On the stabilization of flames on multijet industrial burners

Measurements of velocity and temperature characteristics, together with the analysis of the process of flame extinction, are reported for a range of high-intensity flames stabilized on a model of an industrial oxyfuel burner installed in a divergent quarl. The burner consists of a central axisymmetric jet surrounded by 16 circular jets, simulating the injection of oxygen in practical burners. A laser-Doppler velocimeter was used to measure density-weighted velocity characteristics, and bare-wire thermocouples were used to measure near unweighted temperature characteristics. Experiments were carried out to improve knowledge of the flow in the near field of multijet burner heads, which is essential to design further modifications in their geometry and to predict their effects. Isothermal and combusting flows are studied; for the latter, the experiments quantify the effect of quarl geometry, fuel-to-air ratio, swirl number, and central-to-peripheral jet velocity ration on the flame characteristics.

The results show that flame stabilization occurs in the vicinity of the quarl and is affected by its geometry owing to changes in the rate of entrainment of cold air. Increasing the swirl level and decreasing the peripheral airflow improves flame stability by promoting the mixing of fuel and air along the annular stabilization region. Turbulence measurements show common features with and without combustion and suggest the absence of large-scale mixing in the present flames. Although the laminar flamelet concept may represent most of the features of the flames investigated, the local quenching of burning flamelets is shown to preclude the internal ignition of flame gases in a way that influences the process of flame stabilization.