Investigation On Heat And Mass Transfer Characteristics Of Photo-thermal/Joule-heating Membrane Distillation Process

With the increasingly tense water-energy nexus,it is a top priority to explore low-energy and high-efficiency freshwater acquisition methods.Compared with other desalination methods,membrane distillation has attracted widespread attention due to its high separation efficiency,mild operating conditions,low requirements on membrane mechanical properties,and availability of low-grade heat sources.However,the existence of problems such as temperature polarization effect leads to high energy consumption and low efficiency of membrane distillation process,hindering its further development and application.Although applying self-heating membrane distillation processes,such as photo-thermal and Joule-heating processes,can effectively alleviate the above problems.At present,there is still a lack of high-quality composite membranes that can achieve stable and efficient freshwater output.The heat and mass transfer characteristics within the membrane modules under the action of multi-physical fields,such as photo-thermal and Jouleheating conversion,are still unclear.Therefore,this thesis conducts numerical simulations and experimental researches on the heat and mass transfer process of photo-thermal/Joule-heating membrane distillation,and intends to provide a theoretical basis for the guidance and optimization of the related membrane separation industrial processes.PDMS/MWCNT/PVDF composite membranes were prepared by a series of methods,such as electrostatic spinning,spraying,and w etting.The structure,morphology,and composition of the membranes were characterized,and the changes in the structure and properties of the membranes under different working conditions were analyzed.The light absorption characteristics of the membrane under different MWCNT contents were analyzed by exploring the absorption spectrum of the membrane.Simultaneously,the electrochemical characteristics under different current frequencies were explored to analyze the corrosion resistance and thermal conversion performance of the membrane in salt solution.The studies show that the three-layer structure of the composite membrane cooperates to achieve good membrane distillation performance.The PVDF layer at the bottom allows the penetration of water vapor but prevents the penetration of salt ions.The intermediate layer composed of MWCNT provides excellent light absorption capacity and Joule-heating conversion capacity.And the top PDMS layer prevents the shedding of the MWCNT and improves the service life of the membrane.A numerical model of direct contact photo-thermal membrane distillation was applied to conduct numerical simulations on the seawater desalination performance of photo-thermal membrane distillation and the variation law of membrane temperature,and explore the heat concentration and freshwater production rate under different focusing degrees of sunlight.Simultaneously,based on PDMS/MWCNT/PVDF composite membranes,an experimental system of air-gap photo-thermal membrane distillation was established to further analyze the effects of different membrane treatments,different MWCNT contents and different light intensities on the air-gap photo-thermal membrane distillation.The results reveal that the observable phenomenon that the membrane temperature was above that of the main body of the feed liquid proves the realization of heat concentration and local heating by the photo-thermal membrane distillation process.The performance of membrane distillation can be improved effectively by using size eff ect and increasing light focusing ratio.Based on the composite membrane,the experimental system of air-gap Jouleheating membrane distillation was established;the membrane distillation processes under the action of a thermal management system that combi nes multi-stage heat concentration and multi-stage heat recovery device was experimentally studied.Joule-heating conversion characteristics of composite membrane under different current frequencies,input power,and energy concentration were analyzed.Furthermore,the effects of membrane treatments,current frequencies,and input power on the Joule-heating membrane distillation were studied.The enhancement effects of multi-stage heat concentration and multi-stage heat recovery was investigated.The results show that,based on the nonlinear relationship between the vapor flux and the energy concentration,multi-stage heat concentration could be achieved by adjusting the area of the membrane conductive layer,thereby improving the membrane distillation performance and reducing the system cost,and achieving the further enhancement of membrane distillation performance.A composite membrane-based photo-thermal/Joule-heating coupling membrane distillation experimental system was built to conduct experimental res earches on the membrane distillation process when photo-thermal/Joule-heating conversion work simultaneously.Compared with independent photo-thermal and Joule-heating processes,the coupling performance of photo-thermal/Joule-heating membrane distillation process was analyzed.Simultaneously,a numerical model of photothermal/Joule-heating membrane distillation was established to analyze the difference between photo-thermal and Joule-heating membrane distillation processes,and then to investigate the influence of related factors,such as heating mode,feed liquid flow rate,and input power,on the heat and mass transfer characteristics of the membrane distillation process.The results show that compared with the pure photo-thermal membrane distillation process,the photo-thermal/Jouleheating membrane distillation process can not only obtain a stable freshwater output but also achieve a more efficient membrane distillation process by maintaining a higher power density and higher temperature due to the use of higher quality electrical energy.The freshwater production rate increased exponentially with temperature due to the heat concentration characteristics,indicating that obtaining a higher membrane temperature through heat concentration can effectively i mprove the freshwater production rate and thermal efficiency in the membrane distillation process.Aiming at the evaporation and diffusion process of water molecules in membrane distillation process,a molecular model of water molecule moving in the pore structure of CNT membrane was established and simulated from the perspective of reducing the mass transfer resistance and effective evaporation enthalpy.The effects of temperature,CNT structure and CNT surface groups on the vapor flux of water molecules in the membrane distillation process were analyzed.The changes of hydrogen bonding and effective evaporation enthalpy of the composite film under the interaction of CNT and its surface groups with water molecules were also investigated.The results show that the vapor flux of water rises exponentially with temperature,which is consistent with the experimental results under macroscopic conditions.In logarithmic coordinates,the vapor flux of water shows a linear decreasing relationship with the number of C NT layers.After the introduction of surface groups,the number of hydrogen bonds formed between water molecules changed less with the increase of CNT diameter and the decrease of spacing.However,the average hydrogen bond length increased,resulting in the weaker interaction between water molecules and the decreasing effective evaporation of the composite membrane.