Buoyancy-Driven Flow
Fire generates a buoyant column of hot combustion products that rises due to density differences with the surrounding air. This fire plume is the primary mechanism for transporting heat, smoke, and toxic gases from the fire source to the upper regions of a compartment. Understanding plume behavior is essential for designing smoke control systems, positioning detectors, and predicting the thermal environment that occupants and structures will experience.
Heskestad's Correlations
Gunnar Heskestad developed widely-used empirical correlations that describe the axisymmetric plume above a fire. His model divides the plume into three regions: a persistent flame zone near the fire, an intermittent flame zone where flames appear and disappear, and a buoyant plume region above the flames. The correlations predict centerline temperature rise, velocity, and mass entrainment rate as functions of height, heat release rate, and fire diameter.
Entrainment and Dilution
As the plume rises, it entrains ambient air at a rate proportional to the plume's perimeter and velocity. This entrainment is crucial: it determines how quickly the hot gases are diluted and cooled. At the ceiling level, the plume may contain 10-100 times more entrained air than combustion products. Higher ceilings mean more entrainment, lower temperatures, and lower concentrations of toxic gases — one reason tall atriums provide better smoke management.
Engineering Applications
Plume models are the foundation of performance-based fire design. They determine the required capacity of smoke exhaust systems, predict sprinkler activation times, establish detector placement criteria, and inform structural fire resistance requirements. The Heskestad model is embedded in design guides worldwide, including NFPA 92 for smoke management and BS 7346 for smoke ventilation design.