Structure of equilibrium reaction rate fields in turbulent jet diffusion flames

The structure of instantaneous chemical reaction rate field ω_i(x,t) in turbulent jet diffusion flames is examined from nonreacting flow data in the limit of full chemical equilibrium. The equilibrium reaction rate fields are obtained from fully-resolved planar Rayleigh scattering imaging measurements of the instantaneous conserved scalar field ζ(x,t) in the self-similar far field of a turbulent jet flow. The resolution attained reaches below the finest local lengthscales λ_v and λ_D on which gradients in the vorticity and scalar gradient fields, respectively, can be sustained by the competing effects of strain and diffusion in the flow. Together with the signal quality achieved this allows accurate differentiation of the instantaneous scalar field data to determine the corresponding structure of the instaneous scalar energy dissipation rate fields (ReSc)^-1∇ζ·∇ζ(x,t). The self-similar far-field scaling of this flow allows these data to be mapped to any downstream location, permitting a composite construction of the instantaneous equilibrium reaction rate fields throughout a turbulent jet diffusion flame. Results presented for hydrogen-air chemistry indicate that the individual species reaction rate fields are composed entirely of layer-like reaction structures. The strain-diffusion balance which sets the thickness of these structures precludes the presence of any turbulent motions within them. Moreover, whereas direct imaging measurements of OH concentration fields have suggested the presence of broad homogeneous reaction zones near the flame tip, the present OH reaction rate fields show that the reactions remain confined to these locally layer-like structures throughout the entire length of the flame.