Thermal Losses in a Coaxial Pulse Tube Cryocooler

Conductive and radiative losses in pulse tube cryocoolers are difficult to predict, particularly in a coaxial configuration. The static losses in a pulse tube must be accounted for and factored when determining the cooling power and the thermal losses as a result of radiation must be quantified to establish their significance. Studies have previously shown that there is a discrepancy between computationally simulated cooling performances and experimentally measured values. This could be attributed to the under-prediction of thermal losses in numerical models. A lumped parameter model using ESATAN-TMS was created to determine the thermal losses in a coaxial pulse tube cryocooler in order to independently investigate the role the estimation of these losses may play in the discrepancy between experimental and computed values. The conductive losses simulated closely matched those determined by the Sage numerical model, and the radiative losses appear to be negligible. An analysis of an independent method for simulating thermal losses of a coaxial pulse tube and its impact on cooling performance is presented. INTRODUCTION Pulse tube cryocoolers provide active cooling for space instruments, superconducting devices, and other terrestrial equipment to cryogenic temperatures. The pulse tube cryocooler operates by running helium gas through thermodynamic cycles to achieve cooling at the cold head. This cooling depends on a well-designed interplay between the mass flow and pressure pulse within the pulse tube and regenerator [1]. Pulse tube cryocoolers are often designed in a coaxial configuration in order to reduce dead volume and enhance the ease of integration between the cooling interface and the cold head [2]. The thermal losses in the pulse tube cryocooler can be difficult to predict, especially when in a coaxial configuration, and should be quantified accurately as the losses affect the overall cooling performance and efficiency of the cryocooler. It is further important to ascertain the significance of any radiative losses of the system. Numerical models can be used to predict the performance of the pulse tube cryocooler. A coaxial Stirling pulse tube cryocooler (SPTC) with an active displacer was previously numerically studied [3] and experimentally investigated [4]. A discrepancy was observed between the experimentally measured performance and the numerically computed output. Some attempts were made to improve the model validation by tying certain regions to known measured temperatures from the experimental findings, in order to minimize the discrepancy. Given that it is difficult to fully replicate the complex coaxial geometry in the numerical Sage model, there is a possibility that the conductive losses are underestimated, thereby impacting the numerically simulated cryocooler performance. As such, it is of use to conduct an independent investigation of the conductive losses in the cryocooler and investigate the magnitude of any radiative losses. Cryocoolers 2 1 , edited by R . G. R oss, J r. , J . R . R aab and S.D . M iller © I nternational Cryocooler Conference, I nc. , B oulder, CO , 2021 263