OCR Text |
Show 7 emission were studied changing the location of the fuel injection nozzle. The variation of emission index of nitric oxides versus inlet air temperature and inlet air velocity is shown in Fig.4. The inlet air velocity was calculated by the air flow rate and the inlet area of the chamber taking into account of the volumetric increase of air with temperature. For fuel injection nozzle 1, it is quite natural that the emission index of nitric oxides showed a monotonous increase with the temperature rise of the inlet air. However, high emission indices for high inlet velocity, hence for high excess air ratio, was an unexpected tendency. It can be ascribed to the observed fact that a block of flames grew at the center of the recirculating flow with the increase of the inlet air velocity. For fuel injection nozzle 2, the emission level was extremely high. The large fractions of the injected fuel formed a block of flames at the center of the recirculating flow, emitting strong radiation, where the heat release rate and nitric oxide formation looked concentrated. The general inclination of nitric oxides emission behaved adversely in terms of excess air ratio, defined by the supplied fuel and air flow rates. It means that the high production rate of nitric oxides around the center of the recirculating flow dominated the emission level even in large excess air ratios. For fuel injection nozzle 3, the relatively low nitric oxides emission showed a quite different tendency in terms of global excess air ratio or inlet air velocity in contrast with the former cases. The emission level decreased with the excess air ratio. A long jet-like flame with a blue portion near the fuel nozzle was observed when the inlet air velocity was low, while it gradually faded away with the increase of the inlet air velocity. Nevertheless, the dependence of nitric oxides emission on the inlet air temperature was unchanged, the higher the inlet air temperature, the more nitric oxides was emitted. Finally, for fuel injection nozzle 4, the emission index of nitric oxides showed scarce dependence on either inlet air velocity or its temperature. Adding this fact, the emission level was extremely low for any tested conditions. The typical feature of the flame differed from those in the previous cases, that is, distributed combustion seemed to occur over the combustion chamber because vague and weak flames like a fog spread everywhere, probably "flameless oxidation" named by Wunning [14]. Judging from the intensity of radiative |