Measurements and Predictions of the Aerosol Dynamics of Smoke
Amy Mensch, Haley Hamza, Thomas Cleary
Better understanding and ability to predict the aerosol dynamics of soot can improve life safety predictions generated by fire modeling tools. NIST's fire modeling tool, Fire Dynamics Simulator (FDS), is commonly used by the international fire protection community for design of smoke handling systems and smoke detector activation studies, as well as fire reconstructions . FDS includes sub-models for aerosol transport, deposition and coagulation based on well-established correlations for spherical particles –, while soot from flaming fires is generically characterized as a fractal structure of agglomerated 0.02 µm – 0.04 µm diameter primary particles. The appropriate characteristic size of a soot agglomerate depends on what is being characterized, i.e. inertial drag, coagulation, or thermophoresis. For coagulation, the number of bins specified to represent the actual distribution of particle sizes also requires consideration of the needs for prediction accuracy and computational efficiency. The soot dynamics sub-models in FDS and the soot parameters specified for the simulation all impact soot-related predictions, which include surface deposition, smoke alarm activation, visibility and tenability. Aerosol transport mechanisms that are often present and important in fire environments include gravitational, turbulent diffusion, and thermophoresis. In fire experiments where soot deposition was measured , , contributions from multiple mechanisms make it difficult to use the results to validate any one mechanism. However, for a standard heptane fire (EN 54 part 9 ), Rexfort  was able to show an improvement in the FDS predictions of soot concentration with particle coagulation modeled, particularly after the fire was extinguished. Another approach to isolate deposition mechanisms is to introduce post-flame soot into a non-reacting environment with well characterized flow and thermal conditions. The contributions from thermophoretic deposition were isolated using a laminar flow channel with opposing hot and cold surfaces in Mensch and Cleary . Within the uncertainties, the measurements matched the FDS predictions when using the soot's primary particle diameter in the thermophoretic equations. Mensch and Cleary  also constructed a 1.5 m3 cubic isothermal enclosure with a fan in the center directing flow upward to evaluate gravitational deposition, turbulent deposition and coagulation. Initial experiments, conducted for spherical particles of known size distributions, monitored the decay in aerosol concentration over time. The results showed good agreement for low fan flow rates and high particle concentrations, cases when coagulation dominated over deposition. The current study builds upon the previous spherical particle study  to quantify the concentration decay of two different aerosols, which simulate smoke from smoldering fires and soot from flaming fires, under quiescent flow. The experimental scenarios are simulated in FDS to demonstrate a possible approach for handling the coagulation of smoke in practical fire scenarios. The FDS predictions are compared to the experimental results to evaluate the limitations of the current FDS models and the proposed treatment for soot agglomerates.
Suppression, Detection and Signaling Research and Applications Conference (SUPDET 2022)
, Hamza, H.
and Cleary, T.
Measurements and Predictions of the Aerosol Dynamics of Smoke, Suppression, Detection and Signaling Research and Applications Conference (SUPDET 2022) , Atlanta, GA, US, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=935503
(Accessed December 6, 2022)