Numerical And Experimental Study On Plate Falling Film Multi-effect Seawater Desalination

Fresh water is one of the most important basic resource for human.At present in the world, there are50%rivers shrunk or being seriously polluted. If so, over55%of the world’s population will face a crisis of water shortage in the next30years,.Now16provinces of China have to confront the fresh water shortage and6provinces are in urgent need of fresh water. Since seawater is abundant in coastal offshore areas, it is the best choice to gain fresh water from sea using seawater desalination technology. This technology has showed its significance in sustainable utilization of water resources and economic development of coastal areas. Introducing water and power cogeneration, the cost of fresh water can be largely reduced Obviously, seawater desalination can alleviate the problem of water shortage and transform the power plant into water producer instead of fresh water consumer.In this paper, a new low-temperature multi effect seawater desalination technology combined with the plate falling film evaporator was developed. With numerical simulation method (FLUENT), the dynamic characteristics of falling film along the plate were simulated. The results showed that the flow of the falling film had strong three-dimensional fluctuation characteristics. A correlation for falling film thickness was fitted based on simulation results:δ=0.2805Re0.5017(v2/g)1/3, R2=0.99742. In addition, the break-up characteristic and evolution process of the liquid film were studied. Critical flow rate of rupture with two different feeding methods were predicted and the values were in good agreement with corresponding experiment results. At the same time, the experiment for heat transfer was also conducted and a heat transfer coefficient correlation under different flow conditions was gained. The turning point of laminar flow and turbulence:Recr=5760Pr-1.093; Laminar flow: h+=0.823Re-0.352,Re<Recr; Turbulence:h+=0.0038Re0.42Pr0.66,Re>RecrConsidering the variation of temperature and concentration, physical properties of water, steam and seawater were different for plate falling film low-temperature multi effect desalination system. A model based on mass and energy balance was proposed including all these differences. Due to the complexity and nonlinear property of multi effect desalination system, traditional algorithm was not suitable for the calculation. With the help of MATLAB, a new calculation program was designed to overcome disadvantages of long computing time and poor convergence of traditional algorithms.A three effect of low-temperature multi effect seawater desalination experiment setup was built according to the parameters calculated from the designed program. Operating performances (water yield, temperature distribution, pressure distribution, water producing ratio) under different conditions(the heating power, feed temperature, feed flow rate, cooling water temperature) were observed and analyzed. Considering the heat loss, the revised model could fit well with the experiment.These operating data provided reliable support for the design of seawater desalination setups.