Computational Studies On Membrane Distillation In Hollow Fiber Membrne Devices

Membrane distillation is an energy-saving membrane separation technology by using a hydrophobic porous membrane as barrier and thermal energy as separation force. With the development of membrane distillation, the corresponding equipments trend to be large-scale. Design theory and method of membrane devices will be very important. However, the researches of transport phenomena in membrane devices are insufficient, especially for those which involve the momentum, heat and mass transfer. To date, few theories have been published for this complex transfer phenomenon. The paper addresses theoretical analysis and experiments to study the membrane distillation process in the hollow fiber membrane devices. The study focuses on the influences of the irregular array fibers on momentum, heat and mass transfer phenomenon in the shell side of hollow fiber membrane devices.A numerical simulation method (scalar model) and the Fluent software are used to study the tubeside heat and mass transfer of vacuum membrane distillation in hollow fiber membrane devices. The fluxes of water and ethanol obtained from the simulations agree well with the experiment results which are obtained in distilled water and ethanol-aqueous solutions. Some regressive equations of local Nusselt number and Sherwood number are obtained by numerical simulation.The mass transfer of longitudinal laminar flow of Happel free surface cell model under two boundary conditions is solved. One boundary condition assumes the wall has a certain mass flux and the other one assumes a constant concentration. For different void ratioes, the analytical solution of Sherwood number in fully developed region, the strict Graetz analytical solution of Sherwood number in the developing region, and approximate Leveque solution are obtained. In Happel free surface cell model, the mass transfer coefficient decreases with the increasing void ratio. The void ratio affects the position of fully developed region. When Reynolds number, Schmidt number and radius of fiber are certain, the position of fully developed region will move forwards with the increasing void ratio.The effect of fiber irregular array is discussed in this paper. According to laminar flow theory, a numerical method is employed to simulate the mass transfer in the shell-side of random distributed hollow fiber bundles under two boundary conditions. The fiber surface has a constant mass flux in one B.C., and a constant concentration in the other B.C.. The effects of the packing ratio<Φ> on the flow volume and concentration distribution, the coefficient f·Re and Sherwood number are discussed. For the packing ratio range 0.1≤Φ≤0.7, the relational expressions of the f·Re and Sherwood number are given in the paper. The flow area of the sub-cell tessellated by Voronoi method has a certain relationship with the flow volume distribution. However, the packing ratio<Φ> shows little effect on this relationship. At the developing region, the packing ratio not only affects the coefficient A in the general expression Sh=ΛReaScb f(de/L) but also the exponent a of Re and exponent b of Sc in it.An approximate method is proposed to address the longitudinal laminar flow of in the shell side of hollow fiber membrane devices. According to the velocity field of fiber bundles, the analysis based on the Voronoi tessellations method which is used to describe the shellside flow is effective. Therefore, the approximated method based on the integral of infinitesimal cell of Happel free surface cell model is set up. And a new concept of local porosity which described the non-uniformity of fiber bundles is introduced.Based on the numerical simulation in Voronoi free surface cell, the mass transfer phenomena in the shellside of hollow fiber membrane devices is analyzed. The results show that mass transfer phenomena in shell side can be divided into three region, fully developed region, developing region A and developing region B. Under constant wall flux B.C., a method of analyzing the flow field and mass transfer in the Voronoi cell with complex shape is presented and validated. Meanwhile, the influences of the fiber center eccentricity are analyzed by this method. The result shows the Sherwood number decreases remarkably due to the fiber eccentricity. An approximate calculation method of Sherwood number in the developing region A is presented and validated. Comparing the non-dimensional concentration distribution in Voronoi cell of fiber bundles with Voronoi free surface cell, it is found that Voronoi free surface cell model cannot be used to describe the mass transfer of shellside in hollow fiber devices. The simulation results of random distribution of fiber bundle presented in Chapter 4 is suggested to establish a simplified mass transfer model in shellside in hollow fiber devices.