Study of ignition and extinction of small-scale fires in experiments with an emulating gas burner

- Ignition and extinction of condensed-phase fuels are emulated with a gas burner.
- The critical mass flux increases with decreasing heat of combustion.
- The critical energy flux is constant in a buoyant region and varies when Fr>1.
- Stagnant layer theory is suggested to give approximate results for the fire point.
- An anchor point is proposed to establish when quasi-steady burning is initiated.

The objective of this study is to explore mechanisms for ignition and extinction for condensed-phase fuels via the use of a gas-fueled burner. Flames were generated with a porous 25 mm circular burner using mixtures of methane and propane with nitrogen. The procedure was to specify a set of mass fluxes of nitrogen-fuel mixture that corresponded to the flash- fire- and extinction points and for the minimum mass flux where steady burning was achieved. The results show an increase in the critical mass flux with a decreased heat of combustion. The data fall into two regimes depending on the mixture flow rate; one buoyancy-driven (Fr<1) and one induced by momentum jet forces. The buoyancy-driven regime is geometrically consistent with the definitions of flash and fire points under natural convection conditions. The results for the momentum regime align reasonably with existing stagnant layer theory. Extinction theory is also suggested to give approximate results for the fire point. This argument is based on similar flame geometries for fire point and extinction and theoretical reasoning. An anchor point is proposed as the end point of ignition. Produced anchor point data result in a flammability diagram, below which quasi-steady burning occurs.