| Results of experimental and numerical investigations of steady turbulent jets from round ribbed tubes are presented. The round ribbed tubes are smooth tubes with wire coil inserts. Experimental investigations were carried out with a constant air volume flow rate at a maximum jet outlet velocity of 28 m/s which corresponds to a Reynolds number based on the smooth tube inside diameter of 23,700. The ratios of the coil diameter to the tube inside diameter, d/D, were 0.06, 0.08, 0.11, 0.13, and 0.16 and the ratio of pitch spacing to the tube inside diameter, p/D, was 1.2. Turbulence quantities in the radial direction across the jet outlet were measured at six different axial locations, X/D, of 0.1, 1, 3, 5, 10, and 15. Experimental results showed that the coil inserts cause a significant decrease in the maximum axial mean velocity and an increase in the jet half-width, with corresponding increases in the turbulent velocities in the developing region. These are indications of enhanced mixing process with the surrounding air. At the jet outlet, mixing enhancement is increased when d/D < 0.1. However, further downstream, the highest mixing is achieved for the coil with the largest d/D ratio. The effects of Reynolds numbers and coil insert length for the coil-inserted tube with d/D = 0.06 and p/D = 1.2 were also investigated. Results show no significant differences on turbulence profiles for the Reynolds numbers of 13,190 and 23,700. Reducing the coil length delayed the mixing enhancement to a further downstream location. The three-dimensional numerical calculations were performed for the coil insert with d/D = 0.11 and p/D = 1.2. Numerical investigations were carried out for a uniform inlet velocity of 26 m/s which corresponds to a Reynolds number based on the smooth tube inside diameter of 19,810. Numerical results show qualitative agreement with the experimental results. The simulated flow indicates that the generation of streamwise vortices by the coil insert is the mechanism which increases the entrainment and enhances the mixing process. |
