59934 - Investigation on Flowfield and Fuel/air Premixing Uniformities of Low Swirl Injector for Lean Premixed Gas Turbine Combustors 

Due to more and more strigent emission demands, lean premixed combustion is widely employed in aero-derivative and land-based gas turbine combustors at present. However, with the continuous increase of inlet temperature and inlet pressure, auto-ignition delay time is greatly reduced. This requires an optimized design of lean premixed injector, which can realize uniform fuel/air premixing at shortest premixing distance as much as possible, so it is a development tendency that fuel holes are incorporated into swirler-vanes. Besides, Lean premixed combustion is susceptible to combustion-driven oscillations because it operates at very lean conditions closed to the lean blowout (LBO) limits. There has been much effort to investigate the mechanisms driving combustion instabilities, one of which is high swirl flow. Therefore, a new combustion technology called low swirl injector (LSI) is a promising way to achieve good combustion stability. Besides, compared to high swirl injectors, the recirculation zone of LSI is smaller, which decreases the residence time and reduces NOx emissions. Therefore, based on the development trend of combining fuel holes with swirl-vanes and the advantages of wide operating conditions as well as low NOx emissions of LSI, this paper proposes a new lean premixed LSI with a convergent outlet, which comprises of axial swirler, premixing section and convergent outlet. The present research aims to further investigate the flowfield and fuel/air premixing uniformities of LSI by the combination of experimental and numerical methods, which can provide design basis for LSI.

Firstly, at the atmospheric pressure condition, the flowfield of LSI was obtained by particle image velocimetry: The main recirculation zone is detached from the injector outlet and originated at 5mm to 10mm downstream of injector outlet, which is fundamentally different from the main recirculation zone that is closed to the outlet of high swirl injector commonly used for gas turbine combustors, and is beneficial to reduce the risks of auto-ignition and flashback. The radial dimension of the main recirculation zone increases first and then decreases along the axial distance. The reason is that after ejected from the injector, swirl air gradually develops to the wall surface with centrifugal force and the radial size range of reverse pressure gradient gradually expands. But after fully developing to a certain level, swirl intensity gradually decreases and radial size range of reverse pressure gradient gradually shrinks because of wall constraints and friction dissipation.

Then, this paper carried out research on the influence of key structural parameters (positions and diameters of fuel holes, length of premixing section) on fuel/air premixing characteristics by CFD method at 100% power operating conditions for a certain type of aero-derivative gas turbine. Besides, the fuel/air premixing characteristics were characterized by spatial mixing deficiency (SMD), the smaller the SMD value, the more uniform the fuel/air premixing. The results shows that the fuel penetration depth of blade basin is moderate and the fuel distribution range is wide, which can make full use of the swirling air turbulence and enhance premixing. So the SMD value of blade basin model is 0.0879, which is much lower than the inner hub model SMD value of 0.3901 and the outer hub model SMD value of 0.2735. In the premixing section, different diameters affects the fuel penetration depth by changing the fuel/air momentum ratio, and when diameter of fuel holes is 0.4mm, the fuel penetrates moderately and can premix well with swirl air. In addition, increasing the length of premixing section can promote further fuel/air premixing, but when the length of premixing section is increased to a certain extent, the premixing gains are relatively reduced.

This paper verifies the flowfield characteristics of LSI whose main recirculation zone is far away from the injector outlet, and obtains the influence law of positions and diameters of fuel holes, and lengths of premixing section on fuel/air premixing uniformity. The results can provide technical basis for LSI design and application in aero-derivative and land-based gas turbine combustors.