A Numerical Study Of Longitudinal Vortex Induced Enhancement Of Mass Transfer In A Membrane Channel

Membrane separation as a newly emerging and high efficiency separation method has been widely used in many industrial fields. In the membrane separation process, the existence of concentration polarization phenomenon has a great influence on the mass transfer. Traditional two-dimensional (2D) flow disturbers can considerably weaken the concentration polarization, but they simultaneously cause substantial pressure drops.There exist many similarities between mass transfer and heat transfer in the view of governing equations and physical phenomena. For the heat transfer process, using longitudinal vortex generators to induce the longitudinal vortexes to destroy the boundary layer to enhance the heat transfer is an attractive method. With the theory of heat transfer enhancement by longitudinal vortex for reference, the present research used the CFD software FLUENT 6.3 to simulate the process of separation in a narrow membrane channel to investigate the effects of longitudinal vortex generators on concentration polarization phenomena. The simulations used the k-εRNG turbulent flow model.The concentration polarization phenomena occurring in a narrow membrane channel with and without flow disturbers were studied. For the process of membrane separation without flow disturbers, the concentration polarization occurs mainly in a very narrow range near the membrane surface, and the degree of concentration polarization increases along the fluid flow direction but decreases with Reynolds number (Re). When the membrane channel is equipped with the longitudinal vortex generators including the delta and rectangular winglets, and the traditional 2D disturbers including the tri-prism, four-prism and circular cylinders, for a given Re the 2D disturbers can considerably reduce the concentration polarization factor, i.e., considerably enhance the mass transfer, but they simultaneously cause substantial pressure drops, while the longitudinal vortex generators as the 3D disturbers can effectively enhance the mass transfer with much less pressure drop penalty as compared to those for the 2D disturbers. Overall performance comparison of the abovementioned five disturbers suggests that under equal pressure drop and equal pump power conditions, the rectangular winglets yield the best mass transfer enhancing effect while the tri-prism gives the worst mass transfer enhancing effect.The effects of the geometric parameters of the delta and rectangular winglets on their enhancement of mass transfer in a membrane channel were investigated. The parameters include the height, attack angle, aspect ratio, and interval. For the delta winglets, under the conditions of equal pressure drop and equal pump power conditions, higher height and more narrow interval will lead to an increased mass transfer enhancing effect, while the aspect ratio shows a minimal influence. The attack angle exhibits optimal values of 10°and 40°at the equal pressure drop and equal pump power conditions, respectively. For the rectangular winglet, under the equal pressure drop and equal pump power conditions, the height and interval yield optimal values of 2/3 and 3/2 of the channel height. The mass transfer enhancing effect increases when the aspect ratio is increased from 1.2 to 2.0, but remains almost unchanged when the aspect ratio is further increased to 2.8. The attack angle possesses optimal values of 10°and 30°under the equal pressure drop and equal pump power conditions, respectively. Based on the above results and further analyses, the values are recommended for the abovementioned four parameters for both the delta and rectangular winglets.