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This study addresses the influence of several operating conditions (composition and ambient pressure) on biogas diffusion flame structure and NO emissions with particular attention on thermal and chemical effect of CO2. The biogas flame is modeled by a counter flow diffusion flame and analyzed in mixture fraction space using flamelet approach. The GRI Mech-3.0 mechanism that involves 53 species and 325 reactions is adopted for the oxidation chemistry. It has been observed that flame properties are very sensitive to biogas composition and pressure. CO2 addition decreases flame temperature by both thermal and chemical effects. Added CO2 may participate in chemical reaction due to thermal dissociation (chemical effect). Excessively supplied CO2 plays the role of pure diluent (thermal effect). The ambient pressure rise increases temperature and reduces flame thickness, radiation losses and dissociation amount. At high pressure, recombination reactions coupled with chain carrier radicals reduction, diminishes NO mass fraction.

A computational study of mixing process and air entrainment in hydrogen turbulent non-premixed flame characterized by strong gradients of velocity and density at the inlet section is presented. Different approaches for turbulence–combustion interactions are evaluated in the framework of RSM (Reynolds Stress Model) turbulence model and the computational results are compared to experimental data. The combustion models investigated are SLFM (Steady Laminar Flamelet Model) and EDC (Eddy Dissipation Concept). Mixing is described by oxygen atom mixture fraction and air entrainment is characterized by gas mass flow rate. Computational results are compared to measurements in physical space at two locations (the first one represent the near-field region and the second one the far-field region). At the first station, the results showed an overestimation of mixing and air entrainment and an inaccurate consumption of O2 and H2. In addition, the predictions are found to be sensitive to combustion modelling. At the second station, the description of mixing and air entrainment is improved and the predictions are in reasonably agreement with experimental data. Less dependency to combustion modelling is noticed in this location. Further analysis of the near-field region based on the turbulence time scales revealed that turbulence is not well developed in this region of the flame.

The aim of the present study is to assess numerically the capability of two turbulence approaches, Reynolds averaged Navier-Stokes equations (RANS) and large eddy simulation (LES), in predicting mixing in hydrogen–hydrocarbon turbulent non-premixed flame. The near field region of this flame is characterized by a high density ratio and a high velocity ratio between the air coflow and the fuel which makes difficult mixing modelling in this zone. This is important as flame stabilization and pollutant formation occurs in this region. In addition, differential diffusion effects may exist. The turbulence RANS models selected are Realizable κ–epsilon and Reynolds stress model, respectively. LES is performed with Smagorinsky sub-grid model. Mixing is described using mixture fraction and its variance. Mixture fraction characterizes the mixing state between fuel and oxidizer while the scalar fluctuation intensity reflects the effect of unmixedness. The effect of differential diffusion on mixing is not taken into account as the target here is mixing modelling. The predictions indicate that the RANS turbulence models applied fail to predict the initial stages of mixing while reasonable description is achieved with LES.