Microwave attenuation in forest fuel flames

Mphale, K.M., Luhanga, P.V.C., and Heron, M.L. (2008) Microwave attenuation in forest fuel flames. Combustion and Flame, 154 (4). pp. 728-739.

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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 0.32–3.21×1016 m−3 and collision frequencies of 1.1–4.0×1010 s−1 were observed for flames with temperature in the range of 730–1000 K.

Item ID: 6105
Item Type: Article (Refereed Research - C1)
Keywords: forest fire; active remote sensing systems; flame conductivity; weakly ionized plasma; attenuation; thermal ionisation
ISSN: 1556-2921
Date Deposited: 07 Jan 2010 01:40
FoR Codes: 10 TECHNOLOGY > 1005 Communications Technologies > 100505 Microwave and Millimetrewave Theory and Technology @ 50%
10 TECHNOLOGY > 1005 Communications Technologies > 100501 Antennas and Propagation @ 50%
SEO Codes: 87 CONSTRUCTION > 8799 Other Construction > 879999 Construction not elsewhere classified @ 50%
86 MANUFACTURING > 8616 Computer Hardware and Electronic Equipment > 861603 Integrated Circuits and Devices @ 50%
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