Assessment of Semi-Empirical Soot Modelling in Turbulent Buoyant Pool Fires from Various Fuels

Abstract

The objective of this work is to assess the accuracy and limitations of two different semi-empirical soot models: the Laminar Smoke Point (LSP) and soot-yield approach. A global soot formation model based on the LSP concept is embedded within FDS6.7. Quantitative comparisons were made from turbulent buoyant pool fires between several computational results and well-instrumented experimental databases on the soot volume fraction, mass loss rate, heat release rate and gas temperature. The LSP model in combination with soot oxidation and surface growth is validated for most of the methane, ethylene and heptane turbulent buoyant pool fires, covering a wide range of fuel likely to form soot. This paper aims to broaden the scope of the validation of the available semi-empirical soot modelling. For the porous methane and ethylene burner, the LSP model was found to provide a better description of the soot volume fraction. The overall visual soot distribution is also numerically reproduced with the soot-yield approach, but as expected, there are some differences between the prediction and the measurement regarding the magnitude of soot volume fraction. The computed radiant heat flux was compared with experimental data for heptane flame, showing that predictions using both the LSP and soot-yield models were found to be twice the value of experimental data, although the measured HRR (Heat Release Rate) is reliably reproduced in the numerical simulation. For the heptane buoyant pool fires, a sufficient accuracy of the numerical model is confirmed only in some of the locations as compared to the experimental results. It is demonstrated that neither the temperature nor the soot volume fraction can be reliably calculated in the necking flame flapping region when the pyrolysis rate of condensed fuel (heptane) is coupled with radiation/convection heat feedback. This implies that an accuracy of prediction on the turbulent buoyant pool fires depends on the studied fire scenario regardless of the semi-empirical soot models.