| In the rapid freezing industry,there are a variety of refrigeration technologies,ammonia refrigeration and fluorine refrigeration are widely used at present.When the freezing temperature is low and the freezing rate is high,ammonia refrigeration and fluorine refrigeration have certain limitations.Therefore,it is one of the key points of current research to seek a new environmental protection and fast low temperature refrigeration method.The low temperature air refrigerator based on the reverse Brayton cycle has the advantages of lightweight,high efficiency,fast cooling,high reliability and wide cooling temperature area.It is widely used in various fields such as food refrigeration,low temperature and superconductivity.The working medium is air,and the air does not change its set state in the circulation system.The key cooling component of low temperature air refrigerator is the turbo expander.The turbo expander has the characteristics of high speed,large temperature drop at inlet and outlet and fast cooling speed.The performance of the turbo expander has an important influence on the efficiency and reliability of the whole reverse Brayton air refrigerator.Therefore,the cryogenic turbo expander,a key cooling component of the air refrigerator,is studied in this paper.The main research contents and conclusions are as follows:(1)Structural design of low-temperature turbo expander based on reverse Brayton cycle.The reverse Brayton cycle system is designed,calculate the system flow of the refrigeration cycle,determine the cycle parameters of each state point in the system,complete the cycle thermodynamic state table.Exergy analysis of exergy is carried out to calculate exergy efficiency of all components in the system based on parameters of each state point,which provides a basis for the design and optimization of the recycling system.The principle of cryogenic turbo expander is introduced.According to the parameters of the turbo expander designed by the circulation system,the thermal design and basic size determination of the turbo expander are carried out by using the quasi-ternary flow design method based on the streamline curvature method.A program for rapid design and calculation of cryogenic turbo expander based on NIST is successfully developed.(2)Numerical simulation of key flow parts of cryogenic turbo expander.Based on ANSYS Turbogrid software,the flow parts of turbo expander were divided into grids,and ANSYS CFX was used for numerical simulation.The simulation results of the Φ10 mm spindle low-temperature turbo expander show that the flow lines of temperature,pressure,airflow density and velocity in the flow field vary evenly,and the Mach number of the flow field is basically less than 1,which meets the aerodynamic performance requirements.The turbo expander has higher efficiency without diffuser,and the tip clearance size has a great influence on the performance of the turbo expander.The simulation results of Φ22 mm spindle low-temperature turbo expander show that: Under normal inlet conditions(inlet temperature = 290.15 K)and low inlet conditions(inlet temperature = 223.15 K),the total static efficiency of the turbo expander can reach 85%,and the total total efficiency can reach77%.The low temperature turbo expander under different inlet conditions has higher speed,larger cooling capacity and higher efficiency.It can meet the high cooling demand in the field of low temperature refrigeration.(3)Experimental study on foil gas bearing,a key component of low-temperature turbo expander.The Φ10 mm spindle cryogenic turbo expander is supported by dynamic and static pressure gas bearings,and the Φ22 mm spindle cryogenic turbo expander is supported by full dynamic pressure foil gas bearings.The bearing stability is studied experimentally.The results show that the vibration frequency of the foil-gas bearing with Φ10 mm dynamic and static pressure is mainly 1 frequency,and the amplitude of the turbo rotor decreases first and then increases with the increase of the rotating speed.The track of the shaft center of the turbo rotor with the aspect ratio of 1 bearing is clear and regular,the shape is elliptical,and the difference between the short and short axes is not much,and it has good stability.The research results show that the maximum main amplitude of the bearing rotor is not more than1.5 um,the bearing rotor system has a slight low-frequency vortex,the low-frequency amplitude is not more than 1 um,the rotor stable running speed can reach 132972 rpm,theΦ22 mm full dynamic pressure foil gas bearing has good stability.(4)Performance experiment and simulation of cryogenic turbo expander.The comprehensive performance test platform of reverse Brayton cryogenic air refrigerator was built,and the data acquisition and processing system based on Labview was developed,which can collect,process,display and control the data in real time.The performance of two turbo expander with different specifications was studied on the experimental platform.The experimental results of Φ10 mm spindle low-temperature turbo expander show that the outlet temperature of the expander decreases from 20 ℃ to-13 ℃ within 2 min.The maximum speed of the turbo expander is 278000 rpm when the inlet pressure is 0.32 MPa,the system flow rate is up to 0.023 kg/s,the cooling capacity is up to 1.21 k W,and the maximum efficiency is 65.5%,which fully reflects the superior performance of the turbo expander in fast refrigeration.The experimental results of the Φ22 mm spindle cryogenic turbo expander show that when the expansion ratio is 2.47,the system flow rate is up to0.218 kg/s,the cooling capacity is up to 8.45 k W,and the efficiency is 65.21%.By comparing the experimental and simulation results of the Φ22 mm spindle low-temperature turbo expander,the research shows that the maximum relative error of efficiency at each state point is 4.89%,the average relative error is 3.41%,and the error value is small.Therefore,the simulation results have high accuracy and reliability,which provides a reliable basis for the subsequent research of low-temperature turbo expander. |
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