Theoretical Analysis And Experimental Study On The Influence Of Expansion Ratio On The Single Screw Expander

In the recovery utilization of medium and low temperature waste heat,the Organic Rankine Cycle(ORC)was regarded as the most potential technique.For small and medium-sized ORC system,the expander as output power equipment limited its system performance further.The effect of expander on the properties of ORC system mainly reflected on of expander efficiency and expansion ratio.Therefore,how to improve the efficiency of expander and expansion ratio has become the key to realize ORC system practicality.The single screw expander has various advantages on high expansion ratio,stress balance,small vibration and wide application in pressure range.Therefore,the single screw expander as power output equipment of the ORC had good application prospect.In this dissertation,the theoretical analysis and experimental research on the expansion ratio of the single screw expander was carried out.The main research works were summarized as follows:The influence of the expansion ratio on the thermodynamic performance of ORC system was researched.The thermodynamic model of ORC system was established.The expander efficiency,expansion ratio and working medium pump efficiency and mutual coupling effect of three factors on the properties of ORC system were analyzed,and the expansion ratio on the system performance is very obvious.Increasing the expansion ratio is an important means to improve the performance of the ORC system though the efficiency of the expander and the efficiency of the pump are reduced slightly.Combined with the existing experimental research and technical level,the expansion ratio of the expander of the small ORC system should be increased to about 6-10,and the thermal efficiency of the system can reach 8% to 10%.The expansion ratio of single screw expander under variable working conditions mainly including variable speed and variable pressure were studied.The change trend and mechanism of internal and external expansion ratio under two conditions of variable speed and variable inlet pressure were analyzed.The experimental results showed that when the intake pressure was 0.4-0.65 MPa,the internal expansion ratio was about 5-8,and the external expansion ratio was about 3-4.Under the different working conditions,internal and external expansion ratio of single screw expander is very different,it can be seen that expansion ratio is obvious affected by working conditions,with the increase of intake pressure,expansion ratio gradually increasing,and no obvious under expansion.The optimization analysis of the expansion ratio of single screw expander was carried out.The influence of single screw structure parameters on the basic unit volume and maximum internal volume specific ratio of screw was analyzed.The results show that the volume of screw groove increases first and then decreases,the maximum internal volume specific ratio decreases gradually and the gradient decreases with the increase of center distance.When the screw and the star wheel were unequal diameter structure,with the increase of the unequal diameter ratio,the relative center distance of the maximum value of the basic unit volume increases gradually,and the maximum internal volume specific ratio also increases gradually.When the center distance was the same,the screw groove volume increase first and then decrease with the increase of unequal diameter ratio,when the unequal diameter ratio was 10%,compared with diameter structure,the basic unit volume increased about 36%,the maximum internal volume ratio increased about 80%.Thus,high basic unit volume and the maximum internal volume specific ratio could be obtained through adjust the center distance and unequal diameter ratio,which can provide theoretical basis for the design of high expansion ratio single screw expander.The research and development of high expansion ratio single screw expander was carried out.The basic structure design parameters of high expansion ratio single screw expander were determined.The design of the screw diameter was 117 mm,and the unequal diameter ratio was about 10%(the diameter of the star wheel was 128 mm),the center distance was 87.75 mm,and the design of internal volume specific ratio was 8.Then the design of screw shaft,screw rotor,star wheel and prototype shell was carried out,and the strength and rigidity of the screw shaft were checked.Based on this,the prototype of the single screw expander was fabricated and the initial performance test of the high expansion ratio single screw expander was carried out.The experimental results showed that when the intake pressure was 1.06-1.87 MPa,the internal expansion ratio was about 12-26,and the external expansion ratio was about 7-8.The optimum design of structure parameters is the technical means to achieve high expansion ratio.This study provides theory and data support for the optimization design of the single screw expander.