Understanding heat/mass transfer mechanisms in turbulent flow is essential for solving many engineering problems. From the view point of heat/mass transfer control, surface roughness is popular for enhancing heat/mass transfer in turbulent pipe flow, relevant to heat exchangers, mixers, and nuclear reactors to name a few. Such a surface roughness is often modeled with a rib. In the current investigation, the flow and concentration fields from the LES data base were analyzed in detail. The profiles of mean velocity components, mean concentration, root-mean-squares (rms) of concentration fluctuations are presented at the selected streamwise locations downstream of a rib. In comparison with the straight-pipe case at the same Re and Sc, the effects of the ribs are clearly identified, leading to overall enhancement of turbulent mass transfer. The budget of concentration variance was computed and presented in the form of contours. Our LES results shed light on a complete understanding of the mass-transfer mechanisms in turbulent ribbed-pipe flow which has numerous applications in engineering.

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