An electrochemical investigation of mass transfer in a pipe and a simplified bifurcation model.

摘要

A novel technique was used to fabricate nickel flow models of a straight pipe and a Y-bifurcation. These were used to obtain integral mass transfer coefficients by the electrochemical technique with the ferri-ferrocyanide system at Reynolds numbers ranging from 250 to 8600, at the four Schmidt numbers of 1310, 2470, 3100 and 5655. For the straight pipe, good agreement was obtained with previously reported mass transfer correlations. As the Schmidt number increased, the effect of transition from laminar to turbulent flow on mass transfer was delayed to progressively higher Reynolds numbers. With the Y-bifurcation model, possible flow separation and the formation of a new mass transfer boundary layer in the daughter branches significantly influence the mass transfer behavior.; The electrochemical technique has been used to obtain transient mass transfer coefficients for smooth pipes. In contrast to the analytical solution of the corresponding heat transfer problem, two distinct transient periods are observed. The first is controlled by chemical reaction kinetics at the surface, followed by the diffusion-controlled period in agreement with the heat transfer solution. The transfer rate for laminar flow is then proportional to (D/t)1/2, in accordance with Higbie's penetration theory. In turbulent flow, the transient mass transfer rate, during the second transient period, is proportional to Re1/4(D/t)1/2, higher than in laminar flow.; The laser photochromic tracer method provided velocity and wall shear stress values in the plane of symmetry of a UV-transparent Plexiglas bifurcation model similar to that used in the mass transfer experiments at Reynolds numbers 500, 600, and 750. A novel copper electrodeposition technique has been used to obtain time-averaged convective local mass transfer coefficients in a straight pipe and a simplified bifurcation model. Laminar flow results for the pipe are in good agreement with the analytical Leveque solution. In the bifurcation, higher mass transfer coefficients along the inner wall and lower ones along the outer wall were observed. Both coefficients converge towards the same value further downstream. Within the branches, mass transfer and wall shear stress follow similar patterns both on the inner and outer walls. It was found that StSc2/3 and C_f/2 demonstrate analogous behavior.; The lower transfer phenomena, both momentum and mass, along the outer wall of the branches are coincident with the localization of atherosclerotic lesions and arterial plaques.