Modeling with WFDS Combustion Dynamics of Ornamental Vegetation Structures at WUI: Focus on the Burning of a Hedge at Laboratory Scale

ABSTRACT

This study is part of an ongoing project on modeling vegetation fires and their impacts on dwellings at wildland-urban interfaces. As a first step, fire tests were conducted at laboratory scale on both wooden wool litter and rockrose non-natural hedge. To this end, a 0.5 m x 0.5 m x 1 m hedge reconstituted from rockrose branches was burned under a Large Scale Heat Release (LSHR) apparatus. A litter of wooden wool was used before and under the hedge to mimic an ignition from an underneath burning herbaceous layer. These fire tests provided experimental data for mass loss and heat release rate, which were used as a basis for comparison with the simulations performed by using the Wildland-urban interface Fire Dynamics Simulator (WFDS). To numerically model the rockrose hedge, the distribution of the different particle classes (leaves, twigs of various diameters) was determined as well as their physicochemical properties. The comparison of the experimental data and predicted quantities showed a good agreement, whether for wooden wool burnings alone or mixed wooden wool and rockrose hedge.

KEYWORDS:

• Wildland-urban interface
• hedge combustion
• WFDS
• numerical model of vegetation

Disclosure statement

No potential conflict of interest was reported by the author(s).

Nomenclature

A	=	Cross sectional area of the exhaust duct (m2)
$A_{Char}$	=	Pre-exponential factor for char oxidation (m⋅s−1)
$A_{H_{2}O}$	=	Pre-exponential factor for desiccation (K1/2⋅kg−1)
$A_{pyr}$	=	Pre-exponential factor for pyrolysis (s−1)
BD	=	Base diameter of the branches (m)
BH	=	Branch height (m)
BRD	=	Diameter of the branch (m)
CFD	=	Computational Fluid Dynamics
$c_p$	=	Specific heat (J/kg.K)
$D^{\ast }$	=	Length scale characterizing the diameter of the pool fire (m)
$d_{x_b}$	=	Grid cell size in the fuel bed (m)
$d_{x_g}$	=	Grid cell size in the gas phase (m)
e	=	Thickness (m)
E	=	Heat release per unit mass of O2 consumed (J/kg)
$E_{Char}$	=	Activation energy for char oxidation (K)
$E_{H_{2}O}$	=	Activation energy for desiccation (K)
$E_{pyr}$	=	Activation energy for pyrolysis (K)
$F_{C_D}$	=	Drag coefficient factor (-)
FDS	=	Fire Dynamic Simulator
FH	=	Flame height (m)
FIGRA	=	Fire growth rate index (W/s)
FkW	=	Flank width (m)
FL	=	Flame length (m)
g	=	Gravitational acceleration (m/s2)
HRR	=	Heat release rate (W)
$k_t$	=	Constant determined via a propane burner calibration (-)
$k_p$	=	Constant of the bi-directional probe (-)
L	=	Length (m)
l	=	width (m)
LHV	=	Lower heating value (J/kg)
LSHR	=	Large-Scale Heat Release apparatus
m	=	Mass (kg)
MC	=	Moisture content on dry basis (%)
MLR	=	Mass loss rate (kg/s)
${\dot{n}}_{O_2}^{°}$	=	Molar flow rates of O2 in incoming air (mol/s)
${\dot{n}}_{O_2}$	=	Molar flow rates of O2 in the exhaust duct (mol/s)
OCC	=	Oxygen consumption calorimetry
PHRR	=	Peak of HRR (W)
$\dot{Q}$	=	Volumetric heat release rate (W/m3)
RMSE	=	Root-mean-square error
ROS	=	Rate of spread (m/s)
T	=	Temperature (K)
TGA	=	Thermogravimetric analysis
tfl	=	Flame residence time in the hedge (s)
tign	=	Ignition time of the hedge (s)
th	=	Duration of the hedge combustion (s)
tPHRR	=	Time corresponding to the peak of HRR (s)
THR	=	Total Heat Release (J)
$\dot{V}$	=	Standard flow rate in the exhaust duct (m3/s)
w	=	Fuel load (kg/m2)
$W$	=	Molecular weight (kg/mol)
WFDS	=	Wildland-urban interface Fire Dynamics Simulator
X	=	Mole fraction (-)
Greek symbols	=
α	=	Expansion factor for the fraction of the air that was depleted of its oxygen (-)
β	=	Flame separation angle (°)
${\beta }_{char}$	=	Constant in char oxidation rate equation (-)
${\beta }_k$	=	Volume fraction of the solid-phase (-)
${\delta }_f$	=	Fuel depth (m)
${\delta }_R$	=	Extinction length (m)
$\mathrm{\Delta }{h}_c$	=	Heat of combustion of gases (J/kg)
$\mathrm{\Delta }{h}_{char}$	=	Heat of char oxidation (J/kg)
$\mathrm{\Delta }{h}_{pyr}$	=	Heat of pyrolysis (J/kg)
$\mathrm{\Delta }{h}_{vap}$	=	Heat of vaporization (J/kg)
$\mathrm{\Delta }P$	=	Pressure drop across the bi-directional probe (Pa)
γ	=	Flame tilt angle (°)
Γ	=	Mass proportion of the particle class (-)
ϕ	=	Oxygen depletion factor (-)
${\nu }_{O_2,char}$	=	Stoichiometric constant for char oxidation (-)
${\rho }_0$	=	Density of dry air at 298 K and 1 atm (kg/m3)
${\rho }_{bk,dry}$	=	Bulk density (kg/m3)
${\rho }_{k,dry}$	=	Mass density of the fuel elements (kg/m3)
σ	=	Surface area to volume ratio (m−1)
${\chi }_{Ash}$	=	Fraction of char converted to ash (-)
${\chi }_{\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}}$	=	Fraction of the dry vegetation converted to char (-)
${\chi }_{\mathrm{s}}$	=	Soot emission fraction (-)
Subscript	=
air	=	Air
CO2	=	Carbon dioxide
$D^{\ast }$	=	Relative to $D^{\ast }$
dry	=	Dry
${\delta }_R/3$	=	Corresponding to ${\delta }_R$ divided by 3
${\delta }_R/5$	=	Corresponding to ${\delta }_R$ divided by 5
H2O	=	Water
k	=	Fuel element
max	=	Maximum
min	=	Minimum
O2	=	Oxygen
TK1	=	Thermocouple 1 in the exhaust duct (-)
$\mathrm{\infty }$	=	Ambient air
Superscript	=
°	=	Incoming air
a	=	Ambient

Additional information

Funding

This research was supported by the projects “MED-STAR” (Strategie e misure per la mitigazione del rischio di incendio nell’area Mediterranea) and “INTERMED” (Interventions pour gérer et réduire le risque d’incendie à l’interface habitat-espace naturel) financed by the fund PC IFM 2014-2020 (http:// interreg-maritime.eu/fr/web/med-star).