(177c) State of the Art of Transport Phenomena in Channels with Turbulence Promoters

It has been known for a long time that insertion of obstacles to flow in channels or tubes results in higher heat and mass transfer rates. These obstacles, commonly called turbulence or "eddy" promoters facilitate the mixing between the bulk of fluid and the fluid adjacent to the solid interface thus reducing the thickness of the boundary layer and increasing local shear. The result is enhancement of transport rates. Various turbulence promoters have been used: baffles, packing materials used in packed beds, corrugated surfaces, attached rods or bars of other geometric configurations placed in the channel at an angle to the flow direction and net-type promoters. The latter promoters, also called spacers are used in spiral-wound membrane modules to define the feed channel.

In this presentation we will review the work of several researchers with different kinds of promoters for various applications to elucidate the transport mechanisms in spacer-filled channels for Ultrafiltration. Due to the complexity of such channel geometries, most of the early research done on hydrodynamics and heat and mass transfer were experimental, with only few authors attempting to solve the problem numerically. However, this trend has changed. With recent advances in computational science, the use of numerical techniques has been widely adopted for simulating fluid flow since the early 2000s, particularly the technique so called Computational Fluid Dynamics (CFD). This technique allows to analyze the flow field for a variety of channel geometries without disturbing the flow and eliminating the need to fabricate physical models. Are laboratory experiments "a thing" of the past? We will discuss the main results reported in the literature of 2D modeling, which have provided valuable insights into interactions between the flow and mass transfer in spacer-filled channels and, will discuss the evolution into 3D modeling, which is underway to better represent real-world applications.

Furthermore, we will also describe our experimental study of fluid flow and mass transfer in spacer-filled channels for ultrafiltration, which examined the performance of several commercially available spacers and their modified versions. This study introduced a new spacer characteristic: the hydrodynamic angle; proposed the transport mechanism in a spacer-filled channel; developed semi-empirical mass transfer correlations that take into account the high Schmidt numbers at the membrane surface (in ultrafiltration on the order of 10⁶) and the zig-zag type flow paths, indicating laminar flow mass transfer behavior; and, developed semi-empirical pressure drop correlations, which show the contributions of both skin friction and kinetic losses.