The effects of differential diffusion in counter-flow premixed flames with dilution and hydrogen enrichment

Abstract

The effects of differential diffusion on local flamelet velocities, turbulent burning rates, and structure of lean turbulent premixed flames in the thin reaction zone regime are investigated using aerodynamically stabilized flames in a counter-flow apparatus. Various fuel–oxidizer–inert mixtures with different transport properties, representative of distinct effective Lewis numbers, are studied. In order to minimize the effects of mixture reactivity in these experiments, unstretched laminar flame speed is kept constant during mixture dilution, and hydrogen enrichment of hydrocarbon flames, through changing the mixture equivalence ratio. Furthermore, bulk-flow properties and stagnation surface temperature are kept constant; hence, the study focuses on the effects of differential diffusion, which is the change in transport properties of the mixture, i.e., fuel and heat diffusivities, in the context of fuel flexibility. Highly strained laminar flame measurements are also reported as a reference of comparison. Local instantaneous statistics of various flame parameters within the imaged plane, such as flame location, flame velocity, and flame-front topology, are quantified using high-speed particle image velocimetry (PIV) and Mie scattering flame tomography at two levels of turbulence intensity. These parameters are presented as probability density functions using sufficiently large data sets to ensure statistical accuracy. The results for various flame-parameter statistics, which are measured over a wide range of Lewis numbers, show that the effects of differential diffusion are important in turbulent flames in the thin reaction zone regime. At constant turbulence intensity, differential diffusion increases the burning rates of turbulent flames in thermo-diffusively unstable mixtures through two main mechanisms: (1) increasing local flamelet displacement velocity, and (2) increasing flame-surface area. The relative contribution of these two parameters in increasing turbulent burning rates is approximately 76% and 24%, respectively, which is not dependent on the fuel, oxidizing-gas mixture, or turbulence intensity, and the results overlap over a wide range of Lewis numbers.