A pool fire is a turbulent diffusion flame that burns above a horizontal pool of liquid fuel, fed by the fuel’s own evaporation. It is one of the most studied fire scenarios in safety engineering because it represents a common and dangerous outcome of industrial spills, storage-tank failures, and transportation accidents. The behavior of a pool fire depends on the fuel type, the size and shape of the pool, the surrounding airflow, and the surface beneath the liquid, and those variables interact in ways that make pool fires both deceptively simple in appearance and genuinely complex in practice.
What Drives a Pool Fire
The core cycle of a pool fire is straightforward: heat from the flame radiates and convects back down to the liquid surface, causing more fuel to evaporate. That vapor mixes with surrounding air, ignites, and sustains the flame, which in turn sends more heat back to the surface. This feedback loop is what keeps the fire going until the fuel runs out or the cycle is interrupted. The relative contributions of radiation, convection, and conduction to that feedback shift depending on the fire’s size, the fuel’s chemistry, and whether wind is blowing across the pool.
For small pool fires (roughly under 20 cm in diameter), conduction through the pool rim and convection from the flame base dominate the heat transfer back to the fuel. As the pool gets bigger, radiation from the flame becomes the primary mechanism feeding energy back to the liquid surface. This transition matters because radiation scales aggressively with flame size: a pool fire twice as wide doesn’t just deliver twice the radiant heat, it delivers far more, which is why large pool fires burn more fiercely per unit area than small ones once they cross a certain size threshold.
Experiments with square pool fires ranging from 10 to 25 cm burning ethanol and heptane showed that the balance among these three heat pathways shifts with both pool size and airflow. With no wind, radiation dominates for sootier fuels like heptane. For cleaner-burning fuels like ethanol, convection plays a bigger role even at relatively calm conditions.1Proceedings of the Combustion Institute. Evolution of heat feedback in medium pool fires with cross air flow and scaling of mass burning flux by a stagnant layer theory solution
How Pool Size Shapes the Flame
Pool diameter is the single most influential variable in pool fire behavior. Small pools produce laminar flames that barely flicker. As the diameter grows, the flame transitions to turbulent flow, becoming taller, more chaotic, and far more energetic. This transition from laminar to turbulent has been mapped experimentally using ethanol fires at increasing pool scales, and the shift tracks well with buoyancy-driven flow calculations.2International Journal of Heat and Mass Transfer. Interpretation on heat transfer characteristics and mass burning rate of liquid fuel-immersed porous media fire: Effect of pool scale and particle size
Flame height is closely tied to pool diameter and the rate at which fuel burns. For circular pools, well-known correlations relate flame height to the pool’s heat release rate. Rectangular pools are trickier because their aspect ratio matters: a long, narrow trench fire behaves differently from a square one of the same area. Researchers have modified standard flame-height correlations by introducing an equivalent diameter that accounts for the rectangle’s shape, producing predictions accurate to within about 9% of measured values across a wide range of experiments.3Fuel. Simple flame height correlation for buoyancy-controlled diffusion plumes generated by rectangular sources fire with different aspect ratios
The burning rate, meaning how quickly the fuel is consumed per unit area, also depends on size in a non-obvious way. For small pools, burning rate can actually decrease as the pool gets slightly wider, because the flame geometry shifts in ways that reduce feedback to the surface. Beyond a certain diameter, the trend reverses and the burning rate climbs as radiation feedback takes over. This U-shaped curve catches some people off guard because intuition says “bigger equals faster,” but the transition zone in the middle can be a low point.4International Journal of Heat and Mass Transfer. Interpretation on heat transfer characteristics and mass burning rate of liquid fuel-immersed porous media fire: Effect of pool scale and particle size
The Puffing Rhythm
If you watch a pool fire for a few seconds, you’ll notice it doesn’t burn steadily. The flame pulses rhythmically, bulging outward and then pinching inward in a repeating cycle. This puffing is a fundamental hydrodynamic instability driven by buoyancy: hot combustion gases rise, drawing in cooler air at the base, which rolls up into toroidal vortices (think smoke rings stacked on top of each other). Those vortices climb the flame and pinch it periodically.
The puffing frequency depends almost entirely on the pool diameter and is largely independent of the fuel flow rate. Across a wide range of experimental data, the relationship follows a simple inverse-square-root law: frequency scales with the diameter raised to the power of negative one-half. A widely cited approximation puts the puffing frequency at roughly 1.68 times the inverse square root of the pool diameter in meters.5Fire and Materials. A Review of Research and an Experimental Study on the Pulsation of Buoyant Diffusion Flames and Pool Fires Helium plumes (non-reacting buoyant flows) puff at similar frequencies for the same diameter, which confirms that puffing is fundamentally a fluid dynamics phenomenon, not a combustion chemistry one.6Combustion and Flame. Experiments on the periodic instability of buoyant plumes and pool fires
Puffing matters for more than academic interest. The periodic surges in flame height and radiation output mean that objects near a pool fire experience fluctuating heat loads, which complicates both structural protection design and human exposure estimates. Computational models that fail to capture puffing tend to under- or overpredict the peak radiative heat flux hitting nearby surfaces.
Thermal Radiation and Soot
The dominant hazard of a pool fire to people and structures beyond the flame boundary is thermal radiation. How much radiation a pool fire emits depends heavily on the fuel. Clean-burning fuels like methanol radiate relatively little energy because their flames produce almost no soot. Sooty fuels like diesel, kerosene, and heavier hydrocarbons generate luminous yellow flames packed with incandescent carbon particles, and those particles are powerful radiators.
Measurements of eleven liquid fuels with different sooting tendencies showed that fuels with similar chemical structures produce similar radiation distribution profiles along the flame axis. Alcohols, for instance, cluster together with lower radiant fractions, while alkanes produce higher fractions with more energy emitted from the middle and upper portions of the flame.7Fire Safety Journal. Total radiative heat loss and radiation distribution of liquid pool fire flames
At larger scales, soot’s role becomes even more complicated. In big hydrocarbon pool fires, the outer mantle of soot can become so thick that it actually shields the interior of the flame, absorbing radiation before it can escape outward. This “soot shielding” effect means that beyond a certain pool diameter, the radiant fraction (the share of total heat released that leaves as radiation) can plateau or even decrease. Simulations of a 2-meter JP-8 pool fire found that high soot concentrations near the pool surface created a significant shielding effect, reducing the radiation that reached the fuel surface compared to what simpler models predicted.8Fire Safety Journal. Effect of soot concentration on radiative transport in a 2 m JP-8 pool fire This is counterintuitive: more soot doesn’t always mean more radiation danger to nearby targets, because the soot can trap energy inside the flame plume instead of letting it escape sideways.
Wind, Altitude, and Other Environmental Effects
Wind is the most impactful environmental variable acting on a pool fire. Even modest crosswinds reshape the flame dramatically. As wind speed increases, the flame tilts in the downwind direction, stretches longer, and drags across the pool surface. Experiments with pool fires exposed to winds up to 10 m/s (roughly 22 mph) documented extremely high levels of tilt, drag, and flame lengthening.9Fire Safety Journal. Wind-blown pool fire, Part I: Experimental characterization of the thermal field At low wind speeds, the plume curves in a smooth arc, making it hard to describe the tilt with a single angle. At higher speeds, the flame flattens out more uniformly.
Wind also changes where the heat goes. Cross-flow pushes heat toward the downwind side and enhances convective feedback while reducing radiation feedback. For heptane, convection can become the dominant heat pathway at higher wind speeds. For ethanol, convection already dominates at calm conditions and stays relatively stable.10Proceedings of the Combustion Institute. Evolution of heat feedback in medium pool fires with cross air flow and scaling of mass burning flux by a stagnant layer theory solution These shifts matter for hazard analysis because a tilted flame can reach targets that a vertical flame wouldn’t, and the radiation footprint moves dramatically downwind.
Experiments examining flames at different crosswind speeds also found that the visible upper flame length initially decreases, then increases as wind speed rises, while the flame tilt angle increases steadily with wind.11Fuel. Experimental investigation and theoretical analysis of effects of crosswind on flame lengths and tilting behaviors of pool fires with large ullage heights The initial shortening happens because moderate wind improves air mixing and combustion efficiency, but at higher speeds, the flame gets stretched out laterally faster than it can burn efficiently.
Altitude changes pool fire behavior too, though fewer people encounter this in practice. Comparative experiments between Hefei (near sea level) and Lhasa (about 3,650 meters, with roughly 65% of sea-level atmospheric pressure) showed that pool fires at high altitude have higher peak flame temperatures, taller flames, and faster puffing frequencies, but emit less radiation. The thinner atmosphere makes the fire more buoyant, so the flame stretches taller and oscillates more vigorously, but the reduced oxygen density means less soot production and less radiant output.12Fuel. Influence of low air pressure on combustion characteristics and flame pulsation frequency of pool fires Flame height at reduced pressure is consistently higher than at normal pressure, and the radiation flux is slightly lower.13Fuel. Burning characteristics of conduction-controlled rectangular hydrocarbon pool fires in a reduced pressure atmosphere at high altitude in Tibet
Fuel Type Makes a Bigger Difference Than Most People Expect
Not all liquid fuels produce the same kind of pool fire. The chemical structure of the fuel determines how much soot the flame generates, how luminous it is, how much radiation escapes, and how the fire responds to wind. Ethanol and heptane are frequently compared in research because they sit at opposite ends of the sootiness spectrum: ethanol burns with a nearly invisible blue flame, while heptane produces a bright yellow, soot-heavy plume.14Combustion and Flame. Flame radiation feedback to fuel surface in medium ethanol and heptane pool fires with cross air flow
This distinction isn’t just visual. It changes the entire heat feedback mechanism. A sooty heptane pool fire sends much more radiation back to its own surface, which accelerates evaporation and increases burning rate. A clean ethanol flame relies more on convective heating from the hot gases directly above the pool. When you add wind, heptane’s radiation feedback drops because the flame tilts and exposes less hot surface area to the pool, while ethanol’s convective feedback is less sensitive to the wind direction.
Heavier fuels like diesel and crude oil bring additional complications. They produce thick black smoke plumes that can obscure the visible flame entirely, making it difficult to assess flame geometry from visual observation alone. Temperature-based measurements (mapping where the gas is hottest) give much better estimates of the true flame boundary than video cameras when heavy smoke is present.15Fire Safety Journal. Wind-blown pool fire, Part I: Experimental characterization of the thermal field The smoke itself carries toxic particulates, and pool fires of diesel and lubricating oils have been studied for their emissions profiles under both open-air and compartment conditions.16University of Leeds White Rose eTheses. Toxic Species And Particulate Emissions From Wood And Pool Fires
Boilover
Boilover is one of the most dramatic and dangerous phenomena associated with pool fires of heavy hydrocarbons stored in tanks with water at the bottom. As the fire burns, a heat wave propagates downward through the fuel layer. When that thermal front reaches the water sublayer beneath the fuel, the water superheats and erupts into steam, violently ejecting burning fuel from the tank. The result is a sudden, massive fireball that can hurl flaming liquid hundreds of meters.
Experimental work has confirmed that boilover occurs only after the temperature at the fuel-water interface reaches the boiling point of water. It is the violent seething of water at that boundary, not a gradual process, that triggers the event.17Fire Safety Journal. Study and prediction of boilover in liquid pool fires with a water sublayer using micro-explosion noise phenomena The intensity of a boilover depends on the degree of superheat at the fuel-water interface, which in turn varies with the thickness of the water layer and the ullage height (the gap between the fuel surface and the tank rim).18Petroleum Science. Mechanisms of water layer thickness and ullage height on crude oil boilover: A theoretical model coupling the effects of multiple physical fields
Boilover is primarily a concern for crude oil and heavy fuel oil tanks at refineries and tank farms. Lighter fuels burn off too quickly for the heat wave to reach the bottom, and fuels without a water sublayer don’t produce the phenomenon at all. But when it does happen, it can escalate a manageable storage-tank fire into a catastrophic event with very little warning. Audible micro-explosion noises from the fuel-water interface can sometimes be detected before full boilover, offering a potential early warning signal.
Pool Fires on Water Versus Land
When fuel spills onto water rather than a solid surface, the fire behaves differently in several ways. Water conducts heat away from the fuel far more effectively than soil or concrete, which should slow the burning rate. But water also prevents heat losses through the substrate edges the way a metal pan rim would absorb heat from a land-based fire. In practice, experiments comparing continuously released fuel spills burning on land versus water found that the burning rate was slightly higher on water.19Journal of Loss Prevention in the Process Industries. Experimental study of continuously released liquid fuel spill fires on land and water in a channel
Spill fires on water also spread differently. On land, the fuel is contained by the terrain or by dikes. On open water, a spill will spread continuously until the fuel layer becomes too thin to sustain burning, or until the spreading rate and evaporation rate balance each other. For cryogenic fuels like liquefied natural gas (LNG), the spreading and evaporation dynamics are especially important because the fuel is boiling on contact with the water surface, so the pool area and the burning area change simultaneously as the spill evolves.
These differences make pool fire hazard analysis for marine terminals, offshore platforms, and LNG carriers fundamentally different from onshore scenarios. The fire’s area isn’t fixed by a dike or berm; it’s determined by fluid dynamics, and it can grow much faster than a confined fire on land.
Enclosed Spaces and Oxygen Depletion
A pool fire inside a building or compartment behaves nothing like one in the open air. The enclosed space limits the oxygen supply, and the fire’s behavior becomes tightly coupled to ventilation openings. Experiments using pool fires in a confined compartment with adjustable horizontal openings found two distinct outcomes depending on the balance between the fuel’s demand for oxygen and the air supply through the opening. Below a critical ratio of fuel area to opening area, the fire burns until the fuel is consumed. Above that ratio, the fire self-extinguishes as it depletes the available oxygen faster than air can flow in.20Journal of Fire Sciences. An investigation of horizontal opening effect on pool fire behavior in a confined compartment: A study based on global equivalence ratio
The transition between these two regimes has practical implications for compartment fire safety aboard ships, in storage rooms, and in tunnels. A fire that self-extinguishes due to oxygen depletion might seem like a favorable outcome, but the resulting compartment is typically filled with a hot, toxic, fuel-vapor-rich atmosphere that can reignite explosively if a door is opened and fresh air rushes in. Fire crews dealing with enclosed pool fires have to account for this possibility.
Suppression Strategies
Putting out a pool fire means breaking the feedback loop: either cool the fuel surface below its evaporation point, smother the flame by cutting off oxygen, or disrupt the vapor-air mixing zone. Different suppression agents attack different parts of the cycle.
Foam is the standard tool for liquid-fuel fires, and it works by flowing across the burning liquid surface to form a blanket that cuts off fuel vapors from the flame. For alcohol fuels like methanol, ordinary firefighting foam breaks down on contact because the alcohol dissolves the foam’s water content. Alcohol-resistant foams are formulated to resist this breakdown. Tests on large-scale methanol pool fires showed that alcohol-resistant aqueous film-forming foam (AR-AFFF) and alcohol-resistant fluoroprotein foam could both flow more than 30 meters across a burning methanol surface and extinguish the fire successfully. AR-AFFF spread about two and a half times faster than fluoroprotein foam and achieved better extinguishment performance overall.21PubMed. Experimental investigation of foam spread and extinguishment of the large-scale methanol pool fire
Water mist is an alternative approach, especially in enclosed spaces where foam application is difficult. Fine water droplets cool the flame zone and the fuel surface simultaneously. But the relationship between mist performance and fire size is not straightforward. Tests on diesel pool fires found two distinct regimes: at some injection pressures and fire sizes, the water mist suppressed the fire effectively by cooling the fuel surface. At other combinations, the mist actually intensified the fire, likely by disrupting the flame structure in ways that enhanced air mixing without delivering enough water to the fuel surface to offset the increased heat transfer. Fuel surface cooling was identified as the dominant extinguishment mechanism at the boundary between success and failure, and the main cause of suppression failure was simply not getting enough water onto the liquid surface.22SAGE Journals (Journal of Fire Sciences). Extinction Limit of Diesel Pool Fires Suppressed by Water Mist
Modeling Pool Fires Computationally
Predicting pool fire behavior by computer simulation is a major area of ongoing work in fire safety engineering. Designers of refineries, chemical plants, LNG terminals, and aircraft hangars all need to estimate how much heat a potential pool fire would radiate to surrounding structures, personnel, and equipment. Running a full-scale experiment every time isn’t practical or safe, so engineers rely on computational fluid dynamics models to simulate the fire.
Modern large-eddy simulation approaches can capture the unsteady, turbulent nature of pool fires with reasonable accuracy. A modeling framework validated against experimental data for pool fires predicted the radiative heat flux hitting the pool surface, the location of the peak flame temperature, and the flame puffing frequency all within about 10% of measured values.23Physics of Fluids. Predicting large-scale pool fire dynamics using an unsteady flamelet- and large-eddy simulation-based model suite When the computational mesh is too coarse, the model tends to underpredict the radiative heat flux, meaning it gives a conservative (safer) answer rather than an optimistic one, which is preferable from a design standpoint.
Still, modeling pool fires accurately at very large scales remains difficult. The interactions among turbulence, soot formation, radiation transport, and fuel evaporation create a tightly coupled system where small errors compound. Models that work well for a one-meter fire don’t automatically scale to a 30-meter tank fire. Most validated simulations in the literature cover pool diameters up to a few meters, and extrapolation beyond that range carries increasing uncertainty. This gap between laboratory-validated models and real-world industrial scales is one of the persistent challenges in fire safety engineering.