| Miniature pulse tube cryocoolers(MPTCs)inherit the advantages in low vibration,high reliability and long life of pulse tube refrigerators,and perform the capability of rapid cooling rate with compactness and low mass.These characteristics of MPTCs can achieve the compact space layout to satisfy the long-term missions in military applications and aerospace applications.MPTCs have a great prospect in the field of cryogenic refrigerator.Through long-term studies of MPTCs,Radebaugh proposed that increasing frequency and charging pressure can reduce the volume of pulse tube cryocoolers.Generally,restricted by the strength limit of using materials,charging pressure in tube is rarely considered to be greatly increased.Therefore,increasing frequency to an approximate hundred of Hertz becomes the most effective way to miniaturize these components of pulse tube refrigerators.However,simply increasing the frequency leads to large losses.Lots of researches referring to the theory and applications of MPTCs have been reported and discussed in promoting the efficiency of MPTCs.One of these significant works was the MPTC with well-performed cooling performance and low mass developed by NGAS,and they still remained its key parameters and designing process as secrecy.The other institutions,which mainly focused on the geometrical optimization of cold head assembly,rarely reported the impedance relationship between the compressor and cold head assemblyThis thesis aims at developing a high efficient MPTC for aerospace applications,the theoretical and experimental investigations on the MPTC are conducted in this paper.The research work is shown in underneath content which concludes:1)The ideal thermodynamics of MPTC is discussed by using enthalpy,entrppy and acoustic power flow.Based on the ideal thermodynamics and high frequency characteristics,the losses of MPTC operating over hundred hertz are analyzed.The results show that as the frequency increases,the thermal penetration of gas decreases,resulting in the finite heat transfer.In order to provide sufficient heat transfer between the gas and the regenerator material,the reduced volume of regenerator should be accompanied by a decrease in hydraulic diameter,however,it's a contradiction in demanding hydraulic diameter for minimum pressure drop.Therefore,it is necessary to comprehensively consider the heat exchange and flow dynamic between the gas and material,choosing the regenerator material which has thinner wire diameter and higher porosity can promote the efficiency of regenerator operating over hundred hertz.2)By analyzing the phase characteristics of MPTC with high frequency,it is indicated that increasing in the charging pressure can compensate for the partial losses resulting from an increasing frequency.The effect of the increasing frequency on phase shifting capability can be mitigated by reducing the volume of the pulse tube,but relative large volume of pulse tube leads to large loss.The increasing frequency will significantly improve the phase shifting capability with appropriate size of the inertance tube and the volume of the gas reservoir.3)The effects of the structure and operating parameters of regenerator on cooling performance are summarized,which show the correlation between the operating parameters and geometric dimensions.And the analysis above guides the designing and optimization of geometrical regenerator.The effects of the length,the diameter and the ratio of the length to the diameter of pulse tube on cooling performance and adiabatic efficiency are analyzed.The frequency is positively related to the phase shifting capability.The phase shifting capability is sensitive to the dimensions of inertance tube,and a small changing in length leads to a large variation of phase angle,and the optimal length of the inertance tube(corresponding to maximum phase angle)does not vary significantly with respect to the inner diameter.The performance differences by using the 400 mesh,604 mesh and 635 mesh stainless steel screen as the regenerator material are proposed.The results show that using the 604 mesh stainless steel screen as the regenerator material which has 17?m in wire diameter and 0.68 in porosity can remarkably improve the efficiency of cooling performance operating at 120 Hz.Based on SAGE model and the above analysis,the dimensions and operating parameters of MPTC were determined.4)Based on the governing equations of compressor,rules for the matching mechanism between the compressor and the cold head assembly were summarized.Through the gas load and the thermal-dynamic joint simulation,the electromagnetic and dynamic parameters of compressor were well designed and optimized,respectively.Through the analysis of the influence of operating frequency,charging pressure and cold end temperature on the cooling performance,the structural parameters and operating parameters of the whole cryocooler were determined.The joint acousticmechanical coupling phasor was established,and the coupling characteristics of compressor and cold head assembly operating at 120 Hz was determined.5)Through comparative experiments,when the cooling power is 1W at 80 K,the cryocooler with 604 mesh stainless steel mesh as the regenerator filler requires 16 W electric input power less than that of 635 mesh.When operating frequency increases to 120 Hz,the inertial effect of the gas increases.The effect of roughness and coiled type inertance tube on the cooling performance are almost negligible.For the same cold end temperature and cooling power,the required electric input power using the copper as the cold exchanger material is less than using the 4J32 alloy.As the temperature decreases and cooling power increases,the difference in cooling performance caused by the two materials becomes more pronounced.6)A 1.22 kg miniaturization of coaxial MPTC has been designed,fabricated,and tested.The MPTC operating at 120 Hz,with 3.8MPa charging pressure,293 K reject temperature and 604 mesh screen as the filler of regenerator,achieves a no-load temperature of 53.5K and takes 4mins for cooling down from 293 K to 80 K with 55 W electric input power.When electric input power is 34 W and 55 W,the corresponded cooling power of 1W and 2W can be obtained at 80 K,respectively,and the relative Carnot efficiency is 7.83% and 9.68%,respectively.The experimental results of pulse tube orientation show that when electric input power is larger,the phenomenon of natural convection in pulse tube is not obvious with operating frequency changing from 60 Hz to 140 Hz.When electric input power is small,the increasing of operating frequency can restrain the natural convection in pulse tube to a certain extent.In practical applications,when the MPTC operates at 120 Hz and the electric input power is relative small,the inclination angle of pulse tube need to be carefully concerned. |
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