Heat transfer

Heat transfer is the movement of energy caused by a temperature difference. Energy passes from hotter material, gases or surfaces to cooler ones until the driving difference is removed or balanced. In fire safety, heat transfer connects combustion with heated fuel, smoke, building boundaries, structural members, people and neighbouring objects.

The three mechanisms used to describe this movement are conduction, convection and thermal radiation. They can occur simultaneously and influence one another. A flame, for example, emits radiation while hot gases rise convectively and surrounding solids conduct energy away from their heated surfaces.

Heat transfer determines both how a fire grows and how its effects spread. Feedback heats fuel and supports further combustion, while losses cool the reaction region. The same energy can therefore be part of fire development at one location and an unwanted exposure at another.

The three heat-transfer mechanisms

Conduction transfers energy through molecular interactions within a material or between contacting materials. It allows heat to pass through a wall or structural section without bulk movement of that section. Temperature difference, material conductivity, thickness and area influence the result.

Convection transports energy through the movement of gas or liquid. Hot smoke carries energy as it rises and travels, and it also exchanges heat with surfaces along its path. Natural buoyancy, wind and mechanical equipment can all contribute to the flow.

Thermal radiation transfers energy by electromagnetic waves. It can cross a vacuum and does not require a transporting medium between the source and receiver. Flames, hot soot, combustion gases and heated surfaces can all participate in emission and absorption.

Atreya notes that convection is not a fundamental molecular mode in the same sense as conduction and radiation because it combines their effects with medium motion. In practical fire analysis, however, treating all three as separate pathways helps organise the problem. Their relative importance depends on scale, temperature and geometry.

Three heat-transfer pathways in a fire. Based on Van Rossum (2027), §5.1.

The dominant mechanism can change during a fire. Small flames can lose a larger relative share through convection, while radiation becomes increasingly important at larger scales and higher temperatures. Van Rossum uses approximately 70% convection and 30% radiation as a rule of thumb for an open fire, while stressing that actual proportions vary.

Approximate open-fire energy split, not a universal ratio. Based on Van Rossum (2027), §4.3.

Heat flux and energy balances

Heat flux expresses heat transfer per unit area and time, commonly in watts per square metre. It describes the intensity of exposure at a surface rather than the total energy released by a fire. Total heat release rate, total heat flow through an object and local heat flux must be distinguished.

A surface energy balance accounts for energy arriving, leaving and being stored. Radiation and convection may heat the exposed side, while conduction carries energy into the solid and other losses cool it. The surface temperature changes according to the net result rather than according to one incoming term alone.

A receiver experiences an energy balance, not the entire fire output. Based on Rockett and Milke (2002), p. 1-29.

Temperature difference drives heat transfer, but equal air temperatures do not guarantee equal surface exposure. Different flow conditions alter convection, and different source geometry or surface properties alter radiation. A complete assessment therefore includes each relevant pathway.

Heat loss also controls whether combustion can continue. Near a flammability limit, losses can exceed heat production and prevent a self-sustaining flame. For a solid, insufficient feedback can reduce pyrolysis and cause the fire to decline even while fuel and oxygen remain.

Storage and changing temperatures

A steady heat-transfer calculation assumes that temperatures do not change with time. Real fires are transient: heat release rate, smoke temperature and receiving-surface temperature all change. Steady calculations can estimate an order of magnitude under selected conditions, but do not reproduce the entire fire history.

Materials store energy as their temperature rises. Density and specific heat capacity determine how much energy a given volume can store for a temperature increase. Conductivity and geometry then influence how quickly that energy moves to deeper regions.

Thermal inertia describes the resistance of a material's surface temperature to rapid change under heat exposure. It is associated with conductivity, density and heat capacity acting together. Materials with different thermal inertia can therefore respond differently to the same short-duration heat flux.

The initial temperature distribution matters in a transient calculation. Surface temperature or heat flux must also be specified over time, together with relevant internal changes such as moisture evaporation. These inputs make the result specific to the construction and exposure being analysed.

Heat transfer in fire development

Heat feedback supports evaporation and pyrolysis, supplying gaseous fuel to flames. Flames then release more energy and can increase feedback. This loop can sustain combustion, accelerate spread or decline depending on the balance between incoming heat and losses.

Fire spread occurs when heat transfer enables combustion to extend to new fuel or new locations. Direct flame contact, convective heating, radiation and conduction through boundaries can each contribute. The route and distance affect both timing and the conditions required for ignition.

Within a compartment, hot gases collect near the ceiling and transfer energy to boundaries and contents. Radiation from flames, hot gases and heated surfaces can preheat other fuel. Conduction through walls and ceilings can expose the unexposed side even without smoke passing through.

Fire dynamics brings these processes together with fuel production, combustion, buoyancy and ventilation. Heat release rate influences plume flow and gas temperature, while the resulting heat transfer changes fuel response and heat release rate. This feedback prevents a fire from being understood as a fixed heat source isolated from its surroundings.

Scale changes the relative pathways. Reduced-scale experiments can misrepresent radiation because the proportion of radiative transfer changes with flame size. Tien and colleagues report radiation as dominant in flames with characteristic lengths above 0.2 m in the discussed fire context, while convection is more significant in smaller flames. This is a broad source observation, not a universal boundary for every flame.

Engineering applications

Heat-transfer analysis supports estimates of ignition exposure, gas temperature, structural heating and conditions at the unexposed side of a barrier. The output should match the decision: heat flux for exposure, internal temperature for material response or transported power for smoke and energy flow.

For a wall, a steady layered-resistance method can compare constructions under selected boundary temperatures. For changing fires, transient calculations track energy storage and internal gradients. Composite details, thermal bridges and direction-dependent properties may require more detailed geometry.

For smoke, plume methods estimate how much air and energy rise from the fire. Surface convection methods then address heat delivered to ceilings or devices. These models share inputs but are not interchangeable, because transported energy and local surface heat flux answer different questions.

For radiation, geometry determines how much emitted energy reaches a receiver. Surface orientation, separation, emissivity and participating gases or soot can all matter. A source temperature alone therefore cannot define the heat flux at another object.

Source, path and receiver

A useful heat-transfer description identifies a source, a path and a receiver. The source may be a flame, hot gas layer or heated surface. The path can include radiation through gas, convection along a surface and conduction through a solid. The receiver can be fuel, a construction, a person or a device.

This separation prevents source heat release from being confused with receiver exposure. Not all energy released by combustion reaches one object, and energy arriving there can be reflected, absorbed, stored or transmitted onwards. Receiver heat flux is therefore a result of the whole configuration.

The receiver's initial condition also matters. Its temperature, moisture and material properties influence how much energy is required for heating and how deeply that heating penetrates. Two receivers at the same location can consequently follow different temperature histories.

Paths can change as the fire develops. Rising smoke can reach a ceiling and spread horizontally, new surfaces can become hot radiators, and increasing solid temperatures can reduce subsequent heat absorption. A model with one fixed pathway may therefore apply only to a limited stage.

Time provides the final connection between these elements. A large heat flux for a short period and a smaller flux for a long period do not automatically produce the same result. Thermal storage, decomposition and changing boundary conditions make the sequence of exposure important.

Selecting models and data

Model selection begins with the required output and timescale. A steady resistance model can compare heat flow under fixed boundary temperatures, while a transient model is needed for temperature development during growth and decay. Choosing the simpler model does not remove the need to test its assumptions.

Scale deserves particular attention in experiments. Tien and colleagues explain that reduced fire scales can reduce the proportion of radiation compared with other mechanisms. A small test can therefore represent convective behaviour more strongly than the full-scale fire it is intended to inform.

Material data should match the expected temperature range and direction of transfer. Conductivity, specific heat and emissivity can vary, and composite materials may not behave identically in every direction. Using one ambient value throughout a severe exposure is an assumption, not a property of every material.

Geometry enters through several routes. Conductive area and thickness, convective orientation and boundary layers, and radiative view between surfaces all affect the result. Precise material properties cannot compensate for an incorrect physical arrangement.

Rounding should reflect uncertain inputs and the safety decision. Van Rossum recommends retaining precision during calculation, rounding only the result and recognising that fire inputs can carry substantial uncertainty. Where times are compared, available times are rounded down and required times up to avoid false precision in the safe direction.

Limits and common misunderstandings

Heat and temperature are not the same quantity. Temperature describes thermal state, while heat transfer is energy movement caused by temperature difference. A small hot source can transfer little total energy, and a large flow at a lower temperature can transport substantial power.

No mechanism is always dominant. Temperature, scale, geometry and material properties can shift the balance between conduction, convection and radiation. The 70% convection and 30% radiation rule of thumb should not replace an assessment of these conditions.

An intact barrier does not necessarily prevent thermal exposure. Conduction can continue through the solid, and radiation can heat its unexposed surface. Evaluating only flame or smoke penetration misses this energy pathway.

Finally, heat transfer is not only harmful. Heat losses can suppress ignition and extinguish combustion, while controlled removal can reduce structural and occupant exposure. The engineering question is where the energy goes, at what rate and with what effect over time.

Peak heat flux and accumulated energy describe different aspects of exposure. A short peak may occur before a thick section has responded fully, while prolonged moderate heating can raise deeper temperatures. The assessment period should therefore match the failure or ignition mechanism being considered.

Contributions can be combined only on a consistent basis. Radiative and convective fluxes must refer to the same receiving surface and period, and conductive loss must use the corresponding solid boundary. Mixing instantaneous values from different fire stages would not form a valid energy balance.

Experimental comparisons can test whether the chosen mechanisms are represented adequately. Disagreement may come from material data, geometry, source description or omitted pathways, not only from numerical accuracy. The whole model chain therefore needs examination.

Documentation should make the selected fire stage, boundary temperatures, material properties and geometry traceable. This allows another analysis to establish whether the result applies to the same source and receiver. It also prevents a rule of thumb from being reused without its original limitations.

The most useful result is often a time history rather than one isolated number. It shows when exposure begins, when it peaks and how long a receiver remains hot after the source declines. This supports assessment of delayed ignition, structural response and residual hazard without assuming that the end of flaming means the end of heat transfer.

A time history also makes conservation easier to check. Energy entering a receiver should appear as storage, onward transfer or losses under the model's assumptions. Large unexplained imbalances can reveal inconsistent areas, units or boundary conditions before the result is used for a safety decision.

See also

• Conduction
• Convection
• Thermal radiation

References

• Atreya, A. (2002). Convection heat transfer. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Rockett, J. A., & Milke, J. A. (2002). Conduction of heat in solids. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Tien, C. L., Lee, K. Y., & Stretton, A. J. (2002). Radiation heat transfer. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.