Geometric Modeland Thermodynamic Simulation On The Performance Of Wet Steam Twin Screw Expander

Owing to energy shortage and concerns over environmental issues,interest in power recovery from geothermal energy and industrial medium and low temperature waste heat have increased all over the world.The use of twin screw expanders instead of turbines offers a number of potential advantages.A new geometry-based thermodynamic analysis method was presented in this paper.Then the mathematical model of the screw expander was established to research the influence of inlet pressure,dryness of inlet wet steam and rotate speed on the leakage,isentropic efficiency and power output.Firstly,a 3D geometrical model of twin screw expander was developed,and the model consisting of male and female rotors and of inlet and outlet ports.A new calculation method of some geometric parameters such as groove volume,suction port area and leakage area was presented based on the geometrical model.The new calculation method was applied in calculating geometric parameters of SEPG500 screw expander.Secondly,based on the equations of conservation of mass and energy,a thermodynamic model of twin screw expander driven by wet steam was built.In the model,the leakage is treated as two-phase layer flow for leakage through contact line,rotor tip,inlet end face and blow hole.In the suction process,the flow through the inlet port is treated as two phase flow through orifice plate.Using the model,temperature,pressure,dryness and leakage mass of wet steam in the control volume as well as isentropic efficiency and power of expander were obtained.The developed model is verified by comparing predicted macroscopic performance parameters with operational data.At last,the influence of inlet pressure,inlet dryness and rotate speed on the performance of the expander were analyzed.The results showed that throttling losses,pressure drop losses and leakage losses during the working process are the main influence for the isentropic efficiency of twin screw expander driven by wet steam.Leakage in the suction process and the early stage of expansion process are most serious,while the contact line and the rotor tip are two main leakage paths.The increase of inlet pressure and the decrease of inlet dryness and rotational speed will increase the mass of leakage.The throttling losses increases with increasing rotate speed and inlet pressure,and has little to do with inlet dryness.Power increases with increasing inlet pressure and rotate speed but has a slow ascending trend at high pressure.Power decreases withincreasing inlet dryness,but the variation trend begin to flatten out at high inlet dryness region.The isentropic efficiency of wet steam screw expander increases with increasing inlet dryness and decreasing rotate speed.But the change of isentropic efficiency with inlet pressure has a peak value.The research results will lay theoretically foundation for the further optimizing of wet steam screw expander system,and are of considerable realistic significance and important theoretical value for promoting the development and utilization of geothermal energy and industrial medium and low temperature waste heat recovery in China.