Abstract
Wildfire simulations can help to better understand the dynamics and effects of forest fires. The basis of wildfire simulation is the tree-burning simulation. In this paper, the fire simulation of 7 different geometry Hungarian trees in the case of arson is presented. It was observed that the trees were burned down fast. The maximum mass loss rate and maximum heat release rate were larger as the tree was larger. The largest intensity fire could be observed in the case of the smallest tree. The maximum temperature was higher in the case of a large crown diameter. The maximum aerosol reached high pollutant concentrations. In the case of large crown height, the maximum CO2 concentration was higher. The results presented in this paper can be the basis of the following forest fire simulations.
1 Introduction
Tree fire and burning simulation is the basis of forest fire simulation, however in the literature there is only some examples of a single tree fire simulation. Most of the scientific literature deals with forest or grassland fire simulation [1].
Paper [2] presents a novel, interactive to model the combustion of botanical trees. Physical and biological properties are stored in connected particles. These properties include the dynamic branch motion and exothermic combustion reaction, which includes the evaporation of moisture, the formation of char, and the reduction of wood and material properties. For fire simulation the Navier-Stokes equations are approximated through a grid-based fluid solver. In [3] Wildland urban interface Fire Dynamics Simulator (WFDS) is presented. The model is based on the large-eddy simulation approach. The governing equations of heat transfer, fluid flow, combustion, and thermal degradation of vegetative fuels are solved numerically. The used model includes solid fuel pyrolysis and water evaporation. It was assumed that trees are modeled with simple geometric shapes with uniformly distributed bulk fuel. The simulation was validated with fire experiments with Douglas firs. In [4] the surface-to-crown transitions of the fire in the case of a single tree and 66 Douglas firs on grassland are studied. The developed model is compared to an experimental result of a single Douglas fir. With this research it was shown that physic-based models are useful for crown fire simulations. In [5] an inverse analysis is presented with physics-based simulations, which include a single Douglas fir burning simulation and a forest fire simulation. The aim of the research was to reproduce the generation and behavior of firebrands based on experiments. In [6] the one-at-a-time sensitivity study of a tree burning simulation was carried out. It was studied how variables like the maximum mass loss rate, maximum temperature at the top of the tree, maximum heat release rate, and the percent of the remaining tree are sensitive to the moisture content, the ambient temperature, the crown base diameter, and the tree height. The tree was a pine and the model was based on [3].
It can be seen from the literature review that only some paper deals with the numerical simulation of a single tree fire. The aim of the research is to simulate the burning of native Hungarian trees [7].
2 Materials and methods
For the simulations Computational Fluid Dynamics (CFD) software [8], Fire Dynamics Simulator (FDS) was used. The program includes a thermal degradation model developed specifically for vegetation fires. More details about the thermal degradation model can be found in [9] and in a previous paper [6]. The heat of combustion was 14,516 kJ kg−1, which is already included in the software used.
The branches and foliage were modeled as particles. The trees were modeled as cylinders from the particles with the following parameters: crown diameter, trunk diameter, height, and crown base height (Fig. 1).
The parameters of the trees are collected from databases and from the internet [10–19]. In case of no data, the crown base height was taken as approximately 70% of the total height [20, 21]. The properties of the trees are summarized in Table 1.
Properties of the trees
Tree | Crown diameter (m) | Trunk diameter (m) | Height (m) | Crown base height (m) |
Oak (Quercus robur) | 30 | 1.5 | 24 | 15 |
Beech (Fagus sylvatica) | 21 | 1.5 | 35 | 15 |
Ash (Fraxinus ornus) | 6 | 0.3 | 7 | 2 |
Poplar (Populus tremula) | 19 | 0.3 | 24 | 9 |
Maple (Acer platanoides) | 22 | 1.5 | 18 | 13 |
Elm (Ulmus glabra) | 13 | 2 | 20 | 14 |
Willow (Salix alba) | 15 | 1 | 15 | 10.5 |
Source: edited by Authors from databases [10–19].
The modeled trees are shown in Fig. 2.
The simulation area was 40 × 40 × 100 m; the mesh size was 0.5 × 0.5 × 0.5 m. The simulation time was 10 min except in the case of the ash, where 6 min was sufficient [22]. The ignition source was a circular burner below the tree and a temperature of 1,500 °C to model arson. As the aim of the study was to investigate the geometrical properties of the trees, therefore the moisture content was 20% in each case and the thermal degradation model was also the same. The sensors were placed at the top of the trees. The sensors measured the temperature, the aerosol concentration, and the CO2 concentration. Besides the mass loss, the Mass Loss Rate (MLR) and the Heat Release Rate (HRR) were calculated. The simulation was run on a Message Passing Interface (MPI) cluster using 5 nodes.
3 Results and discussion
The results are shown in Fig. 3.
Maximum values of the variables
Tree | Max. MLR (kg s−1) | Max. HRR (kW) | Intensity of fire (kW m−2) | Max. temperature (°C) | Max. aerosol concentration mol mol−1 | Max. CO2 concentration mol mol−1 |
Oak | 468 | 6.37∙106 | 9,015 | 2,010 | 1.06∙10−6 | 13,400 |
Beech | 323 | 3.48∙106 | 2,267 | 1,964 | 1.31∙10−6 | 14,200 |
Ash | 26 | 3.83∙105 | 13,541 | 1,754 | 1.16∙10−6 | 14,300 |
Poplar | 245 | 3.11∙106 | 1,778 | 2,212 | 1.37∙10−6 | 14,300 |
Maple | 178 | 3.03∙106 | 7,962 | 1,994 | 1.04∙10−6 | 12,900 |
Elm | 98 | 1.56∙106 | 930 | 1,913 | 1.1∙10−6 | 13,300 |
Willow | 130 | 1.91∙106 | 1,151 | 1,848 | 9.6∙10−7 | 14,100 |
Source: edited by Authors.
The temperature reached near 2,000 °C in the case of all trees. The temperature was highest in the case of the poplar followed by the oak and the maple. In the case of the oak, beech, elm and maple the temperature did not decrease back to 20 °C, because these trees has a large diameter trunk, therefore it ignited. The maximum aerosol concentration was more or less the same in every tree, which was over 10−6 almost all of the trees except the willow. It is a high pollutant concentration [24]. The highest concentration was in the case of poplar and beech followed by ash. The aerosol concentration reached a peak value fast then decreased in the case of most trees. In the case of the beech, oak, maple and elm it decreased, but not to zero. The explanation is that these trees have a large diameter trunk, which started to burn. The CO2 concentration reached over 12,000 ppm in all cases. It had the highest value in the case of the ash, which is followed by the poplar and the beach. In the case of trees with narrow trunk (ash, poplar, willow) the CO2 concentration increased then reached a peak, then decreased fast. In the case of the trees with large trunk diameter the CO2 concentration decreased more slowly. In the case of the beech it increased, then became constant then increased again when the trunk ignited. The maximum values of the examined variables are summarized in Table 2.
It can be seen that the MLR was the largest in the case of the oak, as it was the largest tree and the smallest in the case of the ash as it was the smallest. It can be concluded that the MLR is larger as the tree is larger. The HRR was the largest in the case of the oak and followed by the beech and the poplar, as they were the largest trees. The next trees with the largest HRR are the maple and the willow. The smallest HRR could be observed in the case of the elm and the ash, as they were the smallest trees. It can be concluded that the HRR is larger as the tree is larger. The temperature was the highest in the case of the poplar, followed by the oak. These trees have a quite large crown diameter and quite a lot of branches and foliage. The beech, maple, elm, and willow have a similar maximum temperature. The ash as the smallest tree had the smallest temperature. It can be concluded that trees with a larger crown diameter had a larger maximum temperature. The maximum aerosol concentration was the largest in the case of poplar and beech. This is because these trees had a dense crown with a lot of branches and foliage. The other trees had similar value. The lowest value had the willow. The explanation of it is that the top of the tree was modeled as foliage and not as branches. It can be concluded that the maximum aerosol concentration depends on the branches at the top of the tree. The maximum CO2 concentration was not surprisingly similar in all of the trees. The largest value occurred in the case of the ash and the poplar. The explanation is that these trees had a large crown height compared to the diameter of the crown. The smallest values occurred in the case of the maple, elm, and oak. These trees had a large crown diameter compared to the crown height. It can be concluded that the maximum CO2 concentration was higher in the cases, when the tree crown was large in height compared to its diameter.
4 Conclusions
In this paper the fire simulation of different geometry Hungarian trees was carried out. Due to the large ignition source the mass of the trees decreases fast in all cases. In the case of trees with large trunk (oak, beech, maple, elm) not the entire tree burned down. The MLR increased fast, then after reaching a peak it decreased fast in all cases. The HRR was the largest in the case of the largest diameter tree (oak). In most cases it increased fast, then reached a peak point then decreased fast. The maximum MLR and HRR were larger as the tree was larger (oak, beech). The largest intensity fire occurred in the case of the smallest tree (ash), because the ignition source was large compared to the tree. The temperature reached near 2,000 °C in the case of all trees. Trees with a larger crown diameter and a dense crown (poplar, oak) had a larger maximum temperature. The maximum aerosol concentration was around 10−6 almost all of the trees, which is a high pollutant concentration. The maximum aerosol concentration depends on the density of the branches at the top of the tree. The aerosol concentration reached a peak value fast then decreased in the case of most trees. The CO2 concentration reached over 12,000 ppm in all cases. The maximum CO2 concentration was higher in the cases, when the tree crown was large in height compared to its diameter (ash, poplar). It can be concluded that most of the simulation results approximate the real processes, but there are some differences. Further research task is to develop a model, which takes into account the differences between the trees burning properties. Also the tree geometry is planned to be improved using BlenderFDS. A mesh sensitivity study is also planned. The main task of further research is to create forests from the examined trees and carry out the fire simulation of them.
Acknowledgements
The Author's would like to thank to Dr. William Mell for sharing FDS example files.
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