Theory questions answered here?

paul fellows · Post by **paul fellows** » Thu Mar 18, 2010 12:55 pm

I am starting this new thread as a place where people, including myself, can ask questions about the theory behind valveless pulse jets. And hopefully get answers without being made to feel stupid for not knowing this already!

In doing this I would ask people bear in mind the following guidelines;

when asking questions?

(1)there is no question about valveless pulse jet theory that is to stupid to be worth asking!
(2)Ask one question at a time that way others looking for answers can see if a question as already been asked and answered!
(3)If you need a lot of answers post multiple posts each containing just one question!
(4)This is not a place for saying my theory about pulse jets is this, and inviting comments from others. Start a new thread for that! (I have )

When answering questions

(a)I know your time is finite, so keep your answers short, and in plain language if possible.
(b)If you think you have the answer but are not certain, post your answer anyway, but make it clear that there is some doubt. As this will hopefully incurage someone with a better theoretical grounding to clarify things. That way both you and the person asking the original question learn something.
(c)Do not say that something has to be that way, because that is the way everybody as always done it! The only thing that can be learned from such a statement is, that you probably don't know the theory yourself?
(d)use the quote button when answering so that the thread can be easaly followed.

In stating this thread I am hoping it will grow to be a simplified overview of valveless pulse jet theory in short easily digestible chunks.

To that end I would beg the moderators to look in from time to time and delete any posts that do not belong here or are wrong or misleading.

paul fellows · Post by **paul fellows** » Thu Mar 18, 2010 1:01 pm

do pulse jets have to be noisey?

paul fellows · Post by **paul fellows** » Thu Mar 18, 2010 1:27 pm

paul fellows wrote:do pulse jets have to be noisey?

it would seem that they do!

thanks to crazycorkey for dircting me here

http://jetzilla.com/topic_003_01.html
GIVES AN EXPLANATION OF WHAT ACOUSTICS DO IN A PULSE JET

in summery the frunt of the sound wave accelorates the gas molicules forward,
after the peek of the wave the presure difrances slow the gas to its its origonal speed,
but the gas as been physicaly moved a short distence in the direction that the wave wastraveling.
the negitive half of the sound wave moves gas molicules the other way,
so in normal air at normal volumes the gas molicles end up back where they started.
the pulse jet is a quater wave osilator so if the forward motion coincides with a pulse and and the restroitive force coincides with the 'breathing' in will give a gain over just kadenacy.

dont take my interpritation a gospel look at the site http://jetzilla.com/topic_003_01.html

paul fellows · Post by **paul fellows** » Thu Mar 18, 2010 1:33 pm

what is the differance between a pulse jet optimised for thrust and one optimised for heat production?

paul fellows · Post by **paul fellows** » Thu Mar 18, 2010 1:42 pm

paul fellows wrote:what is the differance between a pulse jet optimised for thrust and one optimised for heat production?

as i understand it a pulse jet with a exaust pipe that is a long cone in shape is optimised for thrust.the long cone shaped exaust being called a primary thrust augmenter.
and an exaust that is the same width along its lenth as a higher exaust tempriture.

if anyone as more info?

paul fellows · Post by **paul fellows** » Thu Mar 18, 2010 1:48 pm

do the Kadenacy and acoustic waves have to be 90 degrees out of phase with eacg other?

would the jet work better if they could be broght more into phase?

paul fellows · Post by **paul fellows** » Thu Mar 18, 2010 1:57 pm

if the exaust is 1/4 of a wave lenth.

and the inlet is 1/6 of a wave lenth.

is there a standed lenth for a combustion chamber

paul fellows · Post by **paul fellows** » Sun Mar 21, 2010 9:12 am

paul fellows wrote:if the exaust is 1/4 of a wave lenth.
and the inlet is 1/6 of a wave lenth.
is there a standed lenth for a combustion chamber

the figuer of a 1/6th of a wave lenth comes from a quoat i read else were on this sitethat the inlet should be half of the wave lenth of the first odd harmonic.

1/2 of 1/3 =1/6
but if you look at the site logo[img]jets.com/phpbb3/styles/prosilver/imageset/site_logo.[/img]
it is obvious that the intakes are know where neer 1/6th of a wave lenth

Thor · Post by **Thor** » Sun Mar 21, 2010 2:11 pm

paul fellows wrote:
paul fellows wrote:if the exaust is 1/4 of a wave lenth.
and the inlet is 1/6 of a wave lenth.
is there a standed lenth for a combustion chamber
the figuer of a 1/6th of a wave lenth comes from a quoat i read else were on this sitethat the inlet should be half of the wave lenth of the first odd harmonic.
1/2 of 1/3 =1/6
but if you look at the site logo[img]jets.com/phpbb3/styles/prosilver/imageset/site_logo.[/img]
it is obvious that the intakes are know where neer 1/6th of a wave lenth

Greetings Paul.

Just a quick reminder when I looked at your above post where you have made on major error.
Do not at any time mistake actual tube length with that of acoustic length's. As there is a major difference between the two.

Cheers.

paul fellows · Post by **paul fellows** » Tue Mar 30, 2010 9:47 am

i'v just re-read Bruno's page on pulse jets
the corect lenth for the inlets is
1/4 of the lenth of one of the odd harmonics of the frequance that the jet runs at
so ignoring the diferance between tail pipe lenth and one quarter (or an odd multipal) of a wave lenth for the moment we find that the ratios of inlet to tail pipe are as follows
for the 3 harmonic = 1/3
for the 5 harmonic = 1/5
for the 7 harmonic = 1/7
for the 9 harmonic = 1/9
if only life was that simple

but there is the diferance between physical lenth and wave lenth to take into acount.

paul fellows · Post by **paul fellows** » Tue Mar 30, 2010 10:57 am

there are four aspects to the relationship between physical and wave lenths

1) the tail pipe will be one quater of the wave lenth of the finished jet.

2) end corection, the actual wavelenth of the jet is not 4 times the lenth of the tail pipe. but 4 times the lenth of the tail pipe plus 4 times (0.58 times the redious of the pipe. ) as the radious of a pulsejet tail pipe is usualy small compaired to its lenth having that corection calculated to the tenth desimal place is not perticulerly usefull, and some people just use 0.6

3) having messured and made the perfect pulse jet, you then start it up. it gets red hot, the metal expands, and all of the dementions chainge. in some designs where the inlet gets as hot as the tail pipe the expation is less of a problem for these. however for other designs this might be more of a problem.

4) the wave lenth that a given pipe will resonate at is dependant upon the speed of sound of the gas in the tube, starting the jet chainges the chemical composition and tempriture of the gas in the tube,

.
for valved pulsejets this just chainges there pitch from that whitch might have been expected. for valveless pulsejets this means that they will allways have to be tuned after they have been built.
i hope some one with practical experence of tuning a pulsejet will post a few rule of thumb tips in this thread to help

www.pulse-jets.com

Theory questions answered here?

Theory questions answered here?

acoustics?

Re: acoustics?

heat v thrust

Re: heat v thrust

90 degrees

chamber lenth?

Re: chamber lenth?

Re: chamber lenth?

Re: Theory questions answered here?

physical and wave lenths