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Show transfer to the load reached its maximum value. The droplet size distribution in position 25 was within a narrow band with the SMD (D32) size at its lowest level, indicating good fuel atomisation. The favorable influence of finer atomisation on NOx emissions is consistent with quasi steady-state dropletcombustion predictions (7) as well as air-fired spray flame experiments (e.g. ref. 8 & 9). In the current experiments a controlled air leak to the furnace provided nitrogen for thermal NO formation . It is interesting to note that when the atomiser was placed at position 30 nun from the burner tip, the exhaust NO concentration and exhaust temperature were reduced significantly. In addition, heat transfer to the load showed a significant increase. Visual observations indicated a more stable and significantly more luminous flame at the nozzle position of 30 nun in comparison with all other nozzle positions. The experimental results also showed a significant influence of spray angle on NOx formation. The differences in the NOx concentration might be due to the different mixing phenomena of the fuel with oxygen reSUlting in higher flame temperature when 500 angle nozzle is employed for frring. The 250 angle nozzle showed delayed mixing which may result in lower flame temperatures and therefore lower NOx concentrations. The position of the injector inside the burner shows a significant effect on the conversion of NO to N02. It can be seen from Fig. 5b that the 500 angle nozzle at a position of 30 nun inside the burner produced the lowest conversion of NO to N02. The experimental results indicate that the conversion of NO to N02 near the burner region seems to increase with an increase in spray angle, however, it becomes more uniform at the later stage within the furnace. It is expected that close to the burner, rapid cooling of the combustion gases within the sampling probe may have influenced some of the measured data. COMBUSTION EFFICIENCY: The variation of the exhaust temperature with SMD of the droplets produced from the 500 angle injection nozzle are shown in Fig.6a. Experimental results indicate that exhaust temperature reduces as the SMD values for the droplets were reduced. When the SMD of the droplets increased from 55 Jlm to 62 Jlm the exhaust temperature increased by approximately 30 K. Although turbulent spray-combustion phenomena are very complex, nozzle positions which produced droplet distributions with lower SMD values seem to have resulted in more favorable combustion characteristics. In particular, this can be highlighted by examining the variation of heat flux and CO concentration with SMD for the same injector nozzle as shown in Fig. 6b-c. The heat flux for smaller droplets i.e. 55-59 Jlm resulted in approximately 95 kW/m2 heat transfer to the load while firing with larger droplets of 60-63 Jlm resulted in a 10-150/0 reduction in heat flux to the load. Therefore, sprays with finer droplets can generate a stable and high intensity flame lcharacterized by an increase in combustion efficiency. The relationship between droplet size and combustion efficiency was further investigated by examining the variation of CO with SMD for injector nozzle 500 angle in Fig 6c. Here it is seen that CO exhaust concentrations increased significantly as the SMD values increased. The burner was specifically designed to obtain a downwardly positioned flame with uniform heat-transfer characteristics. The velocity field generated by the flame in the test furnace confrrm the computational results. Extensive in-flame measurements were done in the rapid-heating test furnace. The rate of heat transfer to the sectioned thermal load indicated longitudinal uniformity of the heat transfer within approximately 250/0 over four to five flame lengths. Fig. 7 shows the variation of heat flux to the sectional thermal load for the three nozzles under investigation. Here, the heat flux to the load reached its maximum value in the first two segments of the furnace for the 500 angle nozzle. The 250 angle nozzle, however, shows a slight increase in heat flux around the middle of the furnace as a result of longer flame lengths. Consequently, as heat fluxes from this nozzle were relatively low in the first two segments overall thermal efficiency was also lower. Nature of Flames. The flames were found to be highly luminous because of soot formation and oxidation at very high temperatures. Using the measured flame lengths with the computed flow field, overall burning-rate constants can be extracted. The burning-rate constants of the order of 4 nun2/s estimated for the present burner are generally lower than values found earlier for similar fuels (10). The values found here are actually lower than the values found for single gas-oil |
