M1E Preview

[larry cottrill]

Dear Bill.
Another small point.
Viv commented on the Q of the system. I'm absolutely with him on this point and was about to start a thread on this.
Over a small number of samples 'Easy starting' motors have all had a Q less than about 1. Larry’s motor is a fine example of such a soft motor.
Higher Q (Q>1) motors offer better potential performance; the wave structure being more focussed on the combustion area. I hope he does not mind me saying this but Mikes 50mm E motor is a super example of something in this class; needing a bit more care to find the tune initially but once found it goes very well and starts easily. This is all conjecturing based upon little data. I think your motor has a high Q.

Graham.

Graham -

Please go ahead and start your thread on this; I think this is a much overlooked point. It is something I for one understand pretty well in electrical L/C circuits, but not at all well in acoustic systems.

I'd really like to see something on this, as long as it remains comprehensible to the mathematically impaired ;-)

L Cottrill

[Mike Everman]

Mmm, good stuff indeed. BTW Bill, I didn't get to respond to the cone design till now, but that's the ticket. I'd be tempted to make it as long as your intake, and no more than half the entry dia at the fat end, and round that end while you're at it. Perhaps put some welding rod spokes at two places along the length so it will stay put, sort of.

[Graham C. Williams]

Dear Larry.
I'm not too sure of the best way forward yet. Perhaps the Standard Deviation of the pressure measured at the peak of the rebound would be a better approach. Do you have any suggestions?

Graham.

[resosys]

I'm also very interested in some dialog about Q and how it can or should be quantified and measured in an acoustic resonator system.

I've spent some time working with Q in resonant tank circuits and secondary systems on tesla coils. When the secondary coil Q is high, hitting the resonant frequency really makes a huge difference in output spark performance.

In a resonant electrical circuit, I've always used this:

Q = (2 pi) (total stored energy) / (energy lost per cycle)

I'm struggling to regain some of the memory I've re-purposed since I built the last tesla coil....



Chris

[Graham C. Williams]

I said

Dear Bill.
That's exactly how the tapered induction pipe works. Under backflow the induction pipe is pushed up the M curve.
I'm not too happy about this set of curves application to non-steady flow but it illustrates the point.Graham.

RUBBISH, almost.
The Mach number is the ratio of flow velocity to local speed of sound. Squaring both sides; as more of the gas total energy is given to the Kinetic energy of the gas the Mach number increases. It's the Kinetic energy of the inflow that initially opposes the outflow and increases confinement.
Almost? Because if the outflow Mach number =1 then the reflected expansion waves cannot pass upstream back into the combustion chamber, so confinement must increase again. Some time ago I was explaining this to Viv, He said 'So, you loose the pipe'. (I’m sorry Brian Lewis, I cannot see your 'wavelength increase' scenario applying here)

Graham.

[hinote]

RUBBISH, almost.

Almost? Because if the outflow Mach number =1 then the reflected expansion waves cannot pass upstream back into the combustion chamber, so confinement must increase again.

Graham.

Hey Graham:

Do you think some of these (L-H type) engines are approaching M=!.0 in the intake? (I'm assuming outflow)

This would certainly raise new questions/parameters to consider in the equation, --don't you agree?

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[Stuart]

Hey Graham:

Do you think some of these (L-H type) engines are approaching M=!.0 in the intake? (I'm assuming outflow)

This would certainly raise new questions/parameters to consider in the equation, --don't you agree?

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

You can figure it out (more or less) if you know the thrust and the temperatures, from which you can back calculate the mass flows and Mach. Get me some numbers (also cross sectional areas) and I'll figure it out for you. So, I need areas, temps, and the thrust. (I know, some will have to be averages, but ya gets whats ya pays fer)

[hinote]

Do you think some of these (L-H type) engines are approaching M=!.0 in the intake? (I'm assuming outflow)

You can figure it out (more or less) if you know the thrust and the temperatures, from which you can back calculate the mass flows and Mach.

Attached are 2 snapshots of the Uflow worksheet for M1E.

In this run, I have introduced a tapered intake shape. The resutls are quite noticeable--much more effect on the performance than I would have imagined.

You can see the Mach numbers exceeding 1.0 on both outflow (negative values) and inflow. Perhaps the most interesting part is how powerfully the tailpipe shape is inducing inflow--the inflow velocity is exceeding Mach 1.3 for part of the cycle!

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[Stuart]

Hate to be a negative Nelly, but you shouldn't normally be seeing Mach numbers greater than 1 in a flow unless you've got a converging/diverging nozzle to pop the flow over the brink. That makes me verrry suspicious of your numbers (not that they aren't useful in general, but as you hit Mach 1 things happen in the world of strange with shock waves etc.).

[hinote]

Hate to be a negative Nelly, but you shouldn't normally be seeing Mach numbers greater than 1 in a flow unless you've got a converging/diverging nozzle to pop the flow over the brink. That makes me verrry suspicious of your numbers (not that they aren't useful in general, but as you hit Mach 1 things happen in the world of strange with shock waves etc.).

If you look at my post about three boxes back, that's essentially the question I was asking.

BTW the local geometry takes the shape of a convergent/divergent nozzle in this particular sim. I'm not sure it's accurate enough to qualify for supersonic flow, though. I'm asking for more expert opinions than I can provide.

Bill H.
Acoustic Propulsion Concepts


".......some day soon we'll be flying airplanes powered by pulsejets."

[Stuart]

What I need to do for you guys is get you the isentropic flow tables that are based on area ratios, Mach number etc. I haven't seen a set of charts on the net so far, and they would be a bitch to type in from my fluid dynamic textbooks. I have a bunch of splines I programmed to generate these charts, but I haven't had time to look at that stuff for quite a while. Give me some time.

Just eyeballing the Uflow figures, it looks like the mach data that is around 1.01 with density of .6 may just be programmatic rounding error that shows choked Mach 1 flow (what you would expect ). The data where Mach is 1.3 and density is .5 would only be okay if that is the heated reverse flow AND it represented the Mach somewhere close to the exit after a diverging section.

I need to know more about the area ratios where this data is coming from to give an intelligent answer. Also, Mach number is very related to the temperature, so Temp data would be very useful.

I have a vested interest in knowing more about supersonic nozzles, so if I can help on this it would help my understanding as well.

[Bruno Ogorelec]

If I may pitch in from the sidelines, I would be very suspicious of any sign of a nozzle producing supersonic speeds in a pulsating flow.

Are there any transitions that involve flow suddenly moving from a hot to a cool part of the duct? As Stuart says, the Mach number is dependent on temperature.

For instance, in pulsejet the exhaust flow often becomes supersonic upon exiting the tailpipe -- not because it has accelerated, but because it is suddenly dumped into cool ambient air.

Maybe this can also happen inside the engine.

[Graham C. Williams]

I've just posted an e-mail to Dr. Jose Miguel Corberán asking him about this. I'll let you know his reply.


Graham.

[Graham C. Williams]

I've just noticed the picture on this page:
http://capella.colorado.edu/~laney/uflow.htm
It shows just the effect.
Graham.

[Graham C. Williams]

From
Prof. José M. Corberán

I suppose you mean in constant cross section pipes. In that case, also Mach numbers greater than 1 can be possible in unsteady conditions. Why not?

There is very little information on measurements under unsteady conditions. It would be interesting to see experimentally the formation of regions with supersonic flows and the appearing of shock waves downstream.

Thank you for your feedback and nice regards

José Miguel

Dear all
I need to follow this up with the good Doctor but I think the key is in the unsteady conditions. Perhaps if you could gather your questions I'll put them in one email. Please keep them very short and keep the English simple.
Graham.