Pat R. Roach

General pulse tube theory

A brief review of pulse tube thermodynamics is given. The underlying phenomena are described starting with a very simplified model. The concept of phasor analysis is used to show the fundamental behaviour. The concept of enthalpy flow is expanded to include entropy and Gibbs free energy flows and to apply these for the first time to regenerative refrigerators. A simplified model based on enthalpy, entropy and Gibbs free energy flows is presented to illustrate the effects of temperature gradients, thermal conduction and viscosity. As models become more sophisticated, more features are included. Furthermore, it is shown that, contrary to common usage, the concepts of work, flow and heat flow are not useful when applied to regenerative refrigerators. Rather, the Gibbs free energy flow fulfils the role usually assigned to work and heat flows. Recent advances in thermoacoustic and 2-D flow models are also reviewed.

Pulse tube coolers with an inertance tube : Theory, modeling, and practice

We have studied the advantages to be gained by replacing the orifice of a pulse tube cooler with an inertance tube—a long thin tube that introduces the possibility for additional phase shift between pressure and mass flow in the pulse tube section. A case for using an inertance tube is made by employing an electrical analogy, where the ‘inductance’ added by the inertance tube allows for an improved power transfer efficiency at the cold end of the pulse tube. Detailed computer modeling of pulse tube systems with inertance tubes confirms these advantages. Comparison between a laboratory cooler with an orifice and with two inertance tubes is presented; the inertance tubes yield dramatic improvements over the use of the orifice.

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.

Measurements of the Material Properties of a Laminated Piezoelectric Stack at Cryogenic Temperatures

Future NASA missions require cooling of large structures in space. One class of thermal management solutions for providing controlled, distributed cooling would utilize actively controlled micro‐scale valves that are integrated with heat exchangers and sensors in order to provide independent, local temperature control. The most attractive actuation method for these micro‐valves is a multilayer piezoelectric (PZT) stack because this technology is capable of providing large force using reasonable voltages (e.g., < 100 V) with minimal power draw. In order to design a micro‐valve configuration that takes advantage of this actuation technique, it is necessary to obtain information regarding the behavior of piezoelectric materials at cryogenic temperatures. This paper describes a test facility that was designed to achieve precise measurements of the coefficient of thermal expansion (CTE) and PZT stack actuator constant (d33) from 40 K to room temperature. The operation of the facility is validated by measuring ...

Kevlar support for thermal isolation at low temperatures

A cryogenic support has been developed that rigidly attaches two U‐shaped aluminum beams to each other with strands of Kevlar. The Kevlar creates a very strong and stiff coupling between the beams while allowing only a very small heat to flow between them. Measurements at room temperature and at 77 K confirm the stiffness and strength of the support

A Simple Modeling Program for Orifice Pulse Tube Coolers

We have developed a calculational model that treats all the components of an orifice pulse tube cooler. We base our analysis on 1-dimensional thermodynamic equations for the regenerator1 and we assume that all mass flows, pressure oscillations and temperature oscillations are small and sinusoidal. The resulting mass flows and pressures are matched at the boundaries with the other components of the cooler: compressor, aftercooler, cold heat exchanger, pulse tube, hot heat exchanger, orifice and reservoir. The results of the calculation are oscillating pressures, mass flows and enthalpy flows in the main components of the cooler.

Theoretical Analysis of a Pulse Tube Regenerator

A theoretical analysis of the behavior of a typical pulse tube regenerator has been carried out. Assuming simple sinusoidal oscillations, the static and oscillatory pressures, velocities and temperatures have been determined for a model that includes a compressible gas and imperfect thermal contact between the gas and the regenerator matrix. For realistic material parameters, the analysis reveals that the pressure and velocity oscillations are largely independent of details of the thermal contact between the gas and the solid matrix. Only the temperature oscillations depend on this contact. Suggestions for optimizing the design of a regenerator are given.

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.

An optimization program for modeling pulse tube coolers

ARCOPTR is a program developed at NASA-Ames Research Center for modeling pulse tube coolers. The ARCOPTR is a 1-D model that treats all the components of an orifice pulse tube cooler or a pulse tube cooler employing an inertance tube. It analyzes the thermodynamic equations for the regenerator and assumes that all mass flows, pressure oscillations and temperature oscillations are small and sinusoidal. The resulting mass flows and pressures are matched at the boundaries with the other components of the cooler: compressor, aftercooler, cold heat exchanger, pulse tube, hot heat exchanger, orifice/inertance tube and reservoir. The results of the calculation are oscillating pressures, mass flows and enthalpy flows in the main components of the cooler. An optimizer has been built into the ARCOPTR that allows optimization of the cooler parameters to attain maximum efficiency. The parameters that can be optimized are the length and the diameter of the regenerator and those of the pulse tube, the wire diameter and the mesh size of the regenerator screen, and the orifice opening or in the case of an inertance tube, its length and diameter. In this study, pulse tube cooler configurations, employing an orifice or an inertance tube, have been optimized using the ARCOPTR. The optimum parameters are plotted as a function of the regenerator diameter. In the optimization the pressure ratio in the pulse tube is maintained at 1.2 by adjusting the compressor stroke.

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.