Analysing flammability limits of hydrogen-air flames using molecular level simulations*

A novel molecular simulation technique, called the Direct Simulation Monte Carlo (DSMC) method, is used to simulate one-dimensional hydrogen-air flames. In this method, the dynamics of the real molecules are emulated with the help of the simulation particles, and their averages provide the macroscopic flow properties. The Quantum-Kinetic (QK) model, which uses total collision energy, molecular dissociation energy, quantized vibrational energy levels and the principle of molecular reversibility, is employed to account for the reactions. A detailed 21-step reaction mechanism is utilised for describing combustion. Systematic variations in the volume fraction of hydrogen are performed in order to evaluate the flammability limit of the hydrogen-air mixture as well as the corresponding flame speed and flame temperature. The results are compared with the experimental data from the literature. The hydrogen volume fractions corresponding to the lean and rich flammability limits are found to match closely with those from previous experimental data. Also, the temperature and flame speeds for the range of cases show good agreement with the measurements. However, some differences are still present which can be mitigated as more accurate molecular-level and/or ab-initio data become available. A distinct advantage of DSMC is that, because it uses molecular level simulations, it avoids using continuum Arrhenius reaction rates and simplified gradient laws for the diffusivities of mass, momentum and heat. This technique is useful in cases when the continuum assumptions or empirical formulations do not hold true.