Parametric Numerical Study of Off-Axis Pulse Tube Losses

Recently there has been an emphasis on applying numerical modeling, with varying degrees of complexity, to the study of the notorious sensitivity of the pulse tube cryocooler to off-axis operation. Previous work has demonstrated the suitability of applying multi-dimensional CFD simulation to the pulse tube cryocooler both as a whole and at the component level for qualitative and quantitative analysis of system performance and losses. To date there has been no comprehensive study of the pulse tube losses for a wide range of operating conditions consistent with modern coolers currently used in both the military and civilian sectors. This work presents for the first time a comprehensive loss analysis of the linear pulse tube cryocooler operating at common conditions using full three-dimensional CFD coupled with a real gas model for the working fluid. The scope of results presented includes both system level models and a pulse tube/heat exchanger component level model. Results from the system level simulations are presented for two pulse tube aspect ratios at tilt angles of 0°, 91°, and 135°. Component level simulation results are presented and discussed for permutations in tilt angle between 0-180°, length-todiameter aspect ratios from 4-8, cold tip temperatures from 4-80 K, phase angles between -30° and +30°, and an operating frequency range of 25-60 Hz. The entire set of modeling results is compiled and used to show a correlation between the normalized pulse tube cryocooler losses as a function of a non-dimensional pulse tube number. Discussion is also provided regarding the utility of component level simulation compared to system level simulation considering analysis complexity, computational resource requirements, and suitability for cooler design.