The Vuilleumier (VM)-type pulse tube cryocooler is a novel kind of cryocooler used to obtain liquid helium temperature, which has been experimentally verified. However, the overall efficiency is not satisfying. Based on previous work on a low pressure ratio system, numerical investigations that explore the effect of high pressure ratios on cooling performance are presented in this paper. The research system is cryogen-free in which a Stirling type pulse tube cryocooler is used to provide the required cooling power for the thermal compressor and makes it adjustable for pre-cooling temperature to achieve optimum efficiency. First, after increasing the displacer swept volume to increase the pressure ratio, the dimensions of the main components were optimized with the lowest no-load temperature as the optimization target. Then the dependence of system performance on average pressure, displacer swept volume, frequency and cold HX temperature were studied considering the heat transfer temperature difference of the thermal bridge. Compared with the previous low efficiency under low pressure ratio, when the heat transfer temperature difference of the thermal bridge is 0 K, a higher relative Carrot efficiency of 1.29% and a cooling power of 250 mW at 4.2 K were predicted with an average pressure of 3 MPa, a frequency of 3 Hz and a pressure ratio of 1.67. Further optimization is underway.

This paper presents a numerical investigation on a Vuilleumier (VM) type pulse tube cooler. Different from previous systems that use liquid nitrogen, Stirling type pre-coolers are used to provide the cooling power for the thermal compressor, which leads to a convenient cryogen-free system and offers the flexibility of changing working temperature range of the thermal compressor to obtain an optimum efficiency. Firstly, main component dimensions were optimized with lowest no-load temperature as the target. Then the dependence of system performance on average pressure, frequency, displacer displacement amplitude and thermal compressor pre-cooling temperature were studied. Finally, the effect of pre-cooling temperature on overall cooling efficiency at 5 K was studied. A highest relative Carrot efficiency of 0.82 % was predicted with an average pressure of 2.5 MPa, a frequency of 3 Hz, a displacer displacement amplitude of 6.5 mm, ambient end temperature 300 K and pre-cooling temperature 65 K, respectively.