Peter Kittel

Ideal orifice pulse tube refrigerator performance

Abstract Using an analogy to the Stirling cycle refrigerator, the efficiency (cooling power per unit input power) of an ideal orifice pulse tube refrigerator is shown to be T 1 / T 0 , the ratio of the cold temperature to the hot temperature.

Cryogenics and the Human Exploration of Mars

Current studies within NASA involve extending the human exploration of space from low earth orbit into the solar system, with the first human exploration of Mars proposed in 2014. The key cryogenic technology areas to be addressed in human Mars missions are long-term propellant storage, cryogenic refrigeration, cryogenic liquefaction, and zero gravity fluid management. Passive technologies such as advanced multilayer insulation (MLI) concepts, vapor-cooled shields (VCS), and catalytic converters will be combined with the development of active coolers (cryogenic refrigerators). The integration of passive and active technologies will form a hybrid system optimized to minimize the launch mass while preserving the cryogenic propellants. This paper will present a brief overview of the proposed Mars reference mission and the concomitant cryogenic fluid management technology, focusing on active cooling technology.

Steady Secondary Momentum and Enthalpy Streaming in the Pulse Tube Refrigerator

Our study investigates the steady secondary momentum and enthalpy streaming that occurs in the pulse tube refrigerator. The linearized mass, momentum and energy conservation laws that are described by N. Rott1 are applied to a pulse tube, with the phase and amplitude of the axial velocity boundary conditions treated as independent parameters. Heat transfer between the gas and the tube wall is included. Heat transfer is shown to affect enthalpy flow by modifying the dynamic temperature amplitude and the temperature phase angle of the gas. We also calculate the steady mass flow circulation due to Reynolds stresses in the pulse tube. The length scale of the circulation is shown to be of the order of the tube length. Mass flow circulation is a loss mechanism because it results in a direct convection of enthalpy between the cold and hot ends.

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.

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.

Thermal conductance of pressed metallic contacts augmented with indium foil or Apiezon grease at liquid helium temperatures

Abstract The thermal conductance of pressed contacts which have been augmented with indium foil or Apiezon-N™ grease has been measured over the temperature range 1.6–6.0 K, with applied forces from 22 to 670N. The sample pairs were fabricated from OFHC copper, 6061-T6 aluminium, free-machining brass and 304 stainless steel. Although the thermal conductance was found to increase with increasing applied contact force, the force dependence was less than in earlier work. The addition of indium foil or Apiezon grease between the contact surfaces resulted in an improvement over uncoated surfaces ranging from approximately a factor of 3 for stainless steel to an order of magnitude for copper contacts.

The Superfluid Helium On-Orbit Transfer (Shoot) Flight Experiment

The SHOOT flight demonstration is being undertaken to verify component and system level technology necessary to resupply large superfluid helium dewars in space. The baseline configuration uses two identical 210 liter dewars connected by a transfer line which contains a quick disconnect coupling. The helium will be transferred back and forth between the dewars under various conditions of flow rate, parasitic heat load, and temperature. An astronaut Extra-vechicular Activity (EVA) is also planned to manually demate and mate the coupling. A number of components necessary for the flight are being developed. These components are described here.

Cryocoolers for Human and Robotic Missions to Mars

Future missions to Mars will make use of a number of different cryocoolers. These cryocoolers will be used to liquefy and preserve propellants and for transporting liquid hydrogen feed stock. The earliest mission requiring a cryocooler is the 2003 robotic mission, which will demonstrate technology for In-Situ-Consumable-Production (ISCP) as part of the long-term goal of developing In-Situ-Resource-Utilization (ISRU). Later missions may require cryocoolers to preserve propellants on Trans-Mars Injection stages, Martian landers, and Mars return vehicles. In addition, ISCP may require liquid hydrogen feed stock as part of the chemical process to produce propellants from the Martian atmosphere. Cryocoolers will be required to minimize or eliminate the boiloff of this hydrogen in transit to Mars. Propellants produced on Mars will need liquefiers and cryocoolers for storage on the Martian surface until the propellants are used in the ascent vehicle.

Propellant preservation using re-liquefiers

Abstract The long-term storage of cryogenic propellants in space requires the use of active cooling. Previous work has considered using closed cycle coolers to exactly balance the parasitic heat loads on the propellant. An alternative approach using a re-liquefier has been suggested. This paper considers some of the advantages and disadvantages of this approach. A re-liquefier uses the propellant vapor as the working fluid. This may eliminate the need for a mixer in the propellant tank; may reduce the effect of leaks in the cooler; and may allow a more graceful way to accommodate cooler failure. However, the re-liquefier is unlikely to be operating at its optimal operating point; and will require some means of controlling contamination.

Comparison of Dewar supports for space applications

Abstract A simple one-dimensional model is developed to illustrate the relative merits of different Dewar support systems. The model considers how different supports affect the parasitic heat load on Dewars in both strength limited and resonant frequency limited applications. The model is used to compare straps, struts and disconnect supports. The comparison shows that struts are superior in strength limited applications; that straps are superior in resonant frequency limited applications; and disconnect struts are superior when the onorbit resonant frequency requirement is lower than during launch.