Ignition of combustible mixtures by hot particles at varying relative speeds

ABSTRACT

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.

KEY WORDS:

• Spark Ignition
• Hot Moving Particles
• Detailed Numerical Simulation
• Openfoam

Additional information

Funding

This work was supported by the German Research Foundation (DFG) through Research Units DFG-BO693/30-1 and HA3497/2-2 “Ignition by Mechanical Sparks”. This work utilizes resources from the national supercomputer Cray XC40 Hazel Hen at the High Performance Computing Center Stuttgart (HLRS) and the computational resource ForHLR II funded by the Ministry of Science, Research and the Arts Baden-Württemberg and DFG (“Deutsche Forschungsgemeinschaft”).