Ali Kashani

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.

Cooling system for the soft x-ray spectrometer (SXS) onboard ASTRO-H

The Soft X-ray Spectrometer (SXS) is a cryogenic high resolution X-ray spectrometer onboard the X-ray astronomy satellite ASTRO-H. The detector array is cooled down to 50 mK using a 3-stage adiabatic demagnetization refrigerator (ADR). The cooling chain from room temperature to the ADR heat-sink is composed of superfluid liquid He, a 4He Joule-Thomson cryocooler, and 2-stage Stirling cryocoolers. It is designed to keep 30 L of liquid He for more than 3 years in the nominal case. It is also designed with redundant subsystems throughout from room temperature to the ADR heat-sink, to alleviate failure of a single cryocooler or loss of liquid He.

SHOOT flowmeter and pressure transducers

Abstract A venturi flowmeter has been designed and constructed for the Superfluid Helium On-Orbit Transfer (SHOOT) experiment. The calibration results obtained from the SHOOT venturi demonstrate the ability of the flowmeter to meet the requirements of the SHOOT experiment. Flow rates as low as 20 dm3 h−1 and as high as 800 dm3 h−1 have been measured with the flowmeter. The pressure drop in the flowmeter is measured by two Validyne differential pressure transducers. The ranges of the two transducers are 0.86 kPa (0.125 p.s.i.d.) and 8.62 kPa (1.25 p.s.i.d.). The low range transducer measures flow rates less than 200 dm3 h−1, whereas the high range transducer measures flow rates greater than 200 dm3 h−1. The repeatability of the flowmeter at the nominal flow rate of 500 dm3 h−1 is within ±1% of the flow rate. Performances of the SHOOT differential and absolute pressure transducers, which have undergone calibration and vibration tests, are also included. Throughout the tests, the responses of the transducers remained linear and repeatable to within ±1% of the full scales of the transducers.

The x-ray microcalorimeter spectrometer onboard of IXO

One of the instruments on the International X-ray Observatory (IXO), under study with NASA, ESA and JAXA, is the X-ray Microcalorimeter Spectrometer (XMS). This instrument, which will provide high spectral resolution images, is based on X-ray micro-calorimeters with Transition Edge Sensor thermometers. The pixels have metallic X-ray absorbers and are read-out by multiplexed SQUID electronics. The requirements for this instrument are demanding. In the central array (40 x 40 pixels) an energy resolution of < 2.5 eV is required, whereas the energy resolution of the outer array is more relaxed (≈ 10 eV) but the detection elements have to be a factor 16 larger in order to keep the number of read-out channels acceptable for a cryogenic instrument. Due to the large collection area of the IXO optics, the XMS instrument must be capable of processing high counting rates, while maintaining the spectral resolution and a low deadtime. In addition, an anti-coincidence detector is required to suppress the particle-induced background. In this paper we will summarize the instrument status and performance. We will describe the results of design studies for the focal plane assembly and the cooling systems. Also the system and its required spacecraft resources will be given.

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.

CHARACTERIZATION OF AN ACTIVELY COOLED METAL FOIL THERMAL RADIATION SHIELD

Zero boil‐off (ZBO) or reduced boil‐off (RBO) systems that involve active cooling of large cryogenic propellant tanks will most likely be required for future space exploration missions. For liquid oxygen or methane, such systems could be implemented using existing high technology readiness level (TRL) cryocoolers. However, for liquid hydrogen temperatures (∼20 K) no such coolers exist. In order to partially circumvent this technology gap, the concept of broad area cooling (BAC) has been developed, whereby a low mass thermal radiation shield could be maintained at temperatures around 100 K by steady circulation of cold pressurized gas through a network of narrow tubes. By this method it is possible to dramatically reduce the radiative heat leak to the 20 K tank. A series of experiments, designed to investigate the heat transfer capabilities of BAC systems, have been conducted at NASA Ames Research Center (ARC). Results of the final experiment in this series, investigating heat transfer from a metal foil fi...

Hierarchy of two-phase flow models for autonomous control of cryogenic loading operation

We report on the development of a hierarchy of models of cryogenic two-phase flow motivated by NASA plans to develop and maturate technology of cryogenic propellant loading on the ground and in space. The solution of this problem requires models that are fast and accurate enough to identify flow conditions, detect faults, and to propose optimal recovery strategy. The hierarchy of models described in this presentation is ranging from homogeneous moving- front approximation to separated non-equilibrium two-phase cryogenic flow. We compare model predictions with experimental data and discuss possible application of these models to on-line integrated health management and control of cryogenic loading operation.

Optimization of cryogenic chilldown and loading operation using SINDA/FLUINT

A cryogenic advanced propellant loading system is currently being developed at NASA. A wide range of applications and variety of loading regimes call for the development of computer assisted design and optimization methods that will reduce time and cost and improve the reliability of the APL performance. A key aspect of development of such methods is modeling and optimization of non-equilibrium two-phase cryogenic flow in the transfer line. Here we report on the development of such optimization methods using commercial SINDA/FLUINT software. The model is based on the solution of two-phase flow conservation equations in one dimension and a full set of correlations for flow patterns, losses, and heat transfer in the pipes, valves, and other system components. We validate this model using experimental data obtained from chilldown and loading of a cryogenic testbed at NASA Kennedy Space Center. We analyze sensitivity of this model with respect to the variation of the key control parameters including pressure in the tanks, openings of the control and dump valves, and insulation. We discuss the formulation of multi-objective optimization problem and provide an example of the solution of such problem.

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.