Research On Structural Design And Dynamic Performance Of Medium And Low Temperature Twin Screw Expander

With the gradual sharp contradiction between economic growth and environmental protection as well as energy consumption and other issues,the twin screw expander as a new type of waste heat recovery device has been widely concerned.However,the existing domestic screw expander at present is almost transformed on the basis of screw compressor and then matches the system in or-der to put into operation.However,the reformed expander can not fully adapt to the thermal proper-ties of the working medium,which reduces the expander's reliability in high temperature and high pressure steam condition and besides affects the work efficiency of the system.In addition to the defects in the process of equipment transformation,in view of the complexity of the expander's working process and the limitation of the experimental study,the research on the expander for the moment is focused on the system performance,therefore the characteristics of the expander's struc-ture need to be further explored.In view of the above facts,the following contents presented in this thesis are mainly aimed at solving the basic application problems about the twin screw expander driven by Organic Rankine Cycle power in the low and medium temperature range,which are based on the simulation model technology,fluid mechanics theory and numerical simulation method.The screw rotor profile is the core technology of the twin screw expander,the thesis calculates the curve equation of the tooth profile through analyzing the design elements and establishing the mathematical model of the rotor profile and compiles the meshing line program by using MATLAB software,on the basis of the above analysis,the influence of the geometric parameters of rotor pro-file on the area utilization coefficient is analyzed,which can greatly improve the design efficiency of the tooth profile.According to the trajectory of fluid,the position and shape of suction and ex-haust ports are designed,after that,the structure and shape parameters of twin screw expander's main components are determined.Based on the powerful 3D modeling function of Siemens NX,the 3D simulation model of the expander is established,and the assembly analysis is carried out to ver-ify the feasibility of the design model.It is relatively complicated to describe the spiral motion of the twin screw expander's working medium in a three-dimensional space by using mathematical analysis and experimental research methods.In view of this,the simulation analysis of the expander's internal flow field is carried out by combining the finite element model of expander working channel with computational fluid dy-namics as well as the use of professional CFD simulation software CFX,obtaining the distribution of pressure field and velocity field of the moving fluid,along with the structural pressure distribu-tion of expander,subsequently revealing the main characteristics of fluid motion in the twin screw expander.On this basis,the influence of the field variables and the exhaust orifice structure on the convection field is further analyzed,defining the dynamic mechanism of twin screw expander.The dynamic performance is one of the important indexes to judge the working performance of the twin screw expander,and the influence of the screw rotor as the core component of expansion's power transmission on expander's dynamic performance may not be ignored.In this paper,accord-ing to the characteristics of the periodic change in volume pressure of the rotor during the high speed rotation,a simplified model for calculating the force of the rotor is established based on the finite element method from the perspective of the vibration and noise of the expander.At the same time,the ANSYS Workbench software is used to simulate the deformation of the rotor under the force action,discussing the harmonic variation of the screw rotor's stress and deformation.Finally,the vibration characteristics of the rotor in natral frequency and excitation frequency are analyzed.The results show that the twin screw expander is reliable and has good dynamic performance,providing a method to predict and evaluate the performance of the expander.