A Web-based Introduction to Fire Modeling
Zone Models
A zone model is a computer model that divides the room(s) in question into different control volumes, or zones. The most common zone models will split a room into two zones, an upper hot zone and lower cold zone.
To be able to use the governing equations that are the base of these models fire protection engineers must make assumptions. Many of these assumptions are based on observations from experimental tests and models. Below is a list of the major assumptions made in a zone model.
Smoke stratifies into two distinct layers (as can be seen in real fires). Layers are also assumed to be uniform throughout, which is not true, but the differences are so small when compared to the other layer, the differences are negligible.
The fire plume acts as a pump of mass (smoke particles) and heat to the upper zone. However, the plume volume is assumed to be small compared to the upper and lower zones and so is negligible.
The majority of room contents are ignored; heat is lost to the room structure, not the furniture. (Some zone models can determine flame spread to a small number of furnishings)
- Inputs:
Room geometry, room construction (including all walls, floors and ceilings), number of vents (or holes) and their sizes, room furnishing characteristics, and the heat release rate (what is burning) - Outputs:
Prediction of sprinkler and fire detector activation time, time to flashover, upper and lower layer temperature, smoke layer height, and species yield - Limitations:
Zone models cannot accurately take into account re-radiation from the surroundings. The heat release rate is not an output, tests must be done to quantify a size fire an engineer thinks will be adequate for what they want to model. Many of the correlations are empirical in nature; some correlations may not fit the model, but still be used. - Features:
*Zone models are established as a fire protection tool; they have been around for over 25 years.
*Zone models are relatively easy to use and cheap on computer time; most models only take several minutes to compute. - Types of Models:
CFAST, MAGIC, ASET, FIRE-MD, FASTlight, Smokepro
