Space charge generated by wind tunnel fires

Abstract

Plume clouds resulting from wildland fires (pyrocumulus) sometimes produce lightning discharges to ground. These discharges were found to carry positive charge to ground exclusively in several cases. Emission of space charge was observed from woody materials burned in the presence of an external electric field. Measurements done in a large wind tunnel for this study confirm and extend measurements made in open air. The net charge given up by the fire in the wind tunnel has sign appropriate to reduce an applied electric field (negative for the earth's fair weather field), and magnitude directly proportional to the magnitude of the applied electric field and the fuel consumption: Q=0.034+0.0015 E, where Q is the net charge liberated in nC g −1, and E is the applied electric field in V m −1 (positive upward). There is a weak dependence of the net charge on wind speed, probably due to wind tunnel airflow characteristics. The net charge in the smoke is a small difference between large amounts of charged ions of both signs liberated by the fire. For fires burning under fair-weather electric fields, the amount and sign of charge released by the mechanism studied cannot be the direct cause of anomalous lightning from pyrocumulus.