pulse jets... looking for advice 1

[DerrickM]

hi i was wondering if anyone had any knowledge of how to calculate the running frequency of a valvless pulsejet. ive built quite a few, most of them ran pretty well but i would like to learn how to actually calculate the ratios and running frequency intead of just guessing and tuning. i mostly do basic mechanical engineering at my grandfathers machine like rendering and animating prototype gearboxes, and some r and d. i havent gone to college yet (i start in january) so i dont have any experience in regards to fluid mechanics and thermodynamics. then as soon as pulsating combustion is thrown in, i get fairly lost. i would really like to learn more about the theory of pulsating combustion and fluid dynamics, as well as acoustic resonance. so, if you have a little info that you think i might find useful, could you please post it and give a brief explanation? thanks


pulsating combustion rules!

 

[HiPerTEC]

DerrickM,

That is awesome that you are interested in engines in general. The pulse-jet is, as you know, a simple engine to fabricate and test. However, the design can be quite tricky to get reasonable performance. Your question about operating frequency is determined by many factors; the design, fuel used, pressures, spark location, etc.
The short answer is that the equations can be quite involved to get very good estimates. However, an easier, reasonable approach might be to look for flame speeds for the particular air-fuel mixture you are using in the engine, at ambient conditions. That is assuming you want to correlate your calcualtions to what you see on a test-stand. The flame speed will give you an idea of how quickly the mixture burns in ft/sec. Then depending on the size (length of your pulse-jet) you can get an idea of how long it takes to burn the mixture in the engine. You could use that number as the speed of the exhaust out of the engine. Then it will evacuate the engine volume, by a drop in pressure below ambient, and the high ambient pressure will push a new charge of air into the engine for another cycle. Look for the velocity of air based on a certain difference in pressure, or delta pressure. The Bernoulli equation is the place to start on that note. Assume the initial velocity of the air is zero, and the high pressure is ambient (14.7 psia).

This may help get you in the ballpark for frequency of a certain length pulse-jet with a certain fuel burned. There are sources on the internet that will help you get there too. You might peak at NASA Glenn's software/student tools.