### Post by finiteparts on May 29, 2017 11:33:16 GMT -5

Ok, back to combustor design.

I think I will skip right into the calculations and then discuss the theory as we go along. I will be working through the calculations for a can style combustor for my Borg Warner EFR6758 turbos. I gave some of the design point specs on the previous page, but here is a refresher:

The approach that I am using is from Arthur Lefebvre's book,

I highly recommend these books in addition to Lefebvre's book:

Ok, so I will just start with the basics. Once we have some design point numbers and a target combustor pressure drop, how do we figure out how many holes to put in the liner? I am a strong believer in showing by example....so I will work these out for my combustor and you can just change the relevant data to work your design. Also, I highly recommend that you always check your units! They beat this into you in every engineering class and it really does help to catch errors...so you will see in my calcs how I was taught to check units.

First, Lefebvre developed a few reference parameters. The first one is a reference area and it should be noted that it is just the open area inside the combustor casing...we don't have to concern ourselves with the liner size yet. The properties are the compressor discharge temperature and pressure, not the hot gas properties in the liner. This is because what we are basically trying to do is ise the orifice area that produces the required pressure drop across the liner. The hot gases have a pressure loss of their own, called the fundamental pressure loss, which should be very small if the Mach number in the liner is low. We will ignore these for the moment, but may touch back on these later.

Now that we have the reference properties, we can calculate what is termed the pressure loss factor. The PLF is used to get the effective flow area requirements from the reference area.

So we now know that we want to end up with a total effective area across all the liner holes and any other leakage paths to be around 1.838 in^2. Now, there needs to be some logic on how that effective area is distributed between the various zones of the combustor. I will start with the primary zone. Since I have a primary zone equivalence ratio target already, this is relatively straightforward on how much air I will need in the primary zone.

With the slightly rich head end, I will need to put 21.3% of the engine mass flow into the primary zone. Now, it should be understood that this is a rough estimate. When we have recirculation zones, some of the secondary air, the burning products, etc., can get pulled back into the primary zone and change this equivalence ratio. The slightly rich head end is a result of following the work of Bragg and others that have shown that the maximum consumption rate of incoming fuel-air occurs in a slightly rich mixture, not in a stoichiometric mixture. If you were trying to put a stoichiometric ratio in the head-end, you would target around 27.8% of the air flow to be in the primary zone. If you put 30%, you are at slightly lean condition, around a phi = 0.93 (phi is the symbol for the equivalence ratio). Definitely a safe spot...anywhere near a phi = 1 is ok and I will show some of the stability curves next time as well as discussing how other factors play into sizing the primary zone.

We also will need to size the secondary and dilution zones, and then work out how we get the effective areas translated into geometric areas so that we can start drilling holes. Stay tuned!

Chris

I think I will skip right into the calculations and then discuss the theory as we go along. I will be working through the calculations for a can style combustor for my Borg Warner EFR6758 turbos. I gave some of the design point specs on the previous page, but here is a refresher:

**Liner outside diameter = 5.0 inch**

Combustor casing inside diameter = 6.0 inch

Compressor discharge temperature, T3 = 340 F

Compressor discharge pressure, P3 = 47 psia

Pressure drop = 5%Combustor casing inside diameter = 6.0 inch

Compressor discharge temperature, T3 = 340 F

Compressor discharge pressure, P3 = 47 psia

Pressure drop = 5%

The approach that I am using is from Arthur Lefebvre's book,

**", but I will also shamelessly steal techniques from other books.***"Gas Turbine Combustion*I highly recommend these books in addition to Lefebvre's book:

**Aircraft Engine Design, 2nd edition, Jack Mattingly, William Heiser and David Pratt, AIAA Education Series, 2002**

Design of Modern Turbine Combustors, A.M. Mellor, Academic Press, 1990

Design of Modern Turbine Combustors, A.M. Mellor, Academic Press, 1990

Ok, so I will just start with the basics. Once we have some design point numbers and a target combustor pressure drop, how do we figure out how many holes to put in the liner? I am a strong believer in showing by example....so I will work these out for my combustor and you can just change the relevant data to work your design. Also, I highly recommend that you always check your units! They beat this into you in every engineering class and it really does help to catch errors...so you will see in my calcs how I was taught to check units.

First, Lefebvre developed a few reference parameters. The first one is a reference area and it should be noted that it is just the open area inside the combustor casing...we don't have to concern ourselves with the liner size yet. The properties are the compressor discharge temperature and pressure, not the hot gas properties in the liner. This is because what we are basically trying to do is ise the orifice area that produces the required pressure drop across the liner. The hot gases have a pressure loss of their own, called the fundamental pressure loss, which should be very small if the Mach number in the liner is low. We will ignore these for the moment, but may touch back on these later.

Now that we have the reference properties, we can calculate what is termed the pressure loss factor. The PLF is used to get the effective flow area requirements from the reference area.

So we now know that we want to end up with a total effective area across all the liner holes and any other leakage paths to be around 1.838 in^2. Now, there needs to be some logic on how that effective area is distributed between the various zones of the combustor. I will start with the primary zone. Since I have a primary zone equivalence ratio target already, this is relatively straightforward on how much air I will need in the primary zone.

With the slightly rich head end, I will need to put 21.3% of the engine mass flow into the primary zone. Now, it should be understood that this is a rough estimate. When we have recirculation zones, some of the secondary air, the burning products, etc., can get pulled back into the primary zone and change this equivalence ratio. The slightly rich head end is a result of following the work of Bragg and others that have shown that the maximum consumption rate of incoming fuel-air occurs in a slightly rich mixture, not in a stoichiometric mixture. If you were trying to put a stoichiometric ratio in the head-end, you would target around 27.8% of the air flow to be in the primary zone. If you put 30%, you are at slightly lean condition, around a phi = 0.93 (phi is the symbol for the equivalence ratio). Definitely a safe spot...anywhere near a phi = 1 is ok and I will show some of the stability curves next time as well as discussing how other factors play into sizing the primary zone.

We also will need to size the secondary and dilution zones, and then work out how we get the effective areas translated into geometric areas so that we can start drilling holes. Stay tuned!

Chris