Controlling mechanisms for low NO x emissions in a non-premixed stagnation point reverse flow combustor

Abstract

The controlling processes in the operation of a non-premixed, stagnation point reverse flow (SPRF) combustor are investigated. The combustor consists of a central injector at the single open end of a cylindrical chamber. The injector inlet area is much less than the area through which exhaust products leave. The SPRF combustor operates stably at low equivalence ratios without external preheating or swirl, and produces low NO x emissions in both premixed and non-premixed modes of operation. Non-intrusive imaging diagnostics are used to understand the combustor operation. Simultaneous Planar Laser-Induced Fluorescence (PLIF) imaging of OH radicals and chemiluminescence imaging are used to characterize the reaction and heat release zone and the flame products. Separate measurements with Particle Image Velocimetry (PIV) provide the reacting velocity field, and elastic laser sheet scattering from liquid droplets seeded into the fuel characterize its mixing. The velocity and chemiluminescence data indicate the flame is stabilized in a region of low mean and high rms velocity. Together with some product entrainment, this enables stable operation of the combustor at very lean overall equivalence ratios. The non-premixed mode of operation is found to be similar to the premixed case in many ways. Though, in non-premixed operation, the flame is lifted well away from the injector, so that significant air and fuel premixing occurs before combustion. Similar NO x emission for both operating modes is attributed to efficient mixing of nearly all the fuel and air before burning. This is confirmed through a combination of oil droplet results, OH PLIF comparisons and laminar flame modeling. The latter indicates entrainment of products is not directly responsible for low NO x emissions at a given overall fuel–air ratio, but rather is likely to contribute to a slight increase in NO x levels compared to a system with no product entrainment.