Why does increased turbine temperature causes increased efficiency?

[TheBeak]

It has struck me, there are a few things in flying that I know happen but I don't know why in enough detail - knowledge is the next best thing to experience and I don't have a huge amount of the later given the economic climate. I have vague ideas and can make educated guesses but I don't REALLY understand why. Could someone please explain to me why increased gas turbine temps cause an increase in overall efficiency. Is it an after/indirect effect because of increased firing temps back in combustion which cause improved work output?

[Genghis the Engineer]

I'm astoundingly rusty on the maths, but in simplistic terms I think it works like this:

- Gas Turbine engine generates thrust through expansion of hot gases
- Greater the expansion, the greater the propulsive efficiency
- Hotter the gas, the more it's expanded (or will, particularly overall on mixing with bypass air)
- The hotter the gas, the higher the turbine temperature.
- Therefore higher turbine temp = higher efficiency.

To some extent also a higher temperature implies a maximum efficiency / stoichiometric fuel/air mix in the combustion chamber, so less unburned fuel is leaving the engine, which will also increase efficiency.

A crude explanation, lacking any maths or real engine theory, but about right I think.

G

[hetfield]

High Temperature = High Energy

[john_tullamarine]

I probably should dig out the books to check before shooting my mouth off .. but I vaguely recall that efficiency depends on the temperature delta .. ie (Tmax-Tmin) in the cycle ?

[FE Hoppy]

Thermal efficiency can be described as the difference between the highest and lowest temperature in the cycle. As we cannot lower ambient temperature (lowest) then we need to increase the combustion temperature (highest) and thus the turbine temperature to increase the thermal efficiency of the engine. wiki carnot efficiency for more info.

Someone once described it to me as squeezing an orange. The harder you squeeze, the more juice you get.

[RB Thruster]

FE Hoppy sums it up well. Also the trend in large fan engines over the last 30 years has been to move to smaller core driving larger fans which move a higher mass of air for a given size of fan (look at the latest GE90 and Trent 900 fanblades)

So if a core that has a higher TGT can be developed that has the on wing life, then it can be coupled to a larger fan to give an improved engine fuel burn.

Smaller cores running hotter driving larger fans = latest generation of large fan engine.

RBT.

[lomapaseo]

I accept that hotter means more energy, but hotter means you also have added more fuel to the fire per pound of air. So how then how do you rationalize efficiency in lbs of fuel per llb of thrust per hour:confused:

I used to know this answer but have long since forgotten it and need to be able to explain it to my wife today

[salamanderpress]

there are two different temperatures to be considered.

1. Burning temp or flame temp. This decides the Carnot efficiency, the other factor being the exit temperature. (temp difference in Kelvin divided by max temp). this is the efficiency realised by the ability to burn fuel at a high temp and expand the air by accelerating it thereby cooling it. The more you can accelerate it down the jetpipe, the more thrust you get, the lower the temp and the higher the thermal efficiency.

2. EGT is not the same as exit temperature, though it obviously IS corelated. There is a limit to the temperature that turbine blades are designed to withstand, and this decides how much work can be extracted, and how much combustion temperature can be tolerated. Workarounds are to have turbine cooling systems. With improvements in materials (crystal blades etc), higher temperatures can be designed.

The objective of high bypass is to enhance PROPULSIVE efficiency by handling a larger mass of air at lower acceleration rather than a small mass of air at higher acceleration, which is different from Thermal Efficiency. Higher flame temperatures improve the efficiency of the core engine which in turn improves overall efficiency. Overall, you get a higher Compression Ratio, improve thermal efficiency and burn less fuel. Is there an upper limit if we can keep improving the metallurgy of the blades.? This would be the limit of temperature at which the fuel could burn without losing its chemical properties of calorific value and stability (?)

In short, you ARE getting more out of the same amount of fuel, but it's no secret weapon. Isn't that why engines are so bloody expensive..?

[lomapaseo]

From an energy standpoint how does more temperature (more fuel per pound of air) make it more effiecient?

Is it because I can expand it further across the turbine per pound of fuel?

Of course the next question will be why? :)

[vapilot2004]

For a gas, temperature is directly linked to pressure.

[muduckace]

Higher EGT (relative to sensor placement) equals higher burn of the environmental air and fuel, lower temperatures allow less air and fuel ingested to burn = less thrust.

[salamanderpress]

Higher flame temperature is possible with a higher COMPRESSION RATIO, while keeping the same fuel-air ratio. (not quite close to stochiometric combustion in a jet engine.) so the amount of fuel you burn relative to the air is not more. It's just burning in a more densely packed manner. And the high temperature allows you to draw more work (turbine shaft power or exhaust thrust) out of it.

The SFC of modern engines is higher, ie less fuel burn per thrust.

Apart from thermal efficiency, there's improvements in engine control, allowing the engine to operate closer to the "surge line", ie more aerodynamic efficiency.

[MaxHelixAngle]

Hi TheBeak,

Not an expert, however have just asking myself the same question before you asked it and have pulled out the old BGT and Thermodynamics text books.

The Theoretical Thermal Efficency of the engine (theoretical because it does not take into account any practical losses which must occur in running an engine) can be defined as: [work done / heat supplied] or [(heat supplied - heat rejected) / heat supplied]. The easiest way of considering the above is forgetting the calorific effect's of fuel and assuming that the working substance of the engine is just air that is heated by some source, the only addition of energy to the air is heat. A certain amount of heat is supplied to the air and a certain amount is expended, the missing heat must have performed the work. The air is only usable through the turbines and as a propulsive jet, down to a certain temperature, any energy (heat) in the air below that is wasted. Thefore the higher the heat is above that minumum, with the same amount of heat wasted, the greater the efficency.

This can be seen on a PV diagram, where it can be broken down into the work achieved by different parts of the cycle. I don't understand why the Carnot cycle has been mentioned as this is a cycle based of isothermal change (heating the air at a constant temp). Check out the Braynton cycle, this is the theoretical cycle of a turbine engine (heating at constant pressure).

Hope this is of some help, will watch on this thread with much interest to see if some of the more learned contributors can provide a more detailed explination.

Cheers,
MHA

[TheBeak]

Cheers to all for taking the time respond.