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UBC Theses and Dissertations

Optical properties of soot aggregates considering the external mixing hypothesis for non-premixed flames Babaee, Keyhan


Soot is one of the important contributors to climate change and has adverse effects on humans’ health and the environment. Some of these impacts can be modeled based on the soot morphological and material properties that influence the optical properties of the soot. Uncertainties around the optical properties can lead to unreliable climate prediction models; therefore, accurate modeling and calculation of soot optical properties can mitigate this issue. A new model is implemented using existing equations to describe the relationship between aggregate size and their primaries: the external mixing hypothesis (EMH). The EMH model is based on the recent studies for non-premixed flames, which quantifies the relationship between primary particle diameter and aggregate mobility diameter based on the mass-mobility exponent and the effective density. Compare to the constant primary particle size assumption and using the Rayleigh-Debye-Gans theory for fractal-like aggregates (RDG-FA) as the optical model, EMH demonstrates an increase in size-dependent and the total mass-specific scattering cross-section (MSC), which is an essential factor in climate modeling. RDG-FA neglects spherule-to-spherule interactions, which reduce its accuracy. In the next step, the multiple-sphere T-matrix method (MSTM) is used as the optical model. A previously published database of MSTM is coupled with the EMH. EMH-MSTM is able to model the variation of mass-specific absorption cross-section (MAC) for different aggregate sizes, which is not resolved by RDG-FA. The MSTM results show that size-dependent and total MAC and MSC levels are increased relative to the RDG-FA predictions, as expected, due to sphere-sphere interactions. Next, an experiment at the University of Alberta used a laboratory buoyant turbulent diffusion flame to produce soot for the model’s validation. Based on the model results, the best agreement can be expected to be for Dm ≈ 2.6, ρ(eff,100) ≈ 600 kg/m^3 and σp|dm ≈ 1.5 for the model and experiment datasets. The uncertainty around the refractive index and other variables such as mass-mobility exponent and the effective density may have caused this inconsistency between model and experiment MSC results. Providing more accurate input information to the model can increase its ability to estimate the optical footprint of the soot aggregates.

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