Development of electrically charged extinguishant dispersions for use in microgravity

Abstract

Methodologies for electrically charging and spraying dispersions of extinguishant droplets and particles are developed, with a view to subsequent control of particle trajectories by electric fields, for use in microgravity environments. Specific charges on water droplets and sodium bicarbonate particles, both of about 100 µm diameter, are measured at mass flow rates of about 35 mg/s. These substances were chosen for their lack of toxicity, before or after passage through combustion zones, as compared to other extinguishants. The small mass fluxes were geared to the burning intensities at the slow rates of oxygen supply by the ventilation airflow in spacecraft. To assess their effectiveness as extinguishers, the dispersion plumes were traversed by a small test burner stabilizing a diffusion flame, or smoldering combustion in a plug of cellulosic fibers, to map out the extinction regions. Sodium bicarbonate is particularly effective at extinguishing flames, although ineffective for smoldering, which proves to be the more exacting problem--the area of the extinction "footprint" at maximum range is approximately 4800 mm 2 , which is equivalent to less than 0.73 mg/cm 2 at the mass flow rate employed. The mass of water required for extinction of smoldering in less than 2 s, for an earthed burner, corresponds to a water film thickness of less than 30 µm. With m 10 kV applied to the burner, the extinction regimes are extended by 63% along the spray axis. Consequences for zero g were inferred from results for initially horizontal dispersions falling under gravity by subtracting out the average free-fall velocities. The conclusions and circuitry required have been oriented toward the concept of a small handheld, battery-powered extinguisher fitted with a compact gas cartridge to be deployed by sweeping the dispersions over the area of an incipient fire or smoldering zone.