Peter E. Bradley

Energy Flows in an Orifice Pulse Tube Refrigerator

A technique which allows for the instantaneous measurements of mass flow rate and temperature in an orifice pulse tube refrigerator (OPTR) during actual operation has been developed recently. This paper presents the values of enthalpy, entropy, and work fluxes at the cold end of the pulse tube evaluated from these measurements. They are thermodynamically self consistent within 1%. An analytical model describing the operation of an OPTR was developed at the National Institute of Standards and Technology (NIST) in the late 1980’s. This model assumes adiabatic performance of the pulse tube and purely sinusoidal mass flow rates, temperature, and pressure oscillations in the OPTR. The experimentally measured enthalpy flux varies from 60% to 85% of that predicted by the adiabatic model. The experimental work reported, here also gives values for various phase relationships that are needed for some calculations with the analytical model.

Design, fabrication, and assembly of a hollow-core fiber-based micro cryogenic cooler

One of the smallest Joule Thomson (J-T) micro cryogenic coolers (MCC) utilizing a hollow-core fiber-based heat exchanger (HX) is designed, fabricated, assembled and tested. Techniques that control mask material etching are developed to fabricate a multi-layer silicon structure on HX-coupling manifolds. A segmental metal coating is applied on the fibers and the capillary for solderable areas and reduced radiation and conduction heat transfer. With 16∶1 pressure ratio mixed refrigerants, a 140 K stable temperature and 76 K transient temperature at cold head are demonstrated.

Impedance Measurements of Inertance Tubes

The flow impedance of an inertance tube of 5.74 mm inside diameter and 2.36 m long coupled to various reservoir volumes was measured and compared with that predicted by a model based on a transmission line analogy. Though data at other average pressures and temperatures were obtained, this paper focuses on data taken with an average pressure of 2.5 MPa and a pressure ratio of 1.3. Frequencies of 50, 60, 70 Hz, were used, and reservoir volumes were 30, 83, 134, 334 cm3. The amplitudes and phases of the instantaneous pressure and flow at both ends of the inertance tube were measured and the resulting complex impedance and inlet acoustic power were determined. The flow at the reservoir end was determined from the instantaneous pressure in the reservoir, and the flow at the inlet end was determined from a measurement of the pressure drop across the aftercooler consisting of copper wire mesh. The model predicted a resonance associated with the compliance of the reservoir and the inertance of the tube analogous...

Demonstration of an Integrated Micro Cryogenic Cooler and Miniature Compressor for Cooling to 200 K

Joule-Thompson (J-T) based micro cryogenic coolers (MCCs) are attractive because they can provide the cryogenic temperatures needed for small electronic devices while having a low cost and small volumetric footprint. A compressor is a major part of a cryogenic system, but so far J-T based MCCs have not used miniature or micro scale compressors. This work demonstrates a J-T based MCC coupled with a miniature compressor for cooling to 200 K, using a custom hydrocarbon mixture as refrigerant. The compressor is formed by coupling a miniature piston oscillator built for Stirling coolers with a micromachined check valve assembly. The MCC is formed by glass fibers within a capillary forming a counter flow heat exchanger, and a silicon and glass chip forming a J-T valve. Minimum temperatures of 166 K have been observed in transient, and stable temperatures of 200 ±1 K have been observed for >1 hour. Some insight is given into the unstable performance in terms of intermittent liquid accumulation. The coefficient of performance is analyzed for the system, and it is found that most of the inefficiencies arise at the compressor.Copyright

DEVELOPMENT OF A 4 K STIRLING‐TYPE PULSE TUBE CRYOCOOLER FOR A MOBILE TERAHERTZ DETECTION SYSTEM

We discuss in this paper the design and development of a 4 K Stirling‐type pulse tube cryocooler for a mobile terahertz detection system. This system integrates new heterodyne detector technology at terahertz frequencies with advancements of Stirling‐type pulse tube technology that brings the advent of cooled detector sensitivities in a mobile, compact, and long duration operation system without degradation of sensitivity. To achieve this goal we reduced overall system size, input power, and temperature fluctuations and mechanical vibrations in order to maintain the detector sensitivity. The Stirling‐type pulse tube cryocooler developed for this system is a hybrid design employing a He‐4 pulse‐tube cryocooler operating at 60 Hz and 2.5 MPa average pressure that precools a He‐3 pulse tube cryocooler operating at 30 Hz and 1.0 MPa average pressure to achieve 4 K cooling for the terahertz receiver. The He‐4 cryocooler employs stainless steel mesh regenerators for the first stage and ErPr spheres for the seco...

Measurements of Phase Shifts in an Inertance Tube

Phase shifts and mass flows were measured at the inlet of an inertance tube, and the results are compared with transmission line models. The mass flow rates at the entrance to the inertance tube are obtained using a hot-wire anemometer. The hot wire was calibrated in oscillating flow conditions by mass flow rate comparisons at two other locations in the apparatus. Measurements were made and future work will attempt to show that by adjusting the diameters and lengths of the inertance tubes as well as the operating conditions, we will be able to independently identify the effects of resistive, inertance, and compliance components. We discuss the hot wire calibration procedure used to ensure self-consistency among the various transducers and system mass flowrate measurement calculations. The magnitude and phase of the inertance tube complex impedance are measured and discussed. The size of the inertance tube studied here was 1.5 mm diameter by 1.15 m long with frequencies between 30 and 70 Hz.

INVESTIGATION OF FLOW NONUNIFORMITIES IN A LARGE 50 K PULSE TUBE CRYOCOOLER

A single‐stage pulse tube cryocooler was optimized to provide 50 W of net refrigeration power at 50 K when driven by a pressure oscillator that can produce up to 2.8 kW of acoustic power at 60 Hz. The cryocooler was designed with the ability to provide rapid cooldown. The rapid cooling technique makes use of a resonant phenomenon in the inertance tube and reservoir system to decrease the flow impedance and thereby increase the acoustic power and refrigeration power in the system when the cold end is near room temperature. Initial experimental data produced no‐load temperatures of about 100 K and showed large azimuthal non‐uniformities in temperature profiles around the center plane of both the regenerator and the pulse tube. Inadequate diffusion bonding in the initial aftercooler resulted in non‐uniform temperatures in the aftercooler and regenerator warm end where temperatures were as high as 350 K. Jetting into the pulse tube through both the warm and cold heat exchangers also contributed to the poor pe...

Design and Test of the NIST/Lockheed Martin Miniature Pulse Tube Flight Cryocooler

A two-stage miniature pulse tube (PT) cryocooler, designed for a Space Shuttle flight demonstration, was built and tested at Lockheed Martin Astronautics (LMA) at Denver, CO and the NIST Boulder Laboratory. The Miniature PT Flight Cryocooler (MPTFC) was designed to provide 0.15 W of cooling at 80 K with heat rejection at 275 K. It was developed as the smallest cryocooler of its kind for the purpose of demonstrating launch survivability and thermal performance in a zero-g environment. A prototype laboratory version was first built and tested to provide information on component sizing and flow rates for comparison to numerical models. The flight version was then fabricated as a Getaway Special (GAS) Payload. Cost containment and manned flight safety constraints limited the extent of the MPTFC development to achieve performance optimization. Nonetheless, it reached 87 K driven by a commercially available tactical compressor with a swept volume of 0.75 cc. The on-orbit cooling performance was not demonstrated because of low battery voltage resulting from failed primary batteries. The first off-state PT thermal conductance measurements were successful, however, and the MPTFC also demonstrated the robustness of PT cryocoolers by surviving pro-launch vibration testing, shipping, and the launch and landing of STS-90 with no measurable performance degradation.

Chamber for mechanical testing in H2 with observation by neutron scattering

A gas-pressure chamber has been designed, constructed, and tested at a moderate pressure (3.4 MPa, 500 psi) and has the capability of mechanical loading of steel specimens for neutron scattering measurements. The chamber will allow a variety of in situ neutron scattering measurements: in particular, diffraction, quasielastic scattering, inelastic scattering, and imaging. The chamber is compatible with load frames available at the user facilities at the NIST Center for Neutron Research and Oak Ridge National Laboratory Spallation Neutron Source. A demonstration of neutron Bragg edge imaging using the chamber is presented.

Effect of flow-pressure phase on performance of regenerators in the range of 4 K to 20 K

Modeling with REGEN3.3 has shown that the phase between flow and pressure at the cold end of 4 K regenerators has a large effect on their second-law efficiency. The use of inertance tubes in small 4 K pulse tube cryocoolers has limited phase-shifting ability, and their phase shift cannot be varied unless their dimensions are varied. We report here on the use of a miniature linear compressor, operating at the pulse tube warm end of about 30 K, as a controllable expander that can be used to vary the phase over 360°. We also use the back EMF of the linear motor to measure the acoustic power, flow rate amplitude, and phase between flow and pressure at the piston face. We discuss the measurements of the linear motor parameters that are required to determine the piston velocity from the back EMF as well as the measurement procedures to determine the back EMF when the expander is operating at a temperature around 30 K. Our experimental results on the performance of a regenerator/pulse tube stage operating below ...