Firelags

Firelags is a term used in the discipline of combustion science to describe the time lag between an applied heat source and the observable propagation of a flame within a combustible medium. The concept functions as a descriptor of dynamic response in systems where ignition timing, heat release, and geometry interact to influence flame development. In practice, firelags are treated as a time delay parameter that partially decouples ignition chemistry from flame spread in simplified models.

Origin and usage: The term emerged in discussions of transient combustion where rapid ignition does not immediately produce a fully developed flame. Researchers use firelags to compare experimental results with simulations and to evaluate how changes in fuel concentration, pressure, or confinement affect the speed of flame growth.

Measurement and interpretation: Firelags are quantified by measuring the interval between an ignition cue (for example, a transient heat flux peak) and the initial detection of flame surface or peak heat release rate. In experiments, high-speed imaging and plume diagnostics are used, while in simulations they appear as an input delay or as a parameter in reaction progress variables.

Applications and limitations: Firelags inform safety design, such as timing of suppression systems or venting strategies, and support sensitivity analyses in CFD studies. The concept is not universally standardized; definitions vary by apparatus and fuel. It is most meaningful in transient, confined or partially vented configurations where delays alter risk profiles.

See also: ignition delay, flame front, heat release rate, combustion modeling.