Interaction of big particles with flow- A study of the spotting distances of the firebrands by modelling and simulations

The wildfire, which is hard to be suppressed especially in large scale fires such as Forest Fire, is a terrible disaster to human civilisation. Millions of acres of burning land and numbers of deaths due to wildfire are reported each year. As one of the main mechanisms of wildfire, the Spotting phenomenon, which identifies the ignitions by Firebrands (the burning particle such as barks, wood pieces or charcoal) away from main fire zone, plays an important role in fire spread. In details, the new ignitions in front of fire zone are continuously caused by spotting phenomenon. Not only it makes the direction of fire spreading become unpredictable, but also helps fly over breaks. In order to study the spotting phenomenon and predict the spotting distances of firebrands, many models have been developed for the firebrands at different conditions. However, rare study about modelling the spotting of firebrands with lift effects to predict their maximum spotting distance has been reported.
In the thesis, the spotting of firebrands is investigated. A new mathematic model for determining the horizontal propagation of firebrand in certain height is developed to calculate the transporting of firebrands. Meanwhile, a new particle burning model is developed considering the thickness and mass regression due to combustion while its fly path. The models developed in this thesis are further compared with the existing models. The discussion with the analysis of advantages and disadvantages is given.
Based on the modelling of spotting phenomenon, the predictions for the maximum spotting distance of rectangular-shaped firebrands were made, and the spotting differences due to thickness change from different burning models were compared. It is found that the longest propagation of burning firebrand could reach 684 m in horizontal direction for each 100 m height drop. The particle spotting was not only strongly affected by ambient flow velocity, but also importantly enhanced by the incident angle where the peak value of spotting occurred at -7.5° of incident angle. By comparing the spotting distance of firebrands with and without burning, it is found that the particle with variable thickness burning could propagate further than other cases.
Further, the parametric studies for determining the influences of each parameter involved in spotting phenomenon were made. The relative importance of each parameter is documented. Among four different parameters which affect the firebrand spotting distance, including flow velocity, incident angle, density and initial particle velocity, it is found that both the flow velocity and incident angle could enhance the spotting more than other parameters. Based the finding from previous chapter, the spotting enhancement by negative incident was further invested. The parameter optimisation was conducted to reduce the parameters, and corresponding surface fitting equations were obtained. As well the comparisons between two different burning models were made.
More importantly, the effects of aerodynamic lift force on firebrand spotting was investigated systematically in this thesis. It is found that spotting distance of firebrand could be significantly increased by lift force, that 171% of increasement and up to 14 times of horizontal propagation by lift force comparing with the case without lift effect. The surface fitting equations for lift effects at different conditions were obtained. It strongly suggests that the lift force should be considered for more accurate prediction of spotting in further work.