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ItemInertance Tube and Reservoir Modeling: Meshing, Convergence and Friction Factors for Oscillating Flow(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.
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ItemComparison of Thermoelectric and Stirling Type Cryocoolers Using Control Thermodynamic Model(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.
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ItemCryocooler Prognostic Health Management System(Georgia Institute of Technology, 2008-05) Sandt , E. ; Penswick , L. ; Shah, A. ; Dodson, C. ; Roberts, T.The use of cooled sensors in ever more complex, integrated applications has made determination of the cryocooler and related component “health or remaining reliable useful life” a critical factor in successfully meeting mission requirements. Sest Inc. has been actively developing a Cryocooler Prognostic Health Management System (CPHMS) under U.S. Air Force sponsorship to address this issue. Using non-invasive means to measure performance of a cryocooler with limited data availability, a variety of failure mechanisms have been evaluated based on a combination of “physics of failure” assessments as well as the results of extensive cryocooler testing carried out on “healthy” and selectively “degraded” cryocoolers. Used in conjunction with simple models of the fundamental dynamic behavior of linear drive free-piston systems, it is possible to identify the presence and type of potential degradation mechanisms. For a diagnostic system, identifying that a problem exists is half the battle. In the CPHMS with the results from the earlier diagnostic evaluation available, it is possible to carry out prognostic estimation of the reliable remaining useful life (RRUL) by the use of Bayesian statistics. The latter are used to continuously improve the estimations of RRUL for the cryocooler under various failure modes. While the CPHMS is focused on “tactical” class cryocooler, the basic approach is easily adapted to other cryocooler types, as well as more complex integrated systems.
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ItemThermodynamic Comparison of Two-Stage Pulse Tube Refrigerators for Two Different Configurations(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.
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ItemHeat Rejection Capacity in Miniature Thermoacoustic Expanders at Cryogenic Temperature 77 K(Georgia Institute of Technology, 2008-05) Hu, Zhimin ; Roberts, T.Miniaturized thermoacoustic expanders MTAEs are a new type of expander designed to improve the cooling efficiency of recuperative type cryocoolers without sacrificing their reliability and simplicity. Experimental studies have demonstrated the feasibility of MTAEs, including their ability to remove heat from the expansion of a DC cold-stage flow to a high-temperature reservoir. The heat pumping from the cold-stage to a hot sink by an MTAE relies critically on the behavior of nonlinear wave systems created in miniature-scale channels (several hundreds microns) inside resonant tube bundles of MTAEs. This paper presents an experimental investigation of this heat rejection capacity between a low temperature source and a much higher temperature sink and the influence of such heat energy pumping on the cooling performance of an MTAE operated at a cryogenic temperature of 77�K. The objective is to explore this technology�s feasibility in both scaled down and cryogenic temperature conditions. The characteristics of acoustic wave systems and the cooling power achieved using helium in the MTAE (170�mg/s) with different supply pressures and hot reservoir temperatures are reported. The observations and challenges of the thermoacoustic streaming phenomenon and modeling in miniature or micron-scale channels is also discussed.