Flame transitions and stabilization mechanisms in a freely falling fuel droplet encountering a co-flow*

The moving burning droplet is an interesting area of fundamental research that gives insight into spray combustion, where burning droplets encounter various flow velocities. Hence, the flame dynamics of a contactless burning droplet under free fall have been investigated in a drop tower facility, which is subjected to a co-flow. The droplet flame responds to the relative flow coherently but when it encounters a co-flow, the flame is no longer in equilibrium and tries to readjust till it equilibrates with the surrounding co-flow. In the initial readjustment period, the flame is not responding coherently with the co-flow, however, after it equilibrates the flame shape evolves based on the relative velocity. The droplet flame is observed to transition between different configurations like wake flame, reversed wake flame, and enveloped flame. In the wake, the flame is observed to stabilize based on triple-flame or bluff-body stabilization mechanism based on the relative flow velocity. The flame stabilization criteria and shape evolution are characterized based on the flow characteristics and transitions have been explained. Using a mathematical formulation based on the spring-mass system analogy, the evolution of droplet flame is predicted for different co-flow conditions imposed.