Why Use Cooling Air in Gas Turbines Instead of Lowering Fuel/Air Ratios?

[overrider]

Gas turbine fuel/air ratio & cooling, help please!

Hello Dear Colleagues,

I am doing a gas turbine project and I wonder why we use cooling air instead of decreasing the fuel/air ratio after getting the following results from my gas turbine thermodynamic model.

I have a mechanism in my model which adjust the fuel/air ratio for different pressure ratios and ambient conditions to reach the desired TIT (turbine inlet temp.) of 1755 K.

For selected pressure ratio of 20, I get the following results from the combustion of methane :

Using %80 of the compressor air for cooling and the rest for combustion, I can only reach a maximum TIT of 1360 K at the equivalence ratio 1.019 for stoichiometric combustion of methane. In this case the fuel/air ratio is 0.0584.

Using %50 of the compressor air for cooling and the rest for combustion, I can reach my desired TIT of 1755 K at the equivalence ratio 0.835. In this case the fuel/air ratio is 0.0488, a high value for gas turbines which usually have a fuel/air ratio around 0.02.

When I don't use ant cooling air, I can reach my desired TIT at the equivalence ratio 0.458 and fuel/air ratio of 0.0267.

From these results I come to a conclusion that there is no need for cooling air. If there is a need for that I please ask how I can define the minimum value needed.

I can provide any information needed. Please give me a hand, I am really stuck on this.

Thank you in advance.

Kayhan.

 

[willgoat]

Cooling air is required to cool components within the gas turbines (combustor wall, nozzles and rotors), any % of cooling air taken compressor will reduce the amount available to do work, so obviously we will want the minimum amount possible. I am thinking that you approached the problem from the wrong angle. Cooling air need is a requirement, ie someone will tell you that they need x% of cooling air from compressor, say 20% and you do your sum to get to the required TET.

 

[Pkruse]

Willgoat is right. With the turbine inlet temperature hundreds of degrees hotter than the melting point of the super alloys, the vanes and blades simply must be cooled. Any gas turbine engineering department will have a group of engineers to do secondary flow and heat transfer calculations. They will request a specific amount of air from specific stages of the compressor section, and then the mechanical design engineers have to figure out how to get it back to where it needs to go. This is a major part of the cost of designing and building the engine.

 

[bigfooted]

There is another problem. It is true that for lower equivalence ratios your combustion temperature will drop. It might even be that you don't need cooling air at these lower temperatures (doubtful because of reasons mentioned above). However, at lower equivalence ratios you get into stability problems. For methane-air you get combustion instabilities at equivalence ratios below around 0.8 (at atmospheric combustion). At around 0.5 you will reach the lean flammability limit meaning you cannot sustain a stable flame anymore.

 

[Pkruse]

Everyone who designs gas turbines makes the turbine inlet temperature as high as the customer is willing to pay for because that makes the engine more efficient. This means a bunch of cooling air. This does not lean out the methane combustor because the cooling air does not pass thru the flame. Great pains are taken to give the flame the very best mix for the most complete combustion.

 

[bigfooted]

Everyone who designs gas turbines makes the turbine inlet temperature as high as the customer is willing to pay for because that makes the engine more efficient. This means a bunch of cooling air. This does not lean out the methane combustor because the cooling air does not pass thru the flame. Great pains are taken to give the flame the very best mix for the most complete combustion.

No, a common misconception. You should compare the total energy release of combustion to the total energy content of the mixture. You will see that when you compare the heat release of a lean mixture with a lower adiabatic flame temperature to that of a stoichiometric mixture with a high flame temperature, combustion(!) efficiency will not be affected. Remember that what counts is how close you get to the adiabatic flame temperature. There is a tendency going on for about 20 years to go to lean premixed, because of NOx regulation. You actually want to have a low flame temperature! But you also want the flame temperature to be close to adiabatic. Hence lean premixed. See for instance the new lean TAPS combustor in GE's GEnX. It still uses backside cooling though, but thermal stresses are much lower.

 

[willgoat]

Increasing turbine inlet temperature (ie temperature of gases leaving combuster) will increase specific power of a gas turbine, so you get more power out of the same size machine with fixed mass flow rate. This is about the operation of the whole gas turbine, not the efficiency of the combustor itself.