Effect Of Expansion Ratio On Organic Rankine Cycle

Under the dual pressure of heavy dependence on foreign imports for oil resources and global energy conservation and emission reduction,it is imperative to develop and utilize low quality energy.In light of low fuel thermal efficiency of car engines and high heat energy in the tail gas,organic Rankine recycling technology has certain advantages in the recovery of exhaust heat from vehicle engines and rational development and utilization of waste heat is beneficial to the improvement of energy efficiency.In this paper,a simulation study is performed on the expander,a key component of the thermal power conversion system of the organic Rankine cycle residual energy recovery system.The expander is a machine that uses the principle of outputting shaft work to the outside to obtain energy when the high-pressure gas expands and decompresses.The conventional organic Rankine cycle exhaust gas waste heat recovery system uses a fixed-structure expander which cannot adjust the expansion ratio.The expander used in this paper is a six-blade variable expansion ratio expander.The variability of the expansion ratio is achieved by adjusting the initial exhaust angle.The variable expansion ratio expander can change the expansion ratio in real time according to the state of the working medium,improving the system adaptability in different working conditions.The structural parameters of variable expansion ratio expanders(such as blade length,intake angle,motor speed,etc.)and the state of the cycle working medium(temperature,pressure)have a certain effect on the performance of the variable expansion ratio expander.In order to study the influence of related parameters on the performance of the expander,this subject uses the control variable method to simulate the above parameters.In this paper,the expansion ratio is used as the independent variable to analyze and study the output power,plutonium efficiency and recovery efficiency of the expander.Firstly,the mechanical structure of the variable expansion ratio expander is initially designed.The maximum bending stress of the expander blade in one loop is calculated to beσ_w=17.49 MPa,which meets the requirements of blade bending strength and provides theoretical support for subsequent simulation research.The initial exhaust angle of the variable expansion ratio expander is set to 170°~270°,and the corresponding geometric expansion ratio obtained by MATLAB is 2.34~6.11.The GT-POWER software is used to establish a one-dimensional simulation model of the variable expansion ratio expander,and the simulation tests are performed to verify the mass conservation law and energy conservation law on the expander simulation model.Secondly,the control variable method is used to study and evaluate the key parameters of the variable expansion ratio expander simulation model established by GT-POWER,and output power and krypton efficiency are selected as the evaluation index of the expander.The findings are as follows:(1)Regarding the axial length of the blade,in theory,when the axial length of the blade in the expander is larger,the mass flow rate of each cycle will increase,and the friction loss will also increase.The research shows that under constant expansion ratio,as the axial length of the blade increases,the output power of the expander improves continuously,however,when the axial length of the blade increases to above 180 mm,the output power of the expander improves obviously no longer.Under the condition that the axial length of the blade is constant,there is an optimal expansion ratio to optimize the output characteristics(output power and efficiency)of the expander.When the axial length of the blade increases from 60mm to 220mm,the corresponding optimal expansion ratio reduces from 5.51 to 4.07.(2)For the initial angle of intake,in general,when the continuous angle of the intake process remains basically the same,the initial angle of the intake will affect how close the expansion chamber to the TDC at the time of intake completion,which will affect the mass flow of the circulating working medium.The study shows that the output power of the expander increases continuously with the increase of the initial intake angle.As the initial angle of intake continues to increase,the efficiency of the expander increases first and then decreases.There is an optimal value.When the initial exhaust angle is 170°,the expansion ratio is 2.34.When the initial angle of intake is 72°,it reaches the inflection point first.(3)For intake air temperature,the output power of the expander increases with the increase of the inlet temperature,and the radon efficiency decreases with the increase of the inlet temperature.When the inlet temperature increases from 110°C to 230°C,the corresponding optimal output power increases by 4.1%,and the corresponding optimal chirp efficiency reduces by 4.8%.Therefore,the inlet temperature of the expander has a little effect on the output characteristics of the expander.(4)For intake pressure,both the output power and the efficiency of the expander increase with the increase of the inlet pressure.When the inlet pressure increases from 0.6 MPa to 1.1MPa,the optimal output power of the expander increases by 209%,and the optimal radon efficiency increases by 15.3%.Therefore,increasing the inlet pressure is conducive to improving the output performance of the expander.(5)For the speed of the expander,when the inlet pressure is 0.8MPa and the rotation speeds are 1200r/min,1500r/min,1800r/min,and 2100r/min,the optimal output power of the expander is 2.19kW,2.49kW,2.60kW,2.62kW,corresponding optimal unitary efficiency is83.9%、82.7%、80.1%、72.9%.Comprehensive consideration,for the four speeds studied above,the output performance of the expander is better when the speed is 1500r/min.Thirdly,the application of variable expansion ratio expander in the CA6DL diesel ORC system is explored,and the residual energy recovery effect of the simulation model under different operating conditions is studied.The research shows that the matching of the expander simulation model with the operating conditions of the CA6DL diesel engine meets the law of conservation of energy;the relative error between the calculation flow of the organic working meduim in the evaporator and the simulation flow in the expander simulation model is less than5%,which meets the test requirements;CA6DL diesel engine has a specific expansion ratio under each working condition,which makes the expansion efficiency,output power,theoretical thermal efficiency,and recovery efficiency of the expander optimal at the same time;CA6DL diesel engines are selected to operate separately at the operating conditions of a50,a75,b50,b75,c50,and c75.Among them,the available energy of the exhaust gas under the c75 operating condition is the highest,and the recovery efficiency of the expander under this operating condition also reaches a maximum of 7.094%.Finally,the four-blade and six-blade expanders are compared.In order to comprehensively compare the performances of these two types of expanders,simulation analysis is performed by using variable operating conditions to determine which operating conditions that these two types of expanders are suitable for,and provide theoretical support for subsequent product development.Under the two conditions,that is,constant temperature and changed pressure,and constant pressure and changed temperature,the variation of each is analyzed to achieve the purpose of improving the performance of the expander.The findings are as follows:(1)The adjustable range of the expansion ratio of the four-blade expander is 1.55 to 3.02,and the adjustable range of the expansion ratio of the six-blade expander is 2.34 to 6.11.(2)When the inlet temperature is set to 200°C and the inlet pressure increases from 0.5MPa to 1 MPa,the growth rates of the unit flow output power of the four and six-blade expanders are 31.2%and 64.8%,respectively.(3)When the inlet pressure is set to 1 MPa and the inlet temperature increases from 110°C to 230°C,the growth rates of unit flow output power of the four-and six-blade expanders are44.9%and 41.7%,respectively.(4)When the inlet pressure is larger than 0.85MPa,the optimal efficiency of the six-bladed expander is higher than that of the four-bladed expander,and vice versa;when the inlet temperature is between 110°C and 230°C,the superior efficiency of the six-bladed expander is higher than the four-blade expander.