Turbines driven by pulse jets

[Viv]

Thinking about PJs and in particular Valveless pulse jets driving turbines I thought it might be fun to have a thought experiment on what happens when you connect a standard turbo from a car or truck to a valveless pulse jet engine.

Lets start by simplifying things so we can see the details,

Lets resolve the valveless pulse jet down to a very basic form i.e. a tube of say 3 inches diameter and lets say three feet in length

To start with, leave inlet and exhaust the same at 3 inches.

place a quantity of fuel and air in the middle and ignite it.

What happens next?

Viv

[Viv]

To Continue

If we take the above 3 foot long 3 inch diameter tube and as suggested put a small quantity of fuel air mix in to it and ignite it, what happens?

Now I said a small quantity meaning the amount we would expect to fill the combustion chamber of an engine under steady running conditions, this is different to startup conditions were the whole engine can be full of fuel air mix along its entire length, obviously that first single bang would give a disproportionately large peak thrust reading and does not reflect a real thrust reading, the steady running thrust will only come from the fuel air charge ingested under running conditions, this is a much smaller fraction.

Now: in our simple engine tube if we ignite this small fuel air charge we will see a local pressure rise that will then travel to each end of the tube, we are going to ignore acoustics and other complications for now

As the tube is is open at both ends the pressure will leak out the ends and fall to the ambient pressure for our experiment that sea level on earth rather than ambient on the moon.

The pressure rise resulted from the expanding gasses of our fuel air mix, again we are keeping it simple

Now lets reduce the diameter at the ends of the tube by fitting some smaller 1 inch diameter tubes, they can be a few inches long,

Lets repeat the experiment, ignite a small quantity of fuel air mix, what happens this time?

Was the pressure higher? lower? the same? did it last for longer or less time?

Viv

[ace_fedde]

Viv,

Is that a "to be continued" question or a "let see how many dummies there are on the forum" question?

Let me try. I would expect a rise in pressure, thus and in temperature and this situation lasts longer.
At the same time we will see an decrease of gas speed in the 3 inch pipe an increase in the 1 inch ends. (opposed to the straigt 3 inch)
If the 1 inch parts are long enough a less than ambient pressure will be created after the escaped gasses.
If the ends are squeezed to much no vacuum (less than ambient pressure) will be produced.

So where are we going now?

Fedde

[Viv]

Viv,

Is that a "to be continued" question or a "let see how many dummies there are on the forum" question?

Let me try. I would expect a rise in pressure, thus and in temperature and this situation lasts longer.
At the same time we will see an decrease of gas speed in the 3 inch pipe an increase in the 1 inch ends. (opposed to the straigt 3 inch)
If the 1 inch parts are long enough a less than ambient pressure will be created after the escaped gasses.
If the ends are squeezed to much no vacuum (less than ambient pressure) will be produced.

So where are we going now?

Fedde

Hi Fedde

Its a to be continued set of questions aimed at developing a basic model for this type of thing, its not aimed at knowing who the dummies are

Yours was a good answer as it shows you thought about what was happening, now take off one nozzle, looks a bit like a valveless pulse jet in simple form

Viv

[ace_fedde]

....., now take off one nozzle, looks a bit like a valveless pulse jet in simple form

Viv

Is that a question again?

Even if it's not, lets try..
gasses traveling in both directions but more to the open end, the following vacuum sucks in fresh air through the nozzle while gasses are still leaving the open end, till the open end also sucks in.

Fedde

[Viv]

Well kinda yes and kinda no to get that vacuum we need some pressure waves floating back and forth but thats acoustics and we are leaving that out of the picture for the moment, remember its all supposed to be simple case stuff and easy to look up in a fluid dynamics text, and yes we could talk about Kadenacy but its altogether too soon for him to enter the discussion

The real thing I wanted to look at was the simple tube with restrictive nozzles on both ends compared to a original simple tube with out any nozzles, then moving on to the case were we had only one nozzle.

You got it right for the tube with two nozzles as you correctly pointed out the pressure would rise higher than ambient and take longer to fall again because of the nozzles, this gives us simple statement that pressure fall time is affected by nozzles, we can also say that pressure rise peak value is affected by nozzles.

But with two nozzles the flow is equal from both nozzles as the pressure is evenly distributed across them!

We can also see that the last case with one nozzle and one open end (3") the pressure may induce the flow to leave via the end with the least resistance rather than forcing its way out of the smaller and more restrictive nozzle.

So now we have cause and effect leading to pressure and flow more later

Viv

[ace_fedde]

Viv,

Really curious where we are going but please keep the last question for me. If I answer that one well the patent is mine!!

Just joking

I have a set of questions for you (or anybody else) as well:
viewtopic.php?f=6&t=4631&p=62889#p62889
If you answer there the last question well the patent is....

Fedde

[Viv]

So too continue with out any of that troublesome wanky wanky maths.

"We can also see that the last case with one nozzle and one open end (3") the pressure may induce the flow to leave via the end with the least resistance rather than forcing its way out of the smaller and more restrictive nozzle."

Now lets look at a typical car or truck type supercharger, the aim is to bolt this turbo on to the end of our valveless Pulse jet and get it to spin up and then do some useful work.

Take note of the nozzles we placed on our simple three inch tube valveless engine model, a simple model of a turbo from say a truck diesel would look the same as a restrictive nozzle.

The turbo scroll, the turbine wheel, the exhaust outlet all add up to flow restriction, thats before we add any thing else, the turbine turns a shaft so we have bearing friction, on the end of the shaft we have the compressor with its attendant losses to add, it does take work after all to turn and compressor and do some thing useful!

Adding all of the losses found in a turbo makes it in this simple case maths free thought model look to all the world like a simple restrictive nozzle.

So the question now is what happens when we place this turbo on our 3 inch tube motors exhaust? don't forget the inlet end still has our simple 1 inch nozzle.

Viv

[ace_fedde]

Viv,

Haha, we are both using turbo's as nozzles/restrictors, see the (your) thread mentioned above.

Back to this thread: Also with the "spinning nozzle" we are back here:

You got it right for the tube with two nozzles as you correctly pointed out the pressure would rise higher than ambient and take longer to fall again because of the nozzles, this gives us simple statement that pressure fall time is affected by nozzles, we can also say that pressure rise peak value is affected by nozzles.

But with two nozzles the flow is equal from both nozzles as the pressure is evenly distributed across them!

Viv

But not quite the same because:
As we regard the turbo just as a nozzle we have as "side affect" that it starts spinning.

Assuming that you meant the turbo to be a turbo, the spinning might be our main goal!!

The difference will be though that the vacuum creating drag will last longer for the spinning turbo than for the 1 inch nozzle.

Fedde

[Viv]

Hi Ace

"The difference will be though that the vacuum creating drag will last longer for the spinning turbo than for the 1 inch nozzle."

What vacuum creating drag would that be then?

That aside you were on the right track again but did not keep to the simple model

If the turbo is like a simple nozzle and our simple model engine has equivalent 2 nozzles, do you remember what happened to the combustion pressure and how long it took to fall back to ambient? (you can think about your vacuum drag! here too;-)

And the killer question is how did 50% of the combustion products leave our simple model with 2 nozzles?

From that you can work out how much of the engines total combustion pressure will "ever" be available to do work in the turbo of this simple model.

Viv
Ps yes I know that pressure and flow are being abused a bit here but its a simple model and we are not using any maths

[ace_fedde]

And the killer question is how did 50% of the combustion products leave our simple model with 2 nozzles?

Uhhm, through the turbine?

From that you can work out how much of the engines total combustion pressure will "ever" be available to do work in the turbo of this simple model.

Uhhm, 50%?

So 50% will be thrust! Mmm! I have this great foreside of my bike with Torque'n'Thrust TnT!

Fedde

[Viv]

And the killer question is how did 50% of the combustion products leave our simple model with 2 nozzles?

Uhhm, through the turbine?

From that you can work out how much of the engines total combustion pressure will "ever" be available to do work in the turbo of this simple model.

Uhhm, 50%?

So 50% will be thrust! Mmm! I have this great foreside of my bike with Torque'n'Thrust TnT!

Fedde

Yes 50 % through the turbine, only 50% though, compared to a normal gas turbine we can see the normal gas turbine sends 100% of its combustion products to the turbine wheel.

Would the remaining 50% be thrust? well? that 50% is coming out of the intake remember so it may not be so useful to us is the 50% of the potential combustion products coming out of the intake as useful as the 50% doing work in the turbine? that would depend on what it was you were trying to build.

What is clear in this no maths minimal thermodynamics thought experiment is that a valveless pulse jet has a thermodynamic efficiency problem, the chamber pressure can never rise to any really high value due the fact that there is a hole in the chamber for the pressure to leak out of, so no high pressures no powerful flow of gasses!

So putting a turbine on the exhaust of a valveless pulse jet with out doing any thing to the intake is not going to get you very far! no matter how hard you try to talk it up and impress people

Now a Valved pulse jet is a very different situation as the valves can shut the intake during the combustion cycle and allow the combustion chamber to generate higher pressures, thermodynamically its better than a valveless engine but still suffers from a compromised intake cycle due to residual chamber pressure, see Johns excellent thread on a valved engine driving a turbine and his conclusions.

A simple intake solution to this problem would be to connect the compressor of our car or truck type turbo to the intake of our valveless pulse jet?

Viv

[ace_fedde]

A simple intake solution to this problem would be to connect the compressor of our car or truck type turbo to the intake of our valveless pulse jet?

So, in fact, create a higher pressure envirement for the PJ? Nice!

But no more TnT

Fedde

[Viv]

A simple intake solution to this problem would be to connect the compressor of our car or truck type turbo to the intake of our valveless pulse jet?

So, in fact, create a higher pressure envirement for the PJ? Nice!

But no more TnT

Fedde

Hi Ace

Not really as it creates a number of other problems, no free lunches

Viv

[Viv]

One of the more important conclusions that can be drawn from this very simple thought experiment is the fact that stuff leaks out of the intake of valveless pulse jets! its a fact we can not escape unfortunately.

Thermodynamics and fluid dynamics do not care what you think or what you wish for, its very impersonal just like the law of gravity.

Adding any thing to the exhaust that is restrictive (like a turbine or a nozzle) will cause more stuff to blow out of the inlet instead of going out of the exhaust as you wanted.

Ok lets try another way (maybe in French too if it will help;-) lets say you have an air compressor and an air tank, lets say you have two valves on the air tank, put a small turbine on one valve, but don't put any thing on the other valve so its just going to blow down to ambient.

Now with the air compressor running (our pressure gain device) open both valves at the same time, what happens? not a hard question now is it? the turbine spins up yes? to lets say 15,000 rpm, but the other valve makes a horrendous hissing noise

What you have there is in effect the same thing as the valveless pulse jet in our simplified example, no matter how much air can be forced in to the tank by the compressor, half of it is going to just blow out of the open valve, so only half is available to do work spinning our turbine.

If history teaches us one thing (and USPTO) it is that valveless pulse jet intakes point backwards for a very simple reason

More later

Viv