Four main contributions are made in this thesis by the author, as follows.
1. Investigations of fire characteristics in a typical lightning fire area, Dalby, in Queensland were carried out. These include data collection and statistical analysis of 57 years’ fire records, a field investigation of lightning initiated fires in the Dalby area, and field fuel samples were also collected.
Statistical analyses were also made on nineteen years (1973-1992) of field fire records for public land of Horsham and Orbost in Victoria.
The comparison of lightning and other fires reveal the main features of lightning fires such as: the occurrence frequency during a year, the cost involved in extinguishing and area burnt in a lightning fire, and the trend of lightning fire percentage over decade.
The fuels of lightning fires, lightning fires in other countries, and the studies on lightning fire ignitions were reviewed and summarized.
2. A comprehensive review of the literature relevant to lightning fire ignition, which includes mainly the characteristics of lightning current and arcs, the properties of fuels, their pyrolysis and ignition at different heating conditions was carried out.
The main lightning characteristics which determine ignition are defined. It is shown that the ignition effects of multiple strokes need to be studied in lightning ignited fires. It is also noted that the transient heating of lightning is different from normal heating.
The review and summary of fuels and their ignition properties include: how the thickness of fuel fibres and moisture content of the fuels affect ignition; how the moisture content is influenced by weather conditions; the composition of the fuels; the pyrolysis of the fuels at different heating conditions; the heat transfer mechanisms and the influence of fuel properties on heat transfer.
The pyrolysis of the fuel at different heating conditions indicates that the pyrolysates and ignition by lightning are different from those of normal heating. The pyrolysates are influenced by moisture content as well.
3. A simple and novel impulse current generator, which combines high voltage, impulse and continuing currents (multiple impulses and continuing currents), was developed. The generator enables arc initiation without a wire, which may influence the ignition, and eliminates complicated triggering switching devices. The characteristics of the generator were studied and proved to be nearly an ideal current source in the tests of lightning ignited fires. This ensures that the main lightning current characteristics can be simulated and that test results are repeatable and comparable.
The techniques for measuring real arc voltage, power and energy of impulse arcs were developed. The power and energy of impulse current arcs were measured for various currents and arc lengths, and their relationships were obtained.
The techniques for measuring the power and energy transfer to the fuels were developed. The measured power and energy were found to be much greater than the power and energy of the arc in air alone, and depend on lightning current and fuel properties. This power and energy transfer characteristic was first noticed in the study of the fire ignition by lightnings. The large power and energy account for the quick heating process, evaporation of moisture, pyrolysis and ignition of the fuels. These make lightning fire ignition mechanisms quite different from normal ignitions.
The power and energy transfer characteristics reveal the mechanisms of the heat transfer from the impulse current arc to the fuels. The measured energy includes the heat transfer through conduction, radiation and convection. The large amount of energy transferred from the arc to the fuels when they are in contact with the arc clearly shows that the cooling effect of the fuel fibres and so the main process for energy transfer to the fuel fibres is through heat conduction for the impulse current arc condition.
Detailed experiments were made on the ignitions of various fuels (from field and artificially made) by different currents. The association of the power and energy transferred to fuels with the ignitions reveals the lightning current characteristics and fuel properties determining the ignitions.
The fire ignition effects of multiple impulses were studied; these have not been studied before. The accumulative heating effects by multiple impulses and the combustion of the pyrolysates during the interval of impulses were studied and compared with that of continuing currents.
4. A model describing the ignition mechanisms has been developed accounting for the distinguishing features of lightning ignited fires. An energy balance equation was put forward to evaluate the heating and ignition conditions.
Temperature distributions in fuel fibres being heated by lightning arcs have been calculated according to the model using both numeric and analytic methods. Calculated temperature distributions, surface temperature gradients, the energy balance equation for the sustained ignition and the analytical solution of the temperature distribution incorporated with the experimental results, such as the power and energy transferred to the fuels and the ignition probabilities, explain the heating and ignition mechanisms.
The calculated temperature distributions of a fuel fibre and the microscopic analysis help to explain the heating and ignition process, and provide answers for such questions as: the transient heating process, the power and energy involved in such a short transient process (based on the experimental results) and the cooling effects of fuel fibres and their energy absorbing ability during and after the heating of lightnings, and their influence on maintaining the ignitions.
The relationship between the surface temperature gradient and factors such as lightning current duration and fuel fibre thickness was obtained from the analytical solution. The temperature gradient determines the cooling or energy absorbing ability of the fuel fibres. The significance of the relationship is:
• It simply and clearly shows how the current duration and the thickness of fuels determine the surface temperature gradient and the ignition.
• For a given surface temperature gradient, or a certain ignition probability, the relationship shows the current duration needed for different fuel thicknesses.
• The relationship was found to be consistent with the test results.
These findings and analyses indicate that lightning current duration, multiplicity of stroke currents, fuel fibre thickness, moisture content, thermal diffusivity, and composition of fuel should be considered in predicting lightning fire ignition probability.
The model and calculated results can be used to direct further experimental study as well as practical applications. The problems that need to be considered for the application of the model to explain the ignition of the field fuels, which have randomly distributed thickness and packing ratio, are suggested.
In addition to the above main work of this thesis, preliminary suggestions are made for predicting the number of lightning fires. One model is based on a back- propagation artificial neural network and is compared with the conventional model to highlight its advantages in its concept, flexibility and sophisticated topology and a method is suggested for its implementation. The comparison of the principle of the two models is aimed at determining the direction of further work. The suggested further work for the implementation and application of the model is also presented.
These comparisons are made to help select the direction of the further work which may involve several years to achieve useful research results.