Numerical Simulation And Performance Analysis Of Single Screw Expander Working Process Based On Semi-empirical Model

Currently,the growing demand for energy and the consumption of fossil fuels are the main concerns of global energy and environmental issues.The efficient utilization of new renewable energy and other traditional energy has become the focus of research.Among them,Organic Rankine Cycle(ORC)is considered to be a practical,energy-saving and efficient low-temperature industrial waste heat utilization technology.However,for small and medium-sized ORC,the poor performance of expander is the main problem of low cycle efficiency.Further systematic theoretical research on irreversible loss of expander is needed.Single screw expander has the advantages of high product efficiency,low noise,low wear and long service life.It is one of the most suitable expanders in small and medium-sized ORC system.Based on the semi-empirical model,this paper carries out the numerical simulation and theoretical research of the single screw expander,predicts the irreversible loss in the expansion process,analyzes the factors affecting the performance of the single screw expander,and provides a theoretical basis for the design optimization of the single screw expander.The main research contents are as follows:Firstly,based on the thermodynamic changes in the working process,a semi-empirical model of single screw expander is established.Based on the geometric structure of single screw expander,the model parameters are reduced to 5,including nominal mass flow,nominal inlet heat transfer coefficient,nominal exhaust heat transfer coefficient,heat transfer coefficient with environment and mechanical loss torque.The optimal value is obtained by genetic algorithm,and other parameters are obtained by geometric structure or working conditions.Secondly,the numerical calculation and analysis of the semi-empirical model of the single screw expander is carried out.The results of the semi-empirical model of the single screw expander are analyzed based on the existing compressed air variable inlet pressure,variable speed experiment and ORC variable inlet temperature experimental data.When the working fluid is compressed air and R123,the maximum calculation error of exhaust temperature is 20.07% and 2.44%,respectively.The results show that the semi-empirical model of the single screw expander is more suitable for the performance analysis of the expander in ORC,which affects the output of the model.The main factors in the results are leakage losses and mechanical losses.At the same time,it is found in the calculation process that the inlet pressure drop calculated by the constant entropy nozzle model is very small between 2 and 6k Pa,which deviates from the actual working process of the single screw expander.Finally,the semi-empirical model is modified for irreversible loss.The irreversible loss mainly includes leakage loss,mechanical loss and inlet and exhaust pressure drop loss.By comparing two physical models for calculating inlet and exhaust pressure drop,it is found that the calculated value of regression analysis method is closer to the actual working process of single screw expander.The final modified semi-empirical model is compared with the experimental results,and the prediction results of the model are good.The maximum deviations between the calculated values and the experimental values are 3.34% for mass flow rate,9.14% for shaft power,and 1.59% for exhaust temperature.The semi-empirical model of single screw expander finally revised in this paper not only improves the flexibility of the model,considers the irreversible loss more comprehensively,but also has higher calculation accuracy,which provides a basic method guidance for the systematic theoretical research of single screw expander.