Experimental Study on Bluff-Body Stabilized Premixed Flame with a Central Air/Fuel Jet

Bluff-body flame holders are commonly employed in many industrial applications. A bluff-body is usually adopted to enhance the downstream mixing of the combustion products and the fresh fuel-air mixtures, thus to improve the flame stability and to control the combustion process. In the present paper, flames stabilized by a conical-shape bluff-body flame holder with a central air/fuel jet were studied. Effects of both a central air jet and a central fuel jet on the structures and lean blowout limits of the premixed annular flames, and on the temperature on the upper surface of the bluff-body were investigated and presented. It was revealed that a central jet led to a considerable reduction of the temperature on the upper surface of the bluff-body. It was proposed to be caused by the alternation of flow structures (in the case with a central air jet) altogether with the flame lifting from the burner (in the case with a central fuel jet). Thus, it might be used to solve the problem of the bluff-body with high heat loads in practical applications. The flame stability characteristics, for example the unstable flame dynamics and the lean blowout limits, varied with the injection of an air or fuel jet through the central pipe. Different blowout behaviors, being with or without the occurrence of flame split and flashing, caused by a central air jet were presented in the paper. In addition, when a small amount of central fuel jet (i.e., U-f/U-a = 0.045) was injected into the flow fields, an unsteady circular motion of the flame tip along the outer edge of the bluff-body was observed as well. Whereas, with an increase in the amount of the central fuel jet, the flame detached from the outer edge of the bluff-body and then became much more unstable. With a central air or fuel jet injecting into the flow field, premixed flames stabilized by the bluff-body became more unstable and easier to blowout.