Research On The Oblique Flow And Transport Phenomena In Hollow Fiber Membrane Modules

Air humidity seriously influences people’s life and industrial production when it is either too high or too low and it is necessary to manage indoor air humidity within a proper range. Recently, hollow fiber membrane modules have been widely used in indoor humidity control. Hollow fiber membrane modules are similar to shell-and-tube heat exchangers. Liquid desiccant in lumen side and air in shell side can exchange heat and moisture through the membrane. Differently from heat exchangers, the membranes usually have selectivities to prevent liquid desicaant from permeating but allow water vapor to permeate. Furthermore the packing density of hollow fiber membrane fibers is much higher than that of heat exchangers and hence the efficiency of heat mass transfer is better. Previous studies are concentrated on the cases of pure parallel flow or cross flow in hollow fiber membrane modules, while the case of shell- side oblique flow in the module which considers the fiber-to- fiber interactions is not studied yet. This study will investigate the shell- side oblique flow and convective transfer phenomena in hollow fiber modules. Furthermore the counter flow and heat and mass transfer in a randomly distributed tube array under conjugate heat mass transfer boundary conditions are modeled with Voronoi tessellation method. To disclose the transport phenomena between the shell- side fluid flow and tube bundles, some mathematical models are established in this work:(1) Oblique fluid flow and heat transfer across a hollow fiber membrane bank under uniform temperature conditions are investigated. The inline and staggered arrays with ten calculating units are modeled. The momentum and mass conservat ion equations in the air side are established. The boundary conditions on the membrane are assumed uniform temperature conditions. A numerical method is conducted to obtain the Nusselt number and friction factor in the developed region. An air heating experiment is conducted to validate the numerical data. The results show that the N usselt number and friction factor in the developed re gion rise with an increase in the oblique angle. Besides the convective heat transfer coefficients and friction factors for staggered arrays are higher than those for inline arrays due to stronger disturbance.(2) Oblique fluid flow and convective heat tr ansfer across a tube bank under uniform wall heat flux boundary conditions are investigated. The inline and staggered arrays in periodic cells are modeled. Mass and momentum conservation equations in shell side and lumen side are established and uniform he at flux boundary condition is applied to tube surface. The N usselt number and friction factor in the periodic cells are numerically obtained. The results show that the N usselt number and friction factor in the periodic cells increase when the oblique angle rises. The impact of bulk flow on the tube array is intensified and the N usselt number and friction factor in the periodic cells rise when pitch-to-diameter ratio decreases. The N usselt numbers under uniform wall heat flux boundary conditions is higher than those under uniform temperature boundary conditions, which is in accordance to the conventional views.(3) The shell- side oblique flow and convective heat and mass transfer in hollow fiber modules under conjugate heat and mass transfer boundary conditio ns are investigated. The inline and staggered arrays in periodic cells are modeled. The mass, momentum conservation equations and heat mass transport equations in both sides are solved by a finite volume method. Fluid fields, temperature fields and humidity fields in both sides are obtained. Then the Nusselt numbers and Sherwood numbers under conjugate heat and mass transfer boundary conditions and friction factor in the periodic cells are calculated. The results show that shell-side convective heat and moisture transfer rates increase when the skewed angle rises. The Nusselt numbers under naturally formed boundary conditions are higher than those under uniform temperature boundary conditions for inline arrays, while they are lower than those under uniform temperature boundary condition for staggered arrays.(4) Counter flow and convective heat and mass transfer in a random array under conjugate heat mass transfer boundary conditions are investigated. Five to seven neighboring fibers in an irregular array is modeled. The calculating domain, whose packing fraction is identical to that of whole module, is obtained through a Voronoi tessellation method. The momentum, heat and mass transfer conservation equations in both flow channels are established with conjugated heat and mass transfer boundary condition applied to membrane surface. The equations are solved with a finite volume method. N usselt numbers and Sherwood numbers under naturally formed boundary conditions and friction factors are obtained. The results show that the data from common free surface model are 10% greater than those from Voronoi tessellation method when the packing fraction is above 0.30. It is better to use hexagonal unit for the case of high packing fraction. The reason is that fiber-to-fiber interactions should be taken into consideration under high packing fractions and the interactions of more fibers are included in hexagonal unit.