Experimental investigation of the ignition process in a separated dual-swirl spray flame

Centrally staged lean premixed pre-evaporation combustion technology has been used in aero engines for its capability in effectively reducing NOx emissions while still providing stable flames. Previous work has shown the “ignition delay” phenomenon in the forced ignition process, revealing that the burner ignition process is complex and needs further exploration. In this work, the spark ignition and flame kernel propagation in a centrally staged optical model combustor are experimentally investigated to further illustrate the interaction between random kernel motion and flow statistics from the perspective of timescales and kernel propagations. Stochasticity involved in each individual phases of the whole ignition is obtained via repeated high-speed imaging tests. Timescales of the successful ignition processes are analyzed in a statistical method, revealing the time randomness of ignition phases. Spatial probability of the flame kernel propagation in turbulent sprays is calculated and analyzed with the combination of time-averaged flow and spray distribution. The flame projected area evolutions show that the burner ignition phase can be further divided into three phases: flame propagation, flame residence and flame growth. Three modes of ignition failure are identified during the burner ignition phase. The flame residence phase features a special period of spark ignition with low light emissions from the highly wrinkled flame kernel in the center, which is the “ignition delay” before the growth of flame. Timescales of ignition stages involve stochasticity and the flame growth phase has a higher proportion in the whole process. Statistical analysis of the flame kernel spatial position shows that in turbulent spray the flame kernel has a greater probability of propagating to the region with low axial velocity, low shear strain rate, and appropriate spray concentration.