| Turbulent mixing, the dominant mixing mechanism in turbulent pipe flow, is important for understanding contaminant dispersion in pipe flow applications. The scalar fluxes, or the covariance velocity and concentration, are important quantities in understanding the underlying turbulent mixing mechanisms because they are present in the turbulent scalar transport equation. However, the scalar flux terms give rise to a closure problem, and cannot be directly prescribed by the governing equation. Currently, limited information exists regarding the scalar fluxes. In order to contribute to the body of knowledge concerning turbulent mixing in pipe flow, scalar flux measurements were made for a centerline injection of a passive scalar. PIV and PLIF were employed to measure the velocity and concentration fields for Re = 12, 700 at x/D ≈ 6.5. The results of mean velocity and concentration fields compared reasonably well with theory and previously published results. Subsequently, scalar flux profiles were extracted from the planar measurements. The normalized axial scalar flux profile features two distinct, positive peaks. The peak emerging in the top half of the pipe is approximately 0.022 in magnitude and occurs at r/D ≈ 0.15. The peak value in the bottom half of the pipe is significantly lower at approximately 0.16 near r/D ≈ --0.25. The normalized radial scalar flux profile features a positive peak in the top half of the pipe and a negative peak in the bottom half of the pipe. These peaks have essentially the same magnitude of approximately 0.2, and emerge r/D ≈ +/-0.15. Both the axial and radial flux profiles tend toward zero away from the peaks near the centerline and pipe walls. These measurements were also used to calculate the profiles for each term in the scalar transport equation. These profiles revealed that the mixing processes for this experiment are likely governed by a balancing between axial advection and radial turbulent diffusion. |
