The study of emission reduction in small-sized combustion chambers

Аuthors

Yousef W. M.^*, Davydov N. V.^**

Kazan National Research Technical University named after A.N. Tupolev, Kazan, Russia

*e-mail: wyasen@mail.ru
**e-mail: wyasen@mail.ru

Abstract

One of the main problems in aviation is developing low emission combustion chambers. The article studied various schemes of the combustion chambers such as:

- traditional scheme of combustion chamber with axial vane swirlers in the front plate;

- straight-through arrangement of combustion chamber with flat flame stabilizers.

Traditional combustion chamber has two vane swirlers in the front area and a swirling air supply in the first belt of the orifices. For better mixing, the main gaseous fuel was fed between the blades of the large swirler, while the small swirler formed a stable pilot burner in the wide range of the combustion chamber operation. Burning in this combustion chamber was uniform over the whole area of the front plate, and being finished before reaching the second belt of orifices. The CO values of more than 3000 ppm were observed herewith at the outlet. The straight-through arrangement employed the flat stabilizers, located in circumferential and radial directions fr om 10 to 15 mm wide, instead of the front plate. The main fuel supply through a tubular manifold with orifices of 1mm diameter, located in at a distance of 10 mm, was arranged prior to the stabilizers. The pilot burner was located at the center (similar to the previous scheme). This scheme demonstrated better combustion in a wide operation range, but burning was non-uniform along the area, with dark zones where the combusting mixture “freezing” may occur.

According to these results, an approach was chosen to emissions reduction (CO, NO_x), namely:

- fuel and air mixing efficiency increase (for example, by swirler);

- combustion scale reduction by combustion chamber separation into smaller modules, the pre-chamber diameter herewith was of d_mcс ≈ 40–50 mm;

- combustion zone partition in several zones:

а) a rich zone (α = 0.6–0.8) with an increased mixture residence time (due to twist). Here, CO is formed in large quantities, but NO_x is not formed due to the lack of O₂, since all O₂ will go to form CO

b) a poor zone (α = 1.0–1.40, T = 2000K) with a rapid oxidation of CO to CO₂ in the high-temperature zone, and with a reduced mixture residence time so that the NO_x yield is being minimal;

c) a subsequent air supply for rapid mixing with high-temperature combustion products (zones b) by diluting the maximum temperature to the temperatures T < 1760 K, which, according to Zeldovich theory, will freeze the formation of NO_x, but will allow complete the CO to CO₂ oxidation

As is known, the combustion chamber is a very difficult part of the engine, and an experimental study of the dynamics of intra-chamber processes is expensive, difficult and time consuming. The more simple and cheap way is simplified models application of numerical calculation methods in ANSYS Fluent software, which also allows optimizing and shaping dynamics of processes and currents.

The optimal combustion chamber was selected by comparing the results obtained by a numerical method performed according to the above said approach with experimental studies.

The scheme with modular (optimal) combustion chamber allows obtain minimum values for emissions. This was achieved by organizing the zones with high burning intensification of the rich mixtures (α ≤ 0.8) with high-degree of mixing and fuel oxidizing to CO. The oxides formation herewith is minimal. Further, behind the second belt of orifices the high-temperature zone (α ≥ 1.25 with T = 1700С°) is formed wh ere CO is burned-out to CO₂. The CO emission level is 200-300 ppm, and NO_x is 20-25ppm.

In conclusion, the results of the work include the following:

- The analysis of ways to reduce CO, NO_x emissions in low-emission combustion chambers of gas turbine engines was performed, and experimental setup for testing the combustion chamber was developed.

- Computational studies were performed, and vector picture of the flow and distribution of temperatures and velocities along the length of the combustion chamber were determined.

- Tests of the combustion chamber modules performed, experimental data on CO and NO_x emissions was obtained for various modes.

- The obtained emission values require further refinement of the combustion chamber in the afterburning zone area, minimum NO_x emissions maintaining herewith is necessary

Keywords:

numerical simulations, experimental research, small-sized combustion chamber, computational grid, the excessive air coefficient

References

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