Numerical investigation of burner positioning effects in a multi-burner flameless combustion furnace

In this paper results are presented of a numerical study performed for four different burner configurations in a furnace equipped with three pairs of flameless combustion burners firing Dutch natural gas. The simulations have been validated against previously published results of an experimental study . The commercial Computational Fluid Dynamics (CFD) code Fluent 6.3 was used for the calculations. Using the Eddy Dissipation Concept (EDC) model for turbulence–chemistry interaction in combination with the realizable k–ε model for turbulence and a skeletal chemistry mechanism, the main furnace performance was consistently reproduced for all the investigated burner configurations. Moreover, it was found that due to relatively low Reynolds numbers in the cooling air flow in the annulus of the cooling tubes, predictions of the heat extraction rates of these cooling tubes were improved by treating the flow in the cooling tubes as laminar. Furthermore, the applied error tolerance of the ISAT procedure was insufficient for accurate species concentration predictions, however, based on analysis of the main species concentrations in the flue gas, this inaccuracy did not influence the overall predictions. The most important experimental results have been investigated using the CFD simulations. Firstly, a longer path length from the firing burners to the stack, compared to the path length to the regenerating burners, explained the lower CO emissions in the flue gas in the stack. Secondly, it was found that a recirculation zone between the upper firing burners and the stack in configurations C4 and C5 resulted in a smaller fraction of the flue gases leaving the furnace via the stack compared to the other configurations. Thus, a larger fraction left the furnace via the regenerating burners and this resulted in higher preheat temperatures of the combustion air. Furthermore, more pronounced recirculation zones in configurations C3 and C4 led to higher temperature uniformities in the furnace. Finally, it was confirmed that the jets of the burners in configurations C1 and C3 showed similar merging behavior, leading to similar NO emissions, as observed in the experiments