Efficiencies of low-momentum jet diffusion flames in crosswinds

Low-momentum propane, natural gas, and propane/CO2 diffusion flames in a cr osswind were studied experimentally using a closed-loop wind tunnel. Flames were established at the exit of a burner tube mounted vertically in the wi nd tunnel and perpendicular to the airflow, a configuration that is relevan t to continuous gas flaring in the atmosphere. Analysis of the products of combustion showed that inefficiencies result from fuel stripping, and photo graphic images link this process to changes in mean and instantaneous flame structure. Flame images also show qualitatively that a maximum mean flame length and the onset of downwash for different fuels are related to momentu m flux ratio (R) of the two streams. Other features of the flame, such as b urning in detached pockets and the disappearance of the flame tail, do not coincide at fixed values of R for different fuels. The measured combustion efficiency data show that increased crosswind speed (U-infinity) adversely affects the efficiency, while increased jet exit velocity (V-j) makes the f lame less susceptible to the effects of crosswind. Consideration of buoyanc y and momentum forces as defined by a Richardson number successfully predic ted the velocity dependency of the combustion inefficiency as being U-infin ity/V-j(1/3) and not R. (C) 2000 by The Combustion Institute.