Experiments were carried out to find out the effect of different TFGR ratios on flame characteristics, pollutant emission, temperature, luminosity, IR radiation and total radiation of natural gas flames. A spiral duct connected tangentially to the furnace inlet linked the furnace to the chimney. A centrifugal fan was installed on the spiral duct to suck the combustion products and a spiral duct back-draft round damper was installed to adjust the FGR volume flow rate. The optical properties of the flame were studied for visible and IR wavelengths by a TES-1332 A digital luminance meter, a BOMEM FTIR and IR flame photography. Also, the total radiation of flame was determined by an SBG01 heat flux sensor. Furthermore, an S type thermocouple was used to measure the flame temperature. The results indicate that TFGR changes the nonluminous natural gas flame to a luminous flame by the dissociation of the recirculated CO2 to CO. TFGR enhances the tangential momentum of flame and generates a voluminous flame, increases the heat transfer area of flame and eliminates the hot-spot region in the core of flame, which decrease the axial flame temperature and thermal corrosion. Furthermore, the concentration and residence time of CO2 and H2O, as the most important radiative species of natural gas flames, increase in the flame reaction zone, which results in the enhancement of the IR emissivity and the radiation heat transfer of natural gas flame. Finally, although the TFGR increases the concentration of CO emission to a level lower than the standard level, it causes natural gas burners to emit significantly less dangerous NOx.

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