Pulse Jet engine - A new design concept

[aryanscarlet]

I`ve had a new idea in designing a pulse jet by adding a valve in it... According to my intuition and logically this increases the output many folds and I`ve given sufficient reasons for suggesting it. I have a powerpoint of my idea that I have made and I request any of you to please go through it and suggest whether it is viable or not!

Thank you.

http://rapidshare.com/files/299024703/Introduction_Of__Post_Combustion_Valve_in_Pulse_Jet.ppt"

 

[Bob S]

Many years ago, a friend of mine tested a ram-jet (I think a type of pulse jet) in his garage. I could hear it from two miles away. Are you talking about a silent pulse-jet?

Bob S

 

[FredGarvin]

It wasn't a ramjet unless his garage was traveling at supersonic speeds.

 

[Bob S]

It wasn't a ramjet unless his garage was traveling at supersonic speeds.

Must have been a pulse detonation jet. See
http://en.wikipedia.org/wiki/Jet_engine#Types
I know we had to start the engine with applied compressed air and (maybe) continuous spark plug ingition. Both the pulse jet and the pulse detonation jet are described in the above URL as "extremely noisy". What the world needs is a silent jet engine.
Bob S

 

[aryanscarlet]

My point is sirs, that it is already used only in very few applications like military and experimental applications. I know pulse jets are noisy! I just say that my project can increase its efficiency and so it will still continue to be used in missiles etc where it is already been used, just with better efficiency and higher thrust.

I need to know if my project is workable or not? Will it work?


Please help me! May be some CFD analysis would give us the result but I don know how to do it. If any of you know how to do one, I beg you all to just give it a try. That's the only thing that is making my project weak. You can have the dimensions of the existing pulsejet from something called Pulsejet calculator 1.4. Its an xl file you can get just by a google search!

Please try it!

 

[FredGarvin]

Believe it or not, pulse jets and pulse detonation engines are still an active area of research. I know of one program in particular.

 

[aryanscarlet]

I'm aware of research going on! I want you to please download my presentation and review my project idea. Please tell me if its feasible or not!

 

[telepresence]

sorry to bring up an old thread, but I thought I'd respond since apparently no one else did. Note that I've never built a pulse jet, though I'm currently in the research stage of building a valveless one. The link you gave is dead now, but I found another thanks to google. As far as I can tell, your ideas and thoughts seem correct. It would be interesting to see this in action. You basically covered all of the things I thought of when I initially saw it in the rest of the presentation. The one thing I would worry about is air flow restriction of the valve causing the vacuum not to form to pull in fresh air. If the exhaust valve can be designed to have minimal aerodynamic impact on the exhaust flow, than I think this could work.

 

[aryanscarlet]

Dear telepresence,

Thank you for your reply. I would like to clarify that the valve idea has been changed. Instead of a flap valve that closes when the pressure gradient turns negative, I came up with a better innovation. I realized that a very suitable system of valving is already available. The Iris shutters used in cameras work at a frequency of around 10,000 and is completely controllable via actuators. Once a research of the time taken for the required vacuum creation is acheived is done, we can use the appropriate frequency for the valve. Also the valve opens in concentric circles which is least disturbing to the flame stabilization and since it can open and close within 1/ 10000 of a sec the impact will be least. The valve could be made of inconel steel which I find would be suitable.

Proof that pulsed combustion is better may be found @ http://serve.me.nus.edu.sg/arun/file/teaching/ME6024/Basics of Pulse Combustion.pdf

Currently I am trying to do an analysis of it via CFD but I have problems in dynamic meshing and transient flow simulation. I am looking for help from experts but I have no clue of whom to approach. I am doing my Pre Final year of my under grad in aero and If I could get any institute or something to help me with this, I`m sure we could make a good research. Any suggestion is welcome.

 

[telepresence]

The iris valve is a good idea. Those things have always fascinated me. As for the cfd modeling, good luck. I can't really be of any help there. In fact, I would appreciate some help, myself. I have a valveless design that I would really like to model before trying to build, but I've never done any cfd modeling before and have no clue how to go about getting pulsed cumbustion to work with it...

 

[telepresence]

I'm sure you probably already know this, but the valves have always been the weakest link in valved pulse jets. Since iris valves don't have parts that flex, they should be able to hold up better than the more traditional petal valves, So I think it might be good if you put them on both the intake and exhaust. This would help the engine get more fresh air, also, which can only increase performance. if possible, though, I would angle the valve in so that the center where the pieces meet points into the combustion chamber. It seems to me that that might help them stand the pressure better than being perpendicular to it.

Thinking about why valved pulse jets tend to be more powerful than valveless, I think I realized why, and it applies to your design, as well. The determining factors are the amount of fresh air and the internal pressure.
A valveless design has the lowest internal pressure because it effectively has twice the exhaust exit area, meaning that it is twice as easy for exhaust to escape. Also, a valveless design does not get as much fresh air as a valved design because before it can obtain fresh air it must first re-ingest the exhaust from the intake which takes up a large part of the space available for the fresh air.
A valved design has higher pressure because it only has one exhaust tube and it has access to more fresh air because it does not have to clear its intake of exhaust, first. Fresh air is available instantly.
However, a valved design still allows exhaust to exit to soon, and it pulls some of that exhaust back with the fresh air from the intake. Your additional valve stops both of those from happening. Because the exhaust tube is sealed off during the intake phase, ONLY fresh air is ingested by the engine and therefore more of it is ingested. This increase in fresh air means that more fuel can be burned per engine cycle and it can be burned more efficiently. Also, exhaust can not escape instantly as it can in other pulse jets, it is contained until the exhaust valve is opened. This means that a higher internal pressure can be reached, once again increasing efficiency.
This means that an engine of your design can produce more power than an engine of similar combustion chamber size, while still being much shorter because it no longer relies on the long tail pipe to function. This does come at a small cost, though. This engine is slightly more complicated and has more potential failing points. I think the iris valve design has the potential to make valve failings less likely to occur, though. Also it needs a power source of some sort and continuous ignition from a spark plug.

That's my take on it. I covered everything I've thought of. All in all, i think this is a great idea. I just wish I'd been the one to think of it. :)

 

[telepresence]

Here's an idea:

instead of trying to model the entire engine with the valves functioning, you could simplify things by creating two or three meshes.

One mesh is just a closed tube (the combustion chamber with both valves closed). Simulate the combustion.

Take the results of the combustion process after a given pressure is reached (when the exhaust valve would open), take the conditions at that time and make them the starting conditions for the second mesh which is the engine with the exhaust valve open. Run that simulation until the vacuum in the combustion chamber peaks.

Take those conditions and use them as the starting conditions for the third mesh (the engine with exhaust valve closed and intake valve open) and run that simulation.

Repeat as many times as you think necessary.

You won't be able to model the valves as they open and close, but you should be able to get fairly accurate results this way. Hope that helps you out some.

 

[JaredJames]

Valved pulse jets have been around for years.

Actuators are heavy pieces of kit.

You need to power the actuator which means you either need to drive something off the engine (loss of thrust) or add a generation system (increase in weight). So you need to ensure the trade off by adding a butterfly valve isn't a loss in thrust or increase in weight.

Have you checked if there are actuators capable of responding in the time frames required to match the combustion cycle?

 

[telepresence]

Valved pulse jets have been around for years.

Indeed they have. I've been studying them them for several weeks. The Germans used them for their V1 "buzz bombs" in WWII.

Actuators are heavy pieces of kit.

You need to power the actuator which means you either need to drive something off the engine (loss of thrust) or add a generation system (increase in weight). So you need to ensure the trade off by adding a butterfly valve isn't a loss in thrust or increase in weight.

Very true, but I think the increase in power from the engine will more than make up for the loss in weight and power if it's designed well enough. For static and test/"proof of concept" runs, it won't be an issue. So, at least for now, this can be ignored.

Have you checked if there are actuators capable of responding in the time frames required to match the combustion cycle?

We've already covered this, though very indirectly. He plans on using iris shutter valves which can function at extremely fast rates. It follows that if iris valves can and do function at those speeds, there must be an actuator that can also function that fast. The problem won't be speed, it'll be endurance. How many times can an iris valve be switched in rapid succession at high temperatures before it gives out? My guess is a lot more than a petal valve which has to physically bend each time it opens or closes and frequently gives out in a matter of minutes in a hot pulse jet. I think the actuator won't be a problem as a simple motor can probably do the job just fine. Since the combustion process in a pulse jet engine occurs at a constant rate, a stepper motor running at that same rate should be able to open and close both valves at the right time. It can be electronically synchronized with the pressure swings inside the engine.

 

[JaredJames]

Very true, but I think the increase in power from the engine will more than make up for the loss in weight and power if it's designed well enough. For static and test/"proof of concept" runs, it won't be an issue. So, at least for now, this can be ignored.

That's pure assumption then? You may want to check this with some figures.

We've already covered this, though very indirectly. He plans on using iris shutter valves which can function at extremely fast rates. It follows that if iris valves can and do function at those speeds, there must be an actuator that can also function that fast.

From what I've read, he compared a camera iris shutter - which can operate at those speeds (sans actuator) - to a heavy duty piece of equipment - which I've never heard of operating at those speeds (or anywhere near them). This is not a valid comparison.

The problem won't be speed, it'll be endurance. How many times can an iris valve be switched in rapid succession at high temperatures before it gives out? My guess is a lot more than a petal valve which has to physically bend each time it opens or closes and frequently gives out in a matter of minutes in a hot pulse jet.

Speed is a major issue, as above. Typical actuator reaction times on the scale of a standard pulse jet aren't even close to 1/10000th of a second.
That aside, the valves I've seen on certain pulse jets don't bend at all. They are the equivalent of pieces of metal on a hinge, using a square inlet.

I think the actuator won't be a problem as a simple motor can probably do the job just fine.

Actuators are tough pieces of kit also. I'm not sure a "simple motor" can replicate them that well. At least not under the conditions you're talking about.

Can I pitch an idea?

Why not have a disc, twice the diameter of the pulse jet. Cut holes in the disc to match the diameter of the pulse jet, leaving a gap of equal diameter between each one.
You can then use a motor and simply spin the disc in time with the combustion cycle - so that at the right times it would either be covered or open as required.

By doing this you remove any problems of stop/start operation. Opening and closing an iris valve quickly is going to have severe mechanical wear, but simply spinning a disc would remove this problem.

Or

Even easier would be to fit a ball valve to the front of the engine (make sure it's one that can do a full 360) and spin it in time with the combustion cycle. Again, it would open / close as appropriate but alleviate as much mechanical stress as possible.

 

[telepresence]

The ball valve is a nice idea. It would be larger and heavier than an iris, but you're right in that it wouldn't wear much and would be very simple to manufacture and operate.

Here's an rough sketch of an idea I had for a dual valve system that might not need mechanical actuators. If done right, I think the changing pressure inside the combustion chamber should be able to move this valve(s) by itself. It wouldn't contain the exhaust before letting it out, but it also doesn't have any extra complexities. Basically a half way point design, I guess. You get some of the advantages without some of the disadvantages.

The inlet portion of the valve has a larger area and so should be affected more by the pressure than the exhaust portion. The increase in pressure do to combustion should shut the inlet, thereby opening the exhaust. Decreased pressure as the exhaust leaves should open the inlet, closing the exhaust. Ram effect should help open the valve in the intake stage.

The question is, will a sufficiently strong vacuum be able to form before the valves switch to intake position...

 

[JaredJames]

If you're certain it wouldn't back fire due to the pressure differences created than it may work. Personally I can see it failing due to the way it operates.

The ball valve (or plate rotation idea) wouldn't weigh more than than the iris design if done correctly and should prove significantly more resilient to the operating conditions.

 

[telepresence]

If you're certain it wouldn't back fire due to the pressure differences created than it may work. Personally I can see it failing due to the way it operates.

Could you explain?

The ball valve (or plate rotation idea) wouldn't weigh more than than the iris design if done correctly and should prove significantly more resilient to the operating conditions.

Your probably right, especially when considering the extra components needed to operate the valves. The ball valves could easily be operated straight off of an electric motor, the iris shutter valve would be more difficult. Also, I have a feeling a ball valve can seal a lot better than an iris valve could. Lots fewer places to leak.

 

[telepresence]

Refined the design a bit.

I think that the ball valve idea is better than this one, but this would be a nice, simple method of putting two valves in a small, light weight engine. Not sure where there would be applications for it that couldn't be filled by one with ball valves, though.

 

[aryanscarlet]

Thank you all for your valuble contributions. The Ball valve is an extremely good idea and I will do an indepth study on it, I am unclear on how the opening mechanism thou. I also thought of the spinning disc. Gonna work on that too. I would love any additions to it!. Thanks again for that. I would like clarify my take on the iris shutter and its necessity. I want the valve to cause least disturbance to flow. When fully open the exit path must be absolutely free of disturbances or even turbulence. This is the reason a petal valve may never be used. In the ppt the diagram of a simple flap valve is described which would work perfectly and without ext actuation. However the flow after a sudden expansion is highly disturbed and i felt the purpose of the innovation would be lost. So it is mandatory to consider flow after the valve opening. I am psyced abt the response and having come up with the idea when i was in my first semester of Undergrad, i am thrilled at the comments. I would like to take it a step above but I am currently helpless here. I know its a very long shot, but if it would be possible that any of you can introduce me to your professors working in the field, it'd be a great help. If I can get them to take me up as my under study, I'd make sure the research is worth it.

Thanks a lot guys. Hope to keep the discussions alive. I`ll keep posting my findings too. You are welcome to add or alter my claims.

 

[JaredJames]

Once again, an iris shutter isn't as good an idea as some people are telling you.

a) You can't compare industrial iris valve opening times to those of a camera.
b) You won't get an industrial one to open / shut in the time of a camera one.
c) You won't get an industrial actuator to activate in the time of a camera.
d) The disturbance will exist regardless of method used (you can't just cut off the airflow and expect it be perfectly laminar). But at the speeds your talking about there won't be major disturbance that can't be overcome.
e) Mechanical stress on an iris valve will be huge and it may not even survive basic operation.

This is why I mentioned the spinning disc and ball valve - unlike your iris valve there is minimal mechanical stress (well virtually none outside of basic motor wear and tear) and it will be a lot easier to time and power.

Personally I'd go with the ball valve as I could see the disc giving a few issues due to the suction created.

 

[telepresence]

Personally I'd go with the ball valve as I could see the disc giving a few issues due to the suction created.

Also, the disc valve would increase the area of the front of the engine and tend to make the engine less aerodynamic, something that isn't usually desirable in a jet engine.

 

[aryanscarlet]

Ya. I agree wit yo guys. However I don need the iris thing to work at 500 or lower frequency only. So actuatin won be problem. Still,I'm going to work on the ball valve idea. However I am unclear abt the working . First the front valve opens. Then when pressure increases both values close. Next how does the front valve remain close and rear valve alone open.

 

[JaredJames]

However I don need the iris thing to work at 500 or lower frequency only. So actuatin won be problem.

You'll be pushed to get an iris valve and actuator working at a frequency of 10hz (1/10th of a second operation time). Trust me, given the nature of what you need you won't be able to achieve this reasonably.

I request you point me to a source for a) an iris valve capable of the speeds required and b) an actuator capable of providing such speeds and c) a power system that doesn't render advantages from the addition of your system irrelevant.

Still,I'm going to work on the ball valve idea. However I am unclear abt the working . First the front valve opens. Then when pressure increases both values close. Next how does the front valve remain close and rear valve alone open.

You clearly have no idea how a ball valve operates. Everything bolded is complete gibberish so far as ball valves go.

Here is an animation that shows how they operate: http://www.opw-ftg.nl/www/pt/europe/products/chemicalindustrial/processvalves/ballvalves/lbv-operation-animation.jsp

You need to have one that can do a full 360 operation and then using that you simply attach and electric motor and then spin it at an RPM that matches the combustion cycle.

 

[telepresence]

You clearly have no idea how a ball valve operates.

I think he may be thinking of a ball in a cage valve, where the ball is loose and can move back and forth a little, opening and closing the valve as it does so.

Here is an animation that shows how they operate: http://www.opw-ftg.nl/www/pt/europe/products/chemicalindustrial/processvalves/ballvalves/lbv-operation-animation.jsp

Nice animation, by the way.

 

[telepresence]

If you're certain it wouldn't back fire due to the pressure differences created than it may work. Personally I can see it failing due to the way it operates.

Really would like to know what you meant, there. I'm know expert, so I have no clue what could go wrong with it.

 

[JaredJames]

Really would like to know what you meant, there. I'm know expert, so I have no clue what could go wrong with it.

Sorry, it's late now. I'll post an explanation tomorrow.

Also, I know it's not important but you quoted the link to the animation incorrectly so it doesn't work in your response post.

 

[telepresence]

Sorry, it's late now. I'll post an explanation tomorrow.

That would be great. Thanks!

Also, I know it's not important but you quoted the link to the animation incorrectly so it doesn't work in your response post.

Thanks for pointing that out. Fixed it.

 

[Phrak]

Refined the design a bit.

That may have potential, telepresence. It may need a retention spring somewhere. The front valve is a ram--so needs additional thought. The shaft should be larger diameter and hollow for air cooling.

 

[JaredJames]

That may have potential, telepresence. It may need a retention spring somewhere. The front valve is a ram--so needs additional thought. The shaft should be larger diameter and hollow for air cooling.

There we go, that's basically what I was going to describe, but I was going to try and explain the 'why' as well - hence waiting for tomorrow.

 

[telepresence]

The front valve is a ram--so needs additional thought. The shaft should be larger diameter and hollow for air cooling.

Shouldn't be to much worse than a petal valve, and doesn't have to bend, but thickness would need to be a consideration to balance weight and durability. Air cooling sounds interesting... Would help with the weight factor, too, I guess.

 

[telepresence]

There we go, that's basically what I was going to describe, but I was going to try and explain the 'why' as well - hence waiting for tomorrow.

I'll be looking forward to that.

 

[telepresence]

It may need a retention spring somewhere.

There we go, that's basically what I was going to describe...

Just a guess, but is this basically what you are referring to?

The question is, will a sufficiently strong vacuum be able to form before the valves switch to intake position...

 

[telepresence]

However I am unclear abt the working.

basically something like this:

 

[Phrak]

Very cool. What might happen with an 'S' shaped orifice?

 

[aryanscarlet]

Got it guys! Sorry, mistook the ball mechanism! Now its clear. Glad people have pitched in ideas. Will keep posted on my work too.

 

[aryanscarlet]

I made a small change to telepresence's idea. In this valving, the flow right after after combustion is straight thru the valve instead of a sudden reduction in area which causes fluid dynamic losses. Also with the smaller valve, when closed, due to smaller area of cross section of the ball, the pressure on the valve and also the hoops stress on the walls of the combustion chamber will be greater. Also when the valve is closed and combustion takes place, the convergent section will also be a part of the chamber and the hoops stress would be pronounced on the walls and the sharp area changes will produce turbulence during combustion and problems in flame stabilization.

The inlet could also be of the size of the second valve or even a normal v- shutter inlet would do.

Any contradictions or improvements guys?

 

[telepresence]

it seems like that would reduce the power of the engine if you use the valve as part of the combustion chamber. I was seeing the combustion happening while both valves were closed to boost the pressure and ensure that the fuel burns while still inside the engine and not after it leaves. I could be wrong, though.
Also, remember that these valves are constantly spinning. the snapshot you have in that image would only be the case for an instant, certainly not long enough for the entire combustion and exhaust processes to take place.

 

[JaredJames]

it seems like that would reduce the power of the engine if you use the valve as part of the combustion chamber. I was seeing the combustion happening while both valves were closed to boost the pressure and ensure that the fuel burns while still inside the engine and not after it leaves. I could be wrong, though.

aryanscarlet's single valve design is along the lines of what I was thinking of. The dual ball valve system is just over complicating things and I don't see any real gain to be had by doing it. Not to mention increasing the chance of backfire / complete detonation.

Also, remember that these valves are constantly spinning. the snapshot you have in that image would only be the case for an instant, certainly not long enough for the entire combustion and exhaust processes to take place.

The 'snapshot' would be the midpoint of combustion - hopefully the point when the exhaust is being expelled to greatest advantage.

It wouldn't have to be there for the whole process, just enough to gain an advantage.

 

[aryanscarlet]

No you guys have misunderstood me. I did not mean that the combustion will happen with one valve open. That is totally against the entire idea itself. I meant, when the valve is fully open, it must be coincident with the combustion chamber and not like telepresence's first diag. The combustion will happen when both valves are closed only. In the initial idea, the nozzle section that led to the ball valve would cause flame instabilities which can be avoided in the changed diag. Also the opening and closing is up to us to decide based on preliminary tests and optimization can be done. The valve can be left closed for a prolonged time also or reduced time also based on the size, thrust req etc. This can be done by a simple cam.

 

[telepresence]

Also when the valve is closed and combustion takes place...

Okay, my bad. Missed that little "and" connection there.

 

[telepresence]

There we go, that's basically what I was going to describe, but I was going to try and explain the 'why' as well - hence waiting for tomorrow.

I really would like to hear this.

 

[JaredJames]

I really would like to hear this.

Sorry, really busy.

During combustion, there is a force applied to the front "sail". This forces it closed, ensuring all exhaust is directed backwards.

Your biggest problem is that you are assuming the thrust acting on the "pyramid" section + suction created will be enough to force the "sail" to open. Remember, there will be air drawn in through the rear during this time which will act against the rear "pyramid" creating a force that will try to counteract the suction acting on the "sail". The net effect is likely to be that the front "sail" section will remain at least partially, if not fully closed. Efficiency will be lost.

RE Bolded: the force acting on the forward "sail" will be far in excess of the force on the rear "pyramid" so it will easily counteract any effects.

So you need a spring to force the rear "pyramid" valve closed and the forward "sail" valve open. That way, the combustion will act against the forward "sail" to close it and direct all exhaust out of the back. Once complete the spring will force the valve to switch and suction to occur from the front.

 

[telepresence]

Sorry, really busy.

Aren't we all...

Thanks for your response. I do see the need for a spring there.