Study On Multiphase Flow Characteristics Of High Temperature And High Pressure Fluid In The Industrial Pipeline

Subcritical / supercritical water oxidation(HCWO / SCWO)is a new technology for industrial treatment of organic waste.This process involves the transportation of complex multiphase fluid,inorganic salt out and its abrasion effect on the wall of the pipeline under high temperature and high pressure,which greatly restricts the development and application of the technology.In the process,the material is usually composed of multi-component gas,liquid and solid phases.At the same time,in the process of flow,a little change of pressure will lead to sharp fluctuations in the physical properties of the working fluid,and then lead to phase transitions,which makes the flow highly nonlinear.Therefore,the design and optimization of pipeline involving complex component transportation or pressure drop control is a key problem that needs to be clarified.In addition,the complexity of transport conditions also makes the experimental study difficult.Based on the multiphase flow model and numerical simulation,it is an effective method to explore the complex flow process.In this paper,based on the calculation of strong nonlinear flow resistance characteristics in subcritical / supercritical water oxidation process,typical industrial pipeline units are selected to carry out three-dimensional unsteady numerical simulation.The model takes into account the strong nonlinear physical property change,decompression phase transformation and inorganic salt precipitation.The following three aspects are carried out:Firstly,a hybrid multiphase flow model with strongly nonlinear physical property change coupled with decompression phase transition is established.Based on the binary electrolyte model in Aspen Plus,the physical parameters of the working fluid were obtained and fitted to the equations varying with temperature and pressure;Then,the equations are coupled into the simulation software through the user-defined function,and the grid independence of the model is tested;Finally,the pressure drop empirical formula of steam water system in literature and the heat transfer coefficient of flow phase change heat transfer process are used to verify the model.The results show that the model established in this paper is accurate and feasible to simulate the flow characteristics of high temperature and high pressure multi-component gas-liquid two-phase in the pipeline.Secondly,based on typical pipeline components,the flow field,phase distribution and resistance characteristics of complex multi-component gas-liquid two-phase flow were obtained,and the evolution rules of pressure,temperature,velocity and gas holdup were investigated.The results show that the temperature in different pipeline units decreases gradually along the flow direction,while the gas holdup increases gradually;The pressure drop in the horizontal equal diameter pipeline decreases with the increase of diameter,but there is a critical diameter of 16 mm.The pressure drop decreases with the increase of expansion ratio;When the expansion ratio is constant,the pressure drop first decreases and then increases with the increase of the length of the pipeline,that is,there is a lowest point of pressure drop.The pressure drop increases with the increase of shrinkage ratio;When the shrinkage ratio is constant,the pressure drop first decreases,then increases and then decreases with the increase of the length of the tapered pipeline,which indicates that the rapid pressure drop can be achieved through the reasonable selection of the length of the tapered pipeline.The pressure drop decreases with the increase of diameter,increases first and then decreases with the increase of curvature radius,but the overall change is not significant.Thirdly,the coupling characteristics of salting out and flow were studied by using mixture combined with DPM model,and the movement trajectory of particles and the erosion characteristics of pipeline wall under high velocity were analyzed.The results show that the erosion effect of particles in the elbow is mainly distributed in the elbow and the connection between the elbow outlet and the straight pipeline,and the maximum erosion decreases with the increase of the diameter;At the same time,with the increase of curvature radius,the maximum erosion amount first remains unchanged,then decreases sharply,and then returns to stable.The serious erosion is mainly distributed in the elbow,and compared with the inner surface,the outer surface of the pipeline is more serious.