ADVANCES IN CRYOGENIC ENGINEERING: Transactions of the Cryogenic Engineering Conference - CEC
823(2006); http://dx.doi.org/10.1063/1.2202394View Description Hide Description
This paper describes progress towards the development of a large‐capacity, single‐stage, Stirling‐type, pulse‐tube refrigerator (PTR) for high temperature superconducting power applications. Specifically, the design and fabrication of an experimental PTR is described followed by a series of design modifications which have focused on optimization of the flow transition components the hot and cold ends of the pulse‐tube. Computational fluid dynamic models are described and have been used to guide the design modifications. The impact of each modification on cooler performance is discussed. The cooler is instrumented with piston displacement sensors, high‐frequency pressure sensors, and thermocouples along the regenerator wall, within the cold heat exchanger gas volume, and along the pulse‐tube wall. These sensors provide some characterization of the flow distribution in the regenerator and pulse‐tube.
Performance Prediction and Experimental Investigations on Integral Pulse Tube Cryocooler for 15 W at 70 K Using Indigenously Developed Linear Compressor823(2006); http://dx.doi.org/10.1063/1.2202395View Description Hide Description
Theoretical model based on cyclic analysis has been extended to design the integral pulse tube cryocooler. Cryocooler is developed to match with the indigenously developed opposed piston linear compressor of 30 cc swept volume. The design is carried for the Stirling type Pulse Tube cryocooler to develop 15 W at 70 K. The Pulse Tube cryocooler has been developed with due attention to the heat exchangers and regenerator. Experimental investigations have been carried out for study of the effect of pulse tube aspect ratio and to confirm the consistency of the model and suitability of the linear compressor. Experimental investigation confirmed consistent performance of the linear compressor as well as the Pulse Tube cryocooler.
823(2006); http://dx.doi.org/10.1063/1.2202396View Description Hide Description
This paper describes performances of a Stirling type pulse tube cryocooler as a nitrogen sub‐cooler. The main objective of this work is a demonstration of a cooling system for High Tc Superconducting (HTS) power applications such as fault current limiters, cables and transformers. Cooling capacity necessary for these applications is more than several hundred watts at 77 K level. High efficiency and high reliability are also required. Developments of several hundred watts class compact cryocooler such as Stirling‐type pulse tube cryocooler are expected for the HTS power applications. A liner‐motor‐driven compressor (model:2S241K) and coldhead, which were developed by CFIC Inc., were used in our system to evaluate efficiency and reliability for our application. A cooling stage of the cold head was designed as a heat exchanger to cool a sub‐cooled nitrogen flow to reduce a temperature difference between working gas of cryocooler (helium) and the sub‐cooled nitrogen. A cooling capacity of 207 W at 77 K was obtained with an input power of 4.33 kW. The Carnot efficiency was 13.8 %, which is about 1.7 times higher than that of commercial GM cryocooler (for reference). The long‐term operation over 0.5 year (4414 hours) without performance degradation was obtained.
823(2006); http://dx.doi.org/10.1063/1.2202397View Description Hide Description
Secondary flow in a double‐inlet pulse tube refrigerator with a double bypass valve configuration was evaluated to determine the advantage of this configuration in controlling the secondary flow over a conventional single valve configuration. In the double‐valve configuration two needle valves was connected in parallel and their respective valve directions were in opposite directions, namely, one valve was directed toward the hot end and the other toward the inlet of the regenerator. The effects of valve configuration on the flow behavior of the secondary flow, on the cooling performance, and on the pressure‐volume (work) diagram for gas were investigated. The secondary flow was visualized by using a smoke‐wire flow visualization technique, and cooling performance was estimated based on the temperature difference between the hot and cold ends. Results showed that the double‐valve configuration allowed the operating conditions of the refrigerator to be optimized to maximize the work and minimize the heat loss by the secondary flow, whereas the single‐valve configuration did not leave room for such adjustment. After optimization, however, the single‐valve configuration still showed fairly good performance, comparable to that of the double‐valve configuration.
823(2006); http://dx.doi.org/10.1063/1.2202398View Description Hide Description
The authors have undertaken basic research and prototype developments on various four valve pulse tube refrigerators (FVPTR) since several years. Systematic studies have been carried out to characterize the basics of the arising loss mechanisms of this type of pulse tube refrigerator with its active type of phase shifting. Recently, a single‐stage FVPTR in coaxial arrangement has been designed for maximum refrigeration power at cooling temperatures below 30 K limited by an available electrical input power of 6.2 kW. At present the single stage cooler provides a cooling capacity of 10 W or 20 W at working temperatures of 22 K or 27 K, respectively. Instead of lead spheres, lead coated screens have been applied in the coldest part of the regenerator in order to decrease the pressure drop. The improvement of the refrigerator overcomes the well known shortcoming of a refrigerator with a regenerator partially filled with lead spheres at working‐temperatures above 50 K. Numerical simulations of the thermodynamical process yield to continuative lead screen parameter studies. The results raise hope for achievable minimum temperatures down to 15 K. We report on the improved performance of the FVPTR due to the use of lead‐coated screens.
823(2006); http://dx.doi.org/10.1063/1.2202399View Description Hide Description
Oxford Instruments Superconductivity has developed a new closed cycle cooling circuit based on a Pulse Tube Refrigerator (PTR) to cool a Cold Probe for NMR applications.
By replacing the existing Gifford McMahon (G‐M) cryocooler with a PTR, levels of vibration at the probe can be reduced by an order of magnitude, essential for NMR experiments. Another benefit comes from the reduction in moving parts due to fundamental differences between the PTR and G‐M coolers. This should lead to improved reliability and reduced down times.
This cooling circuit can supply more than 3.5 W of cooling power to cool RF coils in the probe at 25 K and 2.5 W to the electronics. Cooling the RF coils and the electronics in the probe can improve signal to noise ratio by 3 to 4 times, helping scientists gain a greater understanding of the structure of proteins and speeding drug discoveries.
823(2006); http://dx.doi.org/10.1063/1.2202400View Description Hide Description
A pulse tube cooler (PTR) for shield cooling and for re‐condensation of Helium in MRI magnets has been developed successfully by co‐operation of Forschungszentrum Karlsruhe (FZK) and Siemens Magnet Technology Ltd. (SMT, formerly Oxford Magnet Technology, OMT). Refrigeration power of 1.1 W at 4.2 K and 40 W at 45 K were achieved with less than 8 kW of compressor input power. The two‐stage PTR is of a ’4‐valve type on each stage. A special non‐wear rotary valve has been developed at SMT. The design of the cooler is based on investigations carried out at FZK on a test rig, which was versatile enough to allow many different experiments to be carried out. They have been accompanied by numeric studies. The code FZKPTR based on the thermoacoustic theory has proved a very helpful tool. This development work at FZK and SMT lead to a commercial PTR solution for MRI magnets. The industrial partner, SMT, defined the needs for his application, and has modified the FZK design according to these requirements and those of industrial production. Some of the intermediate development steps performed during several years of co‐operation are mentioned. Prototype units have been successfully installed at clinical sites. A preliminary version, a 10 K shield cooler has been running continuously for two years. More than two months of recondensing performance on the 4K unit fitted to an MRI magnet, with a margin sufficient to prevent boil off of liquid He during imaging, has been obtained.
823(2006); http://dx.doi.org/10.1063/1.2202401View Description Hide Description
The efficiency of regenerative refrigerators is generally maximized when the pressure and flow are in phase near the midpoint of the regenerator. Such a phase relationship minimizes the amplitude of the mass flow for a given acoustic power flow through the regenerator. To achieve this phase relationship in a pulse tube refrigerator requires that the flow at the warm end of the pulse tube lag the pressure by about 60 degrees. The inertance tube allows for the flow to lag the pressure, but such a large phase shift is only possible with relatively large acoustic power flows. In small pulse tube cryocoolers the efficiency is improved by maximizing the phase shift in the inertance tube. This paper describes a simple transmission line model of the inertance tube, which is used to find the maximum phase shift and the corresponding diameter and length of the optimized inertance tube. Acoustic power flows between 1 and 100 W are considered in this study, though the model may be valid for larger systems as well. For large systems the model can be used to find the minimum reservoir volume that in combination with the inertance tube provides a phase shift of 60 degrees. This transmission line model is compared with some experimental results on a small‐diameter inertance tube and found to agree quite well provided some heat transfer is taken into account. Design graphs for a frequency of 60 Hz and an average pressure of 2.5 MPa are presented for different pressure ratios and for both adiabatic and isothermal conditions.
823(2006); http://dx.doi.org/10.1063/1.2202402View Description Hide Description
The cryogenic department at the Stanford Linear Accelerator Center is responsible for the operation, troubleshooting, and upgrade of the 1.5 Tesla superconducting solenoid detector for the BABAR B‐factory experiment. Events that disable the detector are rare but significantly impact the availability of the detector for physics research. As a result, a number of systems and procedures have been developed over time to minimize the downtime of the detector, for example improved control systems, improved and automatic backup systems, and spares for all major components. Together they can prevent or mitigate many of the failures experienced by the utilities, mechanical systems, controls and instrumentation. In this paper we describe various failure scenarios, their effect on the detector, and the modifications made to mitigate the effects of the failure. As a result of these modifications the reliability of the detector has increased significantly with only 3 shutdowns of the detector due to cryogenics systems over the last 2 years.
823(2006); http://dx.doi.org/10.1063/1.2202403View Description Hide Description
The Levitated Dipole Experiment (LDX) is an innovative facility to study plasma confinement in a dipole magnetic field, created by a superconducting solenoid (floating coil), which is magnetically levitated in the center of a 5 m diameter by 3 m tall vacuum chamber. The floating coil (F‐coil) consists of a Nb 3Sn magnet installed inside a strong vessel filled with high‐pressure helium gas at room temperature. It is surrounded by a fiberglass‐lead composite radiation shield and by a toroidal vacuum shell. The cryostat design provides the ability to operate the magnet for several hours of wanning while suspended in the middle of the vacuum chamber without electric and cryogenic connections to the coil. For this reason the magnet is charged/discharged inductively in a lower part of the vacuum chamber. The retractable cryogenic transfer lines serve to cool down the magnet to 4.5 K before it is lifted to the operating position. The F‐coil can be re‐cooled multiple times while maintaining its field and current. This paper describes the thermal performance of the F‐coil.
823(2006); http://dx.doi.org/10.1063/1.2202404View Description Hide Description
A series of 1‐m long Nb 3Sn dipole magnets have been built at Fermilab in an attempt to refine the wind‐and‐react technology for Nb 3Sn conductor. Models have been made with MJR and PIT strand with varying degrees of success. Subsequently two new dipole “mirror” magnets based on RRP Nb 3Sn coils were constructed and tested. This paper describes the design, fabrication and test results of those magnets.
823(2006); http://dx.doi.org/10.1063/1.2202405View Description Hide Description
An experimental study was carried out on the heat transfer properties of pressurized superfluid helium in the Gorter‐Mellink heat transfer region. By using channels of hydraulic diameter from 5.6 × 10− through 4.81 × 10−3 m, the heat transfer properties of pressurized superfluid helium were measured in the experiment. The temperature dependence of Gorter‐Mellink parameter, AGM , is revealed from the experimental results. It is also proven that AGM depend only on temperature, and not on the channel size and shape. The effect of quantized vortices on heat transfer of pressurized superfluid helium is discussed in comparison of the channel diameter with the mean vortex line spacing.
Experimental and Numerical Studies on Thermal Hydraulic Characteristics of He II through Porous Media823(2006); http://dx.doi.org/10.1063/1.2202406View Description Hide Description
To improve the stability of superconducting magnets cooled by He II, it is considered that the utilization of the porous media enhances the heat transport capability of He II. In the present studies, the thermal hydraulic characteristics of He II through porous media have been investigated experimentally and numerically. Experimental results are presented about 6 porous media with various porosity, thickness and average pore diameters. Numerical results are reported about 4 porous media among them. In the present numerical analysis, the two‐dimensional calculations have been performed for the turbulent Gorter‐Mellink regime to understand the heat and mass transfer of the He II through the porous media in the case of the large heat input. The present numerical model was based on the two‐fluid model with the Gorter‐Mellink mutual friction and dealt with the classical friction loss and the tortuosity in the porous media. This paper discusses the influence of these additional terms on the heat and mass flow of He II through the porous media at the turbulent Gorter‐Mellink regime.
823(2006); http://dx.doi.org/10.1063/1.2202407View Description Hide Description
Three transitional cryomodules (SL21, FEL03, Renascence) have been constructed as part of an energy upgrade effort at Thomas Jefferson National Accelerator Facility (JLab). Each transitional cryomodule contains eight superconducting radio‐frequency (SRF) cavities. Within the vacuum vessel, waveguides transmit up to 13kW of RF power to the superconducting niobium cavities. The waveguides also provide the thermal transition between the room temperature ceramic RF window and the niobium fundamental power coupler (FPC), a 300K temperature gradient across ∼20cm! The thermal performance of the waveguides is determined in part by the placement of heat stations and bellows. The original 13kW waveguide design incorporated a single 60K heat station and two bellows resulting in a total heat load (static + dynamic) to the FPC of ∼3W per waveguide. To minimize this heat load and stabilize the FPC temperatures, a 2K superfluid helium heat station design was incorporated into the second transitional cryomodule, FEL03, installed in the JLab Free Electron Laser (FEL). The designed heat station is capable of removing up to 1.12W, with a bath temperature of 2.05K, while remaining sub‐lambda. This paper describes the design, analysis and testing of the heat station.
823(2006); http://dx.doi.org/10.1063/1.2202408View Description Hide Description
Experiments were performed on transient heat transfer caused by large stepwise heat inputs to a flat plate heater. The plate is 6 mm‐wide and located in the middle of the inner bottom wall of a horizontal rectangular duct which has 3 mm gap and 100 mm length. When the stepwise heat input is larger than that corresponding to the steady‐state CHF (Critical Heat Flux), the quasi‐steady state exists for a certain lifetime on the extrapolation of steady‐state Kapitza conductance curve. Then the quasi‐steady state rapidly changes to film boiling. The lifetime was systematically measured. The measurements were made for the flow velocities ranging from 0 m/s to 1.5 m/s and for the inlet liquid temperatures from 1.8 K to 2.0 K at atmosphere pressure. The results display the relationship between stepwise heat flux and lifetime. Besides, numerical analysis was performed on two‐dimensional heat and fluid flow in the duct under the same conditions as the experimental ones by using the computer code based on the two fluid model and the theory of mutual friction. It is confirmed that the calculated lifetimes agree well with those measured by experiments. Details of heat transfer mechanisms were clarified by the analysis.
823(2006); http://dx.doi.org/10.1063/1.2202409View Description Hide Description
Transient heat transfer is investigated in forced flow of He II for velocities up to 22 m/s. The flow is generated in a 10 mm ID, 0.85 m long straight test section instrumented with heaters, thermometers and pressure transducers. Rectangular heat pulses are generated in the flow and the temperature is measured at several locations as the pulses are carried by the forced flow with their shape being transformed by counterflow heat transfer. The linear pressure drop in the flow also results in a linear temperature gradient due to the Joule‐Thomson effect and therefore has an influence in the heat transfer process. The effectiveness of the counterflow heat transfer also appears to decrease at the highest flow velocities.
823(2006); http://dx.doi.org/10.1063/1.2202410View Description Hide Description
A small liquid helium test chamber with 1.5 L active volume has been designed and constructed, to make the fundamental measurements of physical properties of electron bubble transports in liquid helium, aimed at developing a new cryogenic neutrino detector, using liquid helium as the detecting medium, for the detection of solar neutrinos. The test chamber is a double‐walled cylindrical container equipped with five optical windows and ten high voltage cables. A LN2/LHe cryostat and a needle valve for vapor helium cooling are used to provide a 1.7∼4.5 K low temperature environments for the test chamber. One of key issues for the cryogenic design and experimental sensitivity of electron bubble tracking is that of keeping a thermally uniform liquid helium bath. The external heat loads to the chamber will generate a buoyancy‐induced convection of liquid helium, which will carry the electron bubbles and accelerate or decelerate their transportation and therefore must be reduced to the minimum, so that the slow motion of the electron bubbles will not be confused by this effect. This paper will present the computational simulation and analysis on thermal convection and uniformity of the test chamber.
823(2006); http://dx.doi.org/10.1063/1.2202411View Description Hide Description
A system for maintaining a sample at a constant temperature below 10 K after deactivating the cooling source is demonstrated. In this system, the cooling source is a 4 K GM cryocooler that is joined with the sample through an extension that consists of a helium pot and a thermal resistance. Upon stopping the cryocooler, the power applied to a heater located on the sample side of the thermal resistance is decreased gradually to maintain an appropriate temperature rise across the thermal resistance as the helium pot warms. The sample temperature is held constant in this manner without the use of solid or liquid cryogens and without mechanically disconnecting the sample from the cooler.
Shutting off the cryocooler significantly reduces sample motion that results from vibration and expansion/contraction of the cold‐head housing. The reduction in motion permits certain procedures that are very sensitive to sample position stability, but are performed with limited duration.
A proof‐of‐concept system was built and operated with the helium pot pressurized to the cryocooler’s charge pressure. A sample with 200 mW of continuous heat dissipation was maintained at 7 K while the cryocooler operated intermittently with a duty cycle of 9.5 minutes off and 20 minutes on.
823(2006); http://dx.doi.org/10.1063/1.2202412View Description Hide Description
In the Fermilab Tevatron cryogenic system there are many remotely located low‐pressure plate relief valves that must vent large volumes of cold helium gas when magnet quenches occur. These valves can occasionally stick open or not reseat completely, resulting in a large helium loss. As such, the need exists for a detector to monitor the relief valve’s discharge area for the presence of helium. Due to the quantity needed, cost is an important factor. A unit has been developed and built for this purpose that is quite inexpensive. Its operating principle is based on the speed of sound, where two closely matched tubes operate at their acoustic resonant frequency. When helium is introduced into one of these tubes, the resulting difference in acoustic time of flight is used to trigger an alarm. At present, there are 39 of these units installed and operating in the Tevatron. They have detected many minor and major helium leaks, and have also been found useful in detecting a rise in the helium background in the enclosed refrigerator buildings. This paper covers the construction, usage and operational experience gained with these units over the last several years.