The Effect of Changes in the Flame Structure on the Formation and Destruction of Soot and NO_x in Radiating Diffusion Flames

摘要

In this study, soot and NO-x production in four cunterflow diffusion flames with different flame structures is examined both experimentally and theoretically. The distance between the maximum temperature zone and the stagnation plane is progressively changed by changing the inlet fuel and oxidizer concentrations, thus shifting the flame location from the oxidizer side to the fuel side of the stagnation plane. One flame located at the stagnation plane is also examined. Detailed chemical, thermal, and ptical measurements are made to experimentally quantify the flame structure, and supporting numericla calculations with detailed chemistry are also performed by specifying the boundary conditions used in the experiments. Results show that as the radical-rich, hig-temperature reaction zone is forced into the sooting zone, several changes occur in the flame structure and appearance. These are the following: (1) The flames become very bright due to enhanced soot-zone temperature. This can cause significant reduction in NO formation the to increased flame radiation. (2) OH concentraitonis reduced from superequilibrium levels due to soot and soot-precursor oxidation in addition to CO and H_2 oxidation. (3) Soot-precursor oxidaiton significnatly affects soot nucleation on the oxidizer side, while soot nucleation on the fuel side seems to be related to C_2H_2_ concentration. (4) Soot interacts with No formation through the major radical species produced in the primary reaciton zone. It also appears that the Fenimore No initiation mechanism becomes more important for low-temperature flames and when N-2 is added to the fuel side, due to higher N-2 concentration in the CH-rich zone.