Modeling of radiative heat transfer in strongly forward scattering media of bubbling fluidized bed combustor with and without recycle

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2017
Şen, Özge
Predictive accuracy and CPU efficiency of geometric optics approximation (GOA) and scattering phase function simplifications in the freeboard of lignite-fired METU 0.3 MWt ABFBC Test Rig were tested by applying them to the modeling of radiative heat transfer and comparing their predictions against measurements and benchmark solutions. The freeboard was treated as 3-D rectangular enclosure containing grey, absorbing, emitting gas with grey, absorbing, emitting, non/isotropically/anisotropically scattering particles surrounded by diffuse grey/black walls. A three-dimensional radiation model based on Method of Lines (MOL) solution of Discrete Ordinates Method (DOM) coupled with Grey Gas (GG) for gases and Mie theory / GOA for fly ash particles in rectangular coordinates was extended for incorporation of anisotropic scattering by using normalized Henyey-Greenstein (HG) for the phase function. The input data required for the model and its validation were provided from the experimental data, which was previously taken from METU 0.3 MWt ABFBC Test Rig operating with and without recycle of fine particles. Assessment of GOA in terms of predictive accuracy reveals that applicability limit of GOA should be based on cumulative cross sectional area distribution rather than surface mean diameter or cumulative weight distribution of particles. From the viewpoints of computational economy, GOA was found to improve CPU efficiency of the solution with increasing optical thickness. Predictive accuracy and computational efficiency of scattering phase function simplifications were tested by comparing their predictions with those of forward scattering with HG. Comparisons reveal that phase function simplifications have insignificant effect on incident heat fluxes whereas source terms were found to be sensitive to phase function simplifications and the sensitivity increases with increasing optical thickness. Improvement in CPU efficiency of phase function simplifications is observed in only combusting systems involving high particle loads; however, in those systems, accurate representation of particle scattering that is forward scattering is vital.