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Faro
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Compression Ratio & Combustion Temp: Relation to Fuel Efficiency

Sat Apr 08, 2017 8:16 am

With jet engines, how are compression ratio and combustion temperature related to fuel efficiency?

    - Does a higher compression ratio automatically lead to a higher combustion temperature?
    - If yes, is this because there is a higher oxygen content in the air entering the combustion chamber?
    - How does a higher compression ratio extract more energy from a give quantity of fuel burnt?
    - How does a higher combustion temperature extract more energy from a give quantity of fuel burnt?


Faro
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Re: Compression Ratio & Combustion Temp: Relation to Fuel Efficiency

Sat Apr 08, 2017 4:20 pm

1. Generally, yes. Look up Avogadro's Law
2. Sort of. Die to the pressure increase, you get more molecules in there, so you can spray in more fuel (same reason some cars use turbos)
3. Not 100% sure, but I think it has to do with #2, you get better combustion when the molecules are smashed up against one another
4. Higher temperatures generally result in more complete combustion, because the hydrocarbon chains split apart more readily in high-heat conditions.

I think there's some other property associated with #3/4 that I'm too lazy to look up right now, but I wager someone with more knowledge will chime in soon.
 
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lightsaber
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Re: Compression Ratio & Combustion Temp: Relation to Fuel Efficiency

Sun Apr 09, 2017 4:29 pm

1. Actually not always. For a given compressor technology the higher the pressure the higher the temperature where T2/T1=P2/P1^(gamma-1/gamma)/compressor_efficiency. The higher the pressure, the more efficient the engine, but also the lower the compressor efficiency. But what is a technological limit is the turbine inlet temperature. Now, there is a trade with more cooling allowing a higher turbine inlet temperature. A trade study is performed to meet maintenance promises. So at higher pressure ratios, the fuel air mixture must be leaned (all engines overall operate lean) to keep turbine inlet temperature from increasing.
2. No higher oxygen content. The higher the temperature, the greater the efficiency as there is more work by the turbines for a given drop in pressure ratio and thus more surplus pressure to power the fan (I'm assuming a high bypass engine). Higher pressure means more work. But for any given limit to turbine inlet temperature, there is a peak optimal efficiency pressure ratio. In many ways, this defines the generation of the engine.
3. Higher pressure means that at each turbine stage dH=V dP (way over-simplified) where dH is the change in enthalpy (potential work to extract). So for a greater pressure ratio, integrating that equation (which has many terms that are variable and thus in the real world complex to integrate) has more potential work.
4. Higher temperature is a higher V (greater volume for the same gas. So the same above equation applies dH= V dP.

H= enthanlpy which is where dH=dCp*T + Cp * dT
V= Volume of gas powering the turbine (higher temperature is higher volume)
dP is the change in pressure.

Now I work with the derivative as the integration is quite complex as I'm not going into all the terms that change as a function of pressure or specific volume (turbine blade clearance cleakage vs. flow area, drag per flow area in a turbine), temperature (mostly Cp, but also due to shape limits at the highest temperatures, turbine efficiency), pressure (at higher pressure, structural concerns impact efficiency due to the forces).

Optimum pressure ratio is a function of engine size (larger engines have lower leak path area to flow path area, so their compressor and turbine efficiency improves slightly and thus the optimum pressure ratio goes up.

Optimum pressure ratio is a function of compressor efficiency. Today's higher mach number compressors with lower blade counts (less blade area drag) are much more efficiency with the PW1100G being the extreame (today), but that is creating bearing/shaft issues.

Optimum pressure ratio is a function of turbine clearance control technology. This technology is always improving, so the optimal pressure ratio increases with time for new engines. The trick is designing for a good efficiency after overhaul (which has improved with dramatically better break in operations prior to delivery to the airlines).

Optimum pressure ratio is also a function of counter or co-rotation. SInce counter rotation really improves the efficiency of the first row of the low turbine, a counter rotating engine will have a higher optimum pressure ratio than a co-rotating.

Optimum pressure ratio is a function of combustor technology. For higher temperatures increase NOx formation and can limit allowed pressure.

Optimum pressure ratio is a function of turbine materials and cooling technology. Every improvement increases the optimum pressure ratio.

Optimum pressure ratio is also a function of combustor technology in that the combustor length keeps decreasing. Since this decreases required cooling, the pressure ratio goes up a little. Same with improvements in diffuser technology.

Variable cycle technology also increases the optimum pressure ratio. Basically, the variable cycle techology reduces the stress on the combustor and turbine during climb. This increases durability allowing for a higher pressure ratio. GE uses variable turbine cooling. Pratt was going to go with a variable fan nozzle, but decided to simplify the engine. But the idea is right, it will just take a higher thrust engine to make that variable cycle technology pay off. Expect more and more variable cycle techology as engines progress, with the payoff being greatest in the larger engines.

For example, I'm seeing some really agressive decisions made in the 777X engines (GE9x). To enable a higher pressure ratio, we're seeing much more agressive fuel injector cooling. It also has fuel cooling of parts that previously didn't need to be cooled due to the higher heats of that engine. (fuel is commonly used to cool parts of the engine as there is so much to use as a coolant).

The next technology shift with be ceramics, CMCs I believe will be next. This reduces required cooling allowing for higher compressor exit temperatures that pushes the technology to the next stage.

For more, you need to understand the thermodynamics of jet engines. Start with the Brayton cycle:
https://en.wikipedia.org/wiki/Brayton_cycle

Then you need to look at propulsion efficiency:
https://en.wikipedia.org/wiki/Propulsive_efficiency

Then it becomes a study into part throttle efficiency, manufacturing and maintenance costs, and risk of development. The part throttle efficiency (idle) is one reason all engines are now two or three shafts as the low spool barely turns at idle. It isn't spinning fast enough to do work on the air at idle (the compressor bleed will be full open to prevent compressor surge during the ramp from idle to takeoff or flight idle to full throttle for an aborted landing). Compressor surge is a poor term that actually describes when the compressor stalls and back flows (fuel/air mixture from the combustor back flows into the compressor and even out the front of the engine, it can thus be spectacular if that fuel ignites).

I hope this helped,
Lightsaber
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Faro
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Re: Compression Ratio & Combustion Temp: Relation to Fuel Efficiency

Mon Apr 10, 2017 9:37 am

lightsaber wrote:
I hope this helped,
Lightsaber



Understatement of the month...of course it did Lightsaber :) :) :)

I just have to sit tight and digest all the detail and intricacies...thanx a million!...

Will probably be back with some follow-on questions...


Faro
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Re: Compression Ratio & Combustion Temp: Relation to Fuel Efficiency

Mon Apr 10, 2017 11:19 am

lightsaber wrote:
The higher the pressure, the more efficient the engine, but also the lower the compressor efficiency.



I think this is the first question I do not understand.

How for example does GE9X's compression ratio of 27:1 lead to lesser efficiency? How does that jive with the fact that the engine will purpotedly have the lowest TSFC ever?

Does higher pressure promote overall engine efficiency despite the lower compressor efficiency? That is, does it render other engine components more efficient in a more-than-proportional way?


Faro
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Re: Compression Ratio & Combustion Temp: Relation to Fuel Efficiency

Tue Apr 11, 2017 11:20 am

Faro wrote:
lightsaber wrote:
The higher the pressure, the more efficient the engine, but also the lower the compressor efficiency.



I think this is the first question I do not understand.

How for example does GE9X's compression ratio of 27:1 lead to lesser efficiency? How does that jive with the fact that the engine will purpotedly have the lowest TSFC ever?

Does higher pressure promote overall engine efficiency despite the lower compressor efficiency? That is, does it render other engine components more efficient in a more-than-proportional way?


Faro

The GE9X has greater efficiency.
I was a bit too precise.

Every smaller stage of a compressor is less efficient than the previous stage as the leak path area is a constant while the flow path area is reduced. To achieve a higher compression ratio, more compressor stages are added. Thus the overall efficiency of the compressor drops while improving the overall efficiency of the engine.

To improve compressor efficiency:
1. Spin the compressor faster (fewer stages to achieve the same pressure ratio). No modern engine is close to the limit, it is just that the faster you spin the tighter the bearing tolerances and the tighter the shaft bowing limits, this creates its own set of issues.
2. Reduce the blade gap clearance. This is a control technology limit.
3. Improve the blade shape, so the earlier blades have fewer losses (harder to shape the smaller blades)

So while one component becomes less efficient, the overall engine is more efficient. But it puts another limit for each generation of engine for pressure ratio versus size. As the leak area for a larger engine has a ratio of (Leak area)/(Flow Area) that is less than a smaller engine, larger engines achieve higher pressure ratios. This is also true of the turbine. If turbine efficiency improves, the optimum pressure ratio increases (even with greater compressor losses).

In school, all the efficiencies are a nice fixed numbers. In reality they are a function of scale (size), secondary system effectiveness (stator profiles, case cooling), technology level (blade design), mach number of the blades (RPM, too low in modern engines), and how many compressor (or turbine) stages you put in. Each level of compressor technology allows a certain pressure ratio across a certain size of blade. Each row in the compressor has its own pressure ratio. The higher that is, the fewer stages needed. But the small stages are always leaky.

This is why large teams of engineers are paid to design engines. The number of trade studies performed is daunting. In particular as increasing fan size has a high cost, so most engines, in particular RR engines, are built with a smaller than optimal fan diameter to keep down production costs. That impacts the optimal design pressure ratio.

Please review the Brayton cycle. While very simplified, it describes the basic physics of a gas turbine engine.

Lightsaber
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Re: Compression Ratio & Combustion Temp: Relation to Fuel Efficiency

Tue Apr 11, 2017 11:51 am

Humbled by your omniscience in all things propulsive Lightsaber...many thanks...will take up Brayton seriously before coming back for more questions...


Faro
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Re: Compression Ratio & Combustion Temp: Relation to Fuel Efficiency

Sun Apr 16, 2017 9:30 am

If you have a physics or engineering background I would recommend the MIT lectures from: Thermodynamics and Propulsion, Prof. Z. S. Spakovszky

http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node85.html

Image

or a book by Nicholas Cumpsty and Andrew Heyes "Jet Propulsion: A Simple Guide to the Aerodynamics and Thermodynamic Design"
https://books.google.co.uk/books?isbn=1316432637
Image
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Re: Compression Ratio & Combustion Temp: Relation to Fuel Efficiency

Sun Apr 16, 2017 11:08 pm

lightsaber wrote:
So while one component becomes less efficient, the overall engine is more efficient. But it puts another limit for each generation of engine for pressure ratio versus size. As the leak area for a larger engine has a ratio of (Leak area)/(Flow Area) that is less than a smaller engine, larger engines achieve higher pressure ratios.


How big is the effect of scale? For example, how much efficiency is lost moving from a wide body to a narrow body engine, or from a narrow body to a large biz jet? Even a rule of thumb would be awesome.

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