Carl S. Kirkconnell

Thermodynamic Optimization of Multi-Stage Cryocoolers

Active Stirling class cryocoolers, including pulse tube coolers, are complex, difficult to optimize machines. The large number of characteristics and properties associated with geometry, materials, gas properties, heat transfer devices, flow manifolds, mechanical mechanisms, and electro-mechanical devices that determine a particular machine’s performance make quick optimization difficult. Single-stage coolers are now sufficiently well understood that design optimization is reasonably straight forward. However, multi-stage coolers compound the design problem by virtue of the dramatically enlarged number of variables, and optimization is still a challenge. Often, multi-stage machines are “optimized” by a brute force search of the design space or design decisions are made based on overly generalized or inaccurate assumptions about relationships between variables. The schedule-constrained time typically available to perform optimization procedures combined with the large number of variables and their complex interaction results in sub-optimal products. This paper presents a concept for optimization that more rapidly converges on an optimal design.

CFD Simulation of Multi-Dimensional Effects in an Inertance Tube Pulse Tube Refrigerator

Two entire Inertance Tube Pulse Tube Refrigerator (ITPTR) systems operating under a variety of thermal boundary conditions were modeled using a Computational Fluid Dynamics (CFD) code. Each simulated ITPTRs included a compressor, an after cooler, a regenerator, a pulse tube, cold and hot heat exchangers, an inertance tube, and a reservoir. The simulations represented fully coupled systems operating in steady-periodic mode. The objectives were to ascertain the suitability of CFD methods for ITPTRs and to examine the extent of multidimensional flow effects in various ITPTR components. The results confirmed that CFD simulations are capable of elucidating complex periodic processes in ITPTRs. The results also showed that a one-dimensional modeling is appropriate only when all the components in the system have large length-to-diameter (L/D) ratios. Significant multi-dimensional flow effects occur at the vicinity of component-to-component junctions, and secondary-flow recirculation patterns develop when one or more components have small L/D ratios.

Experimental Investigation of a Unique Pulse Tube Expander Design

The performance of a pulse tube expander in which the regenerator volume is distributed among three parallel tubes arranged symmetrically around the pulse tube has been experimentally investigated. This “4-tube” expander configuration, which was recently patented by Raytheon Systems Company (formerly Hughes Aircraft), is of interest because it has structural advantages over the more common U-tube design. The improved strength of the 4-tube design permits reduced wall thickness, higher allowable side loads, and related design and system integration advantages. Furthermore, the 4-tube expander does not have the problem of conductive coupling between the regenerator and pulse tube, which is a source of lost refrigeration capacity in the traditional concentric configuration, a competing rigid expander design. Experiments were performed with and without the regenerator tubes linked by conductive straps, and the expander was shown to work more efficiently with the straps. The data reveal that the thermodynamic efficiency of the 4-tube expander is comparable to that of the more common pulse tube expander configurations.

A Novel Multi-Stage Expander Concept

Raytheon has developed a novel two-stage expander for use in long life, high reliability cryocoolers for space and commercial applications. The expander is classified as a Stirling machine and requires a conventional reciprocating piston compressor to drive it. The key feature is a new method for obtaining and controlling expansion at the two stages. Thermodynamic efficiency is higher than existing one and two stage coolers and the mechanical implementation is as simple or simpler. The expander device is described in both thermodynamic and mechanical terms and performance predictions given.

LONGITUDINAL HYDRAULIC RESISTANCE PARAMETERS OF CRYOCOOLER AND STIRLING REGENERATORS IN PERIODIC FLOW

The results of an on going research program aimed at the measurement and correlation of anisotropic hydrodynamic parameters of widely-used cryocooler regenerator fillers are presented. The hydrodynamic parameters associated with longitudinal periodic flow are addressed in this paper. An experimental apparatus consisting of a cylindrical test section packed with regenerator fillers is used for the measurement of axial permeability and Forchheimer coefficients, with pure helium as the working fluid. The regenerator fillers that are tested include stainless steel 400-mesh screens with 69.2% porosity, stainless steel 325-mesh screens with 69.2% porosity, stainless steel 400-mesh sintered filler with 62% porosity, stainless steel sintered foam metal with 55.47% porosity, and nickel micro-machined disks with 26.8% porosity. The test section is connected to a Stirling type compressor on one end and to a constant volume chamber on the other end. The instrumentation includes piezoelectric pressure transducers at b...

Raytheon dual-use long life cryocooler

Raytheon has manufactured closed-cycle cryocoolers for both tactical military and space applications for over thirty years. Tactical and space cryocooler technologies have historically been treated as distinct both at Raytheon and throughout the industry. Differing technical requirements, operating lifetimes, and order quantities have driven these types of coolers to dramatically different design approaches and cost levels. For example, a typical space cryocooler system today costs approximately

A Comparative Evaluation of Numerical Models for Cryocooler Regenerators

2M as compared to roughly

Two Stage Hybrid Cryocooler Development

10,000 for a tactical cryocooler. However, stimuli from both the tactical and space cooler user communities are driving the markets together. Tactical cryocooler requirements are starting to push towards operating lifetime requirements more characteristic of the space coolers (e.g., 20,000+ hours). Space cryocooler users, in particular Missile Defense Agency, are pushing for substantial cost reduction. In response, Raytheon is developing a low cost space cryocooler with an intended dual-use capability to also serve the tactical marketplace. This cooler leverages proven flexure-suspension technology to achieve long life, and a low cost concentric pulse tube cold head design has been developed that can be packaged into the existing Standard Advanced Dewar Assembly, Type One (SADA-I). The cooler meets or exceeds the SADA-I operational requirements (capacity, efficiency, etc.) as well. For the space-version of the cooler, the electronics cost has been reduced by an estimated 80% versus current designs, largely by approaching the vibration cancellation requirement from a dramatically different perspective. Fabrication of the brassboard expander is nearly complete, and the prototype design is well underway. The design approach, development progress, and proposed applications are presented.

THE IMPACT OF UNCERTAINTIES ASSOCIATED WITH REGENERATOR HYDRODYNAMIC CLOSURE PARAMETERS ON THE PERFORMANCE OF INERTANCE TUBE PULSE TUBE CRYOCOOLERS

Various models for predicting the flow and heat transfer in a porous cryocooler regenerator have been proposed in the literature. One such model utilizes a semi-implicit set of equations after making some simplifying assumptions, resulting in a momentum equation that is decoupled from the energy and continuity equations. This work addresses concerns with a semi-implicit model based on scale analysis. An important result is that the pressure gradient term in the energy equation, which has been neglected in the semi-implicit model, is leading order.

Second Generation Raytheon Stirling/Pulse Tube Hybrid Cold Head Design and Performance

Raytheon has demonstrated a two-stage hybrid Stirling/pulse tube cryocooler for long life space infrared (IR) sensor applications. The first expander stage is a conventional Oxford-class Stirling expander. The second expander stage is a U-turn pulse tube mechanically and thermodynamically extended from the first stage Stirling cold end.