Measurements of premixed-flame propagation in intense turbulence

Turbulent burning velocities of methane-air mixtures at atmospheric stoichiometry are measured in a three-dimensional form of fully developed turbulent flow field. The intense turbulence is generated in a new cruciform burner that consists of two cylindrical vessels. The long vertical vessel is used to provide a stable downward propagating premixed flame at one atmosphere via ignition and simultaneous opening of four large venting valves at its top. The horizontal vessel is equipped with two identical counter-rotating eight bladed fans at each end, driven by electric motors which are synchronized to the same speed. The motor-driven fans with controllable frequencies up to 7,620 rpm can generate two intense counter-rotating large vortical streams. Each of both turbulent streams passes through a specially designed perforated plate, intertwining with each other. Thus, a nearly isotropic turbulence with large turbulent intensities (up to 450 cm/s) located in the core region between two perforated plates can be generated, as verified by extensive LDV measurements. Four methods, including a two-camera method, the hot-film anemometer, the photodetector, and an ion-probe method, are applied to measure turbulent burning velocities. Among them, the ion-probe method has least uncertainty within 30%. It is found that when the normalized turbulent intensity (u′/S_L) is less than unity, values of the normalized turbulent burning velocity (S_T/S_L) increase linearly with u′/S_L, where S_L is the laminar burning velocity. The S_T/S_L plots tend to depart from the linearity as u′/S_L>1 and bend gradually towards the horizontal for larger u′/S_L up to 11. These results are compared to earlier premixed gaseous experiments using different apparatuses. Discussion of the similitude and discrepancies among these experiments is offered.