Eric N. Schmierer

Self-Consistent Open-Celled Metal Foam Model for Thermal Applications

Many engineering applications require thermal cycling of granular materials. Since these materials generally have poor effective thermal conductivity various techniques have been proposed to improve bed thermal transport. These include insertion of metal foam with the granular material residing in the interstitial space. The use of metal foam introduces a parasitic thermal capacitance, disrupts packing, and reduces the amount of active material. In order to optimize the combined high porosity metal foam-granular material matrix and study local thermal nonequilibrium, multiple energy equations are required. The interfacial conductance coefficients, specific interface area, and the effective thermal conductivities of the individual components, which are required for a multiple energy equation analysis, are functions of the foam geometry. An ideal three-dimensional geometric model of open-celled Duocell® foam is proposed. Computed tomography is used to acquire foam cell and ligament diameter distribution, ligament shape, and specific surface area for a range of foam parameters to address various shortcomings in the literature. These data are used to evaluate the geometric self-consistency of the proposed geometric model with respect to the intensive and extensive geometry parameters. Experimental thermal conductivity data for the same foam samples are acquired and are used to validate finite element analysis results of the proposed geometric model. A simple relation between density and thermal conductivity ratio is derived using the results. The foam samples tested exhibit a higher dependence on relative density and less dependence on interstitial fluid than data in the literature. The proposed metal foam geometric model is shown to be self-consistent with respect to both its geometric and thermal properties.

Development of the SCRF power coupler for the APT accelerator

The team responsible for the design of the Accelerator Production of Tritium (APT) superconducting (SC) radio frequency (RF) power coupler has developed two 700-MHz, helium gas-cooled power couplers. One has a fixed inner conductor and the other has an adjustable inner conductor (gamma prototype and alpha prototype). The power couplers will be performance tested in the near future. This paper discusses the mechanical design and fabrication techniques employed in the development of each power coupler. This includes material selection, copper coating, assembly sequences, and metal joining procedures, as well as the engineering analyses performed to determine the dynamic response of the inner conductors due to environmental excitations. A bellows is used in both prototype inner conductors in the area near the ceramic RF window, to compensate for thermal expansion and mechanical tolerance build-up. In addition, a bellows is used near the tip of the inner conductor of the alpha prototype for tuning the power coupler after it is installed on the accelerator. Extensive analytical work has been performed to determine the static loads transmitted by the bellows due to thermally induced expansion on the inner conductor and on the RF window. This paper also discusses this analysis, as well as the mechanical analysis performed to determine the final geometric shape of the bellows. Finally, a discussion of the electromagnetic analysis used to optimize the performance of the power couplers is included.

Results of the APT RF power coupler development for superconducting linacs

For the new baseline APT (Accelerator Production of Tritium) linac design, the power couplers are required to transmit 420 kW of CW RF power to the superconducting cavities at 700 MHz. These couplers consist of an airside waveguide-to-coax transition, an air/vacuum break made by two planar, coaxial windows, and a vacuum-side coaxial antenna section. The coaxial antenna allows adjustability of the RF matching to the superconducting cavities. Design, fabrication, and testing of the power coupler/window occurred over the last four years, and room temperature testing of the prototype design is complete. Coupler/window assemblies have transmitted power to 1 MW, CW and have handled full reflected 850 kW, CW over a limited standing-wave phase range. Couplers were tested with a portion of the outer conductor cooled by liquid nitrogen to study the effects of condensed gases. Final results, conclusions, and lessons learned about the coupler design, fabrication, and testing are discussed.

Space-based FPGA radio receiver design, debug, and development of a radiation-tolerant computing system

Los Alamos has recently completed the latest in a series of Reconfigurable Software Radios, which incorporates several key innovations in both hardware design and algorithms. Due to our focus on satellite applications, each design must extract the best size, weight, and power performance possible from the ensemble of Commodity Off-the-Shelf (COTS) parts available at the time of design. A large component of our work lies in determining if a given part will survive in space and how it will fail under various space radiation conditions. Using two Xilinx Virtex 4 FPGAs, we have achieved 1 TeraOps/second signal processing on a 1920 Megabit/second datastream. This processing capability enables very advanced algorithms such as our wideband RF compression scheme to operate at the source, allowing bandwidth-constrained applications to deliver previously unattainable performance. This paper will discuss the design of the payload, making electronics survivable in the radiation of space, and techniques for debug.

Characterization of High Porosity Open-Celled Metal Foam Using Computed Tomography

High porosity metal foams have been the subject of many investigations for use in heat transfer enhancement through increased effective thermal conductivity and surface area. Convection heat transfer applications with these foams have been investigated for a large range of Reynolds numbers. Common to these analyses is the need for quantitative information about the interfacial surface area and the effective thermal conductivity of the metal foam. The effective thermal conductivity of these metal foams have been well characterized, however little investigation has been made into the actual surface area of the foam and its dependence on the foam pore density and porosity. Three-dimensional x-ray computed tomography (CT) is used for determining interfacial surface area and ligament diameter of metal foam with porosities ranging from 0.85 to 0.97 and pore densities of 5, 10, 20, and 40 pores per inch. Calibration samples with known surface area and volume are utilized to benchmark the CT process. Foam results are compared to analytical results obtained from the development of a three-dimensional model of the high porosity open-celled foam. The results obtained are compared to results from previous investigations into these geometric parameters. Results from calibration sample comparison and analysis of the foam indicate the need for additional work in quantifying the repeatability and sources of error in CT measurement process.Copyright

FLOW OF SATURATED LIQUID NITROGEN THROUGH MICRO-SCALE ORIFICES

The flow of saturated liquid nitrogen through micro-scale orifices has been characterized experimentally. Measurements of pressure drop and flow rate were made with liquid nitrogen flowing through orifices ranging in diameter from 50 micron to 370 micron, with orifice length-to-diameter ratios ranging from 1.5 to 10. The design of the experimental apparatus, the instrumentation used, and the experimental uncertainties are presented. Obstacles encountered while attempting to obtain repeatable and reliable results at cryogenic temperatures are discussed. Finally, experimentally measured discharge coefficients are shown to agree with a model for single-phase liquid flow through micro-orifice tubes.

Apparatus for Testing Rotating Heat Pipes

*† ‡ A test apparatus, that will be used to study the heat transfer performance of rotating heat pipes, has been designed and built. The apparatus allows for simultaneous testing of a pair of crank-shaped rotating heat pipes operating near room temperature. The test rig is designed to support heat pipes with an on-axis rotating condenser section, an off-axis eccentrically rotating evaporator section, and a curved adiabatic section. Due to the length of the heat pipes (55”), the distance from the axis of rotation to the off-axis evaporator section (9.5”), and the maximum rotation speed (5500 rpm), care had to be taken in the design of a substantial support structure for the heat pipes, the selection of a drive system, and the design of the mounting frame. These design issues, as well as safety considerations associated with the test apparatus, are discussed here. Preliminary test data for stationary and low-speed tests are also presented.

Nonlinear Modeling for Adaptive Suppression of Axial Drilling Vibration

Vibrations developed during drilling present challenges in an array of industries including mechanical, medical, structural, and oil extraction. Velocity weakening, intracranial vibrations, large amplitude standing pressure waves in material cavities, and failure of drill strings are prominent issues among these fields. Stick-slip (torsional) and bit-bounce (axial) vibrations have been found to be particularly problematic in precision drilling jobs such as machining to tight tolerances, dismantling vibration-sensitive devices, and surgical work. Current technologies to detect and suppress systematic vibrations have several shortcomings including malfunctioning, complete failure, complexity, and high power consumption. This paper proposes a method to suppress vibrations of drilling material surfaces using adaptive positive position feedback (APPF) control for efficient tunable damping. An experiment-based parametric study has been conducted to determine the relationship of force, rotational velocity, and acceleration on both drill vibrations and drilling material surface vibrations. Results of a parametric study and Rational Polynomial Fraction method are used to estimate fundamental behaviors of the drilling system to create a refined numerical model for simulating the drilling process. An APPF controller together with the model provided a method to evaluate new actuator designs for vibration suppression and has shown a 69.8 % reduction of displacement vibrations.

High Efficiency Space-Based Software Radio Architectures: A Minimum Size, Weight, and Power TeraOps Processor

Los Alamos has recently completed the latest in a series of Reconfigurable Software Radios, which incorporates several key innovations in both hardware design and algorithms. Due to our focus on satellite applications, each design must extract the best size, weight, and power performance possible from the ensemble of Commodity Off-the-Shelf (COTS) parts available at the time of design. In this case we have achieved 1 TeraOps/second signal processing on a 1920 Megabit/second datastream, while using only 53 Watts mains power, 5.5 kg, and 3 liters. This processing capability enables very advanced algorithms such as our wideband RF compression scheme to operate remotely, allowing network bandwidth constrained applications to deliver previously unattainable performance.

A ROTATING HEAT PIPE FOR COOLING OF SUPERCONDUCTING MACHINES

A curved rotating heat pipe for use in superconducting motor and generator applications is introduced here. The heat pipe shown here is built so that both the condenser and evaporator sections are parallel to the axis of rotation. The condenser section is concentric with the axis of rotation while the evaporator section can be placed in contact with off-axis heat sources in the rotating machine. The geometry is achieved by incorporating an S-shaped curve between the on-axis rotating condenser section and the off-axis revolving evaporator section. We show that because the heat pipe is a sealed, passive heat transfer device with nearly isothermal operation, the heat pipe concept may be advantageous when considering the overall refrigeration system used with the superconducting machine. High-speed, room temperature test data with this heat pipe geometry indicate that the working fluid in the heat pipe continued to circulate, resulting in heat transfer with a high effective thermal conductivity, with the heat...