FLASH-CAT Model

Overview

FRI3D uses the Flame Spread over Horizontal Cable Trays (FLASH-CAT) method, where applicable, to determine secondary combustion and fire propagation through cables. FLASH-CAT is an empirical method derived from controlled fire experiments conducted by the National Institute of Standards and Technology (NIST).

Applicability Conditions

For this empirical method to provide results approximating the NIST experiments, the following conditions must be met:

• The cable trays are horizontal and stacked vertically with a spacing of less than 0.45 m (18 in.)
• The cables burn in the open (i.e., they are away from walls and well below the ceiling)
• The cables are not exposed to elevated temperature sources except for the ignition source below
• There are no barriers separating the trays, and the tray tops and bottoms are open
• The cables are not protected with coatings, armor shielding, or thermal blankets of any kind
• There is a fire beneath the lowest tray
• Each tray has at least a single row of cables, or roughly 25% of the NRC limit

The FRI3D modeler is expected to model secondary combustibles following these limitations. The results of the secondary combustion simulation will not be accurate if, for example, there are several fire sources beneath a tray, nor will they be realistic if the raceways are relatively close to walls or ceilings.

Fire Propagation and Heat Release

Fire spreads through a stack of trays in the characteristic V-shaped burning pattern observed in the NIST experiments: the fire ignites the tray directly above the source first, and each tray above it ignites over a progressively wider span as the fire propagates upward and outward along the trays.

The FLASH-CAT method divides a cable into several spatial segments. A cable segment combusts when the fire propagates to that segment. When it does, the heat release rate per unit area (HRRPUA) for that segment follows the time profile shown in the figure below, reflecting the dynamics of fire growth and decay. The HRR starts to increase at $t_{ign,i}\left(x\right)$ until reaching the peak HRRPUA one-sixth of $\Delta t$ later, remains stable until $\frac{5\Delta t}{6}$, and then decreases linearly to zero.

FLASH-CAT Time History

Time history of the local HRRPUA for each raceway grid.

Once the HRRPUA profiles for each raceway grid have been calculated, the total HRR of the secondary combustible is calculated by summing the local HRRPUA profiles from all raceways:

$$\dot{Q}\left(t\right)={\dot{Q}}_{fire}+W\sum_{i=1}^{N_{trays}}\left[\int_{x_0}^{x_j}{\dot{q}"\left(t-t_{ign,i}\left(x\right)\right)dx\ }\right]$$

Where $\dot{Q}\left(t\right)$ is the total secondary combustible’s HRR, ${\dot{Q}}_{fire}$ is the HRR of the initial fire, W is the raceway’s width, ${\dot{q}}''$ is the local HRRPUA, and t is the discrete observation time. After the HRR profile from the secondary combustibles is obtained, FRI3D adds it to the initial fire’s HRR and reruns the fire simulation. This loop continues — each rerun can ignite further raceways — until no additional secondary combustibles are found.

Using FLASH-CAT in FRI3D

To use the FLASH-CAT methodology, position a raceway at a reasonably close distance above a fire source as shown in the screenshot below, and add cables inside the raceway to serve as the secondary combustible.

Note that the horizontal fire propagation rate in the FLASH-CAT methodology is relatively slow (in the range of 1–3 meters per hour). Cable raceways are typically long, so estimating the complete HRR profile of a secondary fire over the whole raceway length can require a simulation time of several hours; consider limiting the analyzed raceway length accordingly.

What's next?

• CFAST Simulations and FDS Output — the fire simulations that consume the combined HRR and produce the temperatures driving cable and equipment failures.
• Heat Soak and THIEF — the cable-failure calculations applied to the simulated exposure.