Experimentelle Unteruschung zur Strömungsakustik beschleunigt bewegter brennender Tropfen (English shop)

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This work presents an experimental investigation of the kinematics of accelerating, freely falling burning droplets in air. It combines two fields that are frequently considered independently of one another: aerodynamics in the range 0 < Re < 1500 and flame kinetics including combustion and evaporation. The complexity of the processes arises from a nonlinear interaction between the two domains, in which the relative weighting of their respective influences changes considerably during the falling motion. At the beginning of the fall, the influence of the flame predominates; after the transition to a “wake flame”, the flame merely modifies what occurs aerodynamically at the droplet. The droplet wake, and hence also the flame of a burning falling droplet, passes through a cascade of instabilities on the way from laminar to turbulent flow around the droplet. In the course of the investigations, four different types of flame structure could be distinguished and the characteristic similarity parameters required to delimit them were determined. Contrary to previous publications, the transition from an envelope flame to a wake flame does not occur at a characteristic Reynolds number but at a fuel-specific Froude number, which could be demonstrated with a single experiment. Under certain conditions, a phenomenon not previously mentioned in the literature occurs: the emission of acoustic energy. In addition to the prerequisites for this, the sequence and thus the cause of the process of sound emission could also be clarified.