# Computational Fluid Dynamics of Compartent Fires: A Review of Methods and Applications
## 3 CFD Simulation in Compartment Fires
### Physical Models
#### 3.3.2 Pyrolysis  
Since a flame is categorized in the gas phase, liquid and solid fuels must be converted to the gaseous form in the first stage, in case of combustion [8]. This thermal decomposition process, in which materials break down into smaller molecules when exposed to high temperatures in the absence of oxygen or other oxidizing agents, is called pyrolysis. In other words, pyrolysis helps us to understand how materials break down in a fire and release flammable gases. The process typically produces three main products:  
* Gases: Volatile compounds like hydrogen, methane, carbon monoxide, and other light hydrocarbons. * Liquids: Condensable products such as tar, oil, or other heavy hydrocarbons. * Solid Residue: Char or ash. The main aim of pyrolysis modeling is to determine the mass decomposition of the fuel or the rate of flammable gases produced which can determine main fire characteristics \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**SGS Model** & & **CFD Tool** & & **Weakness** \\ \hline \multirow{3}{*}{Constant Smagorinsky} & \(\bullet\) & FDS & \(\bullet\) & Fixed Coefficient \\  & \(\bullet\) & OpenFOAM & \(\bullet\) & Over-predicts dissipation in regions where turbulence is low (e.g., near walls) and under-predicts dissipation in highly turbulent areas (e.g., fire plume) \\ \hline \multirow{3}{*}{Deardorff} & \(\bullet\) & FDS & \(\bullet\) & Fixed Coefficient \\  & \(\bullet\) & OpenFOAM & \(\bullet\) & Fixed Coefficient \\  & \(\bullet\) & FDS & \(\bullet\) & Over-predicts dissipation in regions where turbulence is low (e.g., near walls) \\ \hline \multirow{3}{*}{Dynamic Smagorinsky} & \(\bullet\) & FDS & \(\bullet\) & Challenges in wall-bounded flows, where the eddy viscosity needs to approach zero at the wall: sometimes fail to properly capture this behavior without additional wall models or corrections. \\  & \(\bullet\) & OpenFOAM & \(\bullet\) & Fixed Coefficient \\  & \(\bullet\) & OpenFOAM & \(\bullet\) & May struggle in strongly anisotropic flows away from walls (e.g., Large Open Compartment Fire) \\ \hline \multirow{3}{*}{Vreman} & \(\bullet\) & FDS & \(\bullet\) & Fixed Coefficient \\  & \(\bullet\) & ISIS & \(\bullet\) & Although in a compartment fire, the hot gases create strong buoyancy-driven flows, it does not inherently incorporate buoyancy effects \\ \hline \multirow{3}{*}{K equation} & \(\bullet\) & OpenFOAM & \(\bullet\) & Fixed Coefficient \\  & \(\bullet\) & OpenFOAM & \(\bullet\) & Assumes isotropic turbulence, which may not be suitable for compartment fires characterized by complex, anisotropic flows due to heat release and wall effects \\ \hline \multirow{3}{*}{Dynamic K equation} & \(\bullet\) & OpenFOAM & \(\bullet\) & Model may not be able to adapt itself to sudden changes in turbulence characteristics such as the ignition/pre-development stage in compartment fires \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of different SGS models. such as HRR. Pyrolysis modeling is fundamentally based on the Arrhenius equation (i.e., finite rate chemistry). As mentioned before, when no experimental data for HRR or MLR is available, modelers should use the predictive capability of the simulation tools to identify the fire characteristics (which is known as free burning without any further constraint). In such cases, the pyrolysis sub-model is utilized. There are two distinct cases in which pyrolysis can be utilized, combustible solid fuels such as wood, polymers, etc., and liquid fuels (e.g., pool fire), which will be explained in separated categories. * Solid Fuels Pyrolysis for solid fuels involves heat transfer into them, leading to the breakdown of their molecular structure and the release of volatile gases, liquids, and char. In this case, simulations should consider conduction, convection, and radiation within and around the solid, as well as the diffusion of product gases.