Prettiest FWE (?) - the 'Lady Anne Boleyn'

Moderator: Mike Everman

Re: re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby larry cottrill » Thu Aug 17, 2006 5:29 pm

Mike Everman wrote:Oh, the plot I posted is not a snapshot of one point in time, nor is it a temporal plot. It indicates the points along the length where the richest harmonic content can be excited as a pressure pulse travels the length.
Maxima, for lack of a better description, are places where you'd want (excitation moving from left to right) pressure changes due to changes in cross-sectional area to occur, and pinches should occur at minima, where gas velocity is highest and therefore pressure is lowest.

Ah, I'm beginning to understand. That's a little bit like plotting "influence lines" for various points on a structure (like a bridge) under a moving load. (You could do influence lines for shear, tension, bending moment or whatever ... if I remember correctly.)

For instance, a small diameter would be positioned at a minima, and the area should increase, pressure increasing, until a maxima, where the area must stop rising, and either must move in to a cylindrical section or begin a new pinch. Crudely, the passing pulse has "sucked on the acoustic minimum", amplifying it, and "punched the maximum", amplifying IT.

Makes sense to me.

Sounds like for 1D analysis, the unfolding methodology has got to be dead on for any of this analysis to be meaningful.

Well, uh ... yes. We have to make "just right" assumptions about how to gyrate the intake around ;-) There are various reasons why it is impossible to make an "unfolded" version an exact model of the original "folded" one, but I think the important part is to get it close to spot on "acoustically" and neglect the effects caused by linearizing the mass flow. I mean, what else can we do? A "folded" engine is not a 1D flow system, after all, so reality can only be modeled approximately anyway.

The dimensional layout used for UFLOW modeling was as shown below. The velocity node location shown is just an "eyeball average" over time, chosen by observing the velocity curve as it changes over time in the cycle. How "good" this is can be roughly judged from the dark green line on the "Mach Number" graph in the UFLOW panel presented earlier, which was generated for that exact location, 185 mm in from the front edge.

L Cottrill
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The "unfolded" model used for UFLOW1D modeling. Drawing Copyright 2006 Larry Cottrill
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Graham C. Williams » Thu Aug 17, 2006 6:32 pm

Dear Mike and Larry.
Mike, I've run your analysis on this motor but recognise I'm not as good with this type of analysis as you are. My feelings were that the induction pipe could be a little longer; the start of the induction flair being exactly half way along the 13mm dia straight section this would allow the 23% CofC location along with the possibility of utilising the 37% CofC location. I'll have to go back and look at your tail recommendations; I thought the analysis was telling me that the pinch after the 42mm dia. section should be a little more to the right at the expense of the conical tail section length.
I've also run the Nudis model of the motor as a closed pipe. The re-pressurising was classic for this type of motor.
Tonight I'll unpack the motor and produce a linear version then go on to a combustion run. I'll model both the original and Mikes suggestions if I have time.
Larry, looking at the UFLOW diagrams you've posted reminded me of when I was working on the FWE we ran. As I recall the type 07 likes to be short but tunes to give you very good control over the gas velocity during pressure rise.
'Till then Best Regards
Graham.
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Graham C. Williams » Thu Aug 17, 2006 6:44 pm

Dear Larry and Mike.
Unpacking.
The unpacked drawing above looks as if it's been unpacked around the 37% point or near it. I'll try unpacking it around the 23% point and see what is happening.
Note, I measure these points (23% and 37%)as path lengths from the start of the induction pipe to the expected CofC.The measurements are made on the packed motor, the 23% and 37% are not ment to represent these values in the linear motor. I know there is some debate about the true start of an induction pipe but for the moment I use that indicated by Foa for a lip end pipe.

Regards
Graham
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Mike Everman » Thu Aug 17, 2006 9:50 pm

Cool. I'll look at the unfolded version tonight!
Mike
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Graham C. Williams » Fri Aug 18, 2006 8:22 pm

Time is limiting me so a quick report.
The unpacked model as per Larrys spec above.
Ambient temp for model 289K.
Burning/Heat release Characteristic 1: this Heat release curve seems to fit most pulsejets.
Fuel: Propane.
NUDiS Data Sample Rate: 8
The model started up straight away with combustion taking place every other cycle. I had other things that needed doing so I only ran the model for 3 cycles. No meaningful thrust values, as the model had not yet settled. I'll try a longer run over the weekend but it looked as if it might settle. The late combustion was taking place well down the pipe (towards the tail) this makes me think that the motor is at the limit of shortness for this combustion time or fuel.
I want to try this motor with a faster burning fuel.
A later run with a NUDiS Data Sample Rate of 10 fired once. This may be a problem with my implementation of NUDiS.
I ran a second type of unpacked model assuming that the CofC will be at approximately where the induction pipe enters the model as shown in Larry’s drawing 11 August. The 23% combustion point was set at this location and the induction pipe shortened accordingly.
This model was set-up as the first test above. The model attempted to run every cycle but the end of combustion was running into the next cycle and caused a secondary pressure rise about 50% of the length down the unpacked pipe. In the past this type of pressure rise can stop the motor. I did not run the model long enough to see if it stopped. I want to do this later along with a faster burning fuel again.
I also want to run these models with the modifications as suggested by Mike.
Finally I want to try a third unpacking technique – more later.

Regards
Graham.
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Graham C. Williams » Mon Aug 21, 2006 10:36 am

Dear Larry.
Have you tried reducing the 32mm * 13mm Straight section to 29mm*13mm and what were your findings? I think the massflow zero crossing point stays in the correct region a little longer. The comments below are based upon this modification.
My models are all showing that late combustion (Propane) is extending into the low-pressure part of the cycle. I don't like this personally but in reality it may not stop the motor.
I've looked at alternative fuels: Methanol and Acetylene (it has to be said that my model for Acetylene is not very accurate). The motor appears to run with Methanol and runs with my Acetylene fuel model. I'm not suggesting that you or anyone run a motor of this size on Acetylene the results would most likely be detonation but I don't know for sure.
Another mod I’ve tried: reduce the unpacked induction length to 145mm. I’m not sure about the benefit of this mod.

Had a quick try with Mikes suggestions but need to retry then with the 29mm dia mod, still the comments about combustion time hold.

Regards
Graham.
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby larry cottrill » Mon Aug 21, 2006 2:45 pm

Graham -

Via UFLOW, making the forward throat 29mm (but not moving it longitudinally, nor changing the induction pipe), I show a slight increase in peak chamber pressure, with a corresponding increase in mass flows at the intake and the tail end. On the other hand, the velocity at the tail end is slightly reduced. As one might expect, the engine is a bit more biased toward outflow from the intake. Very small changes, of course, maybe on the order of 2 percent.

Interestingly, the most striking difference is that the velocity node appears to have shifted forward noticeably, closer to the 175 mm station. There is a more definite peak to the pressure wave at that point, and the wave is more rounded (the wave as a whole appears more like a sine curve than in the earlier example).

L Cottrill
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Mike Everman » Mon Aug 21, 2006 3:49 pm

I'm still wrapping my mind around whether this is the proper place to unfold the geometry...
Mike
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby James D » Mon Aug 21, 2006 6:37 pm

Hi all
I've started to build this motor as per Larry's drawing, Is it worth finishing?
I don't mind making modifications but should I run it as drawn first?
Thanks
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Graham C. Williams » Mon Aug 21, 2006 8:02 pm

Dear Mike.
I tend to agree. I'm running a model now (on Methanol) but have changed the unpacked layout. The overall length is the same as Larrys unpacked motor. The induction pipe length is 102mm and approximately equal to the path length along the inside centreline of the packed induction pipe. (Larry, do you think I've got this length about correct?) The 10mm long combustion chamber front cone has been extended to make the length up. All other dims are the same.

Regards
Graham.
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Graham C. Williams » Mon Aug 21, 2006 8:13 pm

Dear James.
If your up for a bit of R and D go ahead or wait a few days. It might run on Propane but things are a lot better with Methanol, can you get some Methanol?
It may help if you have a word with Nick or Steve Wallington before you build, tips and tricks that sort of thing.

Regards
Graham.
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Graham C. Williams » Mon Aug 21, 2006 8:15 pm

Dear Larry.
Thanks for the comments.

Graham.
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Re: re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby larry cottrill » Mon Aug 21, 2006 8:29 pm

James D wrote:Hi all
I've started to build this motor as per Larry's drawing, Is it worth finishing?
I don't mind making modifications but should I run it as drawn first?
Thanks

James -

Anyone working from my first shot at an engine is definitely building at their own risk. ("Now they tell me!", right? ;-)

HOWEVER, I don't think you're in danger of building something that just won't run, ever. Probably, worst case will be that if it just won't start no matter what you do, you'll have to add to or subtract from the engine length (and if you add a little, it could be cylindrical rather than continuing the cone). Remember that you can experiment with lengthening by using a movable sleeve at the tail end (even a loose-fitting one will do!).

On the other hand, if you wait until the theoreticians hash things out to their satisfaction, you'll probably have a much better tuned engine with close to optimal performance, at least for some particular fuel.

Some will be skeptical of such rose-coloured optimism, but don't forget: This engine is not hugely different from the Type 07 motor that Graham and Nick developed (albeit in "linear" form, of course) and THAT was a good-running beast indeed, from their final reports!

L Cottrill
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby James D » Mon Aug 21, 2006 9:43 pm

Thanks Graham and Larry
I'll go ahead and build it, probably be ready to test on friday.
I really don't mind having to modify it, Should be interesting, now if I could just find some methanol...
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re: Prettiest FWE (?) - the 'Lady Anne Boleyn'

Postby Mike Everman » Mon Aug 21, 2006 9:57 pm

Faaaaaascinating!
Try propane first anyway.
Looking good, Wright or Wrong ;-)
Mike
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