Flow instabilities enhancement

Roughness (placing protuberance or corrugation on the surface) These techniques are difficult to scale-up to intermediate or large size modules and are often limited by their high axial pressure drop. The approach, however, does demonstrate that shear stresses developed by flow instabilities enhance membrane permeation rates for difficult feeds. ... 

In the case of a venturi flow, the most economical technique for increasing cavitation intensity would be to reduce the length of venturi, but for higher volumetric flow rates there could be a limitation due to the possibility of flow instability and super-cavitation. A similar argument can be given for the enhancement in the cavitation intensity by reducing the venturi throat to pipe diameter ratio. 

These fluctuations maybe caused by rapid variations in pressure or velocity producing random vortices and flow instabilities within the fluid. A complete mathematical analysis of turbulent flow remains elusive due to the erratic nature of the flow. Often used to promote mixing or enhance transport to surfaces, turbulent flow has been studied using electrochemical techniques [i]. 

In order to get complementary information on the evolution of the shape of the extrudate at the die exit, photographs were taken simultaneously for each flow condition. Examples of free surfaces build establishments are presented in Figs. 7 and 8, respectively for smooth (long dies and low shear rate) and for stronger flow conditions (short die and high shear rate). It is clear that strong flow conditions enhance the differences between LLDPE and LDPE. While LLDPE exhibits rapid evolution towards a constant diameter, instabilities occur for LDPE, which correspond to the onset of melt fracture. 

Most of hydrodynamic methods have focused on increasing the particle back transport from the membrane-liquid interface by increasing the shear rate and the flow instability in the boundary layer. These techniques include secondary flows, spacers and inserts, pulsed flow, high shear rate devices, vibrations, and two-phase flow. The physical methods that are currently been tested to enhance filtration performance of membranes include the application of electric fields and ultrasound. 

Mass Transfer Enhancement Because of Flow Instabilities... 

Our main concern here is to present the mass transfer enhancement in several rate-controlled separation processes and how they are affected by the flow instabilities. These processes include membrane processes of reverse osmosis, ultra/microfiltration, gas permeation, and chromatography. In the following section, the different types of flow instabilities are classified and discussed. The axial dispersion in curved tubes is also discussed to understand the dispersion in the biological systems and radial mass transport in the chromatographic columns. Several experimental and theoretical studies have been reported on dispersion of solute in curved and coiled tubes under various laminar Newtonian and non-Newtonian flow conditions. The prior literature on dispersion in the laminar flow of Newtonian and non-Newtonian fluids through... 

Further details of the mass transfer enhancement techniques in membrane separation processes are reported by Belfort and Al-Bastaki and Abbas. In this entry, the focus is on the flow instabilities produced by Dean vortices in curved and coiled tubes because of their advantages over the other techniques, viz., lower axial dispersion, better radial mixing, residence time distribution closer to plug flow, higher mass... 

Several novel techniques for mass transfer enhancement reported in the literature have been discussed in detail. The present study is focused on the mass transfer enhancement in the rate-controlled separation processes using flow instabilities. There are a large number of examples about the success of flow instabilities produced by Dean vortices in improving the performance by increasing flux and reducing fouling in membrane separation processes. Several curved modules... 

Any action in mitigating flow maldistribution must be preceded by an identification of possible reasons that may cause the performance deterioration and/or may affect mechanical characteristics of the heat exchanger. The possible reasons that affect the performance are [131,147] (1) deterioration in the heat exchanger effectiveness and pressure drop characteristics, (2) fluid freezing, as in viscous flow coolers, (3) fluid deterioration, (4) enhanced fouling, and (5) mechanical and tube vibration problems (flow-induced vibrations as a consequence of flow instabilities, wear, fretting, erosion, corrosion, and mechanical failure). 

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