Stability and Structure of Burner Flames

Abstract

It has been shown that the condition of equality of gas and burning velocities, required for stabilization of a flame above a burner, is established near the rim of the orifice or an obstruction within the stream by the effects of friction and inhibition of the explosive reaction. This condition is maintained between two critical gradients of the gas velocity at the solid surface, the lower gradient bordering on the flash‐back and the upper gradient on the blow‐off range. Values of the gradients in the range of laminar flow were determined by hydrodynamic equations from gas flow and tube dimensions; and their independence of tube diameter, except for extreme sizes, has been demonstrated both for upright and inverted flames. In the latter the critical velocity gradient for blow‐off was also found to be independent of the diameter of the centrally mounted wire within a considerable range. The effect of the surrounding atmosphere on the critical blow‐off gradient has been shown. The gas‐flow pattern was studied experimentally by photographing stroboscopically illuminated magnesium oxide dust particles. No appreciable redistribution of velocities over the cross section of the stream was observed below the combustion zone. The burning velocity was found constant over the surface of the inner cone, except at the tip, where it increases to the axial gas velocity, and at the base, where it decreases to zero. The experimental flame cone outline and flow pattern agree with the theoretical within the limitations imposed by simplifying assumptions. The temperature distribution in the flame was determined by the sodium line‐reversal method, only the center of the flame being colored, and observations of the width and emission spectrum of the luminous combustion zone were made. In natural gas‐air flames C–C and C–H bands are observed in the zone, and the temperature rises gradually behind the zone to a maximum that corresponds to the theoretical flame temperature. In natural gas‐oxygen flames no C–C or C–H bands are observed; and the temperature attains a maximum, exceeding the theoretical, immediately behind the combustion zone. Temperature distribution and flow pattern have been correlated. The flow pattern of inverted flames was also studied with particular attention to the region of flame attachment just above the central wire where the formation of an annular vortex is demonstrated. An interpretation of the formation of polyhedral flame cones, described by Smith and Pickering, has been given.