Research On Heat And Mass Transfer Of Gas-liguid Membrane Contactor

Air humidity regulation has an important impact on people’s life.The traditional liquid dehumidification method is easy to cause droplet entrainment problem because of the direct contact between air and dehumidifying liquid.Air with dehumidifying liquid enters the room,which will cause corrosion to furniture,walls and harm to human health.Membrane liquid dehumidification technology developed in recent years can effectively avoid the above problems.At the same time,the shortage of freshwater resources has seriously troubled human society.The desalination is an effective means to solve this problem.Membrane distillation（MD）is a new separation technology.It has low operating temperature,high water quality and can utilize low-quality energy.Gas-liquid film contactor is the key component to solve the above problems.It uses semi-permeable membrane to isolate air from solution.The membrane has selective permeability,allowing only water vapor to pass through,preventing the permeation of other liquids and gases.Air and solution exchange heat and water vapor through membrane,achieving the purpose of air dehumidification or seawater desalination.The boundary conditions on the membrane surfaces are neither uniform temperature（concentration）nor uniform heat flux（component diffusivity）on membrane surface.It is a natural boundary condition formed by the coupling of air and desiccant liquid.In this paper,the mass,momentum and energy conservation equations of air and solution are established,and the fluid flow,heat and mass transfer characteristics in membrane contactors with different structures are studied by numerical simulation combined with natural boundary conditions.For the flat-plate membrane contactor used for air dehumidification,three different structures are studied.They are side-in side-out quasi-counter flow structure,hexagonal quasi-counter flow structure and side-in side-out quasi-counter flow structure with internal cooling tubes.The effects of channel height,entry ratios,length-width ratio,air Reynolds number,solution Reynolds number,number of internal cooling tubes and external diameter on the basic criterion numbers are analyzed,and compared with counter-flow and cross-flow arrangements.The average Nusselt and Sherwood numbers of air and solution under quasi-counter flow are larger than those under cross-flow.For the hollow fiber membrane contactor used for seawater desalination,counter-flow and cross-flow structures were studied.For counter-flow and cross-flow,laminar flow model and low Reynolds turbulence flow model are used respectively.The resistance coefficient,Nusselt and Sherwood numbers under various random distribution characteristics（dense,middle and sparse distributions）and packing fractions were studied and compared with regular distribution characteristics（in-line and staggered arrangements）.（fRe）_m,a、（Nu_C）_m,a and（Sh_C）_m,a,a with staggered arrangement is larger than random arrangement.The friction factor,Nusselt and Sherwood numbers under the conjugate heat and mass transfer boundary conditions are obtained in this paper.These basic data can provide the fundamentals for future component structural design.