Numerical analysis of the unsteady transition of multiple laminar methanol/air spray flame structures in the counterflow configuration

The counterflow configuration is a well-established setting to study the structure of laminar spray flames. Typically, the similarity transformation is introduced to transfer the two-dimensional gas-phase equations into one-dimensional governing equations. The equations which describe droplet motion are two-dimensional. For spray flames in the counterflow configuration, Continillo and Sirignano suggested that more than one numerical solution of these equations may exist, which meanwhile was confirmed by several studies. Up to three different structures were found for the same boundary conditions for the combustion of fuel sprays carried by air directed against an air stream. The first structure shows two chemical reaction zones one of which resides on the fuel side and the other one on the air side of the configuration. The other two spray flame structures show single chemical reaction zones that reside on one but different side of the stagnation plane. The present paper investigates the unsteady transition mechanisms between multiple methanol/air spray flame structures in the counterflow configuration. Perturbations are introduced to key parameters including gas strain rate, initial droplet size, and the equivalence ratio, to systematically analyze the physicochemical mechanisms that underlie the transitions between the different flame structures. The transitions, in general, result from the complex interplay between the spray vaporization and motion, which includes droplet reversal and oscillation, and the chemical reactions. The timescales of the transitions between the three different spray flame structures are analyzed in detail in the present paper. • Unsteady transitions of multiple counterflow spray flame structures are analyzed. • The interplay of vaporization, droplet motion, and reactions drives the transitions. • Transitions between single and double chemical reaction zones and vice versa may occur. • Transition timescales depend on the characteristic flow time or inverse strain rate.