Figures 9(a) and 9(c) show that when using SFOR the reactions occur in a well defined envelope enclosing the internal recirculation zone and further downstream pockets of cold unreacted gas.
This large flame volume can be explained by several factors, namely, : (1) given the low energy density in mass base of biomass, considerable mass flows of transport air need to be used to supply the furnace with a certain thermal input; therefore particles will be fed with a high momentum that allows the particles to penetrate through the internal recirculation zone; (2) large biomass particles, as compared to coal, typically show higher volatile yields and longer devolatilization times, spreading the reaction of the biomass particles over a larger volume; (3) the high volatile yields of biomass create local fuel-rich volumes in a section of the furnace where mixing is more limited due to the low gas velocities, which limits the homogenous gas-phase reactions.
The standard K-epsilon turbulence closure model was used for economical flow solutions, and the more rigorous Reynolds stress turbulence model was used in areas where accuracy was critical, as in highly swirling internal recirculation zones
. Fluent's preprocessor, Geomesh, generated a 100,000-cell model of the existing burner.