OCR Text |
Show NOx-reduction effectiveness of FIR over conventional FOR in practical applications. All conditions run were free from any heat losses through the boundaries. From these investigations, we draw the following conclusions: • When the nozzle exit velocities are maintained at fixed values as N2 is added to either the air or fuel streams, fuel-side dilution results in significantly higher NO emission indices than for air-side dilution. Detailed analysis of the results indicate that the higher NO is a consequence of the N2 addition to the fuel causing the flame to lie closer to the stagnation plane in a region of lower velocity. The lower velocities, in turn, result in longer residence times for NO fonnation. • When the fuel mass flux is maintained constant as diluent is added to the fuel, and the air stream velocity is simultaneously increased to match that of the diluted fuel stream, NO emission indices are considerably lower than for equivalent air-side dilution. This case more realistically represents the situation in practice than does the fixed-velocity case above. For these conditions, all major factors contributing to NO fonnation are affected: temperatures are lower, residence times are shorter, and the detailed chemical pathways producing NO are altered. Experiments NOx and CO emission indices, N02-to-NOx ratios, and visible and soot-free flame lengths associated with laminar Cf4 -air jet flames were measured for a variety of flame conditions. Experiments were conducted in which either the air stream or fuel stream was diluted with N2, with the range of diluent fractions nominally the same as in the numerical simulations. Conclusions drawn from these experiments are the following: • Fuel-side dilution results in somewhat greater NOx emission indices than air-side dilution. The effect is more pronounced at the higher reactants temperature investigated (400 K versus 293 K). Peak temperatures also are higher with fuel-side dilution. • The higher NOx emissions with fuel-side dilution are likely a consequence of the higher peak temperatures. The higher temperatures, in tum, are thought to result from decreased radiation from the fuel-diluted flames since in situ soot formation is greatly diminished compared to equivalent air-diluted flames. Experiments conducted with flames having no in situ soot 15 |