Tuning Pulsejets- Valve Freq. VS. Pipe Freq.

cbromano · Post by **cbromano** » Fri Dec 19, 2003 6:43 am

For those of you diehards that enjoy also doing the math and theory behind pulsejets i have one question that really is whopping me in the bum trying to solve.

As a beginer in pulsejets i have much to learn on theory but i am not a beginer on waves and fluid dynamics. I am trying to figure how waves in the jet effect the valves.

It is my conception that valve have their own resonance freq. I would assume it is directly related to the size, shape, and material used, along with how far the valve is allowed to travel. With that said i also assume that the length must be tuned to a freq. that allows the standing wave to return at the same moment the valve would want to open back up.

Is there anything wrong with thes assumptions?
In addition how would i figure out this freq? I can find the speed of sound through the pipe and figure out the speed of the standing wave but figuring the right valve freq is giving me alot of trubble.

thanks,

CB Romano

Bruno Ogorelec · Post by **Bruno Ogorelec** » Fri Dec 19, 2003 9:37 am

I don't think you need to worry about this. The approach is the same as in the car engine valves -- what you want is a valve frequency that is far removed from the engine operating frequency. You don't want the valve to resonate with the engine. You want it to follow the dictate of the pressure in the front end of the combustion chamber very closely, rather than having a mind of its own.

Fortunately, that is exactly what generally happens in a pulsejet -- the size, shape, material and other constraints dictate that the valves will have a much higher natural frequency than the pipe and the two will not interact.

Viv · Post by **Viv** » Fri Dec 19, 2003 1:31 pm

Bruno if you remember this was mentioned by Paul Scmidt in the Schmidt Gosslau papers that Andrew sent us.

the relavant page should appear below.

Viv

cbromano · Post by **cbromano** » Fri Dec 19, 2003 6:59 pm

I get the impression that it would be quite impossible fo me to figure out the the valve freq. accuratly enough to make a diffrence. True?

I also have another question though. i said in the first post that i could figure out the speed of sound through the pipe and i found that i was horribly wrong. I cannot predict the heat the thing will produce. I plan on using liqiud propane just because i though it would be alot simpler to use. I cannot figure out how much fuel i can be able to pump into the jet. I am thinking that the inside diameter of the pipe i am going to use is between 3/8-1/4 in. With that said will I be have an excess of fuel or air? I live at about 5000ft above sea level and it is quite cold here. How will that effect combustion?

Thanks
CB Romano