Quenching Combustible Dust Mixtures Using Electric Particulate Suspensions (eps): a New Testing Method for Microgravity

The Electric Particulate Suspension (EPS) is a combustion ignition system being developed at Iowa State University for evaluating quenching effects of powders in microgravity (quenching distance, ignition energy, flammability limits). Because of the high cloud uniformity possible and its simplicity, the EPS method has potential for ‘benchmark’ design of quenching flames that would provide NASA and the scientific community with a new fire standard. Microgravity is expected to increase suspension uniformity even further and extend combustion testing to higher concentrations (rich fuel limit) than is possible at normal gravity. Two new combustion parameters are being investigated with this new method: (1) the particle velocity distribution and (2) particle-oxidant slip velocity. Both walls and (inert) particles can be tested as quenching media. The EPS method supports combustion modeling by providing accurate measurement of flame-quenching distance as a parameter in laminar flame theory as it closely relates to characteristic flame thickness and flame structure. Because of its design simplicity, EPS is suitable for testing on the International Space Station (ISS). Laser scans showing stratification effects at 1-g have been studied for different materials, aluminum, glass, and copper. PTV/PIV and a leak hole sampling rig give particle velocity distribution with particle slip velocity evaluated using LDA. Sample quenching and ignition energy curves are given for aluminum powder. Testing is planned for the KC-135 and NASA’s two second drop tower. Only 1-g ground-based data have been reported to date. EPS Design An electric particulate suspension (EPS) utilizes a high voltage electric field to disperse a powder of semi-insulating or conductive material in a cloud of (oppositely) charged particles. The resulting steady state suspension is subsequently ignited by a spark discharge from a stationary (or moving) needle electrode, Fig.1. Particle motion in the direction of the electric field is confined between two parallel plate electrodes with diffusive motion in the horizontal direction confined by a Pyrex cylindrical retainer. High voltage capacitors provide the necessary energy for the spark. Wall quenching tests involve adjusting the height distance between the parallel plate electrodes and testing for flame propagation following the spark. Quenching by inert powders is accomplished by suspending various amounts of copper or glass particles with a combustible powder or gas mixture and igniting the mixture. Acoustic vibration is utilized to aid the suspension of cohesive particle such as coal dust or other fine particles (Fig. 1). The centered spark kernel produces a cylindrically outward propagating flame. Fig. 3 shows a photograph of quench (batch) test of aluminum powder 25-35 μm using the system in Fig. 2. Powder quenching Powder (wall) quenching studies carried out to date at 1-g include the measurement of quenching and ignition energy curves for aluminum powder and coal dust in various admixtures of oxygen, nitrogen, and carbon dioxide at ambient conditions of temperature and pressure. Example quenching and ignition curves for coal dust and aluminum powder are shown in Figs. 3 and 4 respectively, using the setup of Fig. 1. Aluminum powder 25-30 μm in 173 NASA/CP—2003-212376/REV1 https://ntrs.nasa.gov/search.jsp?R=20040053552 2018-03-25T17:37:07+00:00Z