Measurement of Electrical Conductivity for a Biomass Fire

A controlled fire burner was constructed where various natural vegetation species could be used as fuel. The burner was equipped with thermocouples to measure fuel surface temperature and used as a cavity for microwaves with a laboratory quality 2-port vector network analyzer to determine electrical conductivity from S-parameters. Electrical conductivity for vegetation material flames is important for numerical prediction of flashover in high voltage power transmission faults research. Vegetation fires that burn under high voltage transmission lines reduce flashover voltage by increasing air electrical conductivity and temperature. Analyzer determined electrical conductivity ranged from 0.0058 - 0.0079 mho/m for a fire with a maximum temperature of 1240 K

Radiowave propagation measurements and prediction in bushfires

Australian vegetation is fire-prone. Every year, wet and dry sclerophyll forests of Western\ud Australia, southeastern Australia and grassland ecosystems of the northern part of the continent\ud are subject to high intensity fires. The sclerophyll vegetation contains up to 2.71 % of the\ud element potassium. The element exists in plants’ organic matrix: attached to the oxygen containing and carboxyl functional groups; in aqueous form such as potassium (K+) ions\ud surrounded by water; and as discrete particles in dried plant parts. Theoretically, temperature in\ud the conflagrations can be as high as 2000ºC. During the high intensity bushfires, potassium\ud atoms and salts are released from the plant structure as it crumbles into the combustion zone\ud where the species are ionized. Up to 20 % of the potassium present in plants is ionized in a\ud bushfire environment.\ud \ud During suppression of the threat, high frequency (HF) - ultra high frequency (UHF) radio\ud communications systems are in constant use by suppression crews in firegrounds. Despite their\ud use, HF and very high frequency (VHF) radio communications are reported to be failing in\ud extreme bushfire conditions. The reports of radio communication failure tend to be anecdotal and\ud therefore warrant an investigation. This study aims at carrying out field and laboratory radio wave\ud attenuation and phase shift measurements at HF to X-band frequencies in moderate intensity fires.\ud Very high intensity bushfires often spread very fast and change direction rapidly therefore it is\ud unsafe to set up equipment for measuring attenuation and phase shift in the fires. Consequently,\ud numerical experiments were used to study radio wave propagation in very high intensity fires.\ud \ud Propagation measurement data at radio wave (HF-VHF) frequencies through fire are scarce and\ud that which is available lacks precision. It is also the purpose of the study to produce attenuation\ud and phase measurement data at these frequencies. The field and laboratory measurements were\ud carried out using a Radio Wave Interferometer (RWI) and Vector Network Analyzer (VNA - HP\ud 8277C). RWI uses the same principles as Microwave interferometer (MWI) except that RWI\ud works at radio frequencies. Electron density and momentum transfer collision frequency in\ud moderate intensity bushfire plumes were estimated from the attenuation and phase shift\ud measurements.\ud \ud Laboratory and field measurements using a VNA - HP 8277C and RWI respectively in moderate\ud intensity fires (700-1000 K) have revealed that electron density in the plume could range from\ud 1014-1016 m-3. Theoretical calculations based on local thermal equilibrium in grassfires flames\ud with temperatures up to 1200 K suggest that electron density could be up to 1017 m-3 if up to 20 %\ud of the inherent potassium atoms are ionized.\ud \ud There are at least two possible mechanisms that could lead to a significant signal strength\ud reduction (attenuation) in bushfire environments. They are signal refraction due to thermal bubble\ud and ionization-induced signal absorption in the plume. Electrons, which result from thermal\ud ionization of potassium in the fire, transfer energy from the incident radio wave to the fire plume\ud through collision with inherent neutral particles. The transfer of energy can significantly attenuate\ud and induce a phase shift on radio wave signals. Experimental work carried out in the project\ud suggest that radio wave attenuation is significantly higher at UHF and X-Band frequencies than at\ud HF. Field radio wave propagation measurements at 1.50 m above the seat of a moderate intensity\ud grassfire revealed that 30 MHz signals can be attenuated by up to 0.03 dB/m while 151.3 MHz\ud signals were attenuated by up to 0.05 dB/m. An intense cane fire attenuated 151.3 MHz signals\ud by 0.05 dB/m. The attenuation effect was observed to increase when X-band (10.0 -12.5 GHz)\ud signals were considered. Attenuation coefficients up to 4.45 dB/m were measured.\ud \ud Phase shift induced on the signals was also observed to increase with the increase in frequency\ud band (low for HF and high for X-band). A 30 MHz signal suffered a 3.08º phase shift in the\ud moderate intensity grassfire whereas in the X-band frequencies, a phase shift of up to 29.31º was\ud observed in a fire of about the same intensity.\ud \ud Numerical experiments have shown that signal loss due to refraction is frequency dependent in\ud very hot regions of bushfire plumes. X-band waves are more affected than VHF waves.\ud Numerical experiments predicted maximum attenuation coefficients of 0.11 dBm-1 for 150 MHz\ud and 0.31 dBm-1 for 3 GHz radio waves when they propagate about a meter above fuel-flame\ud interface of a 90 MWm-1 bushfire with fuel potassium content of 0.50 %. Theoretical studies also\ud revealed that; for potassium content of about 0.20 %, a collimated beam of radio signals (10 cm\ud wide) propagating at grazing angles to the fuel-flame interface of a very high intensity bushfire\ud (1600 K) could suffer attenuation coefficients of about 1.45 dBm-1. This effect is calculated to\ud decrease with the increase in height above the combustion zone. For very high intensity bushfires\ud in fuels with high potassium content (e.g., up to 3.00 %), attenuation by refraction is likely to be\ud the most significant form of radio wave energy loss for collimated beams propagating at grazing\ud angles to the fuel-flame interface.\ud \ud The Line-Of-Sight (LOS) radio wave propagation measurements in moderate intensity vegetation\ud fires (700-1000 K) have shown that signal attenuation is plume temperature and frequency\ud dependent. Transmission through hottest region of the fire (combustion zone) suffers significant\ud signal strength loss whereas low attenuation has been observed at cooler regions of the plume.\ud This could be explained by the fact that “collisional plasma effect” on radio waves is more\ud pronounced at combustion zone than at the thermal plume region of the fire as the effect is\ud temperature dependent.\ud \ud Bcontinent with a potential combustion zone temperature of 2000ºC. These bushfires have a\ud potential to adversely affect LOS radio wave communications when transmission is through the\ud hottest part of the fire. It must be noted that radio wave communication blackout could even\ud occur at temperatures as low as 1300 K provided that the fire is sufficiently ionized

Absorption and transmission power coefficients for millimeter waves in a weakly ionised vegetation fire

A vegetation fire plume is a weakly ionised gaseous medium. Electrons in the plume are mainly due to thermal ionisation of incumbent alkali impurities. The medium is highly collisional with free electron - neutral particle been the dominant particle interaction mechanism. Signal strength of an incident millimetre wave (MM-Wave) may be significantly attenuated in the plume depending on the extent of ionisation. A numerical experiment was set to investigate signal power loss of a MM-Wave incident on a simulated weakly ionised fire plume with flame maximum (seat) temperature ranging from 1000–1150 K. The simulated fire plume had alkali impurities (potassium) content of 1.0% per unit volume. MM-Wave frequency range investigated in the experiment is from 30–60 GHz. The simulation has application in the prediction of MM-Wave propagation in a crown forest fire and may also be applied in remote sensing studies of forest fire environments. Simulated attenuation per unit path length for the MM-Wave frequencies ranged from 0.06–24.00 dBm−1. Phase change per unit path length was simulated to range from 2.97–306.17°m−1 while transmission power coefficients ranged from maximum of 0.9996 for a fire plume at 1000 K to a minimum value of 0.8265 for a plume at a temperature of 1150 K over a plume depth of 1.20 m. Absorption power coefficient ranged from a minimum value of 0.0004 to maximum value of 0.1585 at a seat temperature of 1150 K over the plume depth

Wildfire plume electrical conductivity

Wildfires are weakly ionized gas. The ionization is mainly due to plant's inorganic ash content species (more especially potassium), that are emitted from thermally decomposing plant structure into the flame during combustion. The amount of ionization in flames with potassium impurities is influenced by both the temperature and the amount potassium impurities in the flame. A numerical experiment was conducted using a local thermal equilibrium-based model to study the influence of inorganic wildfire contents on wildfire electrical conductivity. Simulated very high intensity wildfires (21–90 MWm−1) were used to quantify steady-state electrical conductivity. Its variation with wildfire plume height is important for high voltage power flashover research. In the simulation, vegetation potassium content was varied from 0.50% to 3.0% on dry weight basis, a reflection of its content in natural vegetation. The model predicted a maximum conductivity of 0.053 mhom−1 in 90 MWm−1 crown fire in vegetation with 3.0% potassium content. A 90 MWm−1 crown fire in vegetation with potassium content of 0.5% was predicted to produce a maximum conductivity of 0.022 mhom−1. Electrical conductivities were lower for a shrub fire with an intensity of 21 MWm−1. The model predicted conductivities of 0.0021 and 0.0009 mhom−1 for potassium content of 3.0 and 0.5% in vegetation, respectively

Mesoscale Convective Systems: A Case Scenario of the ‘Heavy Rainfall’ Event of 15–20 January 2013 over Southern Africa

Southern east Africa is prone to some extreme weather events and interannual variability of the hydrological cycle, including tropical cyclones and heavy rainfall events. Most of these events occur during austral summer and are linked to shifts in the intertropical convergence zone, changes in El Niño Southern Oscillation signatures, sea surface temperature and sea level pressure. A typical example include mesoscale convective systems (MCSs) that occur between October and March along the eastern part, adjacent to the warm waters of Mozambique Channel and Agulhas Current. In this study we discuss a heavy rainfall event over southern Africa, focusing particularly on the period 15−20 January 2013, the period during which MCSs were significant over the subcontinent. This event recorded one of the historic rainfalls due to extreme flooding and overflows, loss of lives and destruction of economic and social infrastructure. An active South Indian Convergence Zone was associated with the rainfall event sustained by a low-level trough linked to a Southern Hemisphere planetary wave pattern and an upper-level ridge over land. In addition, also noteworthy is a seemingly strong connection to the strength of the African Easterly Jet stream. Using rainfall data, satellite imagery and re-analysis (model processed data combined with observations) data, our analysis indicates that there was a substantial relation between rainfall totals recorded/observed and the presence of MCSs. The low-level trough and upper-level ridge contributed to moisture convergence, particularly from tropical South East Atlantic Ocean, which in turn contributed to the prolonged life span of the rainfall event. Positive temperature anomalies favored the substantial contribution of moisture fluxes from the Atlantic Ocean. This study provides a contextual assessment of rainfall processes and insight into the physical control mechanisms and feedback of large-scale convective interactions over tropical southern Africa

Nondestructive Measurement of Momentum Transfer Collision Frequency for Low Temperature Combustion Plasma

Accurately measured momentum transfer collision frequency and electron density for fire plasma enable correct simulation of electromagnetic wave propagation in the medium. The simulation is essential for designing high-performance systems suitable for the environment. Despite this, momentum transfer collision frequency for fire plumes has always been an estimated quantity and/or crudely determined. There are anecdotal reports of severe line-of-sight (LOS) radio frequency signal degradation on firegrounds. The problem has implications on safety of fire-fighters during wildfire suppression hence the need of high performance communication systems. In the experiment, a nonintrusive and direct method for measuring momentum transfer collision frequency in a fire plume was carried out. Using an automatic network analyser, x-band microwaves were caused to propagate combustion zones of eucalyptus and grass litter fires to measure the flames, scattering parameters. The parameters were then used to determine average collision frequencies for the plumes. The average collision frequencies for the eucalyptus and grass fire plumes were measured to be 5.84×1010 and 5.92×1010 rad/s, respectively

Use of multivariate techniques to regionalize rainfall patterns in semiarid Botswana

Abstract Monthly precipitation data from 58 synoptic stations throughout Botswana, spanning 1981–2016, were used in this study. The data were examined using multivariate analysis to determine regions exhibiting distinct precipitation variability patterns and regimes. To accomplish this, the T-mode of principal component analysis was applied to the correlation matrix of the data. Based on the maximum loading values of the rotational principal component scores, the T-mode indicated three separate subregions with varying precipitation patterns over time. Four clusters with distinct rainfall patterns were identified when cluster analysis was performed on the principal component scores. An assessment of the homogeneity of the clusters was performed using L-moment’s heterogeneity measure (H). Statistical analysis was employed to model annual rainfall data using five commonly used rainfall analysis probability distribution functions: normal, lognormal, gamma, Weibull, and Gumbel. The probability distributions with the greatest fit were determined based on the maximum overall score, which was calculated by adding the individual point scores of three chosen goodness-of-fit tests. Each cluster exhibited distinct probability distribution functions, with the gamma, Gumbel, lognormal, and Weibull distributions providing the most accurate descriptions

Prediction and measurement of electron density and collision frequency in a weakly ionised pine fire

Pine litter flame is a weakly ionised medium. Electron-neutral collisions are a dominant form of particle interaction in the flame. Assuming flame electrons to be in thermal equilibrium with neutrals and average electron-neutral collision frequency to be much higher than the plasma frequency, the propagation of microwaves through the flame is predicted to suffer signal intensity loss. A controlled fire burner was constructed where various natural vegetation species 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 wave attenuation. Electron density and collision frequency were then calculated from the measured attenuation. The parameters are important for numerical prediction of electromagnetic wave propagation in wildfire environments. A controlled pine litter fire with a maximum flame temperature of 1080 K was set in the burner and microwaves (8–10.5 GHz) were caused to propagate through the flame. A microwave signal loss of 1.6–5.8 dB was measured within the frequency range. Based on the measured attenuation, electron density and electron-neutral collision frequency in pine fire were calculated to range from 0.51–1.35 × 1016 m−3 and 3.43–5.97 × 1010 s−1 respectively