Fire suppression agents that derive their entire effectiveness from physical processes are known as thermal agents. Thermal agents operate by lowering the flame temperature through dilution, heat absorption, and thermal diffusion. Two aspects of these mechanisms are investigated using detailed chemical kinetic modeling of methane opposed jet laminar diffusion flames burning in air mixed with two idealized surrogate agents. X has the physical properties of argon with the exception of a zero heat capacity. Its only flame effects are through dilution. Results show that X lowers flame temperatures by slowing the combustion chemistry and allowing increased oxygen to pass through the flame sheet. Comparison with results using argon reveals that 59 % of argon effectiveness derives from dilution with the remainder due to heat absorption. Y is also similar to argon with the exception that it can react to form Z, which is identical to Y except for having a positive heat of formation. Reaction of Y to Z absorbs heat and lowers flame temperatures. The reaction rate is adjusted using the rate law constants including activation energy in order to control where heat extraction occurs relative to a flame front. Calculated flame temperatures and hence agent suppression effectiveness are found to be insensitive to heat extraction location. The results for X and Y provide strong support for the concept of a well defined limit flame temperature at extinction. It is argued that the conclusions from this modeling study are relevant to coflowing diffusion flames, which are more characteristic of real-world fires.
Citation: Proceedings of the Combustion Institute
Pub Type: Journals
diffusion flames, dilution, fire suppression, kinetic modeling, thermal agents