Graham C. Williams wrote:Dear all.
Nice one James. You don't waste much time!
No, he doesn't!
So, changing the unpacked model as I described above and with the original 32*13mm section in the tail, running on Methanol at full throttle. The model started straight away and ran for the full 10 cycles (over 2500 iterations), the pressure trace showing a good degree of stability after 4 cycles.
Wonderful ... by "changing ... as I described above" I presume you mean just the shortening if the intake duct.
I'm running the model again but with a 29mm *13 section in the tailpipe. Tomorrow I'll run Mikes suggestions to the standard set-up.
I don't want to give you the Thrust figures because I know I have an error in my calculations, my values all are a little on the high side and assume too many things are perfect.
Wonderful, again. Let's hope it doesn't throw things too far off the original layout to cast doubt on James' prototype. Of course, his photos show that he hasn't built the intake yet, so he still has some leeway!
All that's left is to discuss if the unpacked model I'm using is a valid model or not, any suggestions?
You and mike, with your 'center of combustion' theory are a full step out of my league, so I can't think of a thing to offer that would be of help. I have made an interesting UFLOW observation about the rear nozzle, though:
If you "flatten" the rear nozzle, (i.e., shorten it without altering the inner or outer diameters while lengthening the middle cone to preserve the overall engine length), you will of course get a sharper pressure reflection back into the chamber. This means the nozzle location becomes more critical. There is yet another effect that makes it critical, too, that I only noticed just a few minutes ago: Flattening the nozzle makes the tail cone act more independently! That is, you can see it "thrashing" at its own resonant open-tube frequency, just like an intake pipe !!! This is superimposed on the general pressure wave, of course. Thus, by repositioning the flattened nozzle, you can "tune" the tail cone into (or out of) resonance with the whole. In fact, the engine acts somewhat (and I emphasize, somewhat
) as an asymmetrical two-chambered engine - a forward large chamber with small intake, and a rearward small chamber with a huge intake.
This suggests that you could work through tuning of the overall design by flattening the rear nozzle cone temporarily, adjusting its position until you get everything "sync'd up" the way you like it, then lengthening it out again as desired to take the critical "sharpness" out of the tuning (broaden the resonance of the engine, within reason).
The thing that might be really difficult about this is getting the tail cone gas temps close enough to reality. The resonance of the tail cone, like that of an intake pipe, is going to be highly dependent on temperature. I have recently felt that any cone- or bustle-type tailpipe configuration is going to be radically cooler than a straight pipe equivalent, because of its easier breathing. So, the wave traverse through it will be significantly slower than it is deeper in the engine. This MUST be taken into account (by supplying fairly accurate temp data), since it is such a large part of the total wave path length. I have no idea how to get the accurate local temps needed that would let you know you've got it right.
All my gas temps for UFLOW are just guesses - it's amazing that I ever get anything to come out right at all!