Study On The Key Problems Of Improving The Working Temperature Of Single Screw Expander In Organic Rankine Cycle System

Organic Rankine cycle（ORC）system based on single screw expander（SSE）can realize the conversion of heat energy to mechanical energy efficiently.And it is considered to be a feasible technical to develop and utilize the widely distributed medium and low temperature heat energy,and has a broad application prospect.In theory,the efficiency of the system increases with the evaporation temperature,but it will also bring about a sharp rise in the expansion ratio,an increase in the thermo-elastic deformation and friction of the meshing pair for the expander as well.At this time,the structural design of the expander not only affects the efficiency of the prototype,but also relates to its reliability.Therefore,this paper mainly focuses on the coupling mechanism of working fluid and structure in the process of working temperature rising,and provides theoretical support for the structural design of single screw expander in higher working temperature and higher expansion ratio.The specific research contents are as follows.Based on the first law of variable-mass thermodynamics,the working process model of variable internal volume ratio for SSE was established by combining mass conservation equation,gas state equation and heat exchange equation,which considers leakage,friction,heat transfer and pressure loss in the process of intake and exhaust.Taking the shaft efficiency as indicator,the optimized internal volume ratio of the expander corresponding to the four kinds of working fluids in the process of increasing evaporation temperature was obtained.Aiming at the problem that the maximum shaft efficiency decreases with the increase of optimal internal volume ratio,by analyzing the influence of the internal volume ratio on the irreversible loss of intake and exhaust pressure loss,leakage,friction,etc.,it was found that the intake pressure loss is the key factor that limiting the performance of SSE with large internal volume ratio.The effects of internal volume ratio,the shape and clearance height of intake port and exhaust area on reducing intake pressure loss,intake leakage loss and exhaust pressure loss were analyzed by using numerical method.Then the optimization suggestions of properly reducing internal volume ratio and clearance height at intake port,removing the closed helix were obtained and used in the design of a new prototype.Results showed that the filling factor of the prototype was reduced from 125%to nearly100%and the highest shaft efficiency was increased from 56%to 67.7%at 3000rpm.The improvement of the overall performance of the prototype proves the correctness of the optimization direction,and the conclusions obtained were helpful to the development of the SSE with large expansion ratio.Based on the thermodynamic working process model,the temperature calculation model which comprehensively combined the viscous friction heat transfer of working fluid and lubricating oil in each clearance of the main part of SSE was established for the first time.Herein,the temperature of the heat exchange fluid was taken as its time average value.In addition,this paper innovatively obtained the temperature distribution of the casing,screw and star wheel at different working conditions by experimental method.And the temperature distribution model was verified by the experimental dates.Both the temperature calculation model and the measured temperature of each part of the meshing pair lay a foundation for the further study of the coupling heat transfer between working fluid and wall,the thermal deformation and friction of the meshing pair in single screw expander.In this paper,the optional range of elastic modulus and coefficient of thermal expansion of star wheel materials for different temperature conditions was given from the perspective of thermo-elastic deformation.By analyzing the geometric relationship between the key dimensions of each component and the clearance,the path marking method for monitoring the deformation of each component was developed.Based on this,the thermo-elastic deformation of each key position and the correlation of each clearance between the static and running state were obtained.It was found that the deviation of center distance and screw axial position changes obviously with the increase of intake temperature,which reaches 0.1 mm and 0.027 mm respectively at160°C.To eliminate the influence of these two factors,the deflection of star wheel caused by the deviation of center distance and axial position of screw were analyzed from the theoretical meshing relationship.The structural adjustment method of increasing the clearance between the front and rear sides of the teeth to reduce the interference risk was proposed and the adjustment value corresponding to the inlet temperature was obtained.Transmission accuracy model for linear enveloping meshing pair was established by using the method of action line and instantaneous arm.The influence of machining and assembly error,thermo-elastic deformation of each component and clearance adjustment value on the meshing accuracy were analyzed.And the relationship between the meshing error and working temperature was obtained.The dynamic load expressions of the transmission and sealing side of the meshing pair were derived by using the impact theory.The results show that reducing the elastic modulus of star wheel can reduce the sensitivity of impact dynamic load to meshing error.Considering the thermoelastic deformation and impact dynamic load,the suitable elastic modulus for star wheel is 10 GPa.The pressure distribution equation of the lubricating oil in the front and rear side clearances of the linear envelope meshing pair was established,and the expression of the oil film moment of multi-tooth meshing was derived.The influence of factors such as the front and rear roll angle of the star wheel,the position of the star wheel in the screw groove,the total clearance width,and the viscosity of the lubricating oil on the oil film torque were analyzed.The research shows that the roll angle under the meshing point of power side of star wheel（α_L^′）is the key structural parameter that affects the oil film torque.And whenα_L^′=0.6600.67α′_min（the minimum angle between the side tangent under the meshing point and the plane of the star wheel at driving side of the screw）,the oil film torque can effectively balance the static load.Taking the minimum oil film thickness and the critical bonding temperature as the criteria for judging lubrication failure,factors affecting the lubrication state of the meshing pair were studied.Results showed that increasing the viscosity of lubricating oil,reducing the elastic modulus of the star wheel material and the meshing surface roughness can effectively increase the thickness of the oil film and reduce the operating flash temperature.Based on the previous research conclusions,a prototype with an internal volume ratio of 6.00 was developed,and its performance was tested on an organic Rankine cycle system by using R123 as working fluid.Experimental results showed that the maximum intake temperature of the expander reached 163.8°C,and the maximum intake pressure was 1.61 MPa.The temperature and pressure are higher than those of the previous prototype（the intake temperature and pressure were 130°C and 1.2 MPa,respectively）.The stable operation of the prototype at high intake temperature condition verified the rationality of the expander structure design.At 3000±20 rpm,the maximum shaft power and shaft efficiency of SSE was 6.05 and 46.13%,respectively.The obtained test data were used to verify the relevant part of the theoretical research.By comparing the experimental value with the calculated value,it can be seen that the working process model,temperature calculation and thermo-elastic deformation model,the force of the meshing pair and the friction model were correct.For the organic Rankine cycle system,when the evaporating temperature increased from 111.6°C to135.4°C,the maximum cycle efficiency considering the consumption of the pump was increased from 5.3%to 5.6%,which proved the correctness of the idea of improving the cycle efficiency by increasing the evaporation temperature from the experimental point of view.This work helped to further improve the stable operation temperature of single screw expander.