Quenching of particle-gas combustible mixtures using the electric particulate suspension (EPS) method
A new experiment referred to as EPS (electric particulate suspension) designed for quenching distance measurement of combustible powder suspensions has been taken from concept to working prototype. The method is validated in both normal gravity and microgravity.
Particle suspension concentration profiles were determined by either a moving single laser scan (in 1 g) or by an expanded laser sheet scan (in 1 and 0 g). Stratification of suspensions (asymmetry of concentration profiles due to gravity) was found in 1 g. Microgravity experiments carried out in the NASA Glenn Research Center's (NGRC) 2.2-second drop tower showed higher powder concentrations in microgravity compared to suspensions formed in normal gravity. Powder concentration profiles were confirmed to be symmetric at 0 g and a near-constant value under certain conditions. Abnormal suspension instability occurred in microgravity, observed as spatially periodic "cellular" powder flow structures at high electric field intensities.
A new external current method for determining the maximum suspension particle was introduced using EPS. A variety of particles showed a similar two-stage development-an initial increase stage of current followed by a constant current stage. The rising stage lends itself to a linear regression analysis. A new concept of excess electric field intensity was introduced to correlate the normal gravity and microgravity. Suspension concentrations in both normal gravity and microgravity were successfully correlated using this concept.
The range of ignition tests with EPS was significantly broadened to richer powder mixtures in microgravity. The nominal aluminum concentrations were corrected to the actual suspension aluminum concentrations using the excess field intensity concept. Some interesting newfound web/stringer formations, comprised of aluminum particles, were observed following powder ignition during drop tower experiments. The quenching effect of inert copper particles on aluminum/air mixtures was also investigated in normal gravity. It was confirmed that more ignition energy was required in the presence of inert particles. The average additional ignition energy difference was successfully correlated with ND2 (adopted from previous studies) using an exponential equation.