Not sure what the application would be, but here is a concept to take a single duct and divide it to make a much longer ratio engine. Nearly opposite of blending bodies.
If long engines are supposed to be more fuel efficient, then this one should be a daisy.
Not sure if this would work or not.
Fraction bodies
Moderator: Mike Everman
Fraction bodies
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Mike Everman
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Re: Fraction bodies
What I've found in my scaling experiments with straight pipe exhausts is this: scaling a good motor down does not work well, but if it does work and gets a better thrust in lb/sqin cc area, then you get a better motor scaling it back up again from that.
Flame speed is a constant with propane, and we know that combustion initiation is related to the flamespeed from the cc walls inward. This must relate to the frequency of the whole duct. So, I think leaving the length the same and reducing the CC diameter to .707 of what it was (to get your half motor) will screw it up and not get half thrust, unless the original was far off in this respect and scaling the diameter moved it closer to right. Another factor is the increase in wetted surface area per unit cross-sectional area which makes for even more losses muddying the water.
In analyzing my pocket jet (3.5 lb thrust, 30.3" length, 1.93" cc, frequency: 270 Hz)
As I recall, the flame speed according to Kentfield amounted to 8msec/inch CC radius, but I do not know exactly how that relates to the acoustic length of the duct. Trying to make sense of that, using that number and considering the time it takes to got wall to center and back on a 1.93" diameter cc, you get about 65 Hz if it were a continuous cycle, which is almost exactly 1/4 of the fundamental of the duct. So the conclusion is that either the CC rests for 3/4 of the cycle time (sounds about right), or something else is going on.
Either way, given a flamespeed, the calber or other method of scaling is appropriate, and experiments in changing only diameters will probably just point to a better caliber number for cc diameter to length.
Flame speed is a constant with propane, and we know that combustion initiation is related to the flamespeed from the cc walls inward. This must relate to the frequency of the whole duct. So, I think leaving the length the same and reducing the CC diameter to .707 of what it was (to get your half motor) will screw it up and not get half thrust, unless the original was far off in this respect and scaling the diameter moved it closer to right. Another factor is the increase in wetted surface area per unit cross-sectional area which makes for even more losses muddying the water.
In analyzing my pocket jet (3.5 lb thrust, 30.3" length, 1.93" cc, frequency: 270 Hz)
As I recall, the flame speed according to Kentfield amounted to 8msec/inch CC radius, but I do not know exactly how that relates to the acoustic length of the duct. Trying to make sense of that, using that number and considering the time it takes to got wall to center and back on a 1.93" diameter cc, you get about 65 Hz if it were a continuous cycle, which is almost exactly 1/4 of the fundamental of the duct. So the conclusion is that either the CC rests for 3/4 of the cycle time (sounds about right), or something else is going on.
Either way, given a flamespeed, the calber or other method of scaling is appropriate, and experiments in changing only diameters will probably just point to a better caliber number for cc diameter to length.
Mike Often wrong, never unsure.
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