Velocity Characteristics of a Round Offset Jet with Different Offset Ratios in a Counterflow

Abstract

A jet entering a counterflow improves the dilution of contaminant discharged from a chemical factory through pipe. The velocity ratio of jet-to-counterflow and the offset distance of the jet from the boundary significantly affect the dilution efficiency of sewage. A free and an offset round jet, in a counterflow, with ranges of jet-to-counterflow velocity ratios and offset ratios, were experimentally investigated using particle image velocimetry. The time-averaged features, including the mean velocity, penetration length, half-width, and jet centerline were studied. There is a large recirculation region on each side of a free jet in a counterflow. For an offset jet in a counterflow, there is only one large recirculation region on the upper side when the offset ratio is small, due to the wall hinders the entrainment of the counterflow into the jet. As the offset ratio increased, the gap between the wall and jet increased, which resulted in more counterflow entrainment into the jet and the large recirculation region gradually appeared. Accordingly, the penetration length became shorter, as the offset ratio increased for the same jet-to-counterflow ratio. The maximum axial velocity of a free jet in a counterflow decayed more rapidly than that in a quiescent ambient or an offset jet in a counterflow. The profile of excess velocity in the wall normal direction exhibited self-similarity within one half-width of the excess velocity in the established flow region. The half-width of the excess velocity and zero-velocity increased with downstream distance, for a free or an offset jet entering a counterflow. The mixing efficiency of an offset jet entering a counterflow was verified through the distribution of velocity and spreading rate at different streamwise stations.