The strong-jet/weak-jet problem: New experiments and CFD

The strong-jet/weak-jet coupling is basic to a new generation of ultra-low- NOx burners that are diffusion type and rely on lower flame temperatures th rough controlled mixing of the air, the fuel, and recirculated combustion p roducts. Further, to developing a theory on the isolated jet pair in a prev ious publication, where the jet trajectories were predicted and compared wi th experimental data with good agreement, new experimental results of cold modeling by using flow visualization and laser Doppler velocimetry (LDV) ar e reported in this paper. Mathematical modeling by using a commercial compu tational fluid dynamics code (CFD) was employed to predict the how field. C FD is also used to predict a mixing progress variable, R-12 for the three s tream mixing of air, fuel, and combustion products for the first time. Flow visualization reveals very interesting behavior of large scale structures and engulfment as the fuel and combustion air interact and mix. The measure d mean and root-mean-squared velocities on the plane of bilateral symmetry are compared with the CFD predictions. The profiles across the jet cross-se ctions and the behavior along the air jet axis closely follow the experimen tal results. however, the fuel jet trajectory is poorly predicted. Constrai nts in the model due to the assumption of normal or well-developed turbulen ce spectrum in concentration fluctuations are highlighted. (C) 2001 by The Combustion Institute.