An experimental study on thermal and radiative characteristics of natural gas flame in different equivalence ratios by chemiluminescence and IR photography methods

Natural gas, as the cleanest fossil fuel, has been widely applied in central heating systems and industrial burners. From the perspective of energy saving, abundant sources and enormous use of natural gas together with the high costs caused by the widespread use of this fuel make the investigation of natural gas combustion characteristics important. Consequently, in the present work, the thermal and radiative characteristics, and NOx pollutant emission, of a 100,000 kcal/h natural gas burner, widely used in building heating systems, were investigated in the equivalence ratios of 0.42, 0.62, and 0.72. Non-destructive measurements including chemiluminescence method and IR photography were used to investigate the flame characteristics. The results indicated that increasing the equivalence ratio (in the range of 0.42-0.72) moved the peak of flame temperature from flame up-stream to its down-stream and made the flame temperature distribution more uniform. Also it was found that whereas the maximum temperature of flame occurred in small equivalence ratios such as 0.42, the average temperature of flame was 7.8% greater and the average radiation heat flux was 62% greater in large equivalence ratios such as 0.72. The results also showed that increase in equivalence ratio up to 0.72 raised the rate of soot particles generation in the flame and enhanced the average luminosity and radiation of flame in near IR wavelengths and increased the average emissivity coefficient of flame as much as 20%. Moreover, in flame upstream region, the enhancement of radiation heat transfer by increasing equivalence ratio is dominantly as a result of increase in emissivity coefficient of flame, while in flame downstream region, increasing the flame temperature due to gradual combustion will be important in improvement of radiation heat flux.