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ABSTRACT: Computational fluid dynamic (CFD) analysis has been applied by various authors to study the processes occurring in the pulse tube cryocooler and carry out parametric design and optimization. However, a thorough and quantitative validation of the CFD model predications against experimental data has not been accomplished. This is in part due to the difficulty associated with measuring the specific quantities of interest (e.g., internal enthalpy flows and acoustic power) rather than generic system performance (e.g., cooling power). This paper presents the experimental validation of a previously published two‐dimensional, axisymmetric CFD model of the pulse tube and its associated flow transitions. The test facility designed for this purpose is unique in that it allows the precise measurement of the cold end acoustic power, regenerator loss, and cooling power. Therefore, it allows the separate and precise measurement of both the pulse tube loss and the regenerator loss. The experimental results are presented for various pulse tube and flow transition configurations operating at a cold end temperature of 80 K over a range of pressure ratios. The comparison of the model prediction to the experimental data is presented with discussion.
AIP Conference Proceedings. 04/2010; 1218(1):76-83.
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ABSTRACT: This paper reports on a micromachined Si/glass stack recuperative heat exchanger with in situ temperature sensors. Numerous high-conductivity silicon plates with integrated platinum resistance temperature detectors (Pt RTDs) are stacked, alternating with low-conductivity Pyrex spacers. The device has a 1 ?? 1-cm<sup>2</sup> footprint and a length of up to 3.5 cm. It is intended for use in Joule-Thomson (J-T) coolers and can sustain pressure exceeding 1 MPa. Tests at cold-end inlet temperatures of 237 K-252 K show that the heat exchanger effectiveness is 0.9 with 0.039-g/s helium mass flow rate. The integrated Pt RTDs present a linear response of 0.26%-0.30%/K over an operational range of 205 K-296 K but remain usable at lower temperatures. In self-cooling tests with ethane as the working fluid, a J-T system with the heat exchanger drops 76.1 K below the inlet temperature, achieving 218.7 K for a pressure of 835.8 kPa. The system reaches 200 K in transient state; further cooling is limited by impurities that freeze within the flow stream. In J-T self-cooling tests with an external heat load, the system reaches 239 K while providing 1 W of cooling. In all cases, there is an additional parasitic heat load estimated at 300-500 mW.
Journal of Microelectromechanical Systems 03/2010; · 2.10 Impact Factor
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ABSTRACT: This paper reports on design, fabrication, and testing of a piezoelectrically actuated microvalve with integrated sensors for flow modulation at low temperatures. One envisioned application is to control the flow of a cryogen for distributed cooling with a high degree of temperature stability and a small thermal gradient. The valve consists of a micromachined die fabricated from a silicon-on-insulator wafer, a glass wafer, a commercially available piezoelectric stack actuator, and Macor ceramic encapsulation that has overall dimensions of 1.5 x 1.5 x 1.1 cm<sup>3</sup>. A piezoresistive pressure sensor and a thin-film Pt resistance temperature detector are integrated on the silicon die. The assembly process allows the implementation of normally open, partially open, and normally closed valves. At room temperature, gas flow modulation from 200 to 0 mL/min is achieved from 0- to 40-V actuation. Flow modulation at various temperatures from room temperature to 205 K is also reported. The pressure sensor has sensitivity of 356 ppm/kPa at room temperature, with temperature coefficient of sensitivity of -6507 ppm/K. The temperature sensor has sensitivity of 0.29 %/K. The valve and the sensors are tested across a wide range of temperatures, and the effect of temperature on performance is discussed.
Journal of Microelectromechanical Systems 09/2009; · 2.10 Impact Factor
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ABSTRACT: This paper reports a Joule-Thomson cooling system that provides 0.1-1 W cooling power using a micromachined Si/glass perforated plate heat exchanger. The gas expansion is performed through a micromachined valve that is piezoelectrically actuated, or alternatively through a commercial jewel orifice. The modulated J-T system using the microvalve can achieve 254.5 K at a pressure difference of 430 kPa and 5-8 K temperature modulation at a given pressure. With a jewel orifice, the temperature at the expansion orifice drops 76.1 K from the inlet temperature for an inlet pressure of 1 MPa (145 psia) when the ethane mass flow rate is 0.269 g/s. The system can reach a lower temperature at 200.3 K in a transient state. The cooling power of the system is 200 mW at 228K and 1 W at 239 K, in addition to a parasitic heat load of 300-500 mW.
Solid-State Sensors, Actuators and Microsystems Conference, 2009. TRANSDUCERS 2009. International; 07/2009
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ABSTRACT: Over the past few decades, the pulse-tube cryocooler has advanced from a curiosity to one of the most attractive systems for providing reliable cryogenic cooling; it is now used in aerospace, medical and superconductor applications. This technology development has been enabled by the simulation tools that are available for regenerator, compressor, and inertance tube design. However, a dedicated design tool for the pulse-tube component in a pulse-tube cryocooler and the associated flow transitions between the pulse tube and the regenerator and the pulse tube and the inertance network is not currently available.
AIP Conference Proceedings. 03/2008; 985(1):1445-1453.
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ABSTRACT: This paper reports a micromachined recuperative heat exchanger integrated with in-situ temperature sensors. In this design, numerous high-conductivity silicon plates, integrated with platinum resistance temperature detectors (Pt RTDs) are fabricated and stacked alternating with low- conductivity Pyrex spacers. The device is intended for use with Joule-Thomson (J-T) coolers. The fabricated versions of the device have a footprint of 1 x 1 cm<sup>2</sup>, and lengths of up to 2 cm. Tests at cryogenic temperatures (120-210 K) show that the effectiveness of the heat exchanger is as high as 0.785. In a preliminary J-T self-cooling test, the temperature at the expansion orifice dropped 13 K from the inlet temperature for an inlet pressure of 527 kPa (76 psia). The integrated Pt RTDs presented a sensitivity of 0.26%/K in the linear operational range of 150-300 K, but remained usable at temperatures below that range. Details regarding evaluation apparatus are discussed.
Micro Electro Mechanical Systems, 2008. MEMS 2008. IEEE 21st International Conference on; 02/2008
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ABSTRACT: This paper reports on a drug delivery system that provides modulated delivery of liquid-phase chemicals. The device uses silicon torsion springs on a 2times3 cm<sup>2</sup> chip to pressurize a soft polymeric reservoir and regulate flow with a piezoelectricaly actuated silicon microvalve that is 1.5times1.5times1 cm<sup>3</sup>. Using the finished device, regulated diffusion of a fluorescent dye into agar gel was demonstrated. Fluid flow out of the 500 muL reservoir could be regulated from 10-500 muL/min with up to 80 kPa of delivery pressure. Typical regulation consumes 0.136 muW of power. Analysis of the valve, reservoir springs, and a model based on pressure-enhanced diffusion are presented and are validated by experimental data.
Solid-State Sensors, Actuators and Microsystems Conference, 2007. TRANSDUCERS 2007. International; 07/2007
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ABSTRACT: This paper describes a normally open, self-encapsulated, gas valve that has embedded sensors for pressure and temperature monitoring. The valve has been validated at operating temperatures over 80-380 K, and at pressures up to 130 kPa. A perimeter augmentation scheme for the valve seat has been implemented to provide higher flow modulation. Two kinds of suspensions are described for the valve seat. In tests performed at room temperature, the flow was modulated from 980 mL/min. with the valve fully open (0 V), to 0 mL/min. with 60 V actuation, at an inlet pressure of 55 kPa. Cryogenic flow rate tests show similar modulation with flow from 166 mL/min. with the valve fully open, to 5.3 mL/min. with 120 V actuation voltage, at an inlet pressure of 70 kPa. The platinum RTD temperature sensor is independently tested from 40-450 K with sensitivity of 0.23 %/K in its operational range of 150- 450 K. The pressure sensor has sensitivity of 250 ppm/kPa at room temperature.
Micro Electro Mechanical Systems, 2007. MEMS. IEEE 20th International Conference on; 02/2007
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ABSTRACT: This paper describes results from two types of micromachined recuperative heat exchangers intended for Joule-Thomson (J-T) cryosurgical probes, which require high stream-to-stream thermal conductance while restricting parasitic stream-wise (axial) conduction. In design A, rows of fins composed of high conductivity silicon are bonded onto a 100 mum thick base plate composed of low conductivity Pyrex glass. This planar device has a footprint of 6times1.5 cm<sup>2</sup> and 2.5 mm thickness, and is fabricated using a 5-mask process. In design B, numerous high-conductivity silicon plates alternating with low-conductivity Pyrex spacers are stacked together. This has a footprint of 1times1cm<sup>2</sup>, a length of 1.4 cm, and is fabricated using a 3-mask process. Preliminary experiments show that the primary performance constraint for design A is imposed by the compromise between mechanical robustness and transverse conductance of the thin glass base plate that separates the high pressure and low pressure streams. Design B enhances the robustness of the device and can sustain higher pressure.
Micro Electro Mechanical Systems, 2007. MEMS. IEEE 20th International Conference on; 02/2007
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ABSTRACT: Distributed loads are frequently encountered in large deployable structures used in space applications such as optical mirrors, actively cooled sunshades, and on focal plane electronics. One mechanism for providing distributed cooling is via an oscillatory cryocooler such as a pulse‐tube that is integrated with a fluid rectification system consisting of check‐valves and buffer volumes in order to extract a small amount of continuous flow. This continuous flow allows relatively large loads to be accepted over a long distance with a small temperature difference and has advantages relative to vibration and electrical isolation. Also, it is possible to provide rapid and precise temperature control via modulation of the flow rate. The same working fluid, helium, can be used throughout the entire system, reducing complexity and simplifying the contamination control process.
AIP Conference Proceedings. 04/2006; 823(1):1809-1816.
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ABSTRACT: Active magnetic regenerative refrigeration (AMRR) systems represent an environmentally attractive space cooling and refrigeration
alternative that do not use a fluorocarbon working fluid. Two recent developments have made AMRR’s feasible in the near-term.
A rotary regenerator bed utilizing practical and affordable permanent magnets has been demonstrated and shown to achieve a
reasonable coefficient of performance (COP). Concurrently, families of magnetocaloric material alloys with adjustable Curie
temperatures have been developed. Using these materials, it is possible to construct a layered regenerator bed that can achieve
a high magnetocaloric effect across its entire operating range. This paper describes a numerical model capable of predicting
the practical limits of the performance of this technology applied to space conditioning and refrigeration. The model treats
the regenerator bed as a one dimensional matrix of magnetic material with a spatial variation in Curie temperature, and therefore
magnetic properties. The matrix is subjected to a spatially and temporally varying magnetic field and fluid mass flow rate.
The variation of these forcing functions is based on the implementation of a rotating, multiple bed configuration. The numerical
model is solved using a fully implicit (in time and space) discretization of the governing energy equations. The nonlinear
aspects of the governing equations (e.g., fluid and magnetic property variations) are handled using a relaxation technique.
Some preliminary modeling results are presented which illustrate how an AMRR system can be optimized for a particular operating
condition. The performance of the AMRR in a space cooling application with the layered vs non-layered bed is compared to current
vapor compression technology.
12/2004: pages 471-480;
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ABSTRACT: Regenerative heat exchangers define the crucial design component for high frequency cryocoolers such as Stirling and Pulse
Tube refrigerators. To aid in their design, a parametric investigation has been carried out using the NIST code REGEN3.2 to
optimize a single-stage regenerator operating with a warm end temperature of 300 K and a cold end temperature that varies
between 60 and 80 K. The matrix choice in all the calculations is 400-mesh stainless steel. The optimization has been defined
by maximizing the COP as a function of geometry, mass flow, and the phase between the cold-end mass flow and pressure. We
have conducted the optimization investigation over a frequency range of 30 — 60 Hz using a constant average pressure of 20
bar and a pressure ratio of 1.2. Regenerator performance—defined by net cooling power, COP and losses—is presented along with
the optimized parameters of geometry, mass flux, and phase angle as a function of cold end temperature and frequency in order
to provide convenient design charts for single-stage regenerators.
Additionally, first order design approximations are explored such as the dependence of the optimized length on end temperature
conditions alone, and the independence of an optimized mass flux over a wide range of cooling power.
12/2004: pages 463-470;
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ABSTRACT: This paper presents a comparison between the performance predicted by a computational fluid dynamic (CFD) model and experimental measurements taken using a commercially available vortex tube. Specifically, the measured exit temperatures into and out of the vortex tube are compared with the CFD model. The data and the model are both verified using global mass and energy balances. The CFD model is a two-dimensional (2D) steady axisymmetric model (with swirl) that utilizes both the standard and renormalization group (RNG) k-epsilon turbulence models. While CFD has been used previously to understand the fluid behavior internal to the vortex tube, it has not been applied as a predictive model of the vortex tube in order to develop a design tool that can be used with confidence over a range of operating conditions and geometries. The objective of this paper is the demonstration of the successful use of CFD in this regard, thereby providing a powerful tool that can be used to optimize vortex tube design as well as assess its utility in the context of new applications.
International Journal of Refrigeration.
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ABSTRACT: This paper explores the potential of mixed coolants at elevated pressures for Joule–Thomson cryocooling. A numerical model of a Joule–Thomson cryocooler is developed that is capable of simulating operation with mixtures of up to 9 components consisting of hydrocarbons, non-flammable halogenated refrigerants, and inert gases. The numerical model is integrated with a genetic optimization algorithm, which has a high capability for convergence in an environment of discontinuities, constraints and local optima. The genetic optimization algorithm is used to select the optimal mixture compositions that separately maximizes following two objective functions at each elevated pressure for 80, 90 and 95 K cryocooling: the molar specific cooling capacity (the highest attainable is 3200 J/mol) and the produced cooling capacity per thermal conductance which is a measure of the compactness of the recuperator. The optimized cooling capacity for a non-flammable halogenated refrigerant mixture is smaller than for a hydrocarbon mixture; however, the cooling capacity of the two types of mixtures approach one another as pressure becomes higher. The coefficient of performance, the required heat transfer area and the effect of the number of components in the mixture is investigated as a function of the pressure. It is shown that mixtures with more components provide a higher cooling capacity but require larger recuperative heat exchangers. Optimized mixtures for 90 K cryocooling have similar cooling capacity as those for 80 K. Optimized compactness for 80 K is about 50% higher than can be achieved by pure nitrogen. For 90 K, no mixture provides a more compact recuperator than can be achieved using pure argon. The results are discussed in the context of potential applications for closed and open cycle cryocoolers.
Cryogenics.
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ABSTRACT: A computational fluid dynamics (CFD) model is used to investigate the energy separation mechanism and flow phenomena within a counter-flow vortex tube. A two-dimensional axi-symmetric CFD model has been developed that exhibits the general behavior expected from a vortex tube. The model results are compared to experimental data obtained from a laboratory vortex tube operated with room temperature compressed air. The CFD model is subsequently used to investigate the internal thermal-fluid processes that are responsible for the vortex tube's temperature separation behavior. The model shows that the vortex tube flow field can be divided into three regions that correspond to: flow that will eventually leave through the hot exit (hot flow region), flow that will eventually leave through the cold exit (cold flow region), and flow that is entrained within the device (re-circulating region). The underlying physical processes are studied by calculating the heat and work transfers through control surfaces defined by the streamlines that separate these regions. It was found that the energy separation exhibited by the vortex tube can be primarily explained by a work transfer caused by a torque produced by viscous shear acting on a rotating control surface that separates the cold flow region and the hot flow region. This work transfer is from the cold region to the hot region whereas the net heat transfer flows in the opposite direction and therefore tends to reduce the temperature separation effect. A parametric study of the effect of varying the diameter and length of the vortex tube is also presented.
International Journal of Refrigeration.
