Visualization of pilot flame of an optically-accessible coaxially-staged aero-engine lean-burn fuel injector

Abstract

The visualization of the pilot flame of a coaxially-staged aero-engine lean-burn fuel injector, not only downstream but also inside of the pilot nozzle, was successfully performed at realistic aero-engine conditions. Optical access toward the inside of the nozzle was achieved through the inner and outer shrouds, both of which were made of transparent quartz. The image distortion caused by complex contours of the two shrouds was corrected by a method based on optical ray tracing, which realized precise determination of spatial intensity distribution of optical signals. Line-of-sight OH chemiluminescence, cross-sectional OH-LIF, kerosene LIF and kerosene Mie scattering were employed as diagnostic tools. The effects of pilot local air-to-fuel ratio on spray flame structure were revealed, both inside and downstream of the pilot nozzle under stable combustions. As the pilot mixture got rich, the main reaction zone moved from inside of the pilot nozzle to the region near the injector lip downstream of the injector exit. The OH-LIF signal was detected near the central axis surrounded by the fuel spray. It was also observed near the back-step of the pilot nozzle for the rich cases. The experiments under combustion oscillation were also conducted and the correlation of phenomena inside and downstream of the pilot nozzle was captured. It was clarified that the reaction enhancement in the outer part of the lip vortex region was caused by the convection of rich mixture, which appeared near the pilot atomizer lip at 150~210deg earlier oscillation phase angle. KeywordsAero-engine combustor, Optical burner/injector, Staged lean-burn injector, Laser induced fluorescence, Ray tracing, Optical refraction correction, Elevated temperature and pressure IntroductionFor reduction of NOx from aircrafts, coaxial-staging lean burn fuel injection systems have been studied intensively these days [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] and some of them have been already in service. Typically, they consist of a non-premixedmode pilot nozzle at the center for flame stability and a lean premixed-mode main nozzle surrounding the pilot for drastic NOx reductions. For such an injector, both fuel injection and fuel/air mixing start a few centimeter upstream from the injector exit (i.e. entrance of the combustion chamber). Moreover, this is also true for its pilot flame. Therefore, many important phenomena take place in regions enclosed by the metal walls of the injector, where the application of optical diagnostics is not straightforward compared with a conventional combustor. In our previous work, visualization of fuel spray/vapor behavior and fuel/air mixing inside the main nozzle (i.e. the premixing duct) was performed by high speed visualization and/or LIF techniques [10,11,15]. Through a fullycontoured quartz-made transparent outer-shroud, dynamic motion of fuel sprays under combustion oscillations at an elevated pressure and temperature was clearly captured and discussed in relation to the flame dynamics downstream [15]. ...