A One-Dimensional Turbulence (ODT) study of soot formation, transport, and radiation interactions in meter-scale buoyant turbulent flames

Current ability to predict radiation heat release from large fires via numerical simulation is limited by the lack of affordable models for the coupled effects of radiation and soot formation. Predicting soot loadings and temperatures is complicated by the interrelationships between many physical processes which act on the soot. These processes act over a range of length and time scales which is prohibitively expensive to resolve in a transient, 3-D numerical simulation. The computational expense and the complexity of the interactions motivate the development of a simplified reacting flow model in which some of the relevant interactions may be more easily studied. In the present work the flow field is assumed to follow a boundary layer assumption with Boussinesq forcing, gas phase composition is assumed to be a unique function of the mixture fraction and enthalpy, and soot formation rates are assumed to follow a simple two-equation model for particle inception, surface growth and coagulation, and oxidation. A One-Dimensional Turbulence (ODT) simulation is employed to evolve a collection of 2-dimensional mixing states representative of turbulent mixing in the simplified model fire. Stochastic variations between realizations of the simulation are reproduced similar to the variation in instantaneous snapshots of a turbulent flow field; the statistical solution can be found with a Monte Carlo approach. The evolution of soot distributions are explored and factors affecting this evolution are identified, which include the (relatively slow) soot formation rates, the history effect introduced by radiation losses, and molecular transport processes which act differently on soot particles than on the gas phase species. Sensitivities to modeling parameters for these processes are assessed. The evolution of the mixture fraction PDF is identified as a key driver for soot transport in mixture fraction space, whereas thermophoretic transport is shown to be negligible in fully turbulent fires. The opportunities afforded by full resolution of species and temperature fields are demonstrated by a study of spectral radiation emission explaining the experimental observation that radiation from a fire can be fit approximated as that from a gray body