Two stage centrifugal

[syler]

Where did this "thermodynamics model" come from? I'm no engineer, but I had quite a bit of physics and chemistry in college. My understanding is that heat has two measures - intensive meaning how hot, measured in degrees, and extensive meaning how much heat measured in Joules or BTU for us Americans still in the dark ages. My understanding is that Joules are the units that convert to work.

In a piston motor, heat (energy) is transformed into work which is later returned to heat via friction. A piston engine concerns itself with the amount of energy created, not with temperature. That's why they have cooling systems which by the way removes a great deal of heat.

In order to produce energy, rather than high temps, the idea is to pack as many moles of reactants into a confined space and transform the energy stored in the chemical bonds of the fuel into pressure. This is primarily done via molecular conformation changes which result in higher repulsive forces between molecules. Because these products are gasses, they expand further when heated and produce more pressure which is transformed into work. That is the only time actual temperature enters into it.

When we compress the reactants, two things happen. First we pack more moles into our chamber - that is the n in the ideal gas law. Second, collisions between molecules result in faster combustion so more energy per time is produced.

So, on to the point. Clearly, these motors have limitations. What appears to be the case here is that automotive turbochargers just don't allow for much compression in comparison to axial multi stage compressors. To make matters worse, they produce a lot of heat. The gas law tells us that as T rises, N decreases. So, the decreased N (O2 in this case) results in less to combust and less mass being put through the system. More dense air being pumped through would also provide more power to the turbine without increasing temperature. The additional O2 would of course be burned in the AB producing thrust.

I presume the fancy designs have far more surface area in the compressor and dissipate heat much better while creating greater compression. Commercial turbine types may have other design considerations such as size and weight, etc. Our question is how do we get higher compression while keeping the heat low until the AB. It sounds to me like a dual stage compressor - properly cooled would do that. Since these aren't going in airplanes, why not water cooling? Also, since we are really making our power in the AB rather than the combustion chamber which is really just to drive the air pump, why not cool the combustion product to reasonable range before the turbine? That shouldn't take much.

I think if properly done, a two stage compressor would have numerous advantages. I also see a few ways more mass and more energy could be put through these motors without increasing the intensive heat past the range of the materials.

[syler]

One other thing. All these things, plus the movement of a great deal more mass per the same heat unit could be achieved by using some element of bypass air. Even just a modest 1.5 cold pressure increase along with the cooling effect and far greater mass flow should make these engines far more efficient. maybe you shop guys might want to take a shot at fabricating something of than nature.

[racket]

Hi Syler

Have you purchased the book I recommended in an earlier Email ?? www.amazon.com/Model-Jet-Engines-Modellers-World/dp/190037191X/ref=sr_1_1?s=books&ie=UTF8&qid=1391719737&sr=1-1&keywords=thomas%20kamp%20model%20jet%20engines&tag=533643275-20

Have you read the Book ??

This subject is far too complicated for me to waste time typing up the theory , read the book .

As for a 2 stage compression , I started making one about 10 years ago using the TV84 gas producer from my turbine bike build for the first stage , the intercooler was off a very large diesel engine and measured ~3 ft X 2 ft , but after doing some more research on the required second stage I called the project off due to the inability to procure a second stage turbo with the right combination of compressor and turbine sizes , mainly the turbine size , it needs to be rather small due to the high pressure , low volume , going through it .

For a thrust engine theres minimal increase in thrust from the higher pressure ratio , though a reasonable fuel burn decrease ................for a shaft power engine there would have been power improvements , but the added complication isn't worth all the effort .

It'd be far better to make a single stage gas producer , plumb the exhaust through a freepower turbine thats attached to an axial fan and afterburn its output , I've done the calcs and have the bits for a 500 lb thrust engine , but don't have the time or energy to make it :-(

Cheers
John

[syler]

I may pick up a copy. But, I don't think such a publication would be my go to source necessarily. Maybe something more geared toward full scale jet engine design. Why would increasing pressure not increase thrust? I'm not sure on this, but it should increase the speed of the combustion I would think giving more joules per second. Anyway, I'm just going by physics. Admittedly I have no experience with jet engine design.

[syler]

Feb 15, 2014 16:30:48 GMT -5 racket said:

Hi Syler

Have you purchased the book I recommended in an earlier Email ?? www.amazon.com/Model-Jet-Engines-Modellers-World/dp/190037191X/ref=sr_1_1?s=books&ie=UTF8&qid=1391719737&sr=1-1&keywords=thomas%20kamp%20model%20jet%20engines&tag=533643275-20

Have you read the Book ??

This subject is far too complicated for me to waste time typing up the theory , read the book .

As for a 2 stage compression , I started making one about 10 years ago using the TV84 gas producer from my turbine bike build for the first stage , the intercooler was off a very large diesel engine and measured ~3 ft X 2 ft , but after doing some more research on the required second stage I called the project off due to the inability to procure a second stage turbo with the right combination of compressor and turbine sizes , mainly the turbine size , it needs to be rather small due to the high pressure , low volume , going through it .

For a thrust engine theres minimal increase in thrust from the higher pressure ratio , though a reasonable fuel burn decrease ................for a shaft power engine there would have been power improvements , but the added complication isn't worth all the effort .

It'd be far better to make a single stage gas producer , plumb the exhaust through a freepower turbine thats attached to an axial fan and afterburn its output , I've done the calcs and have the bits for a 500 lb thrust engine , but don't have the time or energy to make it :-(

Cheers
John
That's a good idea.  If I did a 2 stage the first would be very modest.

[racket]

Hi Syler
The reason why we don't get as big an increase in thrust with increasing pressure is because of a "square root" component in the calculations determining the efflux velocity .

Another way of looking at it is the reverse side ...........kinetic energy in the exhaust jet , half mass times velocity squared , a small increase in velocity makes a proportionally larger increase in energy ...............when we take the reverse of that in the jet pipe/nozzle with a finite amount of pressure energy we get less and less velocity increase for ever increasing pressure ..................thrust is only mass times velocity , nothing squared , unlike the kinetic energy equation.

Yep , I made the fan engine design produce a PR across the fan of ~1.3:1 , this "consumed" the 150 horsepower from the "freepower" turbine .

Cheers
John

[pictsidhe]

Syler,
A 2 stage system is an order of magnitude harder to design that a single stage. If you can't do the maths yourself for even a single stage system, a double stage system is never going to happen.

Turbo jets have an optimum pressure ratio for a given T3. If you go too high, you're not moving enough air through the engine to make good thrust (F=mv), efficiency drops. There's a really good 50's NACA paper out there full of diagrams that I can't find right now on the effect of different PRs and temps on turbojets. But at out temps, a 2 stage system isn't worth it for a pure turbojet. I have a dim memory of about 5:1 being about optimum at around 1100K. For turboshafts, there is again an optimum PR for a given T3. The range is quite broad once non ideal behaviour is factored in. Not a lot difference between 12:1 and 30:1 in my current spreadsheet, even 30% power shows similar numbers. Compared to a 5:1, a 12:1 system is going to use about 30% less fuel. below 5:1 bsfc increases quite quickly. Us jet builders don't really have to worry about LCF so wheel speeds can be increased a bit.

I've not read the Kamps book, but the Cohen and Rogers Gas Turbine Theory textbook is a good book to start with. That'll give you an idea of what you're getting into.

I'm working on a 2 stage turboshaft. The spreadsheet is already huge and still needs lots of work. There are lots of headaches. I'm going to have to remachine a high pressure compressor wheel to a lower trim to match a suitable turbine. I either need very wide compressor maps (which compromises efficiency) or at least one, probably 2 sets of variable vanes somewhere or it won't run through the rev range let alone make power. There's a reason there don't seem to be many 2 stage centrifugal turbine engines; they are a PITA. The power turbine tip speed is on the ragged edge of what a turbo wheel could be pushed to (maybe), I don't want the complication of a 2 stage power turbine. I'm sure I'll find more headaches before I get more than ink on my hands.

[racket]

Hi Pictsidhe

Another book worth having is Zucrow's

www.amazon.com/Aircraft-Missile-Propulsion-Turboprop-Turbojet/dp/B000QAA64Y/ref=sr_1_6?s=books&ie=UTF8&qid=1405833469&sr=1-6&keywords=zucrow

Lots of graphs and worked examples

Cheers
John

[pictsidhe]

I like the published date. Older books tend to be easier to digest. I nearly recommended this: www.amazon.com/Mechanics-Thermodynamics-Turbomachinery-Seventh-Edition/dp/0124159540 Mine's softback 5th edition and was a lot cheaper than the latest one.

[racket]

Yep , I like the 1950's editions , LOL.....they don't assume you already have a Doctorate in Mechanical Engineering , so much easier to understand and more than adequate for our needs, we don't need to go to 6 decimal places with our calculations , one or two is more than enough for us amateurs :-)

Cheers
John

[Deleted]

Perhaps the new breed of high pressure turbos with back to back compressor might be of use to us?

[james]

Racket.

What you say is true, not disputing it.
There is a point to be made for multi stage compression and expansion approaching the Isentropic ideal more closely though. If one kept the compression ratio and therefore the temperatures to to
a level that the materials could handle, the fuel consumption of a multi-stage would be better than a single stage. The final temperature for a given compression ratio would also be lower due to the improved efficiency.
That's why Rolls Royce and Pratt and Whitney do it .
The problem with multi-stage in that case is that the angles of the inlet and exit angles of the successive rotor and stator stages become very difficult to calculate.
A small error in any angle will make the process or break the process and also make the engine only operate in a very narrow band of mass throughput.

Feel free to shoot ,me down as I'm still learning and want heated debate!

[racket]

Hi James

Yes, there are advantages for multistage compression , but for us DIY'ers the benefits really don't outweigh the complications, certainly theres a fuel efficiency gain but we aren't that concerned about fuel with our short run times.

If its a thrust engine , the extra jetpipe pressure will produce some extra thrust but thats in ever diminishing amounts for the extra pressures produced, for a shaft engine its a better proposition , but again , is it worth all the extra work , easier to find a bigger compressor wheel for more mass flow .

LOL...........heated debate will require a more contentious issue ;-)

Cheers
John

[mitch]

This might be slightly off topic, but has anyone looked into the 2 stage compressor turbo that ford has used on their larger diesels? I'll link later

[enginewhisperer]

it's not actually a two stage compressor - it's just a back to back single stage, so they could get more flow from a small diameter wheel.