On the mechanism of transfer enhancement by eddy promoters

Mass transfer rates in a rectangular duct of low aspect ratio containing semicircular cylindrical inserts attached alternately on opposite walls and perpendicular to the flow direction were measured with the electrochemical diffusion-limited current technique. Local and average mass transfer coefficients determined with arrays of strip electrodes indicate the existence of at least two transfer regimes. In the laminar regime the exponent on the Reynolds number depends on the eddy promoter spacing. In the turbulent regime the data correlate well according to j_D = 0.57 Re^?0.35(d_h/p)^0.40.The local transfer distributions reveal the existence of separated flow regions downstream of, as well as opposite to, turbulence promoters. The flow in the reattachment region is very unstable and this results in earlier transition to turbulence.The local transfer distributions also suggest that boundary layer disruption at, and redevelopment downstream of, separated regions is the dominant mechanism of transfer enhancement in the turbulent regime. An idealized model based on this assumption, and making use, of results obtained for turbulent transfer to a flat plate in an infinite medium predicts a relationship in approximate agreement with the experimental correlation given above.The mass transfer results for fully developed turbulent flow also correlate well according to a mass transfer similarity law.