Identifying a Fuel-Controlled Ventilation State

In the early stages of fire development, the fire grows as a direct function of the amount of fuel involved in the burning, with little or no influence from the compartment or room environment (i.e., size, shape or number of openings). Therefore, fires with more than enough air (oxygen) for combustion are said to be "fuel-controlled." These fires are comparable to free-burning outside fires.

Identifying a Ventilation-Controlled Ventilation State

As a fire grows larger, the demand for air increases, as does the volume of hot smoke produced by the fire. Eventually, this condition leads to a situation where the discharged smoke blocks off the opening and restricts the ingress of air towards the base of the fire. The restricted air track results in an increase in inward air velocity that creates a turbulent, counter-current smoke flow out of the opening.

Figure 19: Ventilation controlled vs. fuel-controlled fire

The image on the left of Figure 19 shows a ventilation-controlled fire. Firefighters must try to assess to what extent the application of air at one or more openings will increase the rate of fire development and spread. The image on the right shows a fuel-controlled fire that is sufficiently "free burning" that the addition of more air is not likely to cause any (or further) RFD.

Laminar or Turbulent?

When viewing the smoke/air track at an opening (neutral plane), a slow and smooth (laminar) smoke/air track (as shown in Figure 20) could indicate that the fire is in the early stages of development and most likely still fuel-controlled. If the smoke/air track is fast and turbulent (often corresponding to situations with a low interface height), this could indicate a working fire that is in the ventilation-controlled phase (Figure 21).