Study On Heat And Mass Transfer Of Porous Media In Air-gap Diffusion Distillation

The global freshwater resources are scarce and the distribution of different regions is not balanced.It is estimated that by 2025,the world's water-scarce population will double,and more than 40 countries and regions will face the problem of freshwater shortage.Although China is rich in water resources,it has a large population and per capita freshwater resources are only a quarter of the world's per capita freshwater resources.Desalination technology is the most important way to solve the current increasingly scarce water resources problem,and various countries are actively conducting related research.The air-gap diffusion distillation desalination method proposed in this paper uses a porous medium with high hydrophilicity to replace the selective permeable membrane in membrane distillation.It combines the advantages of membrane distillation and solar distillation.It operates under atmospheric pressure,has a simple structure and it can use low grade heat energy that costs savings,has a wide applications.In this paper,the effects of operating parameters of the device and structural parameters of porous media on heat and mass transfer of porous media in the air-gap diffusing distillation(AGDD)desalination device were investigated through experiments.The research is carried out by changing the operating parameters such as the inlet temperature of the hot side,the inlet temperature of the cold side,the flow rate and the structural parameters of the porous medium,such as the porosity,permeability,and thickness of the AGDD.The experiment shows that the inlet temperature of the hot side has the greatest influence on the heat and mass transfer of the porous medium,followed by the flow rate.In terms of structural parameters,the porosity of the AGDD has the greatest influence,and the permeability is the second.Therefore,thin porous media with large porosity and permeability are selected to operate under the condition of large flow rate and large temperature difference between hot fluid inlet temperature and cold fluid inlet temperature,heat and mass transfer of the porous media is the best and the AGDD has the best performance.And when the thick and height of porous medium is 10mm and 0.5m,the flow rate is 20L/h,the hot side inlet temperature is 65°C,and the cold side inlet temperature is 15°C,the AGDD has the maximum evaporation flux of 2.02kg/(m~2h).And when he thick and height of porous medium is 5mm and 0.5m,the flow rate is 20L/h,the hot side inlet temperature is 65°C,and the cold side inlet temperature is 15°C,the water recovery rate of the device is excellent,up to 3.6%.