The numerical investigation of quenching distances in laminar flows is mainly concerned with two setups: head-on quenching (HOQ) and side-wall quenching (SWQ). While most of the numerical work has been conducted for HOQ with good agreement between simulation and experiment, far less analysis has been done for SWQ. Most of the SWQ simulations used simplified diffusion models or reduced chemistry and achieved reasonable agreement with experiments. However, it has been found that quenching distances for the SWQ setup differ from experimental results if detailed diffusion models and chemical reaction mechanisms are employed. Side-wall quenching is investigated numerically in this work with steady-state 2D and 3D simulations of an experimental flame setup. The simulations fully resolve the flame and employ detailed reaction mechanisms as well as molecular diffusion models. The goal is to provide data for the sensitivity of numerical quenching distances to different parameters. Quenching distances are determined based on different markers: chemiluminescent species, temperature and OH iso-surface. The quenching distances and heat fluxes at the cold wall from simulations and measurements agree well qualitatively. However, quenching distances from the simulations are lower than those from the experiments by a constant factor, which is the same for both methane and propane flames and also for a wide range of equivalence ratios and different markers. A systematic study of different influencing factors is performed: Changing the reaction mechanism in the simulation has little impact on the quenching distance, which has been tested with over 20 different reaction mechanisms. Detailed diffusion models like the mixture-averaged diffusion model and multi-component diffusion model with and without Soret effect yield the same quenching distances. By assuming a unity Lewis number, however, quenching distances increase significantly and have better agreement with measurements. This was validated by two different numerical codes (OpenFOAM and FASTEST) and also by 1D head-on quenching simulations (HOQ). Superimposing a fluctuation on the inlet velocity in the simulation also increases the quenching distance on average compared to the reference steady-state case. The inlet velocity profile, temperature boundary condition of the rod and radiation have a negligible effect. Finally, three dimensional simulations are necessary in order to obtain the correct velocity field in the SWQ computations. This however has only a negligible effect on quenching distances.

Detailed numerical simulations have been performed to study the ignition behavior of hot spherical particles at atmospheric conditions. The particles move relative to a combustible gas with a velocity of 0–30 m s ${ }^{-1}$, which spans different flow regimes, from creeping flow to unsteady vortex shedding. The temperature of the particles’ surface increases linearly over time and is recorded at ignition $(T_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f},\mathrm{i}\mathrm{g}\mathrm{n}})$ for methane/air and hydrogen/air mixtures. For low relative velocities $v<5$ m s ${ }^{-1}$ or Reynolds numbers $Re<200$, $T_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f},\mathrm{i}\mathrm{g}\mathrm{n}}$ increases proportionally to $R{e}^{1/2}$ or $v^{1/2}$ and the flow field is axisymmetric. For higher relative velocities, an unsteady vortex street forms behind the particle so that three-dimensional simulations are required. A correlation employing the van’t Hoff criterion yields linear correlations based on the Nusselt number and $T_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f},\mathrm{i}\mathrm{g}\mathrm{n}}$ for both the low- and high-velocity ranges. For rich hydrogen flames at high velocities, the flame temporarily stabilizes near the hot particle in the recirculation zone downstream. As the surface temperature increases further, the flame suddenly starts to propagate downstream, leading to two distinct ignition events: local ignition at the particle’s surface and start of the propagation into the surrounding gas. The latter yields a much steeper increase of ignition temperature with incoming flow velocity.

This study presents the simulation and detailed analysis of the ignition of initially quiescent fuel/air mixtures by small, stationary, laser-heated spherical particles. Our simulations cover a wide parameter space by varying the kind of fuel, stoichiometry, heating rate, radical surface destruction efficiencies as well as particle size. The results agree well with experimentally determined particle surface temperatures at the time of ignition over the whole range of parameters. The surface temperatures required for ignition strongly depends on the kind of fuel and increases in the order hydrogen, acetylene, ethylene, ethane, propane and methane. It also increases with decreasing particle size. By contrast, mixture stoichiometry and heating rate have a minor influence on the ignition temperatures. Comparisons with two-dimensional direct numerical simulations show that fast, but fully coupled one-dimensional simulations are sufficient to capture the details of the ignition event, permitting a systematic investigation for large number of conditions. At small particle radii (r≤2 mm) there exists a simple mapping of only two parameters, an apparent activation energy and a factor comprising thermo-physical properties of the gas phase that is able to estimate the particle surface temperature required for ignition. Such a map might be used for the safety assessment of ignition hazards by small hot particles as function of fuel, stoichiometry and particle size.