 Heat release rate

Heat release rate is the rate at which a process releases energy as heat, commonly used to characterize combustion. It has the dimensions of power and is measured in watts, with $1\mathrm{W}=1\mathrm{J}{\mathrm{s}}^{-1}$. Using $Q_{\mathrm{rel}}(t)$ for cumulative released energy,The heat release rate is the height of the curve; total released energy is its area.

For a synthetic fire whose heat release rate rises linearly from zero to $100\mathrm{kW}$ in $60\mathrm{s}$ and falls linearly to zero over the next $60\mathrm{s}$, the area is a triangle:A second fire with a smaller peak but longer duration can release more total energy. A peak value alone therefore cannot describe the full event.

A fuel-based estimate multiplies fuel mass flow rate by the effective released energy per unit mass, under stated assumptions about composition and combustion. For a deliberately stipulated effective value $20\mathrm{MJ}\mathrm{k}{\mathrm{g}}^{-1}$ and fuel rate $0.02\mathrm{kg}{\mathrm{s}}^{-1}$, the estimate is $400\mathrm{kW}$. In oxygen consumption calorimetry, measured gas flow and oxygen depletion instead provide an estimate of heat release rate. NIST uses independently metered reference burners to compare these two inference routes.^[ref]

A fire's heat release rate differs from the heat transfer received by one nearby surface or from the temperature of its exhaust. Released energy can leave by several paths, while a receiving object can store internal energy as well as lose heat. Even a gas-based calorimeter needs a defined measurement boundary, capture conditions and time response. The NIST National Fire Research Laboratory provides an example of using exhaust collection to quantify large-fire heat release rate.^[ref]