Theoretical And Experimental Study On Fluid Pressure Energy Transfer Process Based On Rotary Positive Displacement Principle

Seawater reverse osmosis is a kind of desalination process based on the reverse osmosis principle. It is the effective way to reduce the desalination cost by energy recovery from stream with high pressure.In recent over 20 years, there is ever NO public and sophisticated report on theoretical analysis and experimental research of rotary pressure exchanger (RPE) possibly due to the technological secret and complexity of its own. In addition, although the prominent economic value the relative research on RPE is still very rare in China. Therefore, in this paper the SWRO experimental system with RPE equipment was setup based on the scientific and theoretical analysis on transition mechanism of mass, momentum and energy and on the face seal mechanism and their effects on the energy recovery process. In the study, the theoretical analysis, numerical simulation and experimental research were used to support each other and get better results.The main research and results were as following:(1) The experimental research was very important work in this paper. The real SWRO system with RPE as energy recovery unit was set up and the motor-driven RPE and fluid-driven RPE were designed, manufactured and assembled respectively. All the basic hypothesis, geometry model and physical model were based on and supported by the experimental work. Moreover, the results of theoretical computation were tested and verified through experiments. In this paper, the experiments of fluid mixing, characteristics of face seal and fluid energy recovery were carried out on the SWRO system. In the experiments, the clearance would be reach 0.02-0.05mm, the pressure at inflow of RPE being 1.76MPa, the mixing controlled under 3.5% and system recovery efficiency closed to 70%.(2) The study object was built up as the exchanger process of fluid mass, momentum and energy in the noninterial system. The transition process had the characteristics of highly periodical and unsteady flow.(3) The simple computational model of the fluid mixing zone being treated as fluid piece whose thickness could be neglected. The computational results showed that the effects of flux increasing on the transition process cancel that of rotor speed on it, which made the maximum inflow length keep constant. The results illustrated the RPE's excellent application to flux fluctuation and high rotary speed. (4) Numerical simulation was used to get the comprehensive description of transition process in the ducts of rotor. The 3D computational model was setup on the fluid mixing process. Then, the 3D model was simplified to 2D model on the consideration of computational efficiency and the periodical characteristics of transition process. The main effective parameters and their effects on transition process were carried out through orthogonal test and the analysis results as:â‘ The geometry and dimension of rotor (including length of ducts, diameter of ducts, quantity of ducts and diameter of ducts distribution circle) and the operational conditions (including flux and rotor speed) are the main parameters which had effects on transition process;â‘¡The geometry and dimension of rotor had more effects on the fluid mixing in duct than that of operational conditions. And they had the opposite effects on the liquid piston which was the result of transition process, which verified the computational results of simple model;â‘¢There is the optimization composition of these parameters. In this paper, the conception of velocity ratio of rotor speed to flow velocity was defined. The best operation status could be get when the velocity ration got the optimum point, which was the basis of RPE optimum design principle.(5) The mechanism of face seal between rotor and stator played the very important role in the RPE's energy recovery process. In order to get the mechanism of seal and the way to make it reached, both the simple model of flow in the slot between two plates and 3D numerical simulation on seal zone were used. The relationship of clearance of slot, seal pressure and leakage in seal process was built up. The results of computation showed that the seal pressure decreased rapidly with the increasing of clearance. When the clearance was over 0.03mm the seal pressure decreased almost to zero theoretically.