Microwave attenuation in forest fuel flames

Abstract

The flames of forest fuels form a weakly ionized gas. Assuming a Maxwellian velocity distribution of flame particle and collision frequencies much higher than plasma frequencies, the propagation of microwaves through forest fuel flames is predicted to have attenuation and phase shift. A controlled fire burner was constructed where various natural vegetation materials could be used as fuel. The burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality network analyzer to measure phase and attenuation. The controlled fires had temperatures in the range of 500-1000 K and microwave attenuation of 1.0-4.5 dB m −1 was observed across the 0.5 m diameter cavity. Attenuations of this magnitude could affect active remote sensing systems signals at microwave frequencies in forest fire environments where flame depths of up to 50 m are possible. In the experiment, temperature was not the only controlling parameter for the ionisation; type of fuel burnt also influenced it. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) analysis of the composition of the fuel confirmed that a higher content of alkali (with low ionization potential) lead to higher electron densities. Electron densities in the range of 0.32-3.21 × 10 16 m −3 and collision frequencies of 1.1-4.0 × 10 10 s −1 were observed for flames with temperature in the range of 730-1000 K.

Microwave spectra inside the combustion chamber of a kraft recovery boiler – effects on communications

“…Of the organic compounds found in black liquor, sodium and potassium are alkaline, and thus have low excitation energy. Consequently, they are highly combustible and when ignited create extra ionisation, which affects radio and microwave propagation, both attenuating and changing the phase of the signal [18].…”

“…Attenuation due to the medium, although indirectly affecting the measured spectrum, is omitted from this study. The attenuation of radio waves in flames at lower frequencies has been extensively studied in many articles [16][17][18].…”

Microwave attenuation in kraft recovery boiler

“…With flames burning 10 days dried forest fuel, the attenuation was ranging at frequency band 10-12.5 GHz from 9 to ∼6 dB/m with eucalyptus litter, from 7 to ∼4.8 dB/m with grass, and 4 to ∼3 dB/m with pine litter, respectively. The results were measured at temperatures of 730-1000 K [8]. At higher frequencies from 30 to 60 GHz, at temperatures of 1000 K, the attenuation in scrub fires ranged from 0.06 to 0.7 dB/m.…”

“…The attenuation in combustion environment consists of typical scattering mechanisms such as attenuation due to free space loss [2,3], reflections [4], refraction [5], and multipath propagation [6,7]. In addition to these scattering mechanisms, attenuation due to charge collisions and ionisation exist in high-temperature flames [8].…”

Electrical conductivity of the thermal dusty plasma under the conditions of a hybrid plasma environment simulation facility

“…The conductor-to-ground short-circuiting caused by conductive wildfires is responsible for a number of the power outages in many countries [31]. The failure to maintain radio communication at HF to UHF during wildfire suppression is also a safety concern for fire fighters [32]. In a similar vein, spacecraft and vehicles traveling at hypersonic velocities within Earth's atmosphere become enveloped by a plasma layer which attenuates microwave and radio signals, leading to 'radio blackout' [33].…”