The aerodynamic, chemical and thermal aspects of the mild combustion process have been studied with emphasis on mixing rates, flue gas recirculation and strong shear produced by reactants supplied from discrete jets. Time-averaged and instantaneous structures of turbulent flow were examined by visualization and local measurements within a 5400 W burner operating with methane with an overall equivalence ratio varying from 0.8 to 1.2 and at non-premixed and premixed modes. The results showed that the entrainment of the flue gases into the fresh mixture was very important for the initiation and progress of the reaction, and occurred in two successive mechanisms. Initially, the flue gases were driven with the reverse flow towards the annular exit where, by Biot–Savart induction, they acquired some momentum from the supply streams provided at the center. The resulting mixing process in the close vicinity of the burner was less intermittent and this was evident in relatively lower values of the second order moments of the residence time distribution. Slightly downstream, the second order moments were, however, increased by large-scale turbulent fluctuations and this led to the enhancement of the mixing process and introduced some further intermittency. The latter entrainment mechanism caused the flue gases to partially encapsulate the discrete jets, which resulted in islands of flammable mixture surrounded by the inert gases. Hence, as the instantaneous OH radical visualizations revealed, the reaction was only initiated away from the burner and in disconnected regions where the Rayleigh pictures showed strong temperature gradients. As the distance from the nozzle increased further, the reaction seemed to follow local flow patterns in that it progressed radially outwards with large structures, which resulted in an increased space-averaged temperature. Furthermore, the residence time decreased away from the burner and the flame came close to extinction due to the high stretching rates of the large structures. However, the flue gases entrained up to this point increased the inert content of the fresh mixture with chemical time scales comparable to the time scales of the flow. This allowed the reactants to attain temperatures near to those of the flue gases and to ignite with a small temperature rise, which led to a much lower thermal NO formation. The results also showed that when the equivalence ratio of the non- premixed mixtures was increased, the region where the combustion took place was shifted away from the burner and extended further downstream towards the roof. In the case of premixed combustion, however, the reaction started and terminated earlier and was confined to regions in close proximity to the axis. The emissions of OH radical occurred rather patchily and in relatively high concentrations.
Experiments in Fluids – Springer Journals
Published: Jun 5, 2001
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