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Faro
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Compression Ratio Vs Turbine Temp

Sat Dec 29, 2007 10:29 am

Just to get things straight in me head, what exactly do an engine's compression ratio and turbine entry temperature affect/determine in terms of SFC, thrust and % of fuel combusted?

Faro
The chalice not my son
 
Buzz
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RE: Compression Ratio Vs Turbine Temp

Sat Dec 29, 2007 11:20 am

Hi Faro, Buzz here. I don't know if I can answer "exactly"... in short: "more is better". More compression ratio before you burn the fuel causes a lower SFC. If the turbine nozzle and first stage turbine can withstand a higher temperature, you're able to get more work out of the fuel you burn: more thrust.

But there's a drawback to it all: you have to burn more fuel to get the extra thrust / shaft horsepower. So if you only need 75 horsepower to fly my favorite Aeronca Champ, you might not be able to find a turbine engine that's low powered.
So it seems that piston engines still have a place in the low power / low fuel burn end of the spectrum. Once you get to needing more than 350 horsepower, turbine engines start to make sense.

Percent of fuel combusted... not sure what you're getting at there. It all catches fire (grin) Mixture control on a piston engine?
You should look up some of John Deakin's articles about running "lean of peak" EGT. It's interesting... in a geeky sort of way (if you're a gear-head like many airplane fixer are) Some engines are made to run rich, for internal cooling.

g'day
 
tdscanuck
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RE: Compression Ratio Vs Turbine Temp

Sun Dec 30, 2007 7:13 am



Quoting Faro (Thread starter):
Just to get things straight in me head, what exactly do an engine's compression ratio and turbine entry temperature affect/determine in terms of SFC, thrust and % of fuel combusted?

Turbine entry temperature determines the maximum temperature that the working fluid can obtain. Compression ratio determines how hot the air coming in to the combustor is. The difference between these two temperatures is how much you can raise the working fluid temperature (how much fuel you can add per unit of air). So there is a trade...for a fixed turbine inlet temperature, higher compression ratio means you can't add as much fuel per unit of air.

On the thermodynamic side, the maximum possible efficiency of the Brayton cycle increases with compression ratio, so higher compression ratio means less fuel is required for a particular power output (roughly, thrust).

% of fuel combusted isn't really a function of either...a good combustor will burn effectively all of the fuel.

Holding all other properties equal, increasing compression ratio will lower SFC. I'm not sure about thrust because you have counteracting trends...less thrust due to less fuel added but more thrust due to more efficient power extraction. One of those is an exponential curve so it probably depends on exactly where your operating point is.

Holding all other properties equal, increasing turbine inlet temperature will increase thrust at constant SFC.

Tom.
 
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Faro
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RE: Compression Ratio Vs Turbine Temp

Sun Dec 30, 2007 10:26 am



Quoting Tdscanuck (Reply 2):

Turbine entry temperature determines the maximum temperature that the working fluid can obtain. Compression ratio determines how hot the air coming in to the combustor is. The difference between these two temperatures is how much you can raise the working fluid temperature (how much fuel you can add per unit of air). So there is a trade...for a fixed turbine inlet temperature, higher compression ratio means you can't add as much fuel per unit of air.

I'm a little confused. In a previous thread we had said that denser air coming out of the HP compressor is desirable for efficiency since it represents a higher compression ratio. One way to acheive that is via cooling, though it was established that this is not technically feasible/desirable. But theoretically, if I cool the air coming in to my combustor, I lower its temp and increase the differential with respect to turbine entry temp, and hence the combustion fuel required. Non comprende.

Faro
The chalice not my son
 
Oryx
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RE: Compression Ratio Vs Turbine Temp

Sun Dec 30, 2007 2:24 pm

All three features help to increase the efficiency: higher compression ratio, higher temperatures before the turbine as well as cooling before the compressor or between different compressor stages.
The limits to all are technological feasibility. Higher compressor ratios lead to higher temperatures before the combustion chamber. As the turbine inlet temperature is limited by the used materials (melting temperature or cooling efficiency) this limits the amount of fuel which can be burned leading to lower power density (thrust). Cooling before or inside the compressor requires large heat exchanger which are today to heavy and to bulky. The gain stems from the fact that compression becomes more efficient when the temperature level is lower.

In short is cool as possible before and after the compressor and as hot and as high pressure level as possible before the turbine.
 
tdscanuck
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RE: Compression Ratio Vs Turbine Temp

Sun Dec 30, 2007 6:43 pm



Quoting Faro (Reply 3):
Quoting Tdscanuck (Reply 2):
Turbine entry temperature determines the maximum temperature that the working fluid can obtain. Compression ratio determines how hot the air coming in to the combustor is. The difference between these two temperatures is how much you can raise the working fluid temperature (how much fuel you can add per unit of air). So there is a trade...for a fixed turbine inlet temperature, higher compression ratio means you can't add as much fuel per unit of air.

I'm a little confused. In a previous thread we had said that denser air coming out of the HP compressor is desirable for efficiency since it represents a higher compression ratio. One way to acheive that is via cooling, though it was established that this is not technically feasible/desirable. But theoretically, if I cool the air coming in to my combustor, I lower its temp and increase the differential with respect to turbine entry temp, and hence the combustion fuel required. Non comprende.

I don't think we're disagreeing. I was assuming that we weren't using intercooling in the compressor since, as the other threat discussed, it's technically tricky right now. If you intercool, you could have a higher compression ratio while maintaining temperature margin to the turbine so that you wouldn't have to reduce the power density.

The most efficient possible cycle, as Oryx correctly notes, would be to fully intercool with maximum compression, so your air coming into the combustor is as high pressure as you can achieve but still at ambient temperature. Then you can add the maximum amount of fuel (right up to the temperature limit of the turbine). This gives you the highest SFC and highest thrust for a particular package.

If you want to be really efficient, you use recuperators to transfer the energy extracted by the intercoolers to reheat the exhaust as it's going through the turbines. However, that has even more technical problems in an aircraft setting than intercoolers. Power plants that use gas turbine cycles can do this and get spectacular efficiency.

Tom.
 
Airgypsy
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RE: Compression Ratio Vs Turbine Temp

Sun Dec 30, 2007 11:44 pm

Checkout the website for the ATF-3 engine for a look at an engine designed as close to the "ideal" engine as you can get.
http://www.pocketprotectors.com/atf3/Engine.htm
Cooling in the right places improves mass flow. Heating the right places increases operating pressures/velocities.
 
thegeek
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RE: Compression Ratio Vs Turbine Temp

Wed Jan 02, 2008 12:53 am



Quoting Tdscanuck (Reply 5):
If you want to be really efficient, you use recuperators to transfer the energy extracted by the intercoolers to reheat the exhaust as it's going through the turbines. However, that has even more technical problems in an aircraft setting than intercoolers. Power plants that use gas turbine cycles can do this and get spectacular efficiency.

But cooling the exhaust in a jet engine would reduce the thrust by a fair bit wouldn't it? Might reduce fuel use too, so it is possible that SFC would be increased by this, just doubtful IMO. Different for a land based gas turbine as there is no power derived after the turbine.

Combined cycle power plants achieve their 60% efficiency by driving a steam plant off the exhaust. I highly doubt that a recuperator is used in this scenario, as it would only increase pressure on turbine inlet temperature and the heat is profitably used without one. In fact, I would expect a more modest pressure ratio in this application to allow lower airflow for the same amount of fuel used (wasted energy in the exhaust equals airflow times exhaust temperature increase over ambient). Intercoolers? Not sure. But if you are wasting heat to the atmosphere, that is a loss; perhaps you could use the steam plant's feed water.
 
tdscanuck
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RE: Compression Ratio Vs Turbine Temp

Wed Jan 02, 2008 6:03 am



Quoting Thegeek (Reply 7):
But cooling the exhaust in a jet engine would reduce the thrust by a fair bit wouldn't it? Might reduce fuel use too, so it is possible that SFC would be increased by this, just doubtful IMO. Different for a land based gas turbine as there is no power derived after the turbine.

A recuperator doesn't cool the exhaust, it heats it (using heat taken from the intercoolers in the compressor stage). This lets you get more energy out of the turbines, since the energy extracted by the intercoolers would otherwise be wasted to atmosphere.

You're right that land based turbines don't pull any power after the turbines, but that's what you're going for in a high-bypass turbofan as well. It's more efficient to extract power in the turbine and drive the fan with it than it is to get thrust directly from the core exhaust. Adding turbines is a process of diminishing returns, which is why you don't extract 100% in the turbines on aircraft engines but the trend, for decades, has been increasing power extraction in the turbines and decreased core thrust.

Tom.
 
Oryx
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RE: Compression Ratio Vs Turbine Temp

Wed Jan 02, 2008 7:31 am



Quoting Tdscanuck (Reply 8):
A recuperator doesn't cool the exhaust, it heats it (using heat taken from the intercoolers in the compressor stage).

I think the temperatures after the turbine are still higher than what one would get from the intercooler. In my opinion it would make more sense to use the heat from the exhaust gas to heat the flow after the compressor and before the combustion chamber. This would reduce trust but also the heat (burnt fuel) which has to be added during combustion. This trade off yields only possitive results when the required heat exchangers can be made a lot lighter and cheaper than today.
 
dakota123
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RE: Compression Ratio Vs Turbine Temp

Wed Jan 02, 2008 7:09 pm



Quoting Oryx (Reply 9):
I think the temperatures after the turbine are still higher than what one would get from the intercooler. In my opinion it would make more sense to use the heat from the exhaust gas to heat the flow after the compressor and before the combustion chamber. This would reduce trust but also the heat (burnt fuel) which has to be added during combustion. This trade off yields only possitive results when the required heat exchangers can be made a lot lighter and cheaper than today.

I can only speak of full-power condition from memory, but the power generation version of the CF6-80 has a compressor discharge temp of about 1,015 deg. F and an exhaust temp of only about 850 deg. F

The only recuperated unit on the market (AFAIK) is the Solar Mercury 50, http://mysolar.cat.com/cda/layout?m=41105&x=7

The only factory-intercooled unit available is the GE LMS100, a marriage of the CF6-80 and part of a heavy frame unit. http://www.gepower.com/prod_serv/pro...s/en/downloads/lms100_brochure.pdf

Mike
 
thegeek
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RE: Compression Ratio Vs Turbine Temp

Wed Jan 02, 2008 11:45 pm



Quoting Tdscanuck (Reply 8):
A recuperator doesn't cool the exhaust, it heats it (using heat taken from the intercoolers in the compressor stage). This lets you get more energy out of the turbines, since the energy extracted by the intercoolers would otherwise be wasted to atmosphere.

This disagrees with not only (cough) wikipedia (cough), but also dictionary.com, and my understanding of gas turbines. Even if the figures above are correct that the compressor outlet temp is hotter than the exhaust temp, compressor outlet temp isn't the relevant figure, as an intercooler (by definition) is between the stages, so the intercooler input temp would have to be cooler that 1015 deg F. Very little could be gained, then, by sinking heat into the exhaust at 850 deg F. And your intercooler's usefulness will be s***.

The idea of a recuperator is to recover heat from the EXHAUST that would otherwise be wasted to atmosphere, which is then applied to the compressor outlet air. This is heating achieved for free other than a slight pressure loss and some weight. The weight would be the problem on an aircraft. But really, the idea of the combined cycle gas turbine with a steam plant has made this obselete for large stationary installations (like power stations).
 
tdscanuck
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RE: Compression Ratio Vs Turbine Temp

Thu Jan 03, 2008 12:20 am



Quoting Thegeek (Reply 11):
The idea of a recuperator is to recover heat from the EXHAUST that would otherwise be wasted to atmosphere, which is then applied to the compressor outlet air. This is heating achieved for free other than a slight pressure loss and some weight. The weight would be the problem on an aircraft. But really, the idea of the combined cycle gas turbine with a steam plant has made this obselete for large stationary installations (like power stations).

You're totally right...I've been using the word "recuperator" incorrectly. I can't remember what the thing I'm thinking of is called now, but it's how you get a Brayton cycle to approximate an ideal (Carnot) cycle.

A recuperator would take heat from the exahaust and inject it downstream of the compressor and upstream of the combustor.

Nice catch, thanks for setting me straight.

Tom.
 
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Faro
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RE: Compression Ratio Vs Turbine Temp

Thu Jan 03, 2008 6:09 pm



Quoting Thegeek (Reply 7):
Combined cycle power plants achieve their 60% efficiency by driving a steam plant off the exhaust.

What is the energy efficiency of contemporary high-bypass tubofans BTW?

Faro
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tdscanuck
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RE: Compression Ratio Vs Turbine Temp

Thu Jan 03, 2008 8:46 pm



Quoting Faro (Reply 13):
What is the energy efficiency of contemporary high-bypass tubofans BTW?

The best pressure ratios around right now are in the low 40's, which gives you an idea Brayton cycle efficiency limit in the low 60%'s for normal turbine inlet temperatures. Knock off a little bit for less than ideal compressors and turbines and the actual energy efficiency should be around 60%.

Quoting Tdscanuck (Reply 12):

You're totally right...I've been using the word "recuperator" incorrectly. I can't remember what the thing I'm thinking of is called now, but it's how you get a Brayton cycle to approximate an ideal (Carnot) cycle.

Found it...reheat. Should have known it would be the same term as an afterburner. In the classical case, reheat is doing combustion between turbine stages to bring the temperature back up.

Tom.
 
thegeek
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RE: Compression Ratio Vs Turbine Temp

Sat Jan 05, 2008 8:04 pm



Quoting Tdscanuck (Reply 14):
The best pressure ratios around right now are in the low 40's, which gives you an idea Brayton cycle efficiency limit in the low 60%'s for normal turbine inlet temperatures. Knock off a little bit for less than ideal compressors and turbines and the actual energy efficiency should be around 60%.

This claim is somewhat bold. The theoretical (Carnot) efficiency of a combined cycle power plant with a turbine inlet temperature of 1727 deg C, and a discharge temp of 127 deg C is 80%, but in practice it only achieves 60%. I'd be surprised if a regular high bypass turbofan achieves a 50% efficiency. Can you back that up?

I'd think that the efficiency would vary, with the most efficient point being top of climb, probably at full throttle.
 
tdscanuck
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RE: Compression Ratio Vs Turbine Temp

Sun Jan 06, 2008 1:50 am



Quoting Thegeek (Reply 15):

This claim is somewhat bold. The theoretical (Carnot) efficiency of a combined cycle power plant with a turbine inlet temperature of 1727 deg C, and a discharge temp of 127 deg C is 80%, but in practice it only achieves 60%. I'd be surprised if a regular high bypass turbofan achieves a 50% efficiency. Can you back that up?

I don't have anything to back it up beyond the data I already provided...the Brayton cycle efficiency for normal temperatures and pressure ratios in the low 40's really is in the low 60%'s. How much to knock off for turbine/compressor inefficiencies is open for debate, but I've seen 94% used in numerous preliminary design calculations.

Tom.
 
dakota123
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RE: Compression Ratio Vs Turbine Temp

Tue Jan 08, 2008 12:53 am

Quoting Thegeek (Reply 15):
This claim is somewhat bold. The theoretical (Carnot) efficiency of a combined cycle power plant with a turbine inlet temperature of 1727 deg C, and a discharge temp of 127 deg C is 80%, but in practice it only achieves 60%. I'd be surprised if a regular high bypass turbofan achieves a 50% efficiency. Can you back that up?

Again looking at a land-based unit (instructive here, I think), the power generation equivalent of the CF6-80 is ~36% thermally efficient on a higher heating value (HHV) basis (~40% lower heating value -- I love how turbine manufacturers always quote efficincy and consumption in LHV when fuel is consumed -- and bought! -- HHV). The RR Trent used in power generation is ~1% - 2% better.

Doesn't the efficiency hit compared to ideal come from compressing air that 'just' goes into cooling?

I can provide the raw fuel consumption and power output numbers if somone wants to check my work...

Mike

[Edited 2008-01-07 16:55:22]

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