Boundary-velocity gradient and premixed flame blowoff in U-bend tubes with secondary flow

The effect of a non-uniform boundary-velocity gradient along the rim of a circular nozzle burner on flame stabilization, including partial liftoff and blowoff, has been investigated experimentally by using U-bend tubes as nozzles for both laminar methane/air and propane/air premixed flames. Secondary flow, induced by the imbalance between pressure force and centrifugal force inside the U-bend tube, generated non-uniform and non-axi-symmetric flow. The intensity of the secondary flow was controlled by varying the flow rate and the radius of curvature of the U-bend tubes. Unique features of flames were visualized with direct photography and planar laser-induced fluorescence for OH-radicals. As the flow-rate increases, the flame lifted off partially from the nozzle rim, and the nozzle attached region decreased with increasing flow rate. Finally, blowoff occurred. Stability of flames was mapped as functions of equivalence ratio, nozzle-exit velocity, and the radius of curvature. The flames in the U-bend tubes had larger velocities at blowoff compared to the case with a straight-tube burner. Flow-field measurement using a laser Doppler velocimeter showed that local boundary-velocity gradients at the critical conditions of partial liftoff and blowoff in the U-bend tubes agreed well with those in the straight-tube burner. Also, such conditions can be described with overall flow characteristics in U-bend tubes, represented by the Dean number.