Vortex formation mechanism within fuel streams in laminar nonpremixed jet flames

Abstract

A vortical structure occurring at the fuel stream in laminar nonpremixed jet flames was recently found and shown to have both a fluid-dynamic impact on the flow field and a possible influence on the flame stability and soot formation. We designed a systematic experiment and numerical simulation to investigate the physical mechanisms of this recirculation phenomenon in a coflow system. We hypothesized that a negative buoyancy, caused by the fuel jet being heavier than the ambient air, may play a significant role in the recirculation. Therefore, we experimentally varied the density of the fuel jet using a binary mixture of methane and n-butane, and tested the density of the coflow oxidizer by replacing nitrogen with carbon dioxide. Several fuel jet velocities, flame temperatures, and nozzle diameters were also studied to thoroughly investigate all parameters that might possibly affect the recirculation. As a result, we found that our modified Richardson number, which is based on the cold density difference between the fuel and the coflow, the flame length, and the jet momentum flux, explained the physical mechanism of the recirculation well, with Ri ~ 60 being the critical value for formation of the recirculation. The negative buoyancy was the primary driving force behind the recirculation, while the jet momentum mitigated its formation.

Numerical Study of NOx and Soot Formations in Hydrocarbon Diffusion Flames

“…On the other hand, the increase of the prediction of NO and soot in propane and n -butane flames in the heated nozzle could be partly related to the enhanced formation of the recirculation flow due to their heavier density properties, , where the formation of the vortices in both flames increased in size with heated nozzle condition. This recirculation zone could redirect the flow in the nozzle exit region and increased the heat accumulation, which led to increasing of the overall flame temperature and the subsequent NO and soot formations.…”

“…The thickness of the nozzle wall was set as 1 mm, and the radius of the fuel inlet was set as 5.4 mm. Based on the literature study, , a parabolic velocity profile of the fuel inlet stream was set at 70 mm below the fuel nozzle exit to ensure a realistic flow field could be obtained at the nozzle exit. The coflow air inlet and the fuel inlet of the model burner were set as velocity inlet with the conditions of zero-gauge pressure.…”

Enhancement of biogas/air combustion by hydrogen addition at elevated temperatures

“…It is worth mentioning the importance of the inlet location for fuel/air mixture in the geometry of the model. Based on the main findings from the studies related to the combustion of the bunsen flames, the present study adopted the idea of setting the fuel/air inlet at 70 mm upstream from the nozzle exit. This is done in order to promote a more realistic velocity profile at the nozzle exit, as applying the fuel inlet boundary conditions at the nozzle exit tends to produce overestimation of the results.…”

Experimental investigation of heat transfer characteristics of an inverse diffusion flame in a coaxial tube burner for with and without swirl

“…The emissions of LPG combustion in an IDF burner is carried out with the addition of hydrogen by Miao et al [8]. Cha et al [9] focused on a recirculating flow structure of a normal diffusion flame and compared with numerical simulation.…”