| Seawater desalination has got much attention to mitigate the crisis of freshwater shortage in the world. Low temperature multi-effect seawater desalination (LT-MED) has become one of the most popular desalination technologies due to simple pretreatment, low corrosion and good quality of product. This paper proposed and studied a LT-MED with plate heat exchangers, for high heat transfer coefficient, compact structure, small footprint, easy disassembly and cleaning, called plate type LT-MED.Chevron corrugated plate is the main part of a plate type LT-MED system. Experiments and numerical simulation were carried out to study the flow and heat transfer characteristics of chevron corrugated plate. A numerical simulation model of single phase flow on chevron corrugated plate was established, and the errors of the heat transfer coefficients of the simulation model and experiential data is less than 15%, indicating the model is verified. Later, the simulation model was used to analyze the effects of inclination angle, height and pitch of the corrugation. The results suggest that inclination angle takes the main role on the heat transfer and resistance characteristics; the corburn factory is increased by 60% with the increasing of inclination angle from 30° to 60° and after that the j will decrease if the inclination angle increases further; while the friction factor f increases with the increasing of inclination angle. The influence of corrugation height on the heat transfer characteristics of the plate is relatively small, j and f will be increased by 5% and 100%, respectively, when the corrugation height increases from 3 mm to 6 mm. j and f will be decreased by 30% and 70%, respectively, with an increasing corrugation pitch from 5 mm to 20 mm. For two-phase flow, the non-dimensional heat transfer correlation of evaporation and condensation was fitted by experiment: Nutp= 2.1* ReL-0.4072* ReG0.5832* PrL0.205, where 19.9≤ReL≤51.2,599≤ReG≤1693; simulation results suggest that the corrugated inclination angle is main influence factor on the distribution of gas volume fraction. Less dry spot appears if the angle is great enough even with a great gas volume fraction. The heat transfer coefficient increases with the increase of inclination angle, which is affected by gas volume fraction; the heat transfer coefficient of two-phase is larger than single phase by 20%-100% under the same inclination angle. After all, a inclination angle of 60°, height of 3-4 mm and pitch of 15-20 mm are suggested based on the simulation and experimental results.A rising film method was proposed to form a complete water film flow in chevron corrugated plate, to avoid dry spots. A mathematical model of plate type seawater desalination system was established based on materials and energy balance and taking changes of seawater physical parameters into account. With the help of Matlab, a calculation program was written for a three-effect plate type rising film desalination system of 10kg/h water production.An experimental setup was built to study the influence of effects and main operating parameters on the performance of the system. The experimental gained output ration (GOR) of a two-effect system was 1.56-1.63, while that of a three-effect system was 2.03-2.13. GOR decreases with the increasing of main vapor temperature, while water production increases. GOR and water production increase with the increasing of feed water temperature and cooling water flow rate, or the decreasing of cooling water temperature and feed water flow rate.An steam ejector is introduced to improve the GOR for a four-effect plate type rising film desalination system of 50t/d water production. The GOR of a single effect increased from 0.783 to 1.205 comparing to the original design, and the total GOR of the optimized design reached 4.82. |
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