Identification of droplet burning modes in lean, partially prevaporized swirl-stabilized spray flames

Experimental and numerical investigations of single droplet burning modes in a lean, partially prevaporized swirl-stabilized spray flame are reported. In the experiment single droplet flames have been visualized by CH-PLIF and simultaneous recording of the Mie signal. Two single droplet burning modes were identified: the envelope flame is a spherical diffusion flame burning at near-stoichiometric conditions. The wake flame is a potentially lean, partially premixed flame located downstream of the droplet. The droplet burning mode is of practical relevance, since it has significant impact on NO formation due to incomplete prevaporization.The droplet burning mode is determined by the ratio of chemical and convective time scales. The convective time scale is related to the droplet slip velocity. The impact of turbulent gas phase velocity fluctuations on droplet mechanics and droplet burning is discussed, based on a previous numerical investigation. In the present study the droplet slip velocity was measured with the 3D Phase Doppler (3D-PD) technique. For the measured slip velocities and ambient conditions in the hot gas region of the spray flame, simulations of single droplet burning were performed utilizing detailed models for chemical reaction, diffusive transport and vaporization. An agreement between the droplet burning modes predicted by the simulation and the droplet burning modes observed in the experiments was found.