| 4 K cryocoolers are in demand for applications of superconductor cooling, small-scale helium liquefaction, precooling of dilution refrigerators, cryoelectronic device cooling, cryopumps and so on. In comparison with traditional G-M and Stirling cryocoolers, pulse tube cryocooler (PTC) has the advantages of simple structure, high reliability, long lifetime, low mechanical vibration and low electromagnetic interference (EMI) due to no moving parts at low temperatures, which make possible to replace traditional cryocoolers. In the past few years, although the improvement of phase shifting of PTCs and applications of high heat capacity magnetic regenerative materials at liquid helium temperatures increase the performance of 4 K PTCs significantly, the coefficient of performance (COP) is still lower than those of G-M cryocoolers, which limits their applications in the liquid helium temperatures. In order to increase the efficiency of 4 K PTC and meet the needs for practical applications, research work was carried out on the following sections:Based on REGEN3.3, numerical simulation and analysis on the regenerator performance of single-stage and two-stage PTCs are carried out respectively. Effects of hybrid regenerative matrix structure, length of each matrix, diameter of lead spheres, charging pressure and working frequency on the performance of the single-stage PTC, which works in the temperature range of 20-300 K at a low frequency (1-2 Hz), are discussed. The main losses of the PTC are analyzed and the theoretical lowest attainable refrigeration temperature of a single-stage PTC is predicted. Regenerator performance of 4 K two-stage PTC is also presented. Effects of mass flow rate on the refrigeration performance and regenerator losses, length of each regenerative matrix and cross-sectional area of the regenerator are analyzed. Theoretical investigations on gas distribution and precooling power between stages are performed.62.5% of the total mass flux enters the first stage and the rest enters the second stage. When the mass flux of the second stage is 3 g/s, the precooling power is 8-14 W. As the mass flux of the second stage increases to 8 g/s, the precooling power increases to 14-19 W. Numerical calculation and analytical results provide theoretical direction for the design and optimization of the PTCs.The cold end heat exchanger in the PTC is of great importance for efficient heat transfer between the working fluid and the cold head. Unfortunately, the theoretical investigation on the cold end heat exchanger is rare in literatures and the design method is still not in mature, mainly because of the insufficient research on the transient heat transfer under the condition of oscillating flow. Theoretical investigation on cold end slot heat exchanger of 4 K PTC is carried out. A modified formula about the cooling power and refrigeration temperature under the condition of oscillating flow and an expression about the optimal relation between the fin width and the height of the slot heat exchanger are proposed. Especially, heat transfer area under the oscillating flow and the static flow conditions is compared quantitively. When the timing of the rotary valve is 1.22, the heat transfer area under the condition of oscillating flow is 1.82 times to that of the static flow. Based on the calculation, a new slot heat exchanger is designed with the heat conductance increase from 7.16 W/K to 14.31 W/K. Regenerator is the key component of PTC. The optimal length of stainless steel screen part of high temperature range and optimal matrix structure of low temperature range of the regenerator are investigated. After a number of optimization experiments, a three-layer regenerative matrix composed of Er3Ni, small-diameter lead spheres and 300 mesh stainless steel screens are filled in the regenerator. The experimental investigation on cold end heat exchanger is performed. The total heat conductance increases from 56.1 to 112.6 W/K by lengthening the cold end slot heat exchanger and deepening the cold end flow channel of the single-stage PTC. The experimental results indicate that the cooling power significantly increases in the temperature range of 20-40 K after improvement. A lowest no-load refrigeration temperature of 10.6 K is obtained, which is a new record for the single-stage PTC. The result is close to the predicted lowest attainable temperature of 10 K. The cryocooler can supply 20 W at 20.6 K and 40 W at 29.9 K with an actual input power of 7.5 kW, which may satisfy the precooling requirement of the second-stage regenerator. Test of 200 hours operation is also carried out. The maximum peak to peak temperature oscillation is 0.4 K with the cold end temperature around 22 K, which shows that the operation of the PTC is stable and reliable.On the basis of the design and optimization of the single-stage high power PTC and improvement of the cold end slot heat exchanger of PTC, a separate two-stage 4 K PTC is designed and manufactured. Theoretical and experimental investigations are carried out. Firstly, a new slot heat exchanger is manufactured on the basis of the slot heat exchanger design method. The heat conductance significantly increases and the cooling power at 4.2 K increases from 600 to 700 mW with the same input power of 6.7 kW. Effect of mass flow rate on the performance of 4 K PTC is investigated. Experimental results show that the mass flow rate is one of the most important factors which determine the cooling power. The experiment with different mass flux of the second stage is implemented. A cooling power of 0.7 W at 4.2 K and 20 W at 40 K are achieved simultaneously with the single compressor and 3 g/s mass flux of the second stage. Two compressors together with rotary valves are used to drive the two stages respectively. The cryocooler can provide 1.1 W at 4.2 K and 10 W at 40.8 K with the second stage mass flux of 8 g/s and total actual input power of 11.7 kW, which is the largest cooling power ever obtained with separate pulse tube cryocooer working at liquid helium temperature. |
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