Stabilization Mechanism of Burner-attached Flames in Laminar Non-premixed Jets

The stabilization mechanism of nozzle-attached flames in laminar non-premixed jets was studied experimentally and numerically. The local gas flow velocity and the flame propagation speed within the mixing layer at flame base were experimentally obtained before the blowout limit was reached. The experimental results show that, for burner-attached flames, the local flow velocity is much less than the burning speed, suggesting that the propagation of partially premixed flame within the mixing layer at flame base is stopped. The thickness of the fuel/air mixing layer at the attached flame base is quite narrow (1.1-1.5 mm) and less than the minimum quenching distance caused by conductive heat loss to the tube (or slit) wall in laminar premixed flame. Heat losses by the conduction from the flame base to the burner wall, and the convection between the flame base and the incoming flow (hereafter referred to as local convection) were studied through simulation. The significant roles of these effects in the stabilization of the attached flame were analyzed. While the heat loss by conduction to the fuel tube is neglected, the partially premixed flame at flame base can propagate along a much narrower mixing layer. On the contrary, a critical thickness of the mixing layer (in which the local partially premixed flame can sustain) exists near the flame base with considering the conductive heat loss. The conductive heat loss to the burner wall plays a relatively important role to flame stabilization without (or with a small amount of) coflow air. As the velocity of coflow air increases, the heat loss due to local convection gradually exhibits significant effects on flame stabilization. In addition, a simple theoretical model is provided based on the balance between the heat released by chemical reaction and the heat lost by conduction and local convection, suggesting clear dependence of the thickness of the fuel/air mixing layer on the local gas flow velocity and the laminar flame speed. The stability behaviors of flame base can be reasonably explained and predicted through such relation.

Automated summary

The study on nozzle-attached flames in laminar non-premixed jets found that the thickness of the fuel/air mixing layer at the flame base is narrow, and the burning speed is much greater than the local gas flow velocity, leading to the halt of partially premixed flame propagation within the mixing layer.