Experimental and numerical investigation of shrub combustion

The use of physics-based models to simulate vegetation fires has been steadily increasing over the last decade. These models offer the possibility to study quickly and at a low cost the combustion of vegetation according to a wide set of parameters which are difficult to control experimentally. However, the validation and improvement of these models is still ongoing and coupled experimental / numerical approaches are needed.

In a previous work, a validation of the numerical code WFDS was done for the spreading of fires across pine needles fuel beds. It highlighted the ability of WFDS to reproduce the combustion dynamics and the related physical quantities (heat release rate, mass loss rate, rate of spread) during the growing and steady-state phases of the fire. The objective of the present work is to test WFDS for the prediction of the combustion of a natural shrub, which is a more complex fuel than a fuel bed. As a first step, 3 shrubs of rockroses (Cistus monspeliensis) were characterized. The distribution of the different classes of particles that compose the shrubs was determined and used as input for the shrub model. A series of 41 fire tests were conducted on the shrubs to investigate their flammability. Ignition was performed using radiant panel. The measurements of the main physical quantities were also used as a comparison basis for the numerical approach. A numerical model of shrub was developed from the characterization results. Then, the combustion of the shrub was simulated with WFDS. The grid mesh resolution needed for the simulation was estimated from the extinction length within the shrub crown and from the heat release rate (HRR) during the fire tests. Finally, experimental measurements and predictions were compared to determine the capability of WFDS to reproduce the combustion of the shrubs.

The experimental investigation of the shrub flammability showed four different regimes of combustion governed mainly by the location of ignition. Simulations were run for the flammability regime that exhibits the best reproducibility. The comparison between simulation and experimental results demonstrated a good ability of WFDS to assess the most relevant fire properties, namely the peak of HRR, flame duration and consumption rate. The main differences between the predictions and observations were identified during the ignition and extinction phases. A plateau of HRR is predicted shortly after the ignition, limiting the fire growth. In addition, the duration of the predicted smoldering phase is too short which causes a total extinction of the fire few seconds after the end of the flaming phase.