| As a typical positive displacement machine,screw pumps are widely used in petrochemical,shipping,marine,energy and food industry due to their reliability and excellent performance in single-phase or multiphase operation.Compared with other pumps,they have higher volumetric efficiency and lower operation noise and pulsation.With the advance of high-efficiency precision machining technology,the manufacturing costs of screw pumps are gradually reduced,the application fields are rapidly expanded,screw pumps have become one kind of key components in many high-end equipment.The demand of high-performance screw pumps also grows sharply.Research work on modelling and experimental investigation of working processes in twin screw pumps was reported in many previous studies both at home and abroad over the past three decades.However,Due to the complexity of the chamber geometry and high-speed continuous change of its size,it is a big challenge to achieve precise simulation of the working process and internal flow.Most of the current methods are based on the one-dimensional chamber mathematical models which neglect kinetic energy and the geometrical factors and simplify the analysis of the main and leakage flow,the transient flow field and the cavitation characteristics inside chamber cannot be obtained.Hence,full understanding of the internal working mechanism in screw pump is not accessible which brings the difficulty to achieve precise design and optimization of high-performance screw pumps.The three-dimensional flow inside screw pump is the research object in this dissertation.Fluid dynamics,moving grid technique,gearing theory,differential geometry,multiphase flow theory,bubble dynamics,numerical method,etc.have been used to analyze the three-dimensional transient flow in the chamber,to study the internal working mechanism and the influential factors.Firstly,based on the theoretical analysis of working process,1-D chamber model of screw pump was proposed which gives a mathematical description of the change of mass and energy and the conservation relationship inside pump.The leakage and boundary conditions were discussed.A CFD numerical model of the flow in screw pump was built,in order to solve which,a generation method of structured moving mesh for the rotor fluid domain with conformal mapping interface was elaborated.In order to simulate the transient characteristics of the screw pump during the working process,a three-dimensional CFD simulation of transient flow field in screw pump was carried out based on the structured moving mesh.Reading and updating of the moving mesh is managed by use of a custom procedure.The internal flow characteristics of the screw pump under different speed and different discharge pressure and the performance of the pump were obtained.The comparison and analysis of the experimental data verify the feasibility and accuracy of the calculation model.On the basis of this,the influence of inlet and outlet geometry,the friction power loss,the screw rotor clearances and the viscosity of the working medium on the pump performance were analyzed,the influence of different turbulence models on prediction of pump performance was also discussedGenerally,the design of the screw rotor profile can only take into account its geometric and meshing characteristics,and cannot examine the corresponding pump performance.In order to accurately design and evaluate the rotor profile of screw pump,this dissertation combines the full 3-D CFD numerical simulation with the geometric design of screw rotor profile.In this dissertation,the three-dimensional CFD numerical simulation is combined with the geometric design of the screw-rotor profile line.For the typical 2-3 type rotor profile design,through comparative analysis,the corresponding transient performance characteristics of different rotor profiles were obtained,which provided a new reference and evaluation standard for the precision design and optimization of the screw rotor profile.With the developing trend of screw pumps to high speed,high pressure,large discharge and low noise,the cavitation inside the pump cannot be ignored.The cavitation process,accompanied by the initiation,expansion and collapse of a large number of bubbles,will lead to strong cavitation noise and shock vibration,it will also cause erosion on the surfaces of rotors.It has been difficult to capture and describe the cavitation phenomenon in screw pumps due to the complexity of the working process and limitation of grid technique.Based on structured moving mesh,VOF model and cavitation model,the numerical modelling of cavitating flow in screw pump has been carried out.The dynamic change regularities of cavitation intensity and distribution area under different working conditions were obtained.The mechanism of clearance cavitation was discussed.This study provided theoretical guidance for suppression of cavitation in screw pumps.In consideration that there are a large number of literature resources using static mesh to carry out the CFD modelling of screw pumps,even though which is simple in operation and easy in convergence during calculation,this method neglects the momentum of liquid inside chamber when rotors rotating.Comparison and analyzing of the calculation error between static-mesh method and moving-mesh method was presented.In order to improve the calculation accuracy,a CFD modelling method with momentum compensation based on static mesh and rotating wall was proposed.This method can help to dealing with some novel screw pumps for which the structured moving mesh generation is not easy.The work above improves the comprehending of three-dimensional flow characteristics and cavitation characteristics in screw pump.It also provides technical and theoretical references for precise design and optimization of high-performance screw pumps. |
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