A study of radiant tube flame structure and NOx emissions

The structure and NOx behavior of turbulent natural gas/air flames, confined in a quartz radiant tube, are experimentally studied. The configuration features initially unmixed reactants with fuel admitted through multiple holes and air admitted through mild swirl slots. Visible flame heights and colors demonstrate that two burning modes exist, one involving long, orange flames and another involving short, blue flames. Near-injector species concentrations and exhaust NOx measurements indicate that the short, blue flames exhibit varying degrees of fuel/air premixing. Enhanced mixing is caused by factors including swirl, hot wall stabilization, and acoustic interactions. The present results are compared with those of several past studies to gain further insight into flame structure. Operation of radiant tubes in the long, orange burning mode with a slightly fuel-lean equivalence ratio yields relatively low NOx emission index, more uniform wall temperature, higher heat transfer efficiency, and less audible noise than the short, blue mode.