Proposed Experimental Intake Pressure Measurement

[larry cottrill]

Here's a way I believe we could find the actual pressure points in an intake [let's say, to determine a good point for drafting in liquid fuel]. This set of three attachments would allow you to plot graphs of actual mean pressure / suction for various stations within the intake, using a combined pressure/suction gauge [mine is an automotive engine "vacuum" / fuel pressure gauge - very handy]. Because you have to do this with the engine running, the measuring point should be a little to the side of the intake centerline, so it is less influenced by the fuel vapor flow.

Ben and I found to our satisfaction [well, OK, to MY satisfaction] that gauge measurements inside a pulsejet are possible. I believe that they are also meaningful, especially in terms of something as massive as liquid fuel - the liquid will be affected little by high frequency variation, and will respond mostly to mean pressure. Likewise, the gauge is unresponsive to engine pulses - all you see is a steady reading as a particular point is sensed, because of the 'reservoir' action of the inside of the gauge and the inertial mass of its moving parts. Plots of the mean pressure values should be meaningful, then, in terms of basic pushing or pulling power on a liquid supply. Three values might be of use - the mean static pressure, the mean inward flow pressure [suction phase] and the mean outward flow pressure [explosion phase]. Note that since a gauge is used, any of these values could swing negative at some points of measurement. Note that the index marks are set up from the exact point of pressure entry on the probe, not necessarily the centerline of the bent end of the tube!

Testing should be done on an engine type that is known to run reliably and smoothly on vapor fuel. The camcorder is set up so that it is looking across the intake flare, dead-on. This plane becomes the datum against which the position marks will be read. The camera must be positioned sufficiently far back to pick up the position marks on the probe and the gauge dial face. The engine is brought up to reasonably hard running and the intake is approached by the selected probe and SLOWLY run into the intake as filming proceeds. When the device is fully run in, it is removed and the process is repeated with the second and third probes. The data is analyzed and tabulated later by watching the video.

Although the pressure data is measured in cm from the flare, it would be more meaningful and useful [probably] to interpret this in terms of percent of the intake tube acoustic length, which should not be difficult to determine in most cases. A low-range gauge is necessary for accurate measurements - when Ben and I measured the suction in the venturi of my Dynajet, the suction value was a small fraction of what the gauge showed was expected in the manifold of a piston engine. I certainly would not expect values any better in a valveless engine intake, where we know flow reversal is happening!

Comments? Suggestions? Criticism?

L Cottrill

[Mark]

I remember some paper claiming that a tiny bit of backflow sneaks by the petals on a Dynajet before being re-ingested. I think it was something like 3/4 of an inch or so. There is a fog that you can see it mentioned. It would seem to be plausible, at least you might speculate the mass of the little air molecules could dart by before the heavier mass of the reeds swing shut.
I know on a rare occasion with too much fuel in the throat when starting your jet, you can start a fire in the throat.
So whatever the case, those gases do some fast moving stuff. But mean pressure measuring should be a fair way to determine what you have to work with or how to employ the pressures to your advantage.
Mark

[Mark]

I remember a giant foot tall 60 degree prism made of water, glass, and silicone glue that I had that I shot a slit of bright light through from a slide projector in the dark. I masked a slide except for a verticle slit of light to shine out. It casted a huge rainbow spectrum on the far wall, but if you walked along slowly in front of the wall you could see the beautiful soft colors transition directly in your eye, green turning to blue, etc, rocking your head enhancing the effect.
I wonder if the inside of a valveless pulsejet would do the same with pressure, a varying degree or spectrum of pressures waiting to be understood.
Mark

[larry cottrill]

Mark -

Yes, I can believe that, about the backflow. I've seen odd droplets fly rapidly out the front during some of the bangs you get while applying bursts of starting air. There is just no way a valve like that can move fast enough to beat explosion gases that flow toward and past it. The petal accelerates from its wide-open position via its own spring tension and the pressure of flow from behind it, but it's still essentially mass acceleration from a standing start. It takes time to close, and during most of that time, the lightweight explosion gases have nothing in their way but the low-speed intake mixture more-or-less stalled in the port - a contest easily won by the high momentum of the combustion gas.

L Cottrill

[Eric]

Larry,
As soon as I stuck the water probe into the engine I knew that I was going to have to TLAR it because I didnt have time or patience to do it the right way.

The suction really is crazy inside the intake, you cant just measure the distance into the intake, because if its not centered it might have more or less suction. I actually found a spot right against the intake wall several inches in that if the propane probe is placed there the engine all of a sudden kicks into high gear and really puts out a lot of thrust....

Eric

[Mark]

I was thinking that too Eric, it is probably not only how far in you go with the fuel line but also it might be dependent on whether you stay close to the walls of the intake, perhaps like a butterfly hugging the ground to avoid the wind, or move to the center of the channel along the same plane. On the starting backflow phase of a Dynajet tail, the exiting column of air is choked or squeezed down and even though this slug of air is heading out, fresh air is skirting in around this column, so that there is duel flow going on for a portion of the cycle.
I marvel at my 2.5 gallon tank with snorkel, how the air must be charging in and out in the same fashion as the above concept. It's such a disproportionate route for the fat piglet tank to breathe through a skinny snorkel, how it can be inhaling and exhaling with all the criss-crossing air traffic going on in the long, narrow passageway, faster than we can think.
I wonder what might be happening to the incoming slug of air as it enters the tank chamber, how it differs or is similar to/from/than the exiting slug which gets constricted as it rarifies relative to the ambient air?
Aspiration, I think I have aspired in another direction. I should stay more focused. Fueling is very interesting though/now with this new tactic in valveless pulsejet design. It ought to keep us dwelling for a fair amount of time.
Mark