Combustion Science and Technology vol:137 issue:1-6 pages:149-170
This paper presents a direct Reynolds stress, stretched flamelet model for the premixed swirling turbulent reacting flows. The model adopts a statistical presumed joint pdf of turbulent flame stretching and flame wrinkling, together with an established laminar flame data library for fuel oil. The main virtue of the model is that the application of the model is not restricted to a single combustion regime. It is capable of realizably extending to both the fully chemically controlled combustion condition (the laminar flame model) and the fully mixing controlled combustion condition (the eddy break-up model).
The scale and NOx characteristics of a pre-vaporized, premixed fuel oil burner have been studied using the model. It is found that combustion confinement has a profound effect on the design of such a burner. There exists an optimum chamber/burner diameter ratio, of about 3:1, at which a premixed blue flame could be stabilized and pollutant emissions are minimized. A very short chamber length adopted in the compact design will result in a higher NOx emission. And there also exists an optimum fuel/air equivalence ratio, of about 0.8. Detailed scale analysis shows that such a burner flame is characterized as flamelet in eddies, with both the turbulence and chemical kinetics playing a role in the combustion process.