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Show The Alberta Study Efficiencies of Low-Momentum Jet Diffusion Flames in Crosswinds M. R. JOHNSON and L. W. KOSTIUK Combustion and Environment Group, Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G8 Low-momentum propane, natural gas, and propane/CO2 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 (Vj ) 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!/Vj 1/3 and not R. © 2000 by The Combustion Institute INTRODUCTION A diffusion flame burning in a crosswind is a basic combustion problem that has many appli-cations. A common application occurs in the energy and petrochemical industries where this configuration is relevant to gas flaring-the process of disposing of unwanted flammable gases by combusting them in a flame in the open atmosphere. In 1996, approximately 115 billion cubic meters of natural gas were flared or vented by these industries worldwide [1]. Since methane has a 21 times greater global warming potential than the carbon dioxide produced from its combustion (on a mass basis) [2], it is important that the combustion be as complete as possible. The level of combustion completion of these flames is quantified by the ratio of carbon-mass originally in the hydrocarbon fuel to the carbon-mass in the produced carbon dioxide. This carbon mass balance across the flame is often referred to as the "combustion efficiency" of the flame. Published research on the combustion effi-ciencies of jet diffusion flames burning in an open environment is limited. In a prominent study by Pohl et al., gaseous hydrocarbon fuels issued from 0.076 to 0.305 m diameter vertical pipes and burned in near quiescent conditions [3]. The entire plume of combustion products was collected, sampled, and analyzed to calcu-late the combustion efficiency. They concluded that if the flames were stable (which was a function of the exit velocity and heating value of the fuel gases), the efficiencies were greater than 98%. Other investigators introduced wind that blew perpendicular to the fuel jet as an-other experimental parameter [4, 5]. Siegel [4] subjected his 0.7-m-diameter flare to mild cross-winds and, through multipoint sampling found efficiencies greater than 99%. However, his apparatus only applied the wind to the flame above the stack and not to the stack itself. This flow configuration would not have created vor-tex shedding from the stack or a low-pressure region in the wake of the stack, which has an important influence in the overall flow field [6, 7]. Kuipers et al. [5] used passive Fourier trans-form infrared and differential absorption light detection techniques to remotely analyze the plume of a 0.6-m-diameter flare burning pri-marily methane in the open environment. They found efficiencies greater than 99%, but com-mented that lower momentum jets may have reduced efficiencies when exposed to a cross-wind. In contrast to the works described above, Strosher [8] reported field measurements of a *Corresponding author. E-mail: Larry.Kostiuk@ualberta.ca COMBUSTION AND FLAME 123:189 -200 (2000) © 2000 by The Combustion Institute Published by Elsevier Science Inc. Johnson and Kostiuk did indeed find lower flare combustion efficiency in crosswinds. However... |