Numerical And Open Cycle Experimental Investigation On A Precooled J-T Cryocooler At Liquid Helium Temperature Including The Last Stage Heat Exchanger

The rapid development of space detection drives the progress in cryogenics. Cryogenic detectors are capable of achieving higher precision and capturing weaker signals. Liquid helium temperature (1-10 K) is the working temperature of some cryogenic detectors, which plays an indispensable role in space technology. Precooled J-T coolers working at liquid helium temperature are of compactness, high thermal efficiency, long duration, etc, and are widely utilized in space. Accordingly, the contents of this thesis are divided in the following aspects:1. Consolidating the development history and latest progresses of space-based precooled J-T coolers working at liquid helium temperature. The focus is the comparison of thermal efficiency and the arrangements of key technology such as supporting and sealing. The problems and obstacles in experimental research are extracted, which is instructive in this thesis. The breakthroughs lie in J-T compressors and regenerative cryocoolers of high performance and reliability, the optimization of operating parameters, the stable and reliable operation of cryocoolers, as well as compact recuperators of high thermal efficiency.2. Thermodynamic analysis of J-T cycle based on N2 is carried out. The maximum refrigeration capacity is limited by the minimum isothermal enthalpy difference between high pressure gas and low pressure gas. The minimum isothermal enthalpy difference increases as the high pressure increases. The efficiency of recuperators affects the refrigeration capacity directly. Precooling helps to increase the refrigeration capacity. In terms of J-T cycle based on 4He, precooling is indispensable to achieve refrigeration capacity in liquid helium temperature. Property variation, parasitic heat load, axial conduction and flow maldistribution degrade the performance of cryogenic heat exchanger, which have been discussed in this thesis.3. The numerical model of precooled J-T coolers working at liquid helium temperature is set up using sage. The high pressure of J-T cycle and the last stage precooling temperature are optimized. Besides, the best combination has been analyzed. In order to reduce the irreversible losses caused by property variation in the last stage heat exchanger, a new structure of heat exchanger is discussed and a bypass J-T valve is introduced to change the distribution of mass flow. The heat capacity rate of hot fluid and cold fluid is balanced to reduce the temperature difference. As a result, the irreversible losses are reduced and the performance of the cryocooler is improved.4. The experiment system of precooled J-T coolers is introduced and experiment is carried out. The refrigeration capacity under a series of operating paramenters is achieved. When the last stage precooling temperature is 17.36 K, the pressure ratio is 16 and the mass flow rate is 11 mg/s,82.74 mW is achieved at 4.46 K. There is disagreement between experimental results and numerical results, which is contributed by the fact that the outlet of evaporator is in two phase. Agreement has been achieved by revising the numerical model.