Ignition by Mechanical Sparks: Ignition of Hydrogen/Air Mixtures by Submillimeter-Sized Hot Particles

Abstract

To broaden the knowledge concerning ignition processes, small, hot, metal/ceramic particles were heated in a quiescent mixture of hydrogen/synthetic air mimicking the behavior of mechanical sparks, for example, from grinding processes. Materials under investigation were silicon nitride, tungsten carbide, and steel types 1.3505, 1.4034, and 1.3541. Their diameters ranged between 300 μm and 800 μm. We measured the particle surface temperature required to achieve ignition of the gas phase. Influences of mixture stoichiometry, particle material, and particle size were investigated.

Concerning particle size, it was found that smaller particles require higher temperatures to ignite the gas atmosphere. Also, the particle material had a profound effect on the temperature required to achieve ignition. Mixture stoichiometry was varied between 5%_v and 60%_v hydrogen, revealing minor influence on the particle temperature at ignition, which was attributed to the high thermal conductivity of hydrogen. Numerical simulations of the ignition process yield surface temperatures of similar magnitude. A minimum gas phase ignition energy density of 0.25 MJ/m³, independent of particle size and gas phase composition, has been derived based on the simulated temperature profiles and by defining an “ignition relevant” volume around the particles.

Keywords:

• Hot particles
• Hydrogen
• Ignition
• Mechanical sparks