| In comparison with traditional regenerative cryocoolers such as G-M and Stifling coolers, pulse tube cooler(PYC) with no moving parts in the low temperature range has the advantages of simple structure, low cost, high reliability, long lifetime, low mechanical vibration and low electromagnetic noise, which makes it competent for cooling super-conducting magnets and cryogenic pumps etc. With the development of zero boil-off liquid hydrogen storage technology and high Tc superconductor MgB2, cryocoolers working at 20 K have a wide application prospect. In the past few years, geat progress has been made on the development of single-stage G-M type PTC. Although the lowest no-load refrigeration temperature below 15 K has been achieved, the coefficient of performance (COP) of PTC at 20 K is still lower than that of G-M cooler. In order to further improve the COP at 20 K and meet the needs for practical applications, comprehensive theoretical and experimental investigation of single-stage G-M type PTC has been carried out. Also a 4.2 K separate two-stage PTC has been designed and tested in this article. The present work focuses on the following sections:1. Thermodynamic analysis of PTCBy combining the linearized model with thermodynamic analysis, expressions for calculating the performance of simple orifice and double-inlet type PTC are deduced. Effects of different key components, such as reservoir volume, orifice and double-inlet valve opening, on the cooling performance of PTC are analyzed. The results show that the reservoir volume has a significant influence when the volume ratio of reservoir to pulse tube is smaller than about 10. The ratio is important for determining the minimum reservoir volume. Only when the refrigeration temperature is below the critical temperature and the opening of orifice valve is bigger than the critical opening, the opening of double-inlet valve is able to decrease the relative dissipation and increase the cooling performance of PTC. Thermodynamic analysis can help further understand the refrigeration mechanism of PTC. 2. Numerical calculation and analysis of G-M type PTC by using REGENBy using REGEN 3.2, effects of regenerator length, mass flow rate, charging pressure, frequency and phase angle on the performance of regenerators of G-M type PTC, which works in the temperature range of 20-300 K at low frequency (1～2 Hz) are calculated and analyzed. The calculation results show that the optimum lengths of regenerator are almost the same under different operating conditions. Mass flow rate of helium has significant influence on the performance of PTC. The optimum mass flow rate for maximum COP is 6 g/s. From calculation results the lowest refrigeration temperature of the designed single-stage PTC is estimated to be 9.5 K when the pressure ratio is 2.0. Effects of charging pressure, frequency and phase angle on the performance of PTC are also presented. Numerical calculation and analytical results can provide theoretical direction for the design and optimization of the PTC.3. Design, manufacture and optimization of single-stage G-M type PTCsBased on thermodynamic and numerical analysis, a single-stage G-M type PTC has been designed, manufactured and optimized. We proposed to use Er3Ni, which is normally used in liquid helium temperature region, as part of the regenerative material in the single-stage PTC to increase the regenerator performance under 15 K. Experimental results show that lower refrigeration temperature and higher cooling capacity can be obtained by adding some Er3Ni at the cold part of the regenerator. Theoretical and experimental studies are carried out to study the mesh size effect of woven wire screens on the performance of PTC. The experimental results indicate that appropriately increase the mesh size to 295 can enhance the heat transfer capacity and improve the efficiency of regenerator with the pressure drop increased a little. After series of optimization experiments, with a three layer matrix composed of Er3Ni, lead sphere and 295 mesh stainless-steel screens installed in the regenerator, a lowest no-load refrigeration temperature of 11.1 K is achieved, with an input power of 6.7 kW. The cooling capacity and COP at 20 K are 17.8 W and 2.95×10-3.A new single-stage PTC has been manufactured and tested for liquefaction of hydrogen. With an input power of 7.5 kW,. a lowest refrigeration temperature of 10.9 K has been achieved, which is a new record and is very close to the temperature estimated by theoretical calculation of 9.5 K. A cooling capacity of 18 W at 20 K is obtained. Tests of long-term operation, repetition and starting up with heat load added are also carried out. Experimental results show that the operation of the PTC is stable, reliable and repeatable. In order to further extend the application of single-stage PTC, a novel 4.2 K separate two-stage PTC has been designed and tested. The cooler consists of two separate pulse tube coolers, in which the cold end of the first stage regenerator is thermally connected with the middle part of the second regenerator. The first stage is the single-stage G-M type PTC introduced above. Cooling capacity of 508 mW at 4.2 K and 15 W at 37.5 K have been achieved with an input power of 6.8 kW driven by two compressors with two rotary valves. Cooling capacity of 590 mW at 4.2 K can be achieved when the filling pressure of the second stage is increased from 1.7 MPa to 1.85 MPa. In order to simplify the structure and extend the applications, the method of driving the same pulse tube cooler by one compressor and one rotary valve is proposed. Similar performance has been achieved compared to that of driven by two compressors with two. rotary valves by reasonably distributing mass flow from compressor to both stages. In 2005, some key technologies of 4 K PTC have been transferred to Iwatani Industrial Gas Company of Japan. Based on our technologies, the commercial product of liquid helium PTC made by Iwatani Company has been released to the market and has already led to remarkable economical and social benefit.
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