(Georgia Institute of Technology, 2008-05)
Dodson, C.; Razani, A.; Roberts, T.
Pulse tube refrigerators (PTRs) have made dramatic improvements in reliability, efficiency and usage, with the addition of the inertance tube helping to create the improvements. The combination of the inertance tube and reservoir help to create a phase shift between mass flow rate and pressure that affects the fluid dynamics in the PTR. Current models inadequately predict (in accuracy) the phase shifts in these oscillating refrigerators. Various modeling techniques have yet to address the issue of numerical solution convergence, especially with respect to the mesh size and time step size when using Computational Fluid Dynamics (CFD) models. This study aims to address the issue based on comparisons to a set of experimental results. Along with the CFD correlation, a comparison with a distributed inertance tube model based on new friction factors for oscillating flow will be reported. A comparison of isothermal to mixed surface wall boundary condition is performed.
(Georgia Institute of Technology, 2008-05)
Razani, A.; Dodson, C.; Roberts, T.
New efforts are underway to develop thermoelectric materials for cooling of infrared detectors at cryogenic temperatures. Stirling type cryocoolers routinely produce cooling at cryogenic temperatures with good efficiency, but challenges remain in their miniaturization, reliability and vibration reduction for space applications. In this study, a thermodynamic comparison of Stirling type and Thermoelectric (TE) cryocoolers is made for a typical second stage cryogenic refrigerator (30 K to 80 K). It is assumed that a reservoir at 80 K is available and a cooling load at 40 K is desired. It is shown that under the assumption of availability of TE materials with a reasonably high figure of merit, a multistage TE cryocooler is required. For comparison of the performance of the cryocoolers, thermodynamic models of the Stirling type and multistage TE cryocoolers are developed. The effect of important system parameters on the performance of the cryocoolers is presented. The thermal design challenges of miniaturization of Stirling Type cryocoolers and the development of multistage TE cryocoolers with high efficiency are discussed.
(Georgia Institute of Technology, 2008-05)
Razani, A.; Fraser, T.; Dodson, C.; Roberts, T.
Using exergy analysis, control thermodynamic models of two-stage Pulse Tube Refrigerators (PTRs) are developed for two different configurations. The models assume that phase shift controllers exist that control the phase shift between the mass flow and pressure in the pulse tubes. In one configuration, using thermally-coupled stages, separate gas circuits are used, thus requiring two compressors. This configuration provides flexibility for thermodynamic optimization. In another configuration, a conventional gas-coupled two-stage PTR is used where a constraint exists for mass flow allocations and the implementation of the phase shift control. The models include controllers for flow conductance, heat transfer effectiveness, and conduction heat transfer parameters in the regenerators in both stages for each configuration. The effects of the allocation of the values of flow conductance and ineffectiveness parameters in the regenerators, the mid-stage temperature, and the phase shift in each stage on the performance of the refrigerators are investigated. Important dimensionless parameters controlling the thermodynamic performance of the two-stage PTRs for each configuration is developed and discussed.