Dynamic Characteristics Of Flow Field And Thermal Expansion Analysis Of Medium-low Temperature Twin-screw Expander

With the release of the domestic dual carbon strategy and the deepening of energy conservation and emission reduction policies,low-temperature waste heat recovery and utilization in the energy industry is undergoing rapid development.The twin screw expander,which combines many advantages such as rotary machinery and volumetric expanders,is the main expander type for low-temperature waste heat recovery and utilization units.Based on the working characteristics of a twin screw expander,this paper first uses formula derivation and theoretical analysis to construct the rotor and shell model from a mathematical model into a two-dimensional structure,and then constructs a three-dimensional model from a two-dimensional model through three-dimensional software.After selecting the flange sealing surface structure and flange connection form,the flow field range of the twin screw expander was determined.After analyzing the flange connection bolts and bearing loads,the fixed constraint positions and loads of the thermal expansion analysis model of the twin screw expander were further determined,providing a theoretical basis for the flow field analysis of the twin screw expander and the boundary condition setting for the thermal expansion analysis of the twin screw expander.After importing the model into CFX software and conducting simulation,it was found that:(1)The dynamic characteristics of important parameters such as pressure,temperature,rotor force,and output power in the flow field of a twin screw expander as they change with the thermal process are analyzed.It is calculated that under rated operating conditions,the axial force of the male rotor is about 4.33 times that of the female rotor,and the torque of the female rotor is about 34.6%of the torque of the male rotor.The output power of the female rotor is approximately 23%of that of the male rotor.(2)After comparing and analyzing the off-design characteristics of flow field parameters of twin screw expanders under different exhaust modes,it is concluded that when axial exhaust ports are used,the outlet flow fluctuation is relatively greater.At the same suction pressure,The output power of the Φ300 twin screw expander with radial exhaust port is about 4.03KW higher than that with axial exhaust port.(3)As the amount of tip clearance increases,the axial force exerted by the working medium on the rotor can be reduced.However,the output power loss is 288.53W per 0.02mm increase in tooth tip clearance on average.Increasing the flow rate will reduce the turbulent kinetic energy in a certain area.Therefore,there will be a phenomenon that the turbulent kinetic energy of the flow field first increases and then decreases with the increase of the tip clearance.The overall periodic variation trend of turbulent flow energy dissipation rate is generally consistent with that of turbulent kinetic energy.Finally,after exploring the thermal expansion characteristics of the rotor and cylinder of a twin screw expander,the concept of "effective tip clearance expansion"is derived,which excludes the gap changes in the positions of the cylinder itself that are not affected by thermal expansion changes,such as the radial suction orifice and exhaust chamber positions.Then,the difference between the maximum expansion of the rotor and the minimum expansion of the inner wall of the cylinder on the rotor side is calculated,thereby obtaining the effective tip clearance expansion.This paper focuses on the dynamic characteristics of flow field and thermal expansion analysis of medium and low temperature twin screw expanders.Through theoretical analysis and simulation calculations,key issues such as flow field characteristics and thermal expansion of twin screw expanders have been studied.The conclusions can provide a certain reference and basis for the design and manufacture of twin screw expanders,a core equipment for medium and low temperature waste heat recovery and utilization in engineering applications.