Two stage centrifugal

[jetjay]

Hello,
This is my first post, I've been lurking a few weeks and have gained quite a bit of knowledge. I saw that a couple of months ago a member had suggested a 2 stage setup to increase the compression ratio. It was determined that the power gains were not worth the effort in the end. I personally am far more interested in the increased fuel economy this could net. I plan to construct my first basic turbine engine this February, I'd like my next one to be a marked improvement over that. What are some of the challenges I might face with such a build? Would I calculate the combustion chamber based on the high pressure or low pressure inducer? How much efficiency is there to be gained?

[wolfdragon]

Well for starters, as you increase P2, T2 and T3 will also increase...

Since we already hit a temperature just under the melting point of the turbine wheel with our 2.something:1 P2 values, raising P2 really won't net much more than a rebuild

This is why commercial/military applications with higher P2s have turbines that have cooling holes on the leading edge and other places to put a boundary layer of relatively cool air between the blade and the hot exhaust gasses coming out of the combustor

[racket]

Jo Jetjay

It is possible to use a two stage compression , but you will need to have an intercooler between compressors to reduce the air temperature going into the second stage otherwise the final air temperature will be getting too high for an alloy comp wheel ...............the lower temp going into the second stage after intercooling will mean less power required for compression so more potential power output from the engine.

Fuel burn rates (SFC) will be improved from having high compression and expansion ratios , but its a lot of work for a bit of fuel saving when the engine will still be thirsty .

The combustor volume can be reduced , and yes , work on the smaller second stage requirements .

Your temperatures exiting the combustor and entering the turbine will need to be the same as a single stage engine , there will be less fuel required to obtain that temperature due to a slightly higher temp (~50 deg C higher ) entering the combustor .

For a thrust engine its pointless going to two stages as theres a "square root" component in the thrust calculations , so even though theres more pressure available in the jetpipe for making thrust , that "square root" makes getting the higerh gas velocities harder and harder to achieve .

The situation is different if you intend going to a shaft horsepower engine where theres more opportunity for larger power increases .

Cheers
John

[jetjay]

I plan to design and build a turbo shaft once I've got the gas generator down. Right now I'm still toying with concepts. Would there be any perceivable benefit to driving both turbines directly off the combustion chamber? Or is it necessary to run them in series?

[racket]

Hi Jetjay

Conventionally its a series setup so that the freepower runs at lower rpm due to the lower gas speeds , a parallel system will see much higher rpm/tip speeds due it being relatively small diameter due to the reduced mass flow compared to the series setup. ................go series :-)

Cheers
John

[syler]

In my mind I toy with the idea of driving a bypass fan in front of the compressor so you have some compression going in. Feeding one turbo into another would produce so much heat that you would be pumping heat, but very little O2 which when reacted with your fuel is what gives power. Cooling the air would increase moles of reactants to combust so obviously more power. Think about injecting liquid O2.

[Johansson]

You have a lot going on in your mind my friend.

[ernie wrenn]

Liquid O2 will explode (Not good) when exposed to high temp and any petro based substance ..

[racket]

Hi Syler

Please purchase this book for yourself

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+kamp+model+jet+engines

It'll save you a lot of time coming to grips with what makes a turbine engine work .

Cheers
John

[syler]

I just might buy that book. but, I'm sure there is no section on violating the laws of physics. Commercial jet engines use multiple stages for a reason. Cooler, more dense air has more oxygen in it. While it may just pass through the combustor, it will eneter the afterburner and more gas will be burned to make it stoich. That means more gas produced and more heat (extensive, not intensive).

Exploding is what you want if you are running a liquid fuel rocket.

[Johansson]

Feb 6, 2014 21:06:02 GMT -5 syler said:

I just might buy that book. but, I'm sure there is no section on violating the laws of physics. Commercial jet engines use multiple stages for a reason. Cooler, more dense air has more oxygen in it. While it may just pass through the combustor, it will eneter the afterburner and more gas will be burned to make it stoich. That means more gas produced and more heat (extensive, not intensive).

Exploding is what you want if you are running a liquid fuel rocket.

There is no need to violate any laws of physics.

The third post from above made by John explains perfectly well what those laws will do to a two stage centrifugal, so what are you trying to achieve with your lecturing? If you are truly convinced that liquid oxygen is a good thing to inject into a gas turbine please feel free to try and come back with the results.

[syler]

Johan, I said liquid O2 is used in a liquid fueled rocket - not in a gas turbine. What I'm trying to do is figure out why these engines are so inefficient. Guys are using turbos that make 400 - 1000HP and only getting a small amount of useful energy. Something is obviously wrong.

[Richard OConnell]

Syler, I am one to believe that any result is a useful one. I wish you luck in your experimentation! Play Safe

[Johansson]

Feb 11, 2014 7:43:16 GMT -5 syler said:

Johan, I said liquid O2 is used in a liquid fueled rocket - not in a gas turbine. What I'm trying to do is figure out why these engines are so inefficient. Guys are using turbos that make 400 - 1000HP and only getting a small amount of useful energy. Something is obviously wrong.

Hi Syler,

You won´t get anywhere until you stop comparing an internal combustion engine and a gas turbine engine, they are different animals. Nothing is wrong, believe me.

To take my JU-01 turboshaft engine as an example, the rotor is a Garrett TV94 good for roughly 1500hp on an IC engine, the compressor pulls 230hp from the turbine shaft to produce 3.2PR while the entire engine "only" produce an estimated 120-130hp or so at the rear wheel at that PR. Perfectly normal behavior.

(now that I started thinking about it, I wonder how much power that could be theoretically produced by an IC engine properly sized and supercharged by an axial compressor from a J79? *drool*)

Cheers!
/Anders

[racket]

Hi Syler

OK .....lets get this sorted once and for all :-)

We'll use the highly successful Allison /RR 250 engine en.wikipedia.org/wiki/Allison_Model_250 ...........I've got good data in one of my texts for the C20 Model ........it flows 3.6 pounds of air per second - 216 lbs/min , at a compression ratio of 7.1:1 ( a reasonably comparable compression ratio to a piston engine )

An IC engine "generally" produces ~10 HP/lb/min of airflow , so that 216 lbs/min should produce ~2,160 HP if flowing into an IC spark ignition ( SI) piston engine , but the Allison engine only produces a maximum of 420 HP , despite there having been 50 years of development and probably a billion dollars spent on R and D .

Why is this so ??

Firstly , the C20 engine , like most "small??" and relatively inexpensive turbine engines ( <$1,000,000) has a restricted turbine inlet temperature ( T I T ) that is low enough for the turbine wheels to survive, with any sort of long life span ,...........early gas turbine fighter aircraft had 50 hour life spans before an overhaul was required because of the unavailability of high temp resistant materials .

Once you start to do the thermodynamic calculations using our relatively "low" temperatures and low "peak" pressures, it soon become apparent why a gas turbine turns out less power than a SI piston engine which can have gas temperatures prior to expansion 2 to 3 times higher than out T I Ts and pressures 10 - 15 times greater before expansion ...........its EXPANSION that creates power , and its the temperature drop during that expansion that gives us the power produced by the engine , roughly , for every 100 centigrade degree temperature drop per pound of gas flow during expansion , ~70 HP is produced irrespective of the engine type.

A piston engine starting off at 2,000 deg C gas temp prior to expansion on the power stroke , and an exhaust temp of 1,000 deg C at the bottom of the power stroke prior to the exhaust stroke has converted 1,000 centigrade degrees of energy into power , or ~700 HP/lb of airflow ............or 10 times the "specific" power output of a gas turbine.

As Anders said in his Email ...................."they are different animals"

Turbine engines of the size Pressure Ratio we use have low thermodynamic outcomes compared to a SI engine , but they are still working as efficiently as the Laws of Thermodynamics allow them to work .

Turbines work on large airflows at modest temperatures and pressures whereas SI piston engines work on modest airflow rates at high temps and pressures, and is the reason why an Allison C20 engine can turn out 420 hp but weigh only 158 lbs , it can produce a reliable a maximum continuous horsepower rating of 400 hp for hours on end , a 400 hp diesel engine that could do the same would weigh 10 times as much , and a SI engine wouldn't be a lot less if you wanted it to do thousands of hours of work at 400 hp continuous .

The very large commercial fan engines with their >30:1 pressure ratios and T I Ts a few hundred degrees C hotter than smaller engines due to very sophisticated turbine blade cooling and materials , have better thermodynamics outcomes , but they are still only doing exactly the same as our DIY engines , with their higher turbine stage efficiency they will produce a larger temperature drop for an equivalent pressure drop , so more power for each psi of pressure drop across the turbine stage .

I can well understand your "frustration" at the apparent lack of "power output" from our DIY engines , I felt exactly the same when developing my TV84 gas producer engine for my first turbine bike , I was "only??" getting ~110 lbs of thrust from a turbo that fed a 475 hp diesel truck engine , I spent years trying to get more thrust thinking I was doing something wrong , but it wasn't until after going around in circles and having expensive "malfunctions" that I finally decided to "do the maths" ...............only to find out I'd already reached the engines full capability some years previously :-(

I hope this helps explain things, if not , ask away :-)

Cheers
John