Investigation On The Electrical-Mechanical-Acoustic Matching Characteristics Of A High Capacity Stirling Pulse Tube Cryocooler Working At Liquid Nitrogen Temperatures

Cryocoolers have broad application prospects in the fields of military,aerospace and low temperature physics.The high capacity Stirling type pulse tube cryocooler has many advantages in high temperature superconducting and small-sized liquefied natural gas applications,because it has no moving parts at its cold end,which makes it more reliable.The key issue to make it adequate in practical applications is to further improve its cooling efficiency.In the dissertation,a high capacity pulse tube cryocooler working at liquid nitrogen temperatures is investigated.A linear compressor model is proposed.The impedance matching between the compressor and the cold head is studied.The influences of the regenerator and the after cooler on the impedance matching and the cooling performance are studied.The cooling capacity of the cryocooler is improved.The details are listed below.1.A linear compressor model is proposed based on the analysis of the electrical impedance,the mechanical impedance and the acoustic impedance.Based on the model,a method of measuring the acoustic power on the piston surface is developed and the influence of each single parameter on the resonance characterisitcs is quantitatively analyzed.A method of how to obtain the maximum acoustic power under restrictions is proposed,which provides theoretical basis for achieving a better matching between the compressor and the cold head.Starting from the analysis of the electrical impedance,the mechanical impedance and the acoustic impedance,a mass-gas-spring model is proposed.It is used to analyze the oscillating characteristics of the linear compressor.The corresponding equation for the oscillator is deduced.Different from the conventional mass-spring model,which treates the gas as a gas spring,the mass-gas-spring model takes the gas as an equivalent mechanical impedance,making it more accurate in describing the oscillating characteristics of a linear compressor.Compared with the models proposed by predecessors,the mass-gas-spring model does not require calculating the gas spring according to the experimental value every time,so that the model has better predictice ability.Besides,it can be used to calibrate the phase angle,to predict the resonant state,and to obtain the maximum acoustic power.According to the equation for the oscillator,a method of calculating the acoustic power on the piston surface is proposed.The difficulty lies in that the pressure sensor and the displacement sensor are different types of sensors,and the phase angel measured has deviation.Based on the equation for the oscillator,the phase angle can be calibrated,so that the acoustic power on the piston surface can be measured.Compared with other methods,this method is simple and easy to implement,and the time is greatly shortened.The influences of the intrinsic parameters and operating parameters on the oscillating characteristics of a linear compressor are quantitively analyzed.The sensitivity analyses of all the parameters are also conducted.The maximum output acoustic power of the compressor under several limits can be derived and the method of obtaining it is described.The above-mentioned limits refer to the following four: the upper limit of the electrical current,the upper limit of the displacement,the charging pressure determined by the cold head and the operating frequency determined by the cold head.With these constrains,a method of achieving the maximum acoustic power is discussed and verified by experiments.2.Based on the analysis of the linear compressor model and the cold head phasor distribution,a method of optimizing the impedance matching between the compressor and the cold head is proposed.The influences of the regenerator and the after cooler on the impedance matching and the cooling performance are discussed.The efficiency of the pulse tube cryocooler is the product of the compressor efficiency(the ouput acoustic power divided by the input power)and the cold head efficiency(the cooling capacity divided by the compressor output acoustic power).In order to improve the efficiency of the cryocooler,it is necessary to simultaneously satisfy the high efficiency of the compressor and the high efficiency of the cold head.Besides,to improve the cooling capacity of the cryocooler,it is nessary to obtain the maximum ouput acoustic power under the premise of the high efficiency of the cryocooler.The maximum output acoustic power point can be obtained based on the linear compressor model,which is also the extreme point of the compressor efficiency.Under this premise,the performance of the cold head is optimized.Accodring to the amplitude and phase angle of the pressure and the mass flow on the piston surface,the initial phasor of the admittance of the cryocooler is determined,and other phasors of admittance can be determined by the impedance relationship of each components.To aquire the ideal phase angle(±30°),the transfer tube and the inertance tube is analyzed.In the process of matching the phasors,the maximum output acoustic power would decrease a little.The effects of the regenerator and the after cooler on the impedance matching and cooling performance of the cryocooler is investigated.The change of the length of the regenerator would cause the change of the acoustic resistance and capacitance,resulting in the change of the impedance matching.Thus,it is necessary to re-optimize the impedance matching.However,the change of the after cooler(not changing its volume)has little effect on the impedance matching.The heat transfer performance of the regenerator and the after cooler has great effects on the performance of the cryocooler,which is studied experimentally to improve the cooling performance.3.The linear compressor model is verified by the experiments conducted on a Stirling type pulse tube cryocooler.The impedance matching is optimized.And the heat transfer performance of the regenerator and the after cooler is improved.Finally,the cooling performance of the pulse tube cryocooler is improved.Experiments are conducted on a high capacity Stirling type pulse tube cryocooler.The proposed compressor model and the impedance matching theoretical analysis are verified.The regenerator and the after cooler are optimized.The goal is to improve the cooling performance of the cryocooler.The compressor used is a linear compressor CFIC-2S297 W,and the cold head is a in-line pulse tube cryocooler.The experimental results show that the cryocooler achieves a cooling capacity of 300 W at 80 K.The operating frequency is 60 Hz.The charging pressure is 2.5 MPa.And the input power is 7375 W.At the time,the calculated output acoustic power is 4.5 kW,which is 18% lower than the maximum value.The calculated phase angle of the inlet and outlet of the regenerator is ±26°,which is close to the ideal value of ±30°.A better matching between the compressor and the cold head is obtained.A relative Carnot efficiency of 11.2% is achieved,which is 74.5% higher than that before optimization.