Efficiencies of low-momentum jet diffusion flames in crosswinds

Abstract

Low-momentum propane, natural gas, and propane/CO 2 diffusion flames in a crosswind were studied experimentally using a closed-loop wind tunnel. Flames were established at the exit of a burner tube mounted vertically in the wind tunnel and perpendicular to the airflow, a configuration that is relevant to continuous gas flaring in the atmosphere. Analysis of the products of combustion showed that inefficiencies result from fuel stripping, and photographic 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 momentum flux ratio ( R) of the two streams. Other features of the flame, such as burning 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 ∞) adversely affects the efficiency, while increased jet exit velocity ( V j ) makes the flame less susceptible to the effects of crosswind. Consideration of buoyancy and momentum forces as defined by a Richardson number successfully predicted the velocity dependency of the combustion inefficiency as being U ∞/ V j 1/3 and not R.