Study on micro-liquid droplets behavior at the expander of the pulse tube cryocooler under 5 K based on molecular dynamics

Abstract

The alternating oscillations in the expander of the pulse tube cryocooler at liquid helium temperature level were studied by molecular dynamics method. It is found that the densely atom clusters – namely micro-liquid droplets were formed in the cold part of the pulse tube and mostly close to the wall of the tube. The size and the number of the clusters were increasing with cycle progressing. The mass proportion of the liquid droplets was up to 8.0 % in all the gas. The largest droplets contained up to 275 He atoms. The clusters moved around during the compression and expansion process of the cycles. It is known that the formation of the micro droplets had influences on the oscillating waves of the gas, resulting in the amplitudes of the density, temperature, so that pressure waves decreased greatly. The local lowest expansion temperatures were also increased over 4.2 K with the formation of the droplets. This may be one of the reasons that it is very difficult to obtain the temperature below 4 K in the Stirling pulse tube cryocoolers.