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Lean combustion has been applied in gas turbine combustors to meet increasingly stringent regulations for pollutant emissions. As a representative low-emission technology, stratified partially premixed (SPP) combustion is capable of effectively reducing NOx emissions while still providing robust flames. In this work, the flow and spray processes inside an SPP combustor were experimentally investigated to further reveal the role of main stage swirl in flow development and fuel placement. Particle image velocimetry (PIV) and Planar Mie scattering (PMie) techniques were applied to provide the velocity distributions and droplet distributions under isothermal conditions with a relative pressure drop ranging from 1% to 5%. A spray post-processing algorithm was developed to acquire the characteristic parameters of the sprays without clear boundaries. Results show that the strong swirl from the main stage leads to the radial flow of pilot air streams and dominates the formation of central toroidal recirculation zone (CTRZ). With the addition of the main stage swirl, a proper flow structure including jet zones, recirculation zones and shear layers is formed, which promotes the widening of droplet radial distribution and the increase of the spray cone angle. The spray distribution therefore transforms from a bubble shape into a cone shape. The findings and conclusions could promote the understanding of interactions between pilot and main stage swirls for stage combustors and contribute to the optimization of such SPP injectors.

In this paper, the optimization method we obtained from dual-orifice atomizers previously is used to design and optimize new dual-orifice atomizers, whereas there are some differences between the new dual-orifice atomizer and dual-orifice atomizer used in Part I. For example, the mass flow is much smaller, there is an expansion angle at pilot nozzle to regulate pilot stage spray cone angle, and there is no recess length between main nozzle and pilot nozzle. Influences of structure parameters on mass flow, spray cone angle and liquid film fusion and separation are investigated, which are consistent with the expectation. Structure parameters that meet performance requirements of dual-orifice atomizer are analyzed. In addition, a new phenomenon has been found is that liquid film oscillation appears with the increase of Δ P, which should be avoided during the design and optimization of new atomizers. Pilot liquid film oscillation will influence the development of dual-orifice liquid film. Pilot swirling groove depth and expansion angle of pilot nozzle are key parameters that influence liquid film oscillation. Conclusions in this paper can be used to guide the design and optimization of new dual-orifice atomizers.