Effective complex permittivity of a weakly ionized vegetation litter fire at microwave frequencies

Abstract

Thermal ionization of alkali species emitted from thermally decomposing vegetative matter into the combustion zone of a fire makes the zone a weakly ionized gaseous medium. Collision between the medium electrons and neutral flame particles is a dominant form of particle interaction and incident microwave energy absorption process. Electromagnetic wave absorption properties of vegetation fire have implications for the safety of fire fighters during wildfire suppression where communication blackouts have been experienced. Propagation characteristics of electromagnetic waves in a vegetation fire could be deduced from its relative dielectric permittivity. In the experiment, a controlled fire burner was constructed where various dried natural vegetation could be used as fuel. The burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality network analyser to determine effective complex permittivity from scattering parameters. A controlled vegetation fire with a maximum flame temperature of 1050 K was set in the burner and X-band microwaves (8.0–9.6 GHz) were made to propagate through the flame. For the flame, at temperatures of 800 and 1015 K, imaginary and real components of effective complex dielectric permittivity were measured to range from 0.113 to 0.119 and from 0.898 to 0.903, respectively.