Direct numerical simulation of a turbulent lean premixed CH4/H2-Air slot flame

Nowadays, in the context of CO2 reduction and gas turbine fuel flexibility, the interest in acquiring knowhow on lean Hydrogen Enriched Natural Gas (HENG) is growing. This article provides a detailed analysis of two turbulent (Re-jet = 2476, Re-t = 236) lean (Phi = 0.7) CH4/H-2-air premixed slot flames (unconfined and at atmospheric pressure) highlighting the effects of two different hydrogen contents in the inlet mixture (20%and 50% by volume respectively for flames named A and B). The data were generated and collected setting up a three-dimensional numerical experiment performed through the direct numerical simulation (DNS) approach. Finite difference schemes were adopted to solve the compressible Navier-Stokes equations in space (compact sixth-order in staggered formulation) and time (third-order Runge-Kutta). Accurate molecular transport properties and the Soret effect were also taken into account. A detailed skeletal chemical mechanism for methane-air combustion, consisting of 17 transported species and 73 elementary reactions, was used. A general description of the two flames is provided, evidencing their macroscopic differences by means of turbulent consumption speed, flame surface areas and mean flame brush thickness. Furthermore, topological features of the flames are explored by analyzing the probability density functions of several quantities: curvature, shape factor, alignment between vorticity and principal strain rate vectors with flame surface normal, displacement speed and its components. Correlations between the flame thickness and the progress variable and curvature are also investigated, as well as correlation between strain rates and curvature, and equivalence ratio and curvature. An expression of displacement speed with diffusion terms taking into account differential diffusion of progress variable species components is derived. The effects of differential diffusion of several species on the local equivalence ratio are quantified: the maximum variation from the nominal inlet value is similar to 9% and it is due to H-2 and O-2. Increasing the hydrogen content the expected value and the skewness factor of the curvature PDF move towards negative values. The effect of thermal diffusion in thinning the flame front is greater for positive curvatures and at the trailing edge of the flame. The addition of hydrogen reduces the displacement speed at negative curvatures in a range that depends on the local progress variable value, with a maximum variation of -33% between the two flames. However, on average, increasing the hydrogen content increases the displacement speed in a wide range of progress variable values, i.e., the S-d in flame B is, on average, higher than in flame A by similar to 25% in 025 < c < 0.75. The database will also be helpful to validate sub-grid models for Large Eddy Simulation. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.