# Computational Fluid Dynamics of Compartent Fires: A Review of Methods and Applications
## 3 CFD Simulation in Compartment Fires
### Physical Models
#### 3.3.2 Pyrolysis
However, according to FDS User's Guide [12], it is clearly stated that the pyrolysis model of evaporating liquid fuels in FDS is recommended for research use only, due to the strong dependence of the evaporation rate on mesh grid. OpenFOAM, on the other hand, requires custom implementation, offering unparalleled flexibility for modelling complex evaporation dynamics, multi-step reactions, or user-defined empirical laws. ISIS employs the Clausius-Clapeyron equation for evaporation, defined as Equation (4):  
\[X_{f}=exp\left[-\frac{h_{v}W_{f}}{R}\left(\frac{1}{T_{s}}-\frac{1}{T_{b}} \right)\right] \tag{4}\]  
where \(X_{f}\) is the evaporating liquid mass fraction of the fuel, \(h_{v}\) is the latent heat of vaporization (J/mol), \(W_{f}\) is molar mass of the fuel (kg/mol), \(R\) is universal gas constant (8.314J/(mol\(\cdot\)K)), \(T_{s}\) is surface temperature of the liquid fuel (K), and \(T_{b}\) is the boiling temperature of the liquid fuel. An early use of LES in simulating a pool fire was conducted by [PERSON] et al. [70] and they showed that the burning rate depends on the pool size. [PERSON] et al. [71] proposed a modified natural convection correlation instead of the forced convection approach (used in FDS v6.7.5) to simulate the heat and mass transfer for liquid pool fires. The main idea was to depend on a Grashof number (a dimensionless number that characterizes the relative strength of buoyancy forces to viscous forces in fluid flow, particularly in natural convection scenarios) instead of a Reynolds number (indicates whether the flow is laminar or turbulent based on the balance between inertial forces and viscous forces) as the governing parameter in heat transfer through the pool fire. It was shown that although the burning rates were similar in both cases, the fuel surface temperature was predicted more accurately in the natural convection approach. In other work, [72], they adopted the natural convection correlation for estimating the evaporation rate and the Nusselt number for the heat transfer calculations. They realized that the forced convection approach is not capable of modeling the flame extinction. In addition, assuming a low value for the auto-ignition temperature of the fuel (such as \(-273\) K in FDS as default) can predict the burning rate but not the extinction accurately. Therefore, considering the actual auto ignition temperature is necessary. The capability of the FDS v6.7.0 in predictive simulation of a liquid pool fire using pyrolysis sub-models was studied by [PERSON] et al. [73]. Their work on different fuel types showed that a default FDS combustion model (infinitely fast combustion) using fast ignition and burning is sufficient to replicate the experimental measurements in the ignition and fire growth phases for the fuels with low boiling temperature (as illustrated in Figure 6 (left)), while this method is not applicable for the fuels with higher boiling temperature (as illustrated in Figure 6 (right)). In these cases, considering a hot point in the simulation to start the burning process is helpful. As a conclusion, fast chemistry over-predicts the development rate of fire.