Pressure jet idea

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Pressure jet idea

Post by matt512s » Mon Jul 11, 2005 7:58 pm

I have had an idea for a pressure jet and am wondering what you
think. Propane is forced through an annular Coanda slot in the interior
wall of the engine intake duct. The fuel flow entrains air which flows
with the vaporized fuel past a conical centerbody. This has the effect
of compressing the fuel-air mixture.
The mixture exits the intake duct into a TVC formed by an annular
cavity in the combustion chamber wall (or a bluff centerbody) and the
flat rear end of the conical centerbody. The exhaust exits through a
divergent nozzle.
To preheat the fuel prior to injection through the Coanda slot, the
fuel line is coiled within the combustion chamber. I would think that
more air would be entrained by use of the Coanda slot than by a
central nozzle. The TVC should increase fuel efficiency.
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re: Pressure jet idea

Post by pezman » Mon Jul 11, 2005 10:40 pm

There have been a few suggestions in the past about using propane through an air-amplifier as the front end of a thermojet. No reason it shouldn't work, I guess.

I think that making an efficient air-amp, getting the air/fuel mixture tuned to a suitable equivalence ratio etc. might take some doing. Exair makes some stainless air-amps that are tuneable, but they are pretty expensive and the tuning might need to vary as a funciton of several variables, esp. if you want throttleability.

There have been a few posts about trapped vortex combustors as well, but nobody on this site has built one as far as I know. Again, getting suitale TVC geometry work out might take some R&D.

At any rate, not intending to sound discouraging -- I think that a reasonably determined person could build what you are proposing and that it would work reasonably well. My guess is that you could surpass the SFC of any conventional pulse-jet (note that the bar is pretty low). Nice t see someone acknowledging the amazing amount of motive energy in liquid propane even before you burn it -- especially if you pre-heat it (there is a post around here somewhere of propane vapor pressure as a function of temperature)

Since your proposal is basically a Brayton cycle engine, it's upper limit of efficiency should be similar to that of a ramjet of turbine w/o bypass.

Personally, I'm more fascinated by unsteady combustion, particularly blast compression after having built a few small test rigs. The problem is getting the cycle to fire rapidly enough.

I saw site where a guy was using pre-heating of propane to power a turbine and he found that the fuel line started surging. If that could be harnessed to drive your engine in an unsteady mode, it might be a lot more interesting. The air-amp would be more tolerant of surge than a rotary compressor. Or if you could drive it into unsteady combustion some other way ....

As an aside, I read (I think in the Gluhareff patent) that pre-heating turns the propane into propylene under the right circumstances and that the stoichiometric air-propylene ratio is lower than the stoichiometric air-propane ratio.

At any rate, if you build it, it will generat lots of interest.

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