2. System Description
- height: 0.2 m (low/medium height)
- heat of combustion: 18,500 kJ/kg (typical value as indicated in )
- moisture content: 4% (very dry fuel)
- fuel load: 0.5 kg/m2 (medium/high load)
- char fraction: 10% (typical value)
- density of the vegetative fuel: 512 kg/m3 (typical value as indicated in )
- surface over volume ratio is 4950 m−1 (typical value for herbaceous fuel )
- 3D full physical model (block 2). This plays the role of a field experiment;
- entropy generation analysis (block 5). This is the proposed approach;
- 1D model with a vector composition approach (block 8). This is the reference for comparing the proposed approach to, as it is a commonly adopted prediction method.
- collect temperature evolutions and the other data which are needed for calculating the various terms of entropy generation and setting the coefficients for the vector composition analysis (block 3);
- generate the reference terms (the fire propagation velocity) for comparing the results obtained through the compact models, i.e., the entropy generation approach and the vector composition approach (block 4).
- STATE 1: the fire front after 185 s, i.e., when the fire front is fully developed, is used for gathering data for entropy generation analysis and the vector composition analysis.
- STATE 2: the fire front after 230 s is used for comparing the results obtained through the three approaches.
3.1. Entropy Generation Analysis of a Grassland Fire Event
- It is oriented with two faces parallel to the fire front.
- In each volume, a fictitious thermocouple is installed. This allows the detection of the temperature during propagation, providing information in the same way that would be available from an experimental campaign. The thermocouple sampling time, dtTH, is 0.25 s.
- The length l is selected as the space the front travels in a time equal to dtTH.
- Each control volume exchanges mass and energy with the surrounding through five control surfaces: two lateral surfaces, two frontal surfaces and the top surface. The latter is considered only for balancing the mass transport through a contribution which is always exiting the volume.
- Head, where wind and slope influence are high and the fire front propagates faster;
- Flanks (in particular Flank 1 and Flank 2), which are the sectors where the wind and slope contributions are smaller than at head;
- Back, which is the sector where the influence of wind and slope is negligible and the fire front proceeds slowly.
- no-slope condition and wind velocity equal to
- no-wind condition and slope equal to
- An upwind scheme is considered. This implies that the temperature at the boundaries are assumed as the temperature in the upstream control volume.
- Travelling wave assumption for the fire front, considering that an idle condition has been reached. Such assumption is acceptable in the case of fully developed fire in a sufficiently homogeneous fuel. Temperature evolution in a cell is the same recorded at the previous cell, just shifted ahead in time of the period spent by the front to travel the distance between two points. The same assumption is made for the following cell, shifting the time back.
3.3. Vector Composition Propagation
3.4. Fire Evolution Comparison
4.1. Pre-Processing Results
4.2. Main Entropy Generation Results
4.3. Propagation Prediction Comparison
- The fire propagation is considered as a travelling wave. This is a limitation in the case of significantly non homogenous fuels.
- Each cell is described though the mean values of the thermodynamic quantities. This makes the results dependent on the size of the control volume. In particular, this is crucial when the tip of the fire front is examined.
- A 3D model able to accurately predict the fire evolution. This model is used to provide the necessary data for tuning the parameters of other approaches. In addition, it provides the reference fire front propagation to be used as a term of comparison for the other approaches.
- An entropy generation approach, which provides the entropy generated in all the control volumes along the propagating fire front.
- A 1D model using, as the convective velocity, the vector resulting from the summation of the wind and slope contributions.
Conflicts of Interest
|c||specific heat (J∙kg−1∙K−1)|
|G||mass flow rate (kg∙s−1)|
|g||heat generation contribution (W∙m−3)|
|d||fuel height (m)|
|h||convective transfer coefficient, (W∙m−2∙K−1)|
|k||equivalent slope velocity coefficient (m)|
|N||number of cells|
|R||radiative heat exchange coefficient|
|r||equivalent propagation term|
|s||specific entropy (J∙kg−1∙K−1)|
|u||equivalent slope velocity (m)|
|v||wind velocity (m∙s−1)|
|V||total velocity (m)|
|λ||equivalent thermal conductivity|
|𝜇||equivalent ROS factor|
|∑||entropy flux (W∙K−1)|
|Φ||heat exchanged (W)|
|ω||convective wind speed|
|1||related to the first fire front|
|2||related to the second fire front|
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