One of highly appropriate methods for increasing efficiency of natural gas furnaces is the flame heat radiation enhancement which raises the radiation heat transfer rate. In this experimental investigation, the effect of inlet gas temperature on radiative heat transfer rate is studied. For a wide range of inlet gas temperatures from 20 to 450oC, combustion products and exhaust gas temperatures were measured and analyzed. Results revealed that increasing inlet fuel temperature as high as 270oC, increases both NOx concentration and exhaust temperature. But by further increase in fuel temperature, NOx concentration and exhaust temperature are decreased rapidly. Furthermore, flame visualization in the experiments proved that increasing inlet gas temperature for more than 270oC results in a significant change in flame color from blue to yellow which demonstrates flame radiation enhancement that implies greater heat transfer efficiency. Increasing flame radiation not only improves the overall furnace efficiency but also reduces NOx formation as a result of lower flame temperature which is followed by higher radiation rate.

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