This work presents measured fractal properties of lean premixed methane-air flames on two weak-swirl jet burners with 25-mm and 50-mm diameter nozzles at high turbulent Reynolds numbers, ReT = u'LI/ν = 350 ~ 2,500 based on r.m.s. turbulent fluctuating velocities and the integral length scale of reacting turbulence, where ν is the kinematic viscosity of reactants. The nondimensional turbulence intensity (u′/SL) and turbulent burning velocities (ST/SL) were measured via high-speed, high-resolution digital particle imaging velocimetry, where SL is the laminar burning velocity. The fractal dimension (D3) and cutoff length scales were determined by the caliper method from analyses of lifted stabilized wrinkled flame front images obtained by laser sheet tomography. It is found that values of D3 are only 2.22 and independent of u′/SL at least for 3 < u′/SL < 18.5 even at high-ReT reacting flows. This finding differs drastically with most previous studies using different burners that reported D3 ≈ 2.33 when u′/SL > 3, but it supports a recent study by Gülder and his co-workers using a Bunsen-type burner that found D3 ≈ 2.18 for much lower values of ReT ≈ 40 ~ 467. The outer cutoff scale (εo), which is comparable to LI, varies from about 12 mm to 10 mm when u′/SL increases from about 2 to 18.5, while the inner cutoff εi ≈ 0.1εo, suggesting that both εo and εi are rather insensitive to u′/SL. Finally, these fractal parameters obtained at high values of ReT cannot predict measured ST/SL data correctly when the available fractal area closure model is applied, indicating a need for further improvement of existing fractal models.
|State||Published - 2009|
|Event||7th Asia-Pacific Conference on Combustion, ASPACC 2009 - Taipei, Taiwan|
Duration: 24 May 2009 → 27 May 2009
|Conference||7th Asia-Pacific Conference on Combustion, ASPACC 2009|
|Period||24/05/09 → 27/05/09|