Stretched laminar flamelet modeling of turbulent chloromethane-air nonpremixed jet flames

An experimental and numerical investigation of a nonpremixed turbulent flame burning chloromethane (CH3Cl) in air is presented. Finite-rate chemistry plays an important role in halogenated flames due to the inhibitory effect of halogens on hydrocarbon combustion. The objective of the study is to assess the applicability of the stretched laminar flamelet (slf) model that accounts for finite-rate chemistry and differential diffusion effects. The slf approach is a convenient tool to incorporate these effects into computations in view of its largely lenient assumptions, and its ease of inclusion into already existing codes based on the popular k-epsilon-g turbulence modeling. In the experimental set-up, a flame is established at Re = 11700. Velocity measurements are made using laser doppler velocimetry, species measurements by means of gas chromatography, and temperature measurements by thermocouples. A library of computationally obtained laminar flamelet profiles is used in the slf calculations. For sake of comparison, a single flamelet profile is also determined experimentally; notwithstanding the detailed chemical description adopted in the computational model, significant discrepancies are evident, possibly indicative of the weight of neglected effects (sooting, radiation, etc.). The applicability of the slf approach to the turbulent flame is checked by comparing the characteristic time of energetically significant reactions to the characteristic turbulent time scales, and the results show that the flame under study operates in the flamelet regime. Predictions for the turbulent flame indicate that the slf model gives improvement for predictions of the velocity, temperature, and concentrations of reactive species, as well as of the conserved scalar (which is affected by finite-rate chemistry through the effect of flame extinction on the mixture density), with respect to a near-equilibrium model. In particular, the location of temperature and concentration peaks are closely reproduced. An improvement is obtained regarding predictions of the concentrations of the important species CO, HCl, CH3Cl, O-2, and N-2. At any rate, due to remaining discrepancies, further investigation is called for to include insofar neglected effects.