Kgakgamatso Mphale

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

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.

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.

Effect of Wildfire-induced Thermal Bubble on Radio Communication

Horizontal roll vortex pairs are dynamical structures that transfer energy and emissions from wildfires into the atmosphere. The vortices form at the edges of an intense line wildfire and emulate two cylinders, which form two curvatures of a biconcave thermal lens. Wildfire plume provides a dielectric material for the dielectric lens, whose permittivity is influenced by the nature, quantity of constituents (e.g., potassium and graphitic carbon) and variation of temperature with height in the plume. The environment created by the plume is radio sub-refractive with an effect of spreading radio wave beams. A numerical experiment was carried out to quantify loss of Ultra High Frequency (UHF) radio signal intensity when high intensity wildfireinduced horizontal roll vortices intercept UHF propagation path. In the numerical experiment, a collimated radio wave beam was caused to propagate along fuel-fire interface of a very high intensity wildfire in which up to two roll vortex pairs are formed. Maximum temperature of the simulated wildfire was 1200 K. Flame potassium content was varied from 0.5–3.0%. At 3.0% potassium content, a vortex pair imposed a maximum radio ray divergence of 2.1 arcmins while two vortex pairs imposed a maximum divergence of 4.3 arcmins at the fuel-fire interface. The ray divergences caused maximum signal strength loss (in decibels (dB)) per unit path length of 0.154dBm−1 and 1.65dBm−1 respectively.

Microwave measurement of electron density and collision frequency of a pine fire

Pinus caribea (pine) litter flame is a weakly ionized 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 analyser 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.

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 MW m-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 mho m-1 in 90 MW m-1 crown fire in vegetation with 3.0% potassium content. A 90 MW m-1 crown fire in vegetation with potassium content of 0.5% was predicted to produce a maximum conductivity of 0.022 mho m-1. Electrical conductivities were lower for a shrub fire with an intensity of 21 MW m-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.

Weather Forecasting: From the Early Weather Wizards to Modern-day Weather Predictions

Weather forecasting is one the important science applications in our day-to-day planning activities. This is one prominent application that has played a significant role to humankind from long way back. Wherever humans have settled around the world, weather forecasting has always been part of their life for man has always been actively involved in their surroundings. Early humankind relied on their philosophical experience and other reoccurring weather phenomena to predict the weather and infer what was coming their way. This was the knowledge gathered over many years of observations and passed from one generation to another. However, it became evident that natural knowledge was inadequate to precisely gauge the atmospheric changes; a growing number of scientists realised the need for more advanced and better ways of predicting weather. Numerical weather prediction (NWP) is one of the modern-day forecasting techniques developed through simplified systems of physical laws of the atmosphere. This entails atmospheric observations, measurements of initial conditions and computer-based modelling to provide a practical means of predicting weather changes. While this is a positive development, weather forecasting is still faced with significant uncertainties that sometimes lead to erroneous and/or bad predictions. In this study, we trace the science of weather forecasting from early human civilizations through Bjerknes’ and Richardson’s numerical computation to modern day operational NWP. We highlight and discuss the concepts of precision, assessment and evaluation together with factors influencing weather forecasting. Also considered is how uncertainties arise and how the ‘chaos theory’ has shaped weather prediction.

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, -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 and  rad/s, respectively.