I have been involved in a small footprint turbofan for a few years now. While the overall parameters of each engine section are understood, the intricacies of achieving each requirement are mind-boggling. Currently Aerospace Research and Development Group, LLC (AeroRND) is neck deep in the annular combustor design challenges (burner section). While there are many areas that I am not permitted to discuss, I would like to review some of our progress.
Figure 1: The concept annular combustor. (No laughing at the plastic looking liner and casing. I had to make it so you could see in.)
Combustor Design Challenges
My initial thoughts were that the combustor should be little problem, save for the heat and materials. The initiated among you know already that I was a bit naive. I have worked with them for years, repairing various liners, including old can-types from the General Electric J79. That did not mean however, designed one to fulfill our very specific needs would be easy. Combustor design technology is as ill-understood as that of the axial compressor, perhaps more-so. These principles are rather researcher-centric, and governed by coefficients and relationships that cannot be better defined by our current understandings. Jack Mattingly was quite correct when he noted that there are few printed resources on the subject, but was kind enough to confirm that Lefebvre was the best-known author on the subject.
Temperature and Materials
The more Q, or temperature, you can throw at the fluid in the can, the better the performance… until it melts. At such a reduced scale, we understood that this would be one of our biggest combustor design challenges. From the onset, I looked forward to the challenge of ‘creative cooling’, and finding some new ways to keep the combustor target temp away from the liner. Now that the fluid quantities and nozzles are understood, the fun part can begin: CAD and CFD modeling some nifty geometry ideas.
The most common thread in axial flow engine design is that the smaller you get in diameter, the longer the engine becomes, until you reach the point of diminishing returns. The burner can is no different. The first hurdle we encountered was the Diffuser, which reduces velocity entering the outer annulus by reducing the total pressure. After fighting numerous design ideas and variations, we settled into a hybrid diffuser that offered reasonable flexibility at a loss in efficiency that we could choke down.
The initial liner sizing focused on fitting around the fuel nozzle, and then subsequent tuning. This turned out to be a tug-of-way between mass flow requirements, the liner to overall height ratio, and the resulting, overly diminished flow requirements from the outer annulus. Efforts to keep the flame lit in the Primary Zone (PZ) (combustion) resulted in increased needs in the Secondary Zone (stabilization), and a longer burner. It was becoming readily apparent that we needed more pressure drop in the liner to get more mass flow into the game.
That was the key to moving forward, and shortened things up about 25%, but at an unfortunate cost of total pressure. This was balanced as a necessary trade-off of length and flame stability. The topic of pressure loss int he combustor is often glossed over by engine designers looking at the overall 2D design. Typical drops in pressure, even in large, efficient engines can be large enough to cause an adjustment to the entire engine. This really was an eye-opener, and where the pinch was felt the hardest in the combustor design.
Figure 2: Concept fuel nozzle.
Flame Stability and the Damn Nozzle
Among many aspects of flame stability, as mentioned previously, is the shape of the flame in the Primary Zone (PZ). This shape is best described as a toroidal combustion bubble; an undulating donut shaped flame will remain present, at least until it blows-out (this is followed rapidly by our aircraft becoming a very expensive lawn dart). The keys to keeping the PZ a happy, harmonious place, lie largely with the complete combustion of the fuel taking place within the PZ. This is an area of substantial research, with numerous benchmarked mathematical relationships to evaluate during the combustor design process. Two that stand out are the Macromixing Time (time to get a proper fuel/air mist), and the Stay Time (the time the mixture remains in the PZ). Keeping the latter long enough was a challenge.
Another way to provide sufficient mixing and flame shape is the fuel injection nozzle. This topic runs off into the sunset, and is represented by gobs of patents and principles. After considering numerous types, I was torn between an airblast simplex nozzle, and the prefilming airblast type. One represents rugged simplicity, and the other, elegant performance. Unfortunately, both have their drawbacks. We settled on a proprietary hybrid design instead, that I intend to test very soon.
Our combustor design challenges are by no means over, but we are finally over the hump. We have a good fix on our nozzle, diffuser, liner, and cooling needs, but there is so much more involved. This includes, lest I forget, fighting the material thicknesses at every turn!
We cannot disclose much more of the designs, save for citing a couple of the significant references that have been helpful, cited below. I am saving some of that for a planned technical article after much of this research is completed. I hope to return here soon, with some more updates and images as we progress.
Now onto the liner design.
Lefebvre, Arthur H., and Dilip R. Ballal. Gas Turbine Combustion. 3rd ed., CRC Press, Taylor & Francis Group, LLC, 2010.
Mattingly, Jack D., et al. Aircraft Engine Design. 2nd ed., AIAA Education Series, American Institute of Aeronautics and Astronautics, Inc., 2002.