LarryC
Aerospace
- Oct 8, 2001
- 15
I am working on a miniature pulsejet design, and have several problems due, in one way or another, to the small size of the combustion chamber (approx .75 in. diam x 2.0 in. long). The exit nozzle (approx .75 in. diam down to .375 in. diam) is rather short (about .5 in.) but as smooth as I can make it. Material is high-strength steel (sim to "chrome moly"
about .07 in. thick. The engine does not pulse yet, due to inadequate tailpipe length, so all runs so far have required compressed air input (intended for starting only, eventually). This "starting" airstream pulls in fuel by carburetion and also draws outside air in through the intake venturi (i.e. an "ejector" effect). I can get good smooth combustion, and (judging by visible flame temperature), I am achieving lean burning, so I assume I'm running air/fuel ratios of 30 lb/lb or better.
I am not concerned about low thermal efficiency -- I understand that very small combustors are inherently "lossy" due to the large radiating surface for the relatively small mass flows involved. My main problem is that good combustion seems to be mostly happening AFTER the gases get out of the chamber, as evidenced by the red heat of the tailpipe downstream from the chamber exit nozzle (on larger pulsejets, the throat of the nozzle is always the observably hottest zone). Note that, like most pulsejet designs, this is a straight tailpipe equal in area to the nozzle throat, NOT a deLaval nozzle.
Other pertinent data: Assumed maximum achievable exit velocity is approx 1000 ft/sec; assumed combustion chamber velocity approx 200 ft/sec (but this may actually be higher right now due to the "starting" air injection -- smooth running has only been achieved with a crude "flame holder" positioned downstream from the intake venturi to brake the airflow); total engine mass (excluding mounting lugs) 2.5 ounces. The most successful fuel seems to be ethyl ether. Starting ignition is provided by miniature spark plug (Champion V-2); spark is well inside the chamber wall and about 1.25 inch upstream of the nozzle throat (this has no bearing on the problem; after a few seconds, the device runs without maintaining spark ignition).
Any suggestions would be appreciated. One thing I have considered is lengthening the combustion chamber, but of course, that would increase the flying weight.
- Larry Cottrill
I am not concerned about low thermal efficiency -- I understand that very small combustors are inherently "lossy" due to the large radiating surface for the relatively small mass flows involved. My main problem is that good combustion seems to be mostly happening AFTER the gases get out of the chamber, as evidenced by the red heat of the tailpipe downstream from the chamber exit nozzle (on larger pulsejets, the throat of the nozzle is always the observably hottest zone). Note that, like most pulsejet designs, this is a straight tailpipe equal in area to the nozzle throat, NOT a deLaval nozzle.
Other pertinent data: Assumed maximum achievable exit velocity is approx 1000 ft/sec; assumed combustion chamber velocity approx 200 ft/sec (but this may actually be higher right now due to the "starting" air injection -- smooth running has only been achieved with a crude "flame holder" positioned downstream from the intake venturi to brake the airflow); total engine mass (excluding mounting lugs) 2.5 ounces. The most successful fuel seems to be ethyl ether. Starting ignition is provided by miniature spark plug (Champion V-2); spark is well inside the chamber wall and about 1.25 inch upstream of the nozzle throat (this has no bearing on the problem; after a few seconds, the device runs without maintaining spark ignition).
Any suggestions would be appreciated. One thing I have considered is lengthening the combustion chamber, but of course, that would increase the flying weight.
- Larry Cottrill