Hello experts;

Good day. I read one or two articles in which air to fuel ratio formula is given as
A/F = ma/mf
Here
ma = mass air flow of the compressor
mf = mass flow of the fuel

I have calculate with this formula but the result is very strange. For example in our case.
The mass air flow is 35.4 pound per second and the fuel flow is 2257 pounds per hour.

The result is 56.5 (change 2257 into PPS)

what it means?

Help me to calculate the air fuel ratio of the gas turbine if the above formula is wrong?

Regards

 

Samul,

Exactly what are you trying to accomplish?

Are you trying to design a gas turbine? Model a gas turbine?

Are you trying to develop a program to calculate the efficiency of a gas turbine? Or a program to determine when it's appropriate to perform some maintenance activity (off-line compressor water washing, for example)?

I'm certain all of this has been done, many times and in many ways, for many Masters and Doctorate programs over the years. If you searched the libraries of technical universities you would probably find several papers that would be most helpful.

Gas turbines have a LOT of 'excess air,' because a lot of the air is used for cooling and dilution of the combustion gases resulting from the burning of fuel. Those gas temperatures are hotter than the turbine nozzles and buckets can withstand, so they must be cooled/diluted--and that's what a lot of the axial compressor discharge air is used for. Only a portion of the axial compressor discharge air is actually used for combustion (mixed with fuel for burning); the rest is used for cooling and diluting the hot combustion gases before they enter the 1st stage turbine nozzles.

The orifices in the head end of the combustor (where the fuel nozzle is located) and the design of the fuel nozzle and the gas fuel orifices in the fuel nozzle determine the fuel/air ratio for combustion of the fuel. Only a portion of the axial compressor discharge enters the head end of the combustor and the fuel nozzle orifices.

The rest of the axial compressor discharge air enters the combustion liner through cooling and dilution slots and holes in the liner body and reduces the gas temperature to something the turbine nozzle and bucket materials can withstand.

So, just by knowing the mass flow of the axial compressor and the mass flow of fuel it's not possible to calculate the air/fuel ratio because all of the air is not used for combustion; a good deal of the air flow through the compressor is used for cooling and dilution of the hot combustion gases before they enter the turbine section.

I don't know of a way to determine how much air is used for combustion and how much is used for cooling and dilution. I do know that the oxygen content of a gas turbine's exhaust is very oxygen rich, compared to, say, a reciprocating engine's exhaust. I believe that combustion, stoichiometric combustion, occurs at something around 14:1 for many fuels. But I think you will find that the axial compressor of gas turbines flows much more air than this.

Again, if you would tell us what all these calculations (compressor ratio; compressor efficiency; compressor mass flow; air-fuel ratios) were needed for we might be able to help or offer more information or references.

I would suggest you do some more research on combustion turbines in general, and axial compressors and combustors and turbines. Most turbine materials can't withstand flame temperature gases, so they have to be cooled/diluted before they are admitted to the turbine.

One heavy duty gas turbine OEM (Original Equipment Manufacturer) uses some special probes (Kiel probes) to measure the static air pressure in the inlet ductwork upstream of the bellmouth, as well as differential pressure sensors to measure the actual inlet pressure (vacuum) at the bellmouth, and the ambient pressure (Barometric pressure) and compressor inlet temperature to calculate air flow--and that's actually a very crude measurement, I'm told. Only good for trending increases or decreases. And, I know for a fact that maintenance and calibration of the sensors is critical to proper operation of the air flow measurement system (the OEM calls it a Performance Monitoring System, as the values are used in a performance trending system, typically used for determining when to perform axial compressor washing, off-line or on-line).

But, we can't provide much help when we don't know what you're trying to do with all of these formulas and equations.

 

Hello Experts;

Good day. I read your reply and thanks for the detail. Actually we have a Centaur 40 two shaft family Gas Turbine and recently i am deputed for the field performance of the gas turbine.

The centaur 40 family have a 11 stage axial compressor and in the operation manual the 25% of the compressor discharge pressure is used for combustion while remaining is used for cooling air.

In the field performance we can measure following parameters
1. Air inlet differential pressure
2. DP in to install a transmitter at the compressor inlet
3. Compressor discharge pressure
4. Compressor discharge temperature
5. Power turbine inlet temperature
6. Exhaust temperature
7. Exhaust pressure
8. Fuel flow in actual or in mass unit.

and we want to calculate
1. 11 stage axial Compressor efficiency

2. Mass air flow (If possible) other wise calculate the Compression ratio and see from the curve what flow at that shaft speed (If any one provide the axial compressor map).

3. Heat rate

4. Thermal efficiency

5. and you suggest what parameters also be needed for the field test?

If any one have an article regarding field performance with an example of calculation i am very thankful.

Regards

 

Samul,

Have you contacted Solar for help with your project? I think they are pretty helpful, at least in my experience in the past.

As for axial compressor "maps", I think that most gas turbine manufacturers keep that information pretty close--as it's generally considered to be proprietary. Getting as much air flow through an axial compressor is key to producing as much power as possible. And, the way different manufacturers approach the maximum axial compressor air flow--without broaching the compressor limits and creating surge and/or stall conditions--are usually proprietary.

But, you would probably be surprised at the response you would get from Solar if you contacted them for information and assistance. You might get most, if not all, of what you are looking for.

It's worth a try.

Best of luck!

Also, there are lots of "users groups" for most of the major types of gas turbines in use around the world. Sometimes for free, and sometimes for a nominal fee, an owner/operator can join one of these groups and participate in forum discussions with other owners about issues and problems and resolutions--including performance measurement and optimization, as well as references for services and parts. Perhaps you could search and locate a Centaur or Solar turbine users group and see about joining--and participating--in their forums and meetings. I'm not certain there is one for your machine, but I would be surprised if there wasn't. Sometimes the OEMs support the users groups; sometimes they don't--particularly since many owners share information about parts and services from third-party providers.

The last thing I can say is that there are numerous companies around the world that will do gas turbine performance assessments--of course, for a price. But, sometimes that price is well worth it. You can learn a lot from watching a knowledgeable company set up their instrumentation and from the report they issue when they are done.

Please write back to let us know how you proceed and the results of your efforts.

 

Hello Experts;

Good day. I read your reply and thanks for good advice. Now if you have field performance related document in which one or two practical example are given with finishing report please send me to at [email protected] just for reference or study.

Do you have any document in which GE turbine instrumentation positions show for example AFPCS 96CS-1 transmitter?

Thanking you i will remain,

 

It's easiest to test performance by the actual work you are doing. gas compressor performance.

 

Samul,

Exactly what are you trying to accomplish?

Are you trying to design a gas turbine? Model a gas turbine?

Are you trying to develop a program to calculate the efficiency of a gas turbine? Or a program to determine when it's appropriate to perform some maintenance activity (off-line compressor water washing, for example)?

I'm certain all of this has been done, many times and in many ways, for many Masters and Doctorate programs over the years. If you searched the libraries of technical universities you would probably find several papers that would be most helpful.

Gas turbines have a LOT of 'excess air,' because a lot of the air is used for cooling and dilution of the combustion gases resulting from the burning of fuel. Those gas temperatures are hotter than the turbine nozzles and buckets can withstand, so they must be cooled/diluted--and that's what a lot of the axial compressor discharge air is used for. Only a portion of the axial compressor discharge air is actually used for combustion (mixed with fuel for burning); the rest is used for cooling and diluting the hot combustion gases before they enter the 1st stage turbine nozzles.

The orifices in the head end of the combustor (where the fuel nozzle is located) and the design of the fuel nozzle and the gas fuel orifices in the fuel nozzle determine the fuel/air ratio for combustion of the fuel. Only a portion of the axial compressor discharge enters the head end of the combustor and the fuel nozzle orifices.

The rest of the axial compressor discharge air enters the combustion liner through cooling and dilution slots and holes in the liner body and reduces the gas temperature to something the turbine nozzle and bucket materials can withstand.

So, just by knowing the mass flow of the axial compressor and the mass flow of fuel it's not possible to calculate the air/fuel ratio because all of the air is not used for combustion; a good deal of the air flow through the compressor is used for cooling and dilution of the hot combustion gases before they enter the turbine section.

I don't know of a way to determine how much air is used for combustion and how much is used for cooling and dilution. I do know that the oxygen content of a gas turbine's exhaust is very oxygen rich, compared to, say, a reciprocating engine's exhaust. I believe that combustion, stoichiometric combustion, occurs at something around 14:1 for many fuels. But I think you will find that the axial compressor of gas turbines flows much more air than this.

Again, if you would tell us what all these calculations (compressor ratio; compressor efficiency; compressor mass flow; air-fuel ratios) were needed for we might be able to help or offer more information or references.

I would suggest you do some more research on combustion turbines in general, and axial compressors and combustors and turbines. Most turbine materials can't withstand flame temperature gases, so they have to be cooled/diluted before they are admitted to the turbine.

One heavy duty gas turbine OEM (Original Equipment Manufacturer) uses some special probes (Kiel probes) to measure the static air pressure in the inlet ductwork upstream of the bellmouth, as well as differential pressure sensors to measure the actual inlet pressure (vacuum) at the bellmouth, and the ambient pressure (Barometric pressure) and compressor inlet temperature to calculate air flow--and that's actually a very crude measurement, I'm told. Only good for trending increases or decreases. And, I know for a fact that maintenance and calibration of the sensors is critical to proper operation of the air flow measurement system (the OEM calls it a Performance Monitoring System, as the values are used in a performance trending system, typically used for determining when to perform axial compressor washing, off-line or on-line).

But, we can't provide much help when we don't know what you're trying to do with all of these formulas and equations.

Your feedback has also helped me a great deal. It's eternal. I can now know how to handle the fuel-air ratio stuff on my MSc Thesis. It's now I understand why air intake is not really measured. Thank You too.

 

Hello experts;

Good day. I read one or two articles in which air to fuel ratio formula is given as
A/F = ma/mf
Here
ma = mass air flow of the compressor
mf = mass flow of the fuel

I have calculate with this formula but the result is very strange. For example in our case.
The mass air flow is 35.4 pound per second and the fuel flow is 2257 pounds per hour.

The result is 56.5 (change 2257 into PPS)

what it means?

Help me to calculate the air fuel ratio of the gas turbine if the above formula is wrong?

Regards

It sounds late, but I think it's fuel-air ratio, not air-fuel.