Pulsejet internal pressure

[matt512s]

Hi!
Was just wondering how high the pressure pulses average inside a valveless. Does it depend on the type?, The size?, The fuel?, All of
the above? Is there a formula for determining the internal pressure in
psi? I am specifically interested in the Reynst style combustor.
Thanks;
Matt

[tufty]

Was just wondering how high the pressure pulses average inside a valveless. Does it depend on the type?, The size?, The fuel?, All of the above? Is there a formula for determining the internal pressure in psi? I am specifically interested in the Reynst style combustor.

All of the above and more - pressure will vary depending on engine station as well. I don't have any figures for reynst-style combustors - indeed I doubt that any exist - but SNECMA/Messerschmidt saw maximum peaks of around 1.5atm at CC exit on an Escopette with CC diameter of 120mm. (diagram taken from 1978 document)

Simon

[mk]

To answer your question not in a complete manner, but at least as good as figured from reading, please take a look at the following paragraphs.

First, it does not really matter wheather we are looking at a valved or a valveless pulse-jet. Both of them showed about equal internal pressure values (minimum and maximum), even all over the past decades. It might be the case that a temporal pressure plot may look different in one or the other duct, vlaved or valveless, but that rather is a question of optimization, i.e. accoustical "tuning" or the charackteristics of the applied valve construction. Okay, but that should not be an issue here.

Taking a look at pulse combustors in general, the local pressure appears to be a function of the flame front velocity and the location. This means differences are caused by certain combustor geometry aspects (standing wave patterns, con- or diverging segments, size, etc.).

However, going back to common pulse-jets, including the Reynst combustor (the developed Reynst combustor also had a tapered stack with which it was tried to make use of standing waves), it would pretty much seem like the dominant factor for maximum pressure is indeed the combustion velocity. Herein I can tell you that flame front velocities in such non-detonating combustors to be found and reffered to in this forum, the Argus V-1 power plant, the SNECMA Escopette and Ecrevisse engines, are all showing combustion velocities well above the laminar burning velocities of any common fuel. Those, BTW, commonly are not exceeding 2 m/sec and can hardly be increased above 5 m/sec even with tricks. Anyway, the combustors in question are showing combustion velocities of about 60 m/sec in average, and may in part reach up to 250 m/sec, but still well below a typical detonation threshold value of lets say 500 m/sec.

An anotation to the above paragraph:
Reffering to a paper I read once, detonation forced some 2 cm thick glass window of a long test tube to crack (-- large local pressures!). So not only because of safty reasons, detonation does not appear to be an issue for hobbyists.
Commonly the maximum pressure is reached in the combustion chamber.

To summarize it in one sentence, non-detonative pulse-combustors' maximum pressure values do not depend on the fuels employed in first approximation. In some details and for very small, or high-frequency pulse-combustors, fuel might indeed play a role.


Well, another influence can be seen in size, together with the engines' shape. Herein it might be possible to think of someting like "critical mass" or so. Due to large enough masses involved, the largest Schmidt tubes were able to achieve detonative combustion (burning velocities up to or above 500 m/sec in part), whereas the smaller versions and other contured ducts of the same size were not.


Okay now, do give you some ideas for common values to work with, also due I don't know how to calculate estimated peak values for myself:

engine ---------------------------- peak over-pressure in comb.* / kPa (/psi)

Argus 014 (valved) -------------------- up to 240 (up to 34.8)
smaller Argus test engines (valved) -- 140 to 220 (20.3 to 30.5)
SNECMA Escopette (valveless) ------- 140 to 200 (20.3 to 29.0)

( *) 1 psi = 1 lb/in² = 6.895 kPa; variations due to summarizing different stages of development)

Considering that the Argus 014 was pretty large in comparison, and in refference to Graham C. Williams, you might find it worthwhile to work with the both last ranges. Not necessarily push the values to the limits. E.g. an undeveloped engine may only show 100 kpa peak over-pressure.

Sadly I am not in the position to reffer to more data.


Finally, please don't mind I went to detonative combustion issues as well, even it was not part of your question. But here it seemed necessary to me to consider it as well.

Hope this helps.

[HattoriHanzo]

Hi!
Was just wondering how high the pressure pulses average inside a valveless. Does it depend on the type?, The size?, The fuel?, All of
the above? Is there a formula for determining the internal pressure in
psi? I am specifically interested in the Reynst style combustor.
Thanks;
Matt

I am not shure what flow velocity we are talking about in a pulsejet.
But you can try to use the continuity equation and law of conservation of mass.

A1P1=A2P2

I suppose the velocity in a pulsejet is larger than M=0,3 so we can treat the flow as compressible where ρ1 is not equal to ρ2.

ρ1A1P1=ρ2A2P2

ρ=density
A=area
P=pressure

I think this method is plausible to determine the parameters. But it can be very complex because of the nature of pulsating. It would be a nightmare to calculate the pressure characteristic over time in a pulsating 3-d flow...without software.

[akshay shete]

hello,
can anyone provide the approx figures of combustion chamber peak pressure for lockwood 55lbs

[metiz]

hello,
can anyone provide the approx figures of combustion chamber peak pressure for lockwood 55lbs

absolute maximum is 2 bar

[PyroJoe]

2 bar?

[metiz]

Don't know what a bar is or do you disagree?
if first: 1 bar = *10^5 Pascal

No normal pulse-jet engine reaches pressures above 2 bars right?

[PyroJoe]

Sounds about right, just didn't know where it came from. Thats about 29psi, if I'm reading it correctly the Argus above is shown at 34.8psi?

[metiz]

Don't know about the numbers, sounds about right I guess. The 2 bar is probably a ballpark figure anyway. I remember reading it somewhere once. Btw, please use metrics