The relationship between vorticity/strain and reaction zone structure in turbulent non-premixed jet flames

Abstract

Simultaneous velocity and OH field measurements were made in unsteady laminar and turbulent nonpremixed planar jet flames to investigate the relationship between vorticity, principal strain rates, and reaction zone structure. Planar laser-induced fluorescence (PLIF) of the OH radical was used as an approximate reaction zone marker, while simultaneous two-component particle image velocimetry (PIV) provided the velocity field information. It is observed that in a turbulent flame, the thinnest reaction zones tend to align at 45° to the flow direction, which is orthogonal to the direction of the maximum principal compressive strain. Furthermore, the maximum compressive strain rates are typically an order of magnitude larger than the outer-scale strain rate, and it is not uncommon for the reaction zone to undergo strain that is significantly larger than the steady-state extinction strain rate. Diffuse OH zones are generally in regions of low compressive strain and low velocity gradients, and in some cases, they are observed to be aligned normal to the principal extensive strain direction. This suggests that the thick reaction zones result from broadening due to diffusion and extensive strain normal to the reaction sheet. Furthermore, some of the OH zones are associated with elongated regions of high vorticity, particularly in the highly laminarized reaction zones at low Reynolds number.