Investigations of air flow behavior past a conical bluff body using particle imaging velocimetry

An annular jet flow past conical bluff body in an original configuration was investigated using particle imaging velocimetry for Reynolds (Re) numbers between 4900 and 58,800. The studied geometrical configuration finds its application in a novel low NO x burner; therefore, the flow conditions corresponding to burner operation were investigated in detail. It was found that the flow is laminar at Re = 4900 and the transition from a laminar to a turbulent flow regime starts at Re = 9800–14,700. The length of the recirculation zone increases abruptly when the flow becomes turbulent and is independent of the Re number within the turbulent flow regime. The fraction of the total flow rate representing flow circulating in the recirculation zone is relatively constant for the investigated flows. An increase in main stream velocity implies proportional decrease in the residence time in the recirculation zone observed within the turbulent flow regime. Two types of coherent structures shed from the trailing edge of the bluff body are identified using proper orthogonal decomposition technique. The first is vortices that cross the centerline of the bluff body and travel downstream along the centerline. The second is smaller vortex structures that are shed alternately from the trailing edge of the bluff body.