Numerical Study On The Heat And Mass Transfer Mechanism Of Bi-layer Desalination System

In recent decades,with the steady growth of demand and the destruction of water resources caused by industrial development,the scarcity of freshwater resources has posed a threat to the sustainable development of human society.Solar-enabled evaporation is a solar energy harvesting technology which has potential applications not only in seawater desalination,but also in wastewater treatment,medical sterilization and even for electricity generation.Recently,a bi-layered seawater desalination system has attracted widespread attentions due to its efficient utilization of solar energy.At present,many researches have focused on improving and optimizing the material,structure and overall device of the bi-layer system.In this paper,numerical simulation method was used to analyze the heat and mass transfer in the bi-layer desalination system from macroscopic and microscopic perspectives,in order to provide some references for the improvement,optimization and expansion of the bi-layer desalination system.For the macroscopic system,we established,optimized,and experimentally verified the macroscopic heat and mass transfer calculation models based on the finite element method.And then influence of various factors on evaporation efficiency and vapor temperature of bi-layer structure systems were analyzed.Results show that:in order to obtain high temperature vapor,the transport of vapor should be weakened which will reduce the evaporation efficiency,and heat dissipation through boundaries should be attenuated;to obtain higher evaporation efficiency,improving the transport condition of vapor is an effective way.At this time,the produced vapor has a low temperature.In addition,we also found that there is an optimal porosity?about 50%?for enhancing the evaporation efficiency of the bi-layer desalination system.Compared with the thermal conductivity of porous materials,the effective thermal conductivity of bi-layer structure has greater influence on the evaporation efficiency.The use of selective absorber will help to improve the evaporation efficiency and vapor temperature of the bi-layer structure system.In microscopic,for the first layer,a numerical model for photo-thermal conversion of nanoparticles verified by Mie scattering theory was established based on the finite element method.And then,the crowding effects of nanoparticles in the full spectrum of sunlight was studied by the progressive analysis from the zero-dimensional system composed of single nanoparticles.It is found that for the regular distributions of nanoparticles in a plane,the hexagonal lattice distribution is the optimal distribution for solar-thermal conversion,and the inter-particle distance of about 315 nm is optimal to get a good energy absorption.Of course,it makes more sense that the energy absorption of any spatially distributed multi-particle system in the solar full spectrum can be regarded as the superposition of three basic modes.Therefore,we can use the electromagnetic field decomposition model to deal with the energy absorption of any multi-particle system in the solar full spectrum.For the second layer of materials,we have used the finite element method and the Poisson-Nernst-Planck equation to establish a numerical model for the thermal drive ion transport.And then the transport of thermal driven ion in Nano-channels and porous materials was studied and analyzed.Results show that the Nano-channel system with the ion concentration(8(8₄4))of 1 mol·m^-3 and the half width of channel?of 1 nm is an ideal thermoelectric system,and the system has an optimal temperature difference?about 85 K?and an optimal channel length?about 85 h?to maximize its thermal power.The thermal current?about 27.2 mA?,the thermoelectric potential?about 63.7 mV?and the thermal power?about 433.2?W?caused by the thermal driven ion transport in the porous material are enough to drive the commercial electrical components.Therefore,it is feasible to directly utilize the transport performance of thermal driven ion in porous materials for vapor-electricity co-generation in bi-layer desalination system.