Static mixers for water treatment: A computational fluid dynamics model
A Computational Fluid Dynamics (CFD) model was developed that can be used to provide a fundamental understanding of the fluid motion in helical static mixers. The numerical model solves the three-dimensional Reynolds-averaged Navier-Stokes equations closed with the k-omega turbulence model. The gove...
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| Format: | Dissertation |
| Language: | English |
| Published: |
ProQuest Dissertations & Theses
01-01-1999
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| Online Access: | Get full text |
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| Summary: | A Computational Fluid Dynamics (CFD) model was developed that can be used to provide a fundamental understanding of the fluid motion in helical static mixers. The numerical model solves the three-dimensional Reynolds-averaged Navier-Stokes equations closed with the k-omega turbulence model. The governing equations are formulated in generalized curvilinear coordinates and solved using a finite-volume multigrid approach. The model is used to comprehensively investigate the physics of the flow in helical static mixers over a broad range of Reynolds numbers. Simulations are carried out for mixers consisting of up to six, alternating, left- and right-twisting, helical elements placed end-to-end at an angle of 90 degrees. The twisting elements cause a very complex, three-dimensional flow, which is characterized by longitudinal and transverse induced pressure gradients, intense longitudinal vortices, and pockets of axial flow reversal. For a two-element mixer in the laminar flow regime, simulations show that the complexity of the flow increases dramatically as the Reynolds number increases from 100 to 500. Furthermore, the inability of the numerical method to yield converged solutions with high-resolution numerics at higher Reynolds numbers suggests that the flow could bifurcate from a steady laminar flow to an unsteady laminar flow for Re > 500. In the turbulent flow regime, the simulations show that there is a range of Reynolds numbers (5 000 ≤ Re ≤ 20 000) for which the mixer elements appear to stabilize the incoming, fully developed, turbulent pipe flow yielding an almost laminar flow at the exit of the mixer. This laminarization depends strongly on the number of elements where more elements tend to stabilize the flow more strongly. A simulation was also carried out for a Reynolds number of 100 000, which is within the range of Reynolds numbers that is relevant to water treatment plants. The predicted flowfield is used to examine existing theories of rapid mixing and explore various approaches for characterizing mixing in static mixers for water treatment applications. |
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| ISBN: | 0599716142 9780599716148 |