The near field combustion region of the multi-jet CGRI burner has been modelled as a two step methane/air reaction system; finite rate chemistry results correspond well with previously reported experimental data, while the eddy dissipation model produces qualitatively erroneous results. The burner nearfield consists of 14 jets (seven strong oxidant jets and 7 weaker fuel jets) which form a ring around a central pilot. This generates a unique flow field in which fuel jets are quickly entrained with furnace gases into a central core oriented along the burner axis. Such jet arrangement accomplishes high levels of flue gas re-circulation without the need to premix oxidant or fuel streams with the flue gases outside the combustion chamber.
CFD modelling was performed using FLUENT with k-e turbulence model and 2-step chemistry for the methane/air reaction. The results agree well with the flow field and the species concentration data and reveal rather complicated flow pattern. The success in the near field modelling provides confidence in the current attempts to execute full furnace modelling with CGRI burners installed.
Studies to date on this system make it apparent that the simplified model of a single pair of high-momentum oxidant and low-momentum fuel jets issued into the combustion chamber can only partially reveal the key features of a jet-ring mixing patterns. Detailed studies of the full ring of jets revealed the intriguing mixing phenomenon whereby the low-momentum gas jets are entrained not into the high-momentum oxidant jets directly, but rather between them right into the core region of a multiple jet arrangement. This feature of a multiple jet ring greatly enhances the overall jet mixing pattern stability and further postpones the final mixing of the two sets of streams, while each of them is separately highly diluted by the entrained combustion products. The dilution in turn slows down chemical reaction considerably, to the point where concentration maxima of oxygen and methane overlap, indicating slow chemical reaction, even at the temperature around 1200 K.