Single screw expanders have been widely studied in middle-low temperature heat recovery power system of organic Rankine cycle, which is significant for the energy-conservation and environment-protection. However, internal irreversible loss including leakage, heat transfer and friction power loss has great influence on the performances of single screw expanders. Although some present researches of single screw expanders have been carried out, few can directly reflect the effect of internal irreversible loss on the performance of expanders. Therefore, it is necessary to research the internal characteristics of the single screw expanders. In this paper, a numerical study of a single screw expander was carried out to analyze its internal irreversible loss. Based on the theory of engineering thermodynamics and hydrodynamics, a thermodynamics working process mathematical model was presented to calculate the flow rate and efficiency of a single screw expander. A separation approach was proposed to solve the above coupling problem, which could be solved by classical fourth-order Runge-Kutta method through MATLAB language programming. Take the organic working fluid R123 for example, the numerical results were verified by experimental results. The numerical results of flow rate, power output, volumetric and isentropic efficiency were in good agreement with the experimental results at the rotation speed of 3000±10rpm under different intake pressure. Then three organic working fluids R123, R245fa and R134a were chose to simulate the characteristics of single screw expanders. Results show that the highest efficiency is R123, followed by R245fa and the last is R134a at the same rotation speed and intake conditions.
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