame is moving, too, so is it really, and can the CC "know" it?

[Graham C. Williams]

Hi all.
Attached 2 UFLOW Files. The Motor is the same in both. It’s my hope that all those that are interested can follow Bills argument as he develops these models. The Model marked intake has 10 data points arranged along the induction path. The model marked Combustion has ten data points along the combustion chamber.

Only freely available information and commonly held ideas about length ratios were used in the construction of this model.


Looking at the pipe page:

The combustion chamber (station 3) is a bit of pipe 50mm*135mm I have on my desk – it’s a place to start.

The induction pipe (station 1) is stock 20mm tube. Commonly held knowledge tells me that it should be a good bit longer than the combustion chamber so I added 3mm as a starting point.

The Exhaust pipe (station 5). Commonly held knowledge tells us that it should be about 6 to 7 times the length of the combustion chamber. The pipes exit diameter (station 6) is the same as the combustion chamber diameter.
The diameter at the combustion chamber end (station 5) - we are told should be a bit more than the induction pipe; I added 1mm.

The length of the transition between Induction pipe and Combustion chamber (station 2) is the minimum that I know will be stable in UFLOW (you can experience problems when choosing zero lengths)

The internal gas temps are all to be found in the ‘Belfast’ Papers.
The pressure of the initiating combustion is chosen to be a commonly held and reasonable starting point (between 180000Pa and 250000Pa at stations 2 and 3).
I don’t know what the pressures in the rest of the pipe will be so I’ve left these at 100000Pa. (about atmos)

I don’t know what the wall temperatures will be so I’ve chosen some that seem reasonable perhaps a bit low.


I don’t know what the initial velocities should be so I’ve also left these at zero. If I was clever (I’m not) I’d feed numbers from Fig 1 back into the model but I’m sure Bill H. will like to cover this in more detail.

The ‘Belfast’ papers tell me a few other things are needed for this motor to work.

1) Sufficient gas must have got into the combustion chamber before the initiating pressure wave drops the gas velocity in the combustion chamber to below the flame speed. You won’t get a bang if there isn’t enough fuel/air in the motor.
2) After combustion initiation the pressure must rise in the combustion chamber.

FIG 1 (from worksheet item 365) shows:
A, The combustion initiating pressure pulse entering the combustion chamber.
B, Fuel/air mixture entering the combustion chamber. Cold to Hot Interface.
C, The change in gas velocity as the initiating pressure pulse propergates through the combustion chamber.

[Graham C. Williams]

A simple experiment with the model.

Select the model with combustion chamber data points.
Change the initial pressure at stations 2 and 3 on the PIPE page to 250000Pa
Run the calculation (Calculate/Play)
Go to the Graphics Output page and advance the Slider (on the R.H.S.) until you see peak rebound pressure in the combustion chamber. Fig 2 is an example of what you are looking for, it’s that peak in pressure.
Go to the Worksheets Results page and look through the pressure values at the various data points. Note the peak pressure.

Increase the length of the induction pipe in 0.002m increments from the starting value of 0.138m to a final value of 0.178m and note how the rebound pressure responds.

You should notice corresponding changes in rebound pressure. Why?
How might you get an expected operating frequency?
Do you notice any change in expected operating frequency? Why?

You may like to draw a graph of peak rebound pressure against induction pipe length.

Choose 3 or 4 lengths of induction pipe ( perhaps 0.138m, 0.178m and 0.158m) and observe the effects when the initial pressure in station 2 and 3 is first 150000Pa, then 180000Pa and finally 200000Pa. How does this effect point B in Fig 1?



FIG 02.


The stage is set. The floor is all yours.

Graham.

P.S. This thread is all about induction but it is worth noting the effect on the motor if you change the temperature of the gas in the exhaust pipe, you could try 500K.

[Graham C. Williams]

Dear all.
I have a problem uploading the two UFLOW files. It'll have to wait until Tuesday.
For the moment, if you leave your names I'll email them to you.

Sorry about that.
Graham.

[hinote]

Dear all.
I have a problem uploading the two UFLOW files. It'll have to wait until Tuesday.
For the moment, if you leave your names I'll email them to you.

Sorry about that.
Graham.

All I can say is, YAY Graham!!!!!!!!!! That's one of your biggest supporters waving banners in thanks for your efforts.

Thank you for having the cojones to present an educational experiment to play around with. I couldn't bring myself to do it.

I think everyone here should try what Graham is proposing. This is really good stuff.

I'll be glad to help anyone who has questions or problems when Graham can't be here to answer. Remember, I'm a rank amateur (like most of the rest of us), so I can't promise a solution for every situation--but I'll be happy to try. I think Mike E. will be able to contribute a lot to this (when he returns from his fishing trip)--he's spent a lot of time with Uflow.

Bil H.
Acoustic Propulsion Concepts

[hinote]

Hi all.
Attached 2 UFLOW Files. The Motor is the same in both. .

Please see the attached graph.


It's the first point inside the front of the combustion chamber.

I've changed the cycle length to more closely match the probable frequency of the actual engine. The analytical length is now .0057 sec (last number on the lower right); the calculated cycle time for the engine is .00568, which is one divided by the probable frequency of 176 Hz.

The reason I've included this is that the proportions of the engine appear to be slightly wrong.

Notice how the combustion chamber pressure is deteriorating from its max value before the end of the cycle. Ideally, I think the pressure should be near its peak but still building slightly--this imples the stagnation that initiates the next combustion event. It's occurring--but it's slightly off-time.

For purposes of educational analysis it's not important at this point--but would probably yield a non-running engine.

Bill H.
Acoustic Propulsion Concepts

[hinote]

The induction pipe (station 1) is stock 20mm tube. Commonly held knowledge tells me that it should be a good bit longer than the combustion chamber so I added 3mm as a starting point.

Please see the attached.

It's a snapshot of the very front of the intake.

There's lots of things to note. First, the intake is resonating--but the damping of the pressure swings is almost immediate. I believe this is a function of the relatively small intake tube diameter, with its relatively large boundary layer.

BTW I thought I would add an intake flare to try and "sharpen-up" the intake functions; the result was similar to Nanosoft's experience (but more so)--the engine simulation failed to run when any sort of flare was added.

So, you guys building "little" L-H engines can just leave that feature out.

Also, if you look at the 2 right-hand graphs you can see another indication that the engine proportions are off--the intake velocity and mass-flow have actually turned around, before they should.

One of the interesting things you can do is to go into the numerical file and find the 2 peak pressure values that correspond to those on the pressure graph; then, just subtract the time values from each other to find the time interval--then, one divided by that number is the frequency of the intake resonance at that point.

Bill H.
Acoustic Propulsion Concepts

[hinote]

The reason I've included this is that the proportions of the engine appear to be slightly wrong.

Notice how the combustion chamber pressure is deteriorating from its max value before the end of the cycle. Ideally, I think the pressure should be near its peak but still building slightly--this imples the stagnation that initiates the next combusion event. It's occurring--but it's slightly off-time.

Please see attached.

This (pressure/time) graph is again at the front of the combustion chamber; I've done a simple re-proportioning in Uflow, to show how the phasing can be changed.

Note now that the end of the cycle shows the pressure still rising slightly at the end of the cycle. I believe this is a more desirable-looking curve.

Of course there's a number of other factors that would need to be readjusted along with this change--so it's not just a simple single-item mod.

Bill H.
Acoustic Propulsion Concepts

[hinote]

Increase the length of the induction pipe in 0.002m increments from the starting value of 0.138m to a final value of 0.178m and note how the rebound pressure responds.

You should notice corresponding changes in rebound pressure. Why?

The rebound pressure increases, approximately at the same rate the intake tube length is increasing.

Noting the intake velocity at the front of the tube at the peak pressure stations shows a similar steady increase in intake air velocity, up to about 3/4 of the series, after which it stabilizes. Presumably, there is an increase in the plugging effect in the intake, due to a combination of intake velocity and cold-air plug length.

Graham, can you expand on this phenomenon?

Bill H.
Acoustic Propulsion Concepts

P.S.: Graham have you noticed the "rocking" effect of the pressure in the combu. chamber, as it rebounds? Presumably this an acoustic effect--does it make the length of the chamber critical?

You can see this effect if you do a moderately rapid scroll and watch the comb. chamber area on the pressure graph.

[hinote]

?
How might you get an expected operating frequency?

This one is VERY important.

The cyclic operation of this demonstration indicates a certain frequency--but the primary driver(s) of the frequency are indicating another.

It's important to remember that the primary driver of the frequency is the overall length of the engine--and the secondary modifier is the (acoustic) temp of the engine. There's almost nothing else that can influence this number--for this type of layout.

There are certain layouts that can be susceptible to operation at F2--but not this (L-H style) type of engine.

So, we are obligated to make the other parameters operate at the frequency created by the length/temp variables--NOT the other way around.

Your sample engine doesn't want to synchronize with the frequency created by the above--so something else needs to be changed.

Bill H.
Acoustic Propulsion Concepts

[Graham C. Williams]

Dear All.
You can now download the UFLOW Files from the link below.

http://speechrecordings.co.uk/jets/UFLOW

Dear Bill.
I've just got back. I'll try to answer your questions later. The flippant answer is 'Yes, it's all out of tune'.

Perhaps we should give others a chance to catch up first? I fear we'll be having a private conversation when I really wanted to let others have an opportunity to use this tool.

Best Regards
Graham.
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[hinote]

Perhaps we should give others a chance to catch up first? I fear we'll be having a private conversation when I really wanted to let others have an opportunity to use this tool.

Well, I didn't see anybody else contributing--so I thought I would throw some interesting observations out for bait.

Bill H.
Acoustic Propulsion Concepts

[jmhdx]

I'm impressed with the efforts made here and wish I could contribute but I have found Uflow chews up hours of time and I'm often unsure of what I'm seeing. Conclusions are difficult to make and wish you luck in finding a rock solid explanation for the double wave through the intake. Uflow was not really designed for our needs and I'm untrusting of it's results.
One thing it does make clear is that the exhaust pipe needs to be narrowly conical to ensure a recompressive effect.
Mike.

[Mike Everman]

Bill, et al,
I swear I'll be chiming in. I'm under the gun at the moment. Looks like good stuff.