(203b) Influence of Particle Density on Pre-Ignition Dust Cloud Dynamics in a Minimum Ignition Energy Testing Apparatus Using Digital In-Line Holography

The ASTM standard for the measurement of minimum ignition energy (MIE) of a dust cloud in air (E2019) calls for the generation of a uniform, non-turbulent (stable) dust cloud of known concentration at the time of ignition in the 1.2L Hartmann tube. The dust dispersion system employed in the MIE apparatus, generally a Kühner MIKE3 device, uses a fixed pneumatic conveyance with a pressure of 7 bar. Ignition spark delay, which is the time from the initiation of dispersion to the point of ignition, is an adjustable parameter. However, most operators usually keep this value at 120 ms. Combustible industrial dusts have a wide variety of particle density. The cloud dynamics (particle velocity, concentration, turbulence) for dusts of different particle density are expected to vary at the time of igniton when dispersed by the pneumatic system. The change in cloud dynamics can affect the MIE testing conditions.

In this study, soda lime glass and polymethyl methacrylate (PMMA), having a ratio of particle density of 2:1, are dispersed in the Kühner MIKE3 apparatus while decoupling parameters such as particle size distribution and shape. A high-speed digital in-line holography (DIH) imaging system is used to obtain measurements of the velocity field, particle size, and concentration in three dimensions around the ignition zone for each dispersed dust sample.

The quantitative results obtained showcase the pre-ignition cloud dynamics and establish dusts of different particle densities behave differently in the ignition region and at the typically selected ignition delay time. Hence, to obtain a consistent dust cloud for optimum MIE measurements in terms of turbulence and concentration around the ignition zone, use of different ignition delays is warranted for the dusts of different particle densities.