Microscopic mechanism of alternating oscillations in a microchannel pulse tube based on molecular dynamics

Abstract

The non-equilibrium molecular dynamic simulation method was used to analyze the microscopic dynamics process of alternating helium oscillations in a micro-channel pulse tube (MPT). The driving force was provided by a sinusoidal speed piston. A reflect wall was adopted as the power source similar to high-frequency liner compressors. The MPT's instantaneous temperature, pressure, speed mass flow, and time-averaged temperature were statistically analysed. At the micro level, the curve of the MPT's instantaneous properties approximated sinusoidal function. There was an alternating oscillation process with asymmetric property distributions between the MPT's compression and expansion process and the space-time distribution of each parameter's properties were non-uniform. The expansion process had a larger axial pressure gradient than the compression process. The analytical results showed that when the forced oscillation time caused by the piston motion was shorter than the natural oscillation time, the acoustic wave's energy flow density was higher. The cycle's time had little effect on the temperature of the cold end under low charge pressure. When the inflation pressure was higher than 20 bar, the temperature at the cold end was more sensitive to the time. As the time decreased, the temperature at the cold end further decreased and the time-averaged temperature at the cold end increased as the charge pressure increased. The negative effect mainly came from the temperature disturbance during the expansion process. The pressure ratio increased as the time decreased and was not significantly affected by the charge pressure, but it could generate a larger temperature gradient in the MPT's axial direction. In summary, the use of different forms of heat exchangers and phase shifters at the warm end could release or recover extra acoustic power. MPTs with fixed operating modes and dimension parameters should have optimal frequency to obtain the lowest no-load temperature at the cold end. The simulation results improve the understanding of pulse tubes' cooling mechanisms and provide theoretical support for the optimal design of micro-channel pulse tube coolers.