09. Heat and Mass Transfer

Numerical study of the mixing of viscoelastic fluids in static mixers was performed for understanding the geometric parameters for optimization and the relative importance of viscous and elastic effects on mixing, with the aid of COMSOL software. The velocity and pressure were calculated using two constitutive models, the Carreau model and the Giesekus model, and the mixing was accessed through the simulation of mass transport of dilute species. The shear thinning effect is well described by the Carreau model, while the Giesekus model could accommodate more elastic effect. First, the calculation was validated using the experimental results in literatures. The calculations using Giesekus model agree with the mixing experiment within experimental error, while the Carreau model underestimates the mixing by about 4 – 20% for different SMX mixers with different geometries, indicating that the elastic effect could contribute additional mixing. Both models could be applied for mixer design as both models predict qualitatively the same tendency for mixer performance, but much more calculation efforts are required for the Giesekus model. Secondly, parametric study of the geometric parameters of the SMX mixer was performed using various fluids. It was found that some commercial SMX mixer designs are not optimized in general, while the design rule proposed in the literature through optimization could provide better performance. However, more than one set of parameters for better mixing could be found, and the one with fewer cross-bars should be chosen for smaller pressure drop across the mixer. Thirdly, a sudden contraction tube (SCT) segment was proposed here as a mixer element for inducing additional elastic effect. Simulations were performed for different mixers using different combination of SMX and SCT elements, and some designs could provide comparable performance as that of the SMX mixer, one of the most used static mixers in industry.

Mr.

C. F. Chen