Flame stabilization and OH fluorescence mapping of the combustion structures in the near field of a spray jet

Abstract

Phase-doppler velocimetry and two-dimensional laser-induced florescence of OH have been used to investigate flame stabilizating in the near field of a spray jet of methanol above an air-blast injector. Comparison between the size-classified velocity fields and the scatter plot of lift-off locations reveal the essential role played by the large-scale mixing structures, in which the low inertia part of the spray has enough time to vaporize and mix with entrained air before burning. An analysis has been developed within the low Stokes flow to derive large-scale coupling functions in the region of flame stabilization. In this frame the stoichiometric mixture fraction, where combustion is possible, is determined by the curvature of the fuel mass fraction profile which in turn increases with the elevation above the injector. A vaporization regime ( τ ch < τ vap < τ mix) of flame stabilization has been analyzed along with its lower and upper limits, due to atomization and dynamic requirements respectively. In this stabilization regime the two-phase flame develops with the remarkable structure of two diverging fronts. Experimental data on the height of flame liftoff and its statistical distributions along with the structures of the stabilized flames, are in good agreement with predictions. However, this approach does not apply to the main part of the spray, which feeds progressively the combustion at downstream distances and in different regimes.