This paper presents a joint experimental and computational study on the effect of bluff-body diameter on the flow field and residence time distribution (RTD) in the recirculation zone of turbulent non-premixed bluff-body ethylene/nitrogen turbulent flames. The velocity was measured with a two-component particle image velocimetry (PIV) system, with a novel optical design, utilizing polarization filters, to enable successful measurements in highly sooting flames. The measured mean velocity components for a 50-mm bluff-body burner are reported. The central jet diameter of 4.6 mm was supplied with a mixture of ethylene and nitrogen (4:1, by volume) to achieve a bulk Reynolds number of 15,000. The annular bulk velocity of the co-flowing air was kept at 20 m/s. Computationally, a 3-D RANS model was developed in ANSYS Fluent software, and the results were validated against the experimental data. The model was then used to investigate the flame structure, particularly in the recirculation zone, for a set of three bluff-body burners with varied outer diameter namely; 38mm, 50mm and 64mm. All other geometrical, boundary and initial conditions remained the same. A stochastic tracking model was employed to estimate the pseudo-particles residence time distribution in the recirculation region. The PIV measurements show two distinct vortices, an outer and inner, within the recirculation zone, which extends to 84mm above the exit plane. These results are consistent with previous flow field measurements in non-sooting flames, albeit with shorter recirculation zone. The computations reveal that increasing the bluff-body diameter, from 38mm to 64 mm, results in an increase in the recirculation zone length by a factor of two, and tripling of the residence time of the reactants in the recirculation zone. The effect of these differences on soot propensity and transport will be the subject of further investigation.
|Title of host publication
|12th Asia-Pacific Conference on Combustion, ASPACC 2019
|Published - Jan 1 2019