Combustion Design

[CH3NO2]

"the flame temperature is 80% of the peak stoichiometric flame temperature."
Wow. That's very different from anything I have heard before. Very interesting.
80% of the peak stoic flame temperature in ABSOLUTE TEMPERATURE on the fuel rich side?...
Depending on the fuel chemistry used that metric could result in huge differences in the A/F mass ratio.

I gotta ask. How is it that a fuel rich PZ @80% of max temp results in higher combustion efficiency? The resultant combustion products would have to be well stirred into Zone 2. I would have expected a slightly lean condition would have been the way to go.
Interesting.

This is more along the lines of what I would have expected.
en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio#/media/File:Ideal-stoichiometry.jpg


-------

"You want to get the vaporization process started and then use the discharge velocity out of the tubes to create a airblast atomization of the fuel."

This is good to know.  I saw your discussion of it previously.  We are looking for shearing effect created by choked flow at the throat of the vaporizer outlet?  What kind of flow area contraction / diffuser expansion ratios are we targeting?  ~4:1 on both sides or something else?

If I design and size a vaporizer tube around the vaporizer internal FAR and the choked throat area,  the added mass of liquid fuel will greatly resist and reduce  the air mass velocity and flow at the throat.    Any thoughts on how to compensate for the offset?   

Thanks,
Tony

[finiteparts]

Hi Tony,

Yeah, that was the surprising thing about the work that came out from Bragg. The reactor theory is a very simplified method to approach a very complicated problem and so we need to be careful how we use it. There is more to follow on this topic, but I wanted to give the readers a sense of how the primary/secondary zoning came to be and why it makes sense to approach the continuous combustion needed in a gas turbine this way.

The reactor theory is focused on getting the highest reaction rate in the reactor, not combustion efficiency. In the primary zone, a well stirred reactor, in Bragg's model, there is a portion of the incoming fuel/air mixture that escapes the "reactor" and thus needs to be burned out downstream of the PZ...this is the basis for why there is a secondary zone. Obviously, that is not good combustion efficiency. But the reaction rate is temperature driven and so there is a balance on burning in the primary zone and burning in the secondary zone. A lean primary zone drives the temperature down, thus slowing the reaction rate in the secondary zone. As for the stirring in the secondary zone, usually the primary zone has done enough of that so that the secondary is usually modeled as a plug flow reactor. A plug flow reactor has no cross-stream mixing...think of it like a plug sliding through a pipe. Now in reality, there is cross stream mixing and often you can see the reaction anchoring behind the secondary jets, but for modeling to size the secondary, this approach usually works well.

Yes, the temperature is in absolute temperature units, Rankine or Kelvin usually. That is a good point, significant errors can be introduced if the equation is based on absolute temperatures and you plug in a relative scale like Fahrenheit or Celsius.

"We are looking for shearing effect created by choked flow at the throat of the vaporizer outlet? What kind of flow area contraction / diffuser expansion ratios are we targeting? ~4:1 on both sides or something else?"

"If I design and size a vaporizer tube around the vaporizer internal FAR and the choked throat area, the added mass of liquid fuel will greatly resist and reduce the air mass velocity and flow at the throat. Any thoughts on how to compensate for the offset?"

A standard combustor pressure drop is only around 5% of P3, so you will not be anywhere near choking the flow. For 5% you will be only a few hundred ft/s discharge velocity in a vaporizer tube. You could choke the flow by replacing the vaporizer tube with a venturi, but that is a very challenging task for a homemade engines. I am assuming that is what you are referring to when you say contraction / diffuser ratio. If you did try to go to do that, it really would take some experimenting because it is critically dependent on how much pressure recovery you can develop in the diffuser section.

For the vaporizer tube approach, if we get the fuel very evenly spread out over the tube inner surface, the film thickness at the discharge lip will be very thin and thus the energy exchange between the tube discharge flow and the fuel will lead to good droplet formation. The high speed discharge flow from the vaporizer tube and the slower combustor volume air will form a nice shear interface. Shear interfaces set up some really nice turbulence that will destabilize the liquid sheet and the surface tension of the liquid will do the rest of the work to form the tiny droplets. Here is an awesome video of liquid sheet instability formed between two coflowing airstreams. It is awesome and I can watch it for hours!

www.youtube.com/watch?v=XOvk6NumQkw&index=9&list=FLb6bqWBMQBEm2MRIBGnD9lw

You will get additional flow losses due to the presence of the fuel in the vaporizer tube, but it should be pretty small. You could take a SWAG (scientific wild ass guess) at by estimating the change in momentum of the fuel in the tube and then subtracting that momentum change from the air flow in the tube as a loss...

Good luck,

Chris

[CH3NO2]

Nice video. It's reminiscent of an air brush.

[CH3NO2]

Hi Chris,

Its interesting to see the rich side of 80% of maximum temperature occurs at 10% fuel mass flow.
Flame temperature goes from ~2300'K to 1840'K.

[CH3NO2]

Hi Chris,

I did some additional research on burning velocity of different fuels at different lambda ratios and can confirm fuel rich stoichiometry increases burning velocity despite the reduced adiabatic flame temperature. 

This is good information to know.  It makes it easy to understand the reason for targeting a fuel rich primary zone.   Although different fuels have different burning velocities, they all tend to max out at the ~80% mark you discussed previously.

Burning fuel rich increases burning velocity by ~15%.

Tony

Flame Speed Report

[finiteparts]

Hi Tony,

The "Bragg criterion" is not "really" related to the burning velocity. What Bragg found was that the primary zone reaction rate achieved it's highest value at a condition that was not at the stoichiometric FAR. In fact, he found that the primary zone achieves it's maximum heat release just below it's rich blowout limit. The partial reacted products then travel into the secondary zone and react out towards the 100% combustor efficiency.

The reactor model that Longwell made is used to find the required combustor loading that gets sets the primary zone correctly. The jet stirred reactor has such a high mixing rate that it is assumed that in the reactor, only the chemical kinetic reaction times are of interest. This gives the minimal sizing need to react the specific flow of reactants entering.

In real combustors like ours, there is a certain amount of time needed to evaporate the liquid fuel into a gaseous state, then there is a certain amount of time needed to mix the fuel vapor with the air at a molecular level that is needed for the reactions to propagate. Then finally, there is a certain amount of time for the reactants to heat up to a level that supports the reaction propagation. All these times (evaporation, mixing and ignition delay) are ignored in Longwell's reactor and thus simplify the equations for the Bragg combustor.

I am trying to work through this a little more and provide an example that will hopefully make this more clear. I had initially wanted to just discuss the Bragg criterion as a way to explain how the multi-zone combustion system came to be, but your interest has driven me to try to write up a more thorough and hopefully clear description...but, it is taking longer than I hoped due to other things taking priority.

You can read Bragg's paper here:

www.dtic.mil/dtic/tr/fulltext/u2/a955667.pdf

It is interesting to see how his reactor modeling correlates to the Avon and Dart combustors (see page 9, Table II).

I have also been working through two other papers on early combustor design by J.Swithenabnk and Robert Essenhigh. There is a very interesting take on how the dissipation of turbulence through the combustor also aligns to Bragg's model with the well stirred reactor feeding a plug flow reactor.

Enjoy,

Chris

[CH3NO2]

Hi Chris,

That article is a great find. I saved it to my collection of tech articles in the turbine combustion chamber folder.
It's mostly over my head but I'll stay on it.

On page 23 it says
"Ideal Secondary Zone. Before leaving the primary zone, all the fuel is
mixed in, and at the exit the flow is burning at 80%
stoichiometric temperature and twice stoichiometric unburnt
f.a.r. Thereafter, fresh air is mixed in to maintain the
optimum reaction rate as the f.a.r. weakens."

Am I reading this correctly that lambda should be twice the stochiometric value, in the primary zone?
The article emphasizes the 0.8 of max temp maximizes "heat release rate"... Is "heat release rate" different from flame speed?

Is this methodology different from "JetSpecs"? On my first reading, it seems like a substantially different approach.

Thanks,
Tony

[finiteparts]

Hi Tony,

It is a classic paper and well worth reading through...but remember that it is some rather old information and it has some downfalls. Very rich head ends can be very smokey...just look at almost any old engine that probably was designed with this logic. This theory does offer some neat insight on "ideal" combustors..but other design features may drive you to a different airflow partitioning scheme.

The wording of this article does make it rather difficult to read and things like "unburnt f.a.r." can be misleading. What he is saying here is that the portion of all the fuel/air mixture that entered the PZ, but has not been reacted by the time it was discharged from the PZ, will be at a equivalence ratio of 2. No, he is not saying that the PZ equivalence ratio is 2, in fact if you look at Table 1 on page 9, he says the ideal FAR for the PZ is 1.16*stoichiometric FAR. For kerosene type fuels the stoichiometric FAR is around 0.067, so the suggested FAR for the PZ is 0.077 by mass. His suggested air partitioning is also given in that table.

Since lambda is a ratio of the AFR, twice the stoichiometric value (2 X 1) would actually be a lean condition, i.e. Lambda = 2 is a lean condition. The article is written based on fuel/air ratios (FAR), not air/fuel ratios (AFR), so switching between them should be done carefully to not cause errors.

Heat release is not necessarily related to flame speed and the previous papers that show laminar flame speeds should be used carefully. We are not dealing with laminar flames and many of those data sets are derived for premixed fuel-air systems. We are dealing with turbulent flames, with higher flame speeds due to flame wrinkling and those speeds are much harder to give set values for.

This method is partially the basis for how Lefebvre developed his theory and I think that was the basis for Jetspecs (?). He took it further to what was termed a burning velocity model, which I can touch on later. Jetspecs appears to be a rough sizing program which bases the liner air partitioning on an estimate of the mass flow from the inducer area. The program has appeared to be robust enough for many newcomers to get working engines from the very simple inputs. But, my goal here is to provide clarity on why we are partitioning the air into zones, how the hole sizes impact the overall pressure ratio and the basic stability rules of thumb that are used to size the reaction zones.

So far, I am trying to work toward is explaining how you size the primary zone to meet the residence time needed to evaporate the fuel, mix the fuel at a molecular level and achieve a certain percentage of the chemical reaction completion. The work by Longwell and Bragg are stepping stones to achieving this and I was hoping to do was to use these theories to show how the real world effects (recirculation, incomplete combustion, etc) impact the performance of the combustor and its stability limits.

Once I cover this, I will move into Lefebvre's techniques for sizing dilution jets, setting pressure drop, sizing liners and passages, etc. This will probably take a bit, since I have a bunch of other projects that eat up the majority of my time, leaving a few minutes here and there to get on here and post.

-Chris

[smithy1]

Hi Chris,
Really enjoying your thread on combustion, very informative indeed. I'd like to pick your brains if I may.

As you know I have a GT6041 powered afterburning go-kart which was originally conceived by our good friend John....it has been running very well in the higher rpm/P2 ranges but I find it a bit "fluffy" at low rpm/P2, I suspect a poor or mismatched combustor....

It used to be fueled by a set of 6 "J" type evaporator tubes and the combustion was near perfect for many, many hours of running, unfortunately two of the "J" tubes had been damaged/burnt beyond reasonable repair, John and I have discussed this many times, however I have since changed the fuel injection system to a single injector from a Rolls-Royce C20B and I've made some modifications to the flame tube to accommodate it. I suspect the flame tube needs to be a slightly a different design to one fitted with evaporators, this is where I'm lacking in knowledge, I've tried the 30-20-50 setup but it just doesn't seem to make a difference to the poor low P2 combustion.

While the engine does start and run well and temps are good, I'd like to make it as good as it could be....I'd like your thoughts on combustor/flame tube design for such a beast. We're talking 106mm comp inducer...141mm exducer. If you need further info, I'll do my best to give you what you're after.

Cheers,
Smithy.

[CH3NO2]

Right on Smithy,

I'm starting a turbine build using a virtual copy of the GT6041.   The turbo is being built by Competition Turbo with a 106mm comp inducer and 141mm exducer. It should be ready in the next 2-3 weeks.

We have a common goal to optimize the same turbo to a combustion chamber.   To the extent that is feasible lets coordinate and help each other by posting our design approach and test results.  I expect to go through several combustion chamber iterations to converge on a compact and efficient design.   It's all part of the fun.

Data acquisition and analysis will be critical to the optimization process so I'll put a lot of emphasis on the data acquisition system and post the test results.

I can't wait to get the build started.   This Summer will be  jam packed with combustion fun. 

Tony

[finiteparts]

Hi Smithy,

I am not sure I really understand what "fluffy" means when applied to combustion. Ha! My first guess is that you hear or feel some form of instability?

It is common for combustors to become less efficient at lower flow rates due to the fuel/air ratios dropping relative to the full power conditions. If the combustor was just designed for full power conditions, it can experience instability or blow-outs at lower power conditions due to poorly staged air flows in the primary zone. In the design phase there are many operating conditions that are considered, but usually the high altitude relight and the full power take-off conditions are the big players.

The work done by Longwell actually gives the best achievable boundaries for combustor stability due to it only being limited by the chemical reaction kinetics. In reality, our systems will never approach those values because our mixing will be much less effective, but it is a start. Longwell and later, Lefebvre, developed combustor loading parameters that allow us to estimate if we are in relatively stable regions of operation and we can explore those once we get some information.

Do you have a sketch/photo of the liner with hole locations and sizes? We need any gap or hole, so we can accurately calculate the liner pressure drop. The C20B uses a liner pressure drop of around 3.5% dP/p...but for us to get the best chance of good combustor efficiencies, we would probably shoot for a 5% dP/p. Then we would need P3 and T3 (compressor discharge pressure and temperature) numbers if you have them at idle and full power. We can try to estimate mass flow from the compressor map and the combustor temp from the EGT if you have that.

The one thing that does sort of stick out is that you are using a C20B injector in a much lower pressure machine. Those injectors produce some very large droplets at lower flow rates  (> 200 micron SMD). This might be ok in the C20B because the higher discharge pressure also means higher discharge temperatures. The impact of the pressure suppressing the boiling point of the fuel is lower than the impact of increased temperature. Higher temperatures mean higher rates of evaporation and reaction from the droplets. If the droplets evaporate earlier, they are more likely to burn in the primary zone were they are supposed to. Now, you are not using the C20B liner, so you might have done something different to stabilize the flame. I would think that because of the lower temperature impacts, you would need to increase the recirculation strength in the primary zone, to draw in the larger fuel droplets. At ground idle the T63 has a T3 = 285F and a full power T3 = 590F. If you get up to a PR=4 with 2.66 lbm/s airflow, you a getting a peak T3 of around 387F. As you come down in power, the reduction in T3 could drop below the gnd idle T3 of the C20B and you might be experiencing trouble due to the reduced capability of the air to vaporize the fuel. The delay in droplet vaporization could cause combustion instability due to primary reaction location to being moved further downstream in the PZ and thus having less flame anchoring.

Just a theory...hopefully we can work from the data how the combustor is operating.

Good luck,

Chris

[racket]

Hi Smithy

The flametube in my TV84 engine, 3.5" inducer 1.8 lbs/sec, worked OK , heres a couple of scanned photos ( pre digital camera days :-; )



It was 140 mm dia and ~1/3rd cu ft capacity , swirl vanes ( 12 of with 9 X 5 mm flow areas) surrounding simplex spray nozzle , 50 -750 psi fuel pressure , flametube domed cap had 36 X 3 mm holes

Cheers
John

[finiteparts]

John,

That is a beautiful combustor...thanks for sharing the pictures.

What is the valve between the combustor case and the nozzle section for?

~ Chris

[racket]

Hi Chris

The gate valve was part of my bleed air experiments , the air discharged into the jetpipe muff which then had a number of rear facing holes into the jetpipe to provide "thrust".

It was during the time when I was producing similar thrusts despite mixing and matching various scroll A/Rs and jet nozzle sizes , it wasn't until I finally realised it was the turb exducer choking and clipped it back that the engine started performing as it should when different scrolls were installed.

The extra pipe from the main air delivery tube from the comp housing up to the plenum above the primary zone was another "early experiment" that didn't produce any changes/benefits .

I really should have made the flametube from 1mm stainless instead of 0.5 mm as the thinner gauge suffered a bit from the radiant heat in the primary zone . ............but considering it was a 1990's "first edition" attempt at making a flametube I had no complaints , the fuel control gave me most of my grief with that engine , but even that was eventually sorted after a number of years and several variants , it ended up giving the engine a similar acc/deceleration rate as an IC engine , blip the throttle and rpm followed along nicely.

LOL, it was interesting looking through the old "hard copies" of bygone times, my hair has gone a bit greyer and I've put on atad of weight but life is still as interesting and exciting :-)

Cheers
John

[finiteparts]

John,

It's good that you like to share all those learnings with everyone on here. It was a different time back then. We didn't have the internet to copy others ideas. I remember when I "thought" up the idea to add a combustor to a turbocharger, it hit me like a ton of bricks! I thought that I had discovered something that no one else had ever thought of. Ha! I still have the article from the April 1984 Popular Science article that made it all "click" for me.



I used to ride my bike across town to go to the library to keep checking out the only book the library had on jet engines. One day I was looking at the magazine section and there was an image of a turbocharger rotor on the cover and inside, a glowing turbocharger! As soon as I saw that rotor on the cover, it clicked! I was on the hunt for a turbo at the junkyards. My first turbo was a Airesearch turbo off of a 80's carbureted Gran National. It was junk. The shaft was completely coked and my guess is that it had caused the rotor to go out of balance. It rubbed the housing, but at the time I didn't know enough to know it was junk. I bought it for $60 and started down the path of gas turbine building.

My first combustor was a copy of what I thought I saw in the books...a tube with a bunch of holes in it. I drilled a shit ton of holes with no rhyme or reason on why they were there. But hey, I was only 13 yrs old...so I can give myself a break due to inexperience. But that is where it all started. I always had a reason for being interested in the information I was learning in school...it all had some application to my turbine design.

I was disappointed when I found that people had converted turbos to turbojets back in the 1940's with the B-type turbos off the B-17s. I wasn't the first, but I still feel good about thinking it up. It is funny how that passion has survived all these years and it is a neat thing to see it other too.

It's fun looking back!

- Chris