Numerical study of the multi-flame structure in spray combustion

Abstract

The multi-flame phenomenon often seen in spray combustion was studied via consideration of two cases from the Delft Spray in Hot Coflow database which have the same fuel (ethanol) but differ in the type of coflow. The two-phase flow was handled with an Eulerian-Lagrangian approach and Large Eddy Simulation with Flamelet Generated Manifolds method was used to study the turbulence and combustion. Simulation results revealed that four spatially separated reaction regions exist in the flame A_II and two in the flame H_II. These regions are of different types premixed or non-premixed and are formed by different species major fuel (ethanol) or intermediate species e.g. CO. The mechanism underlying the multi-flame structure was investigated in terms of droplet evaporation dispersion convection and reaction. Parametric studies on the effect of spray polydispersity and coflowing air temperature demonstrates an even wider range of flame structures. The 'single flame' structure usually present in the hot-diluted coflow case (H_II) was also generated in a case of room temperature air coflow provided the injected droplets are small since representative droplets in all cases with single flame have similar evaporation time scale.