Determination of cup-burner extinguishing concentration using the perfectly stirred reactor model

A model based on the perfectly stirred reactor (PSR) concept is developed and utilized in the prediction of the minimum extinguishing concentration (MEC) of inert fire suppression agents established in the standard cup-burner test. Both physical and chemical mechanisms of fire suppression are considered in PSR modeling. Transport mechanisms are not modeled. Results are contrasted to experimental data and to a semi-analytical approach that assumes heat absorption by the agent is the primary extinguishing mechanism. Good agreement is achieved validating both the PSR approach and the conclusion that agent heat capacity has the first-order effect in the determination of the MEC. PSR results further capture a second-order chemical effect of oxygen depletion on the MEC. The PSR model is also used in a parametric assessment of the impact of test conditions on the MEC. Results show that cooling of the agent or heating of the fuel have a minor effect on the MEC. Variations on atmosphere conditions, such as temperature, pressure and humidity, have a more pronounced effect on the MEC. These results are interpreted in terms of the changing heat capacity of the inerting atmosphere and the altered chemical reactivity of the combustible mixture.