Turbines driven by pulse jets

[ace_fedde]

Viv,
I was already curious all the time, but now I'm really getting curious beyond possible
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

Hi Ace

I was thinking about that answer I gave you and thought to myself that maybe I should expand it a bit as even though I consider it to be a bit out side this simple thought experiment we could usefully look at it here.

If you replace a typical gas turbine combustor with a pulse jet then what really is happening? we could simply consider a gas turbine as a flow device, air enters, is compressed, heat is added, then work extracted from the expanding flow to drive the compressor, what ever is left over is considered thrust

The gas turbine combustor can be considered a simple ram jet, to see why go see Nye thermodynamics excellent project using a 700 CFM diesel air compressor and an old Rolls Royce turbojet combustor, http://www.gas-turbines.com/hornet.htm

Now as you said, what about that high pressure environment for the pulse jet? well yes but with pressure also comes flow! what about flow and what about time? take the example above with the 700 cubic feet a minute flow from that diesel compressor, looking at the ram jet combustor its mixing air with fuel and burning continuously at what ever flow rate is set by the throttle valve.

But what about a pulse jet in the same situation? its does not burn continuously now does it? its intermittent combustion with an intake cycle and an exhaust cycle, both take time! and they do not happen at the same time (ok some overlap;-) so now we have a pretty basic problem of a continuous flow device having to flow intermittently but flow the same amount of air to get the same performance.

If you want to put a pulse jet in a gas turbine engine then you need to consider time and flow not just pressure, this at what ever frequency you design the pulse jet to operate at, its a simple answer as there's no maths and we are not going near any decent books on the subject, but you can see some simple problems to be considered that are in fact major problems of physical sizing and actual performance.

A simple ram jet combustor as used by current gas turbines is always going to outperform a pulse jet due to its smaller size and inherently lower pressure loss, a pulse jet is always going to flow less air mass per unit size (significantly less) and will have an inherently higher pressure loss in comparison, lets not forget that turbo jet combustors have been in development for over fifty years with multimillion dollar R&D budgets, pulse jets have not

Viv

[ace_fedde]

Viv,

Now as you said, what about that high pressure environment for the pulse jet? well yes but with pressure also comes flow! what about flow and what about time? take the example above with the 700 cubic feet a minute flow from that diesel compressor, looking at the ram jet combustor its mixing air with fuel and burning continuously at what ever flow rate is set by the throttle valve.

Your example with the RR combustor fed by the diesel compressor is not an example of pressurized environment but an example of pressurized feed.
In your example there is no added static pressure, just added velocity pressure (=flow).
If the combustor nozzle would end in a (huge) tank that has the same (static) pressure as produced by the compressor you would have a pressurized environment.

If you would drive a (mechanicly) loaded turbo by a pulsejet added static pressure will be produced in the tailpipe/turbo entrance. Then if you would feed the pulsejet with the same amount of added pressure the effect will be the same as a pulsejet working in a huge pressurized tank. Mmm, denser air!! So not more flow but more mass displacement!
Maybe a new thread about this?

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

So thats flow then right? no?

Sorry but I cant see any tanks there ether moving or standing still don't forget the word simple as it is prevalent in this thread for a reason! if you want to introduce the concept of static pressure you can by all means but the rule is you have to explain it in breathtakingly simple terms with out any maths or thermodynamic references that a backwoods turbine expert could understand


Viv

[ace_fedde]

Sorry but I cant see any tanks there ether moving or standing still

Haha!

don't forget the word simple as it is prevalent in this thread for a reason! if you want to introduce the concept of static pressure you can by all means but the rule is you have to explain it in breathtakingly simple terms with out any maths or thermodynamic references that a backwoods turbine expert could understand

Will do, but I'll open a seperate thread for that one. Please continue with the main track!

Fedde

[PyroJoe]

Viv,
Good thing you have going here.
What are the primary goals of coupling a PJ to a turbine?

Efficiency?
RPM?
Probably not torque?


It occurs to me that a ram jet would be a poor efficiency combustor (depending on variables involved). If we look for a moment at TSFC that approach 1.00 for various VPJs it appears the more efficient engines have a long length to them. Not sure if it is a coincidence or a necessity. What would this also tell us about flow? More questions than answers.

Joe

late added question: Which combustor will give the closest to chemically perfect combustion?

[Mike Everman]

Oh yes it is torque. Shaft power for everything that is not able/practical/willing to be pushed around.

[Viv]

Viv,
Good thing you have going here.
What are the primary goals of coupling a PJ to a turbine?

Efficiency?
RPM?
Probably not torque?


It occurs to me that a ram jet would be a poor efficiency combustor (depending on variables involved). If we look for a moment at TSFC that approach 1.00 for various VPJs it appears the more efficient engines have a long length to them. Not sure if it is a coincidence or a necessity. What would this also tell us about flow? More questions than answers.

Joe

late added question: Which combustor will give the closest to chemically perfect combustion?

Hi Joe

Good questions and they fit the purpose of this thread so lets see if we can answer them.

"What are the primary goals of coupling a PJ to a turbine?"
One would hope for some usable work to be extracted from the Pj but this thread is all about how much usable work can be extracted from a valveless pulse jet, the goal is to get people thinking about the basics first before reaching for the welder first.

"Efficiency?"
Now thats the real issue as you can see from how this thread has developed its simple thought experiments and analogies, look at the examples of the simple tube with two nozzles or better yet the air compressor tank with two holes, that gives you a perfect example of the basic thermodynamic problem all valveless pulse jets face trying to drive a turbine on only one port.

"RPM?"
Some level of RPM is inevitable considering the conditions at the exhaust

"Probably not torque?"
Yes! there will be some torque, how much and is it a usable quantity are the real questions, that will tie in to your second question of efficiency though.

Your last question is the best as it gets down to the real measurable nitty gritty of what we are talking about, usable power extraction, shaft horse power is a nice measurable quantity to quantify an engine and turbine combination.

But as Mike has asked in the past what happens when you apply a load to the turbine shaft and load up the VPJ driving it?

For the combustor questions consider that Rolls Royce and others seem very happy with the combustors they have developed over the years they also operate them at considerably higher pressure levels than our PJs

"late added question: Which combustor will give the closest to chemically perfect combustion?"
For myself I would say the standard gas turbine combustor is pretty good but if I were to go out on a limb I would say the trapped vortex combustor gets my technical vote, but thats off topic

Viv

[Viv]

Moving on,

I think some of the basics have been seen by most people now and hopefully some of the pitfalls are seen before rushing in and trying to fit any old turbo unit to that pulse jet you had laying around the shop gathering dust for a few years.

Now let me direct your attention to a nice book you can read online via the google books service, http://books.google.ca/books?id=ao4OvWy ... 8#PPA30,M1

Page 22 is some were good to start as well as reading from page 35 onwards to see some basic arrangements of possible turbine engines.

Viv

[ace_fedde]

Ha!

Will be quiet now for some time while we are studying the book!

Fedde

[Viv]

Ha!

Will be quiet now for some time while we are studying the book!

Fedde

Hi Ace

You can safely skim this book for the interesting and relevant stuff, it should act as a back-grounder only as its not an in depth study of Gas turbine operation, what it should do however is teach people that the answers to questions are easily available and that even such a simple thought experiment as we are doing in this thread can be easily complemented by a google books search.

You may like the fact the featured book has a short section on turbo compound engines as well as more speculative turbines

Viv

[ace_fedde]

Viv, and everybody else,

In the meanwhile I can’t get my thought’s off the pressurized environment for a PJ.
I might even have a purpose for it or a way to use it.

But my knowledge of pulse jets (specially in numbers) is to little to predict what’s needed to have a PJ working in a pressurized environment.

So I’ll open a thread in the valveless forum to ask for help, please join!
Here's the thread:
viewtopic.php?f=3&t=5377

Fedde

[Viv]

Viv, and everybody else,

In the meanwhile I can’t get my thought’s off the pressurized environment for a PJ.
I might even have a purpose for it or a way to use it.

But my knowledge of pulse jets (specially in numbers) is to little to predict what’s needed to have a PJ working in a pressurized environment.

So I’ll open a thread in the valveless forum to ask for help, please join!
Here's the thread:
viewtopic.php?f=3&t=5377

Fedde

Nice, been playing there too

Viv

[ace_fedde]

Turbines driven by pulse jets, now proven!
http://www.youtube.com/watch?v=DVHJ3qhtVfw

Thanks Mark (for the find)

Fedde

[vturbine]

....the goal is to get people thinking about the basics first before reaching for the welder first.

hmmmmmm, if only I had found this before reaching for the welder

Methinks we are glossing over the inefficiency of the compressor power requirements for a small gas turbojet engine, particularly in a stationary environment, and ignoring, as well, the energy inefficiency of a shaft transmission to work with just about anything except pure vehicular reaction thrust, since the compressor MUST run at a very high RPM to generate the compression needed to run the turbine. This high RPM also places extreme requirements on bearings -- generally pressure oiled in most supercharger types, and requiring an oil pump, That pump is often driven by wall current in the small supercharger gas turbine conversions I've seen -- no accounting for that energy absorption as well, in terms of fuel efficiency. If you drive the oil pump from the turbine shaft, again there will be transmission losses as well as the pump power requirements.

Now it seems by comparison, we've hobbled the pulsejet here by excluding acoustic effects and concentrating solely on nozzle resistance. A bit like considering only DC resistance in circuits when talking about impedance. The simple mechanical route to inlet losses in pulsejet thrust engines has been to bend the exhaust pipe, or re-orient the inlet aft. Yet I'm quite sure these are not the only possibilities when we start to look at pulsejets as stationary combustors for turbine use, and consider more deeply what we might call inlet reactance.

While I'd be the last person in the world to claim pulsejets are "efficient" users of fuel, I think small gas turbines on a similar scale aren't the best examples of fuel efficiency either, particularly if called on to supply shaft power in useful rpm ranges. The constructional costs and complexity are not comparable, either. FADECs, inconel turbine and compressor castings, high rpm transmissions, tigged stainless, flameholders, etc. are not the sort of thing a backyard builder can typically whip up from scratch, while a working pulsejet turbine actually is. I say bring on the welders!

You know Viv, I'm just funning you, what you've written here is a very good explanation of why an automotive supercharger/pulsejet hybrid isn't a very good combination. Automotive supercharger turbines were never designed to run from pulsejets, and you've helped explain some of the basic reasons why not. But that doesn't necessarily apply to a turbine designed for pulsejet output and reduced RPM.

I of course agree with you after the tongue in cheek above, spreadsheet calculators are exploratory tools just like welders , I wouldn't be without either. Thanks for a good read, and I look forward to further explanations as you go deeper into the subject.