John M. Pfotenhauer

Inertance Tube Optimization for Pulse Tube Refrigerators

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. Fo...

Effectiveness-NTU relationship for a counterflow heat exchanger subjected to an external heat transfer

This paper presents the analytical solution for the effectiveness of a counterflow heat exchanger subjected to a uniformly distributed, external heat flux. The solution is verified against conventional e-NTU relations in the limit of zero external heat flux. This situation is of interest in applications such as cryogenic and process engineering, and the analytical solution provides a convenient method for treating differential elements of a heat exchanger in a numerical model.

Double-sided IPEM cooling using miniature heat pipes

Integrated power electronic module (IPEM) planar interconnect technologies offer opportunities for improved thermal management by allowing thermal access to the upper side of the power devices. In this paper, the feasibility of using miniature heat pipes to achieve effective double-sided cooling is investigated by analyzing the complete thermal circuit associated with the power device. A nominal case was modeled using the ANSYS(tm) finite element software in a single-sided and double-sided configuration. The numerical model predicted that the double-sided configuration would result in a 13/spl deg/C reduction in the maximum temperature compared to the single-sided case, for the same 100 W/cm/sup 2/ power dissipation in the semiconductor die. This corresponds to a 15% decrease in the maximum temperature rise relative to ambient or a similar increase in allowable power dissipation. Twenty-eight percent of the heat was removed from the upper side of the IPEM in the double-sided case. An additional benefit associated with double-sided cooling was a significant reduction in the spatial temperature gradients along the surface of the IPEM which would translate to lower thermally induced stress and higher reliability. The sensitivity of the numerical predictions to important parameters; including the dielectric conductivity, contact conductance, and heat sink characteristics are numerically investigated. An experimental fixture was fabricated and used to measure a miniature rectangular heat pipes performance characteristics and the solder joint resistance at its evaporator and condenser interfaces in order to validate the numerical model inputs and demonstrate the required heat pipe capacity. The tested heat pipe was limited to approximately 80 W/cm/sup 2/ heat flux in a vertical, evaporator-over-condenser orientation. This limit was not observed in a vertical, gravity-assisted orientation for applied heat flux up to 125 W/cm/sup 2/. Equivalent heat pipe resistances of approximately 0.12 and 0.08 K/W were measured in these orientations, respectively. The contact resistance of the indium solder joint was measured and found to be approximately 0.1 cm/sup 2//spl middot/K/W.

Quench dynamics in silver coated YBCO tapes

The role of silver in the quench dynamics of RABiTS-processed YBCO tapes was examined. The voltage distribution along the 20 cm long YBCO tapes with silver thickness between two and eight micrometers was measured when different transport current pulses were applied. Measurements on each sample were performed in a conduction-cooling environment at approximately 50 K. After normal regions were induced in the sample by short over-current pulses, an operating current was applied to monitor the sample recovery or quench. When scaled to the lowest critical current, a thermal runaway current was identified and found to increase with increasing silver thickness. A simple one-dimensional model of the system supported this trend. [This paper is also published in Advances in Cryogenic Engineering Volume 47A, AIP Conference Proceedings Volume 613, pp. 449–456.]

Quench tests of a 20-cm-long RABiTS YBCO tape

A 20-cm-long YBCO tape fabricated by the Rolling Assisted Bi-axially Textured Substrate (RABiTS) technique was used to study the quench behavior of a second-generation high-temperature superconductor (HTS). Quench tests of the sample were performed in a conduction cooling condition with a cryocooler. The sample was subjected to a short overcurrent pulse to create normal zones, followed by a longer operating current up to the critical currents of the weaker zones of the tape. Distinctive normal zone propagation was observed for the first time on a YBCO tape. Propagation velocities of 1.4 to 8.3 mm/s were measured, which increase linearly with the current. Stability margins and a minimum propagation current were also observed. One-dimensional thermal modeling showed good agreement with the test results. [This paper is also published in Advances in Cryogenic Engineering Vol. 47A, AIP Conference Proceedings Vol. 613, pp. 457–464.]

A helium based pulsating heat pipe for superconducting magnets

Working Paper Department of Mechanical Engineering, University of Wisconsin - Madison, Estados Unidos

Voltage detection and magnet protection

Voltage detection is routinely used to identify resistive regions within superconducting magnets so that a protection circuit can trigger a safe magnet discharge. Nonresistive voltage signals, for example inductive voltages, can, however, produce false signals, causing the magnet protection system to trip prematurely. An experimental verification is given of a quench detection method which eliminates sensitivity to inductive voltages in multiple magnet systems. In addition, a specific application of these ideas for the University of Wisconsin proof-of-principle experiment demonstrated the need for incorporating a microprocessor in the quench detection system. The shortcomings of the traditional quench detection techniques were demonstrated explicitly.<<ETX>>

Optimization of the Intercept Temperature for High Temperature Superconducting Current Lead

Current leads containing high temperature superconducting (HTS) components have the potential to greatly reduce the heat leak to liquid helium environment. However, significant heat leak reduction can only be achieved by intercepting heat at the joint between the upper (copper) and the lower (high temperature superconducting) sections of current leads. An optimization analysis has been performed to minimize the net room temperature refrigeration power required for operating HTS current leads. The analysis shows that competing factors of cryocooler efficiencies and the influence of HTS material properties on the required helium liquefaction power determine an optimum intercept temperature in the range between 70 and 80 K.

Thermodynamic analysis of active valve pulse tube refrigerators

This paper presents a detailed thermodynamic analysis of the working process of an active valve pulse tube refrigerator which is introduced to enhance the performance of a single stage pulse tube refrigerator. With the aid of several simplifying assumptions, the theoretical refrigeration power and coefficient of performance of this kind of refrigerator are obtained. Simulation results illustrate that the values of two intermediate pressures play a significant role in the system performance.

A Single Stage Five Valve Pulse Tube Refrigerator Reaching 32 K

In order to enhance the performance of a single stage pulse tube refrigerator, a new G-M type pulse tube refrigerator with five valves is introduced in this paper. In this novel configuration, five valves with different flow coefficients are used to connect the compressor, regenerator, pulse tube and reservoir. The compression and expansion processes inside the pulse tube are realized by carefully controlling the opening and closing time of the five valves. The system is optimized by changing two intermediate pressures during the compression and expansion processes. A detailed description of the operating procedure and test results of a prototype five-valve pulse tube is given in the paper. Additionally, a simplified thermodynamic simulation is presented and compared with test results. Measurements of the refrigeration power and minimum temperature indicate that the performance of the new configuration is better than that of a standard two-valve double inlet configuration. A cooling capacity of 8 watts at 60 K is achieved with an input power of 1 kW, equivalent to an efficiency of 3.2% of Carnot. A minimum temperature of 32 K was reached with an operating frequency of 1.75 Hz.