# Computational Fluid Dynamics of Compartent Fires: A Review of Methods and Applications
## 3 CFD Simulation in Compartment Fires
### Boundary Conditions  
Boundary conditions in CFD are essential as they define the behavior of the fluid flow at the boundaries of the simulation domain, which can influence the heat and mass transfer between combustion gases and smoke, compartment walls and openings, and outer space in the case of a compartment fire. Generally, boundary conditions include the following categories:  
* Thermal Boundary Conditions: Thermal boundary conditions directly influence heat losses to walls, floors, and ceilings, which in turn affect fire spread, flame temperature, and product gases. For example, a well-insulated boundary can trap heat, causing higher temperatures and more intense fire growth, while highly conductive boundaries may slow fire development by reducing available thermal energy. Accurate representation of these boundary conditions is therefore essential for capturing realistic compartment fire behavior.
* Material and Surface Properties: Material properties directly influence the boundary conditions in CFD simulations by affecting heat flux, temperature profiles, and the fire spread rate. For accurate fire modeling, these properties must be carefully selected and validated based on experimental data to replicate real-world conditions.  
* Flow Boundary Conditions: Flow boundary conditions at inlets and outlets such as doors, windows, and vents are essential to define how air enters the compartment and how product gases exit. These boundaries control the oxygen supply, smoke evacuation, and pressure gradients within the compartment, all of which impact the fire dynamics and its growth.
* Burning Rate and Pyrolysis Boundary Conditions: Burning rate and pyrolysis boundary conditions describe how combustible materials are set up to burn, including parameters for pyrolysis and feedback mechanisms (e.g., temperature, and heat flux) affecting fuel release rates. This is critical for accurately modeling the growth phase of compartment fires.
* Initial Condition: Initial condition describes how the initial setup, like ambient temperatures and initial pressures, affects boundary conditions over time, influencing fire spread predictions.  
The modelers must ensure they are using proper and reliable boundary conditions on all the surfaces which define the simulation area. For example, in the case of an open simulation of pool fires with available experimental data, measured MLR or HRR are prescribed as boundary conditions on the fire surface (e.g., pool fire) for the simulations [25]. In another example, ventilation or wind effect can be defined as boundary conditions on a wall, window, etc. [17]  
A boundary condition can either be defined explicitly as a separate and direct input, such as a prescribed temperature, velocity, heat flux, etc., or in an implicit way where the boundary condition is applied as part of the overall solution of the equations. While each CFD code has its own way of specifying boundary conditions, the overall procedure is similar. Interested readers are encouraged to refer to CFD tools manual for further information about how boundary conditions are defined in different situations [12; 40; 41].