Oscillatory flow in microporous media applied in pulse – tube and Stirling – cycle cryocooler regenerators

Abstract

Pulse tube and Stirling cryocoolers are widely used in aerospace and other high-demand application. A key component in these systems is the regenerator, which is typically a microporous metallic structure that is subject to periodic flow of a cryogenic fluid. The thermal and hydrodynamic irreversibilities in the regenerator, which play crucial roles with respect to the efficiency of the aforementioned cycles, are poorly understood, however. In this investigation experiments were performed where pressure drop associated with steady-periodic (axial and lateral (radial)) flows of helium in test sections packed with several widely used pulse tube and Stirling cryocooler regenerator fillers were measured under ambient temperature conditions. A computational fluid dynamic (CFD) – assisted method was developed for the analysis and interpretation of the experimental data, whereby the permeability and inertial coefficients that lead to agreement between the data and the predictions of CFD simulations were iteratively obtained. The directional permeability and Forchheimer inertial coefficients were thus obtained for the tested regenerator fillers, and were found to be independent of the frequency of flow oscillations for the frequency range 5–60 Hz. The results also show that the oscillatory flow hydrodynamic parameters are different than steady-flow parameters representing similar flow conditions.