TURBULENT MIXING OF COAXIAL NOZZLE FLOWS WITH A CENTRAL-LOBED MIXER

The turbulent mixing of turbulent, confined, coaxial nozzle flows with a central-lobed mixer were determined by measurement using a two-comp onent fiber-optic laser Doppler anemometer at a Reynolds number of 5.1 x 10(4) (based on an inner circular nozzle diameter D-l = 30 mm and b ulk mean velocity U-r of the two streams at 1.7 m/s). The ratio of the annular mean to the core mean velocity was 2:1. The boundary-layer th icknesses in the nozzle exit plane were about 3% of the inner circular nozzle diameter. Altogether three central-lobed mixer configurations were tested, including four-, five-, and six-lobed nozzles. A blunt le ading-edge circular central nozzle (i.e., a coaxial jet arrangement) w as also measured as a basis for comparison, The results showed that th e streamwise vorticity originated from two sources: the geometry of th e lobe and the gap between the lobe nozzle peaks and the inner annular wall. The former result was important to enhance the mixing between t he two coflowing streams, whereas the latter result was important to t he mixing between the annular flow and the surrounding still fluid. No streamwise vorticity would be generated if the size of the gap was la rger than 15% of the inner circular nozzle diameter. Furthermore, when flow separation appeared at the lobe peaks, the strength of the strea mwise vorticity generated by the gap would be strengthened with a cons equence of enhancing the mixing between the annular flow and the surro unding still fluid; the number of the lobes also played an important r ole in the mixing processes. A center plug was found to be effective i n enhancing the mixing between the two coflowing streams by eliminatin g the flow separation at the lobe peak regions for the six-lobed case, but with a consequence of suppressing the streamwise vorticity genera ted by the gap.