Burning velocity of spherically propagating turbulent flames kept increasing with flame propagation. On the other hand, turbulent burning velocity obtained from burner stabilized flame is constant for a given turbulence intensity. This difference of turbulent burning velocity characteristic was discussed by Abdel-Gayed and Bradley et al. For spherically propagating flames, not all turbulent eddies contribute to the turbulence that affect the turbulent flame. Small flames may be wrinkled only by small scale turbulent eddies. As the flame propagates, it becomes progressively wrinkled by the larger eddies. The turbulence which contributes to turbulent flame may increase with the increase in flame radius. This increase in turbulence may cause the increase in turbulent burning velocity. The effective turbulence intensity was proposed by Abdel-Gayed and Bradley et al. as the turbulence intensity which contributes effectively to turbulent flame. In this study, the turbulence characteristics which are necessary for obtaining the effective turbulence intensity were evaluated by Particle Image Velocimetry (PIV) measurement. Then turbulent burning velocity of iso-octane/air flame was investigated with effective turbulence intensity. The turbulent burning velocity increased with the increase in effective turbulence intensity. The increase in turbulent burning velocity may be caused by the increase in flame front area with increase in the effective turbulence intensity.
Spherically propagating laminar and turbulent flames were studied using iso-octane / air mixtures with and without dilution. The main purpose of this study is to clarify the influence of thermo-diffusive effects on the turbulent flames. In order to examine the thermo-diffusive effects solely by separating them from the effects of flame stretch, turbulent burning velocities were compared at constant flame stretch factors. The mean flame stretch factor acting on turbulent flame front may be represented by the turbulence Karlovitz number. Thus, turbulent explosions were carried out at fixed turbulence Karlovitz numbers. The ratio of turbulent burning velocity to unstretched laminar burning velocity increased with the equivalence ratio for non-diluted mixtures at fixed turbulence Karlovitz numbers. And this ratio for CO2 diluted mixtures was larger than N2 diluted mixtures. The Markstein number that denotes the sensitivity of the flame to thermo-diffusive effects depends on the equivalence ratio and diluents of the mixture. The ratio of turbulent burning velocity to unstretched laminar one increased with decreasing Markstein number. Especially, it changed stepwise around Markstein number of zero. However, the burning velocity ratios did not increase with increasing mixture pressure although the Markstein number decreased with pressure.
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