Ben Helvensteijn

Pressure Drop over Regenerators in Oscillating Flow

Modeling the performance of pulse tube coolers and stirling coolers involves estimating the pressure drop over the regenerator. Such pressure drop estimates are generally based on steady flow correlations which do not necessarily apply to oscillating flow. The present paper contains experimental data on the pressure drop over regenerators subjected to an oscillating flow of helium gas at 300K and a charge pressure of 11 atm. Each regenerator tested consists of a thin wall stainless steel tube packed with just one of the following materials: stainless steel screen of mesh-size 250, 300 or 400, or stainless steel felt made of wires 12 µm or 30 µm in diameter. The oscillating flow is established by means of a linear compressor (15 cc) operating between 40 Hz and 70 Hz connected to the inlet of the regenerator. The mass flow is derived from the measured pressure oscillations in a reservoir connected to the regenerator outlet. A differential pressure gauge is applied over the regenerator. Heat exchangers around and in-line with the regenerator stabilize its temperature. The pressure drop and mass flow data are converted into correlations for the friction factor as a function of Reynolds number at given void fraction and frequency. Steady flow measurements have been applied as a reference in order to note the extent to which the oscillations contribute to the pressure drop.

Development of a Linear Compressor for Use in G-M Cryocoolers

A new compressor for Gifford-McMahon (G-M) cryocooler service has been designed and built using twin Clever Fellows Innovation Consortium (CFIC) STAR linear motors and special friction-free reed valves compatible with oil-free service. This fully-balanced arrangement eliminates all oil and other condensibles in the helium, as well as providing attitude independence and in-use mobility prohibited by conventional G-M compressors. Secondary oil management components and their failure modes are also eliminated. The design operating point matches pressures and flow for a Sumitomo 408 coldhead. Standard Qdrive 5 kW motors are used, giving significant capacity margin over the baseline compressor’s 6 kW rating. Test results to date have demonstrated design-point flow and pressures, with significantly lower input power than the baseline compressor. The design is reviewed and test results are presented in this paper.

Conceptual design of a 0.1 W magnetic refrigerator for operation between 10 K and 2 K

The design of a magnetic refrigerator for space applications is discussed. The refrigerator is to operate in the temperature range of 10 K — 2 K, at a 2 K cooling power of 0.10 W. As in other magnetic refrigerators operating in this temperature range GGG has been selected as the refrigerant. Crucial to the design of the magnetic refrigerator are the heat switches at both the hot and cold ends of the GGG pill. The 2 K heat switch utilizes a narrow He II filled gap. The 10 K heat switch is based on a narrow helium gas gap. For each switch, the helium in the gap is cycled by means of activated carbon pumps. The design concentrates on reducing the switching times of the pumps and the switches as a whole. A single stage system (one magnet; one refrigerant pill) is being developed. Continuous cooling requires the fully stationary system to have at least two stages running parallel/out of phase with each other. In order to conserve energy, it is intended to recycle the magnetic energy between the magnets. To this purpose, converter networks designed for superconducting magnetic energy storage are being studied.

Performance of a new regenerator material in a pulse tube cooler

A new class of alloys has been developed at Ames Laboratory in Ames, Iowa. These alloys exhibit heat capacities that exceed those of all other materials, including lead, over a wide range in temperature (15 K<T<85 K). An effort is underway to employ these alloys in a two-stage pulse tube cooler driven by a linear compressor to achieve cooling at 20 K. The first stage of the cooler will have the conventional stainless steel screen regenerator matrix. The matrix for the second stage regenerator (<60 K) will be made from the newly developed alloys. The performance of one such alloy is being tested in an apparatus that consists of a single-stage pulse tube cooler pre-cooled by a liquid nitrogen stage. The regenerator material used in the tests is in spherical powder from having particle sizes ranging from 90 μm to 106 μm. Preliminary results from tests performed in the pulse tube cooler employing the new regenerator material are presented.

Progress on a microgravity dilution refrigerator

We have developed a shallow, single-cycle, helium dilution refrigerator that contains rather coarse metal sponge to study the ability to control the location of the liquid helium for microgravity applications. We have tested the refrigerator on the ground while tilting to put the mixing chamber either somewhat above or somewhat below the still. We calculated that the system could be tilted between 5° and 10° in either direction without interrupting the cooling. The initial test of this refrigerator gave cooling to below 0.060 K and operation for tilts of ±16°. The insights gained from this refrigerator allow the design of a continuously operating version.

Technologies for Cooling of Large Distributed Loads

In future space applications, large and distributed loads will require active thermal control if their lifetimes are to be extended beyond one or two years. Examples include Zero Boil-Off (ZBO) cryogenic systems for exploration missions, and cooling of widely distributed sensor arrays and large deployable structures, such as mirrors and sunshades, for space science missions. These applications will require efficient means of heat transfer from extended structures or from several discrete elements to one or more remotely located heat rejection packages consisting of active and passive components. More or less stringent temperature control will also be required. We have recently undertaken a program to develop a number of technologies relevant to the issues associated with distributed cooling. These include circulation networks that transfer heat via steady flows of cold pressurized gas; gas rectifiers for use with linear pressure wave generators; and MEMS-based throttling valves for precise temperature control. This paper describes the studies that are underway to establish the performance potential of each.

Development of a Dilution Refrigerator for Low-Temperature Microgravity Experiments

A dilution refrigerator (DR) is the most common precooling stage for sub-millikelvin demagnetization experiments. The usefulness of the DR comes from its ability to provide cooling at 0.02–0.04 K for long periods of time while the heat of magnetization is being rejected by the demagnetization stage. In order to make these advantages of the DR available to researchers who need the microgravity of space for their experiments, we are developing a continuously-operating DR that will function in microgravity.

Adsorption of Helium in Commercially Available Activated Carbons

A newly constructed cryostat has been employed in order to establish the helium adsorption characteristics of several commercially available activated carbons. So far, two carbon samples have been studied. Considered are the steady-state adsorption-pressure-temperature relations of 3He as well as 4He (unmixed). Pressures are measured with room-temperature instrumentation. Applying thermo-molecular pressure difference corrections the cold end pressures are obtained. The helium adsorption is derived from straightforward mass balance computations. The measurements span the range of temperatures between 2 K and 50 K for pressures between 1 Pa and 1 MPa. Interpolation allows presentation of the data in the form of adsorption isotherms as well as isobars (only adsorption isotherms are presented here).

Development of a compact dilution refrigerator for zero gravity operation

A compact dilution refrigerator of unique design using internal charcoal adsorption pumps is being modified and tested for operation in zero gravity. This commercially-available refrigerator is self-contained with no external pumps or gas handling system and provides very reliable operation since it has no moving parts. All operations are performed with heaters and are completely computer controlled. The refrigerator provides many hours of operation at very low temperature before the charcoal pumps must be recycled.

Measuring thermal conductivity of insulators at cryogenic temperatures

Traditionally, launch vehicles have consisted of a structure containing the vehicle engines, propellant tanks, and electronics. NASA’s next generation of space transportation vehicles is planned to utilize the innovative concept of conformal propellant tanks. This concept makes the tanks part of the vehicle structure, thereby, offering significant reduction in launch mass. Key to successful implementation of this approach will be the development of a new material that serves not only as cryogenic insulation but also as a heat shield, protecting the vehicle. This material must be precisely characterized at both elevated and cryogenic temperatures. The present paper discusses the apparatus set up to establish the thermal conductivity of candidate materials at low temperatures. The apparatus enables cryogenic heat flow measurements in the presence of a gas. A finite difference model, accounting for the system configuration, is employed to convert the heat flow measurements into thermal conductivity as a function of temperature.