Parametric performance of circumferentially grooved heat pipes with homogeneous and graded-porosity slab wicks at cryogenic temperatures

A recently developed, potentially high-performance nonarterial wick has been extensively tested. This slab wick has an axially varying porosity which can be tailored to match the local stress imposed on the wick. The purpose of the tests was to establish the usefulness of the graded-porosity slab wick at cryogenic temperatures between 110 K and 260 K, with methane and ethane as working fluids. For comparison, a homogeneous (i.e., uniform porosity) slab wick was also tested. The tests included: (1) maximum heat pipe performance as a function of fluid inventory, (2) maximum performance as a function of operating temperature, (3) maximum performance as a function of evaporator elevation, and (4) influence of slab wick orientation on performance. The experimental data was compared with theoretical predictions obtained with the computer program GRADE

Analysis of a Rotating Spool Expander for Organic Rankine Cycles in Heat Recovery Applications

The increasing cost of energy, coupled with the recent drive for energy security and climate change mitigation have provided the impetus for harnessing renewable energy sources as viable alternatives to conventional fossil fuels. Furthermore, recovering heat that is discharged from power plants, automobiles and various other industrial processes is of growing interest. Nevertheless, technologies attempting to provide small-scale heat recovery solutions have seen very limited commercialization. This is broadly due to two reasons: lack of historical research and development in the area of waste-heat recovery and small-scale power generation due to technical and cost impediments; and technical challenges associated with scaling the technology from utility-scale to commercial-scale, particularly with regard to expansion machines (turbines). However, due to rising primary energy costs and the environmental premium being placed on fossil fuels, the conversion from low-grade heat to electrical energy as well as small-scale distributed power generation is of increasing interest. In this regard, this project focuses on a novel rotating spool expansion machine at the heart of an Organic Rankine Cycle (ORC), which in turn is used as a heat recovery system. A comprehensive simulation model of the rotating spool expander is presented. The spool expander provides a new rotating expansion mechanism with easily manufactured components. Apart from efficiency improvements compared with other rotary machines, the spool expander also has the ability to control the expansion ratio using a novel mechanically-driven suction valve mechanism. Another advantage is the relocation of the face sealing surfaces to the outer radius of the device. The spool expander is also scalable to a size range (50-200 kW) that is too large for conventional positive displacement machines, and too small for dynamic machines with respect to manufacturability, efficiency and cost. A detailed analytical geometry model of the spool expander and the suction valve mechanism is presented. This geometry model forms a part of a comprehensive model that includes submodels for friction, leakage, and heat transfer. The results of the comprehensive model are validated using experimental data from a 50 kW prototype expander in an ORC system. Given the promise of the technology, this paper explores the design space using both a simulation based approach as well as an experimental prototype for concept validation

Pilot Performance with Advance Sensor Technlogies Considerations

Research on human performance indicates people may discretely shift modes as the difficulty in tasks changes. These modes are referred to as “cognitive control modes.” Cognitive control modes are ways people operate and handle their process of thinking during a series of tasks. However, past work has been confined to subjective reports of these mode changes - objective markers in data of cognitive control modes, which should appear if these mode changes are truly discrete, have not be identified. This work will attempt to identify markers of cognitive control modes in data collected on pilots flying instrument approaches. Specifically, a simulated flight experiment is being conducted in which control movement, aircraft state, and eye movement data is being collected. If there are markers of discrete cognitive control mode changes, it should appear in one or more of these sources of data. Finding markers of cognitive control mode changes would provide future research with objective evidence rather than subjective reports on such changes. Being able to rely on objective evidence, rather than subjective evidence, is crucial due to reliability and experimental issues with subjective reports

Sensitivity Analysis of a Comprehensive Model for a Miniature-Scale Linear Compressor for Electronics Cooling

A comprehensive model of a linear compressor for electronics cooling was previously presented by Bradshaw et al. (2011). The current study expands upon this work by first developing methods for predicting the resonant frequency of a linear compressor and for controlling its piston stroke. Key parameters governing compressor performance – leakage gap, eccentricity, and piston geometry – are explored using a sensitivity analysis. It is demonstrated that for optimum performance, the leakage gap and frictional parameters should be minimized. In addition, the ratio of piston stroke to diameter should not exceed a value of one to minimize friction and leakage losses, but should be large enough to preclude the need for an oversized motor. An improved linear compressor design is proposed for an electronics cooling application, with a predicted cooling capacity of 200 W a cylindrical compressor package size of diameter 50.3 mm and length 102 mm

A Comprehensive Model of a Miniature-Scale Linear Compressor for Electronics Cooling

A comprehensive model of a miniature-scale linear compressor for electronics cooling is presented. Linear compressors are appealing for refrigeration applications in electronics cooling. A small number of moving components translates to less theoretical frictional losses and the possibility that this technology could scale to smaller physical sizes better than conventional compressors. The model developed here incorporates all of the major components of the linear compressor including dynamics associated with the piston motion. The results of the compressor model were validated using experimental data from a prototype linear compressor. The prototype compressor has an overall displacement of approximately 3cm3 ,an average stroke of 0.6 cm. The prototype compressor was custom built for this work andutilizes custom parts with the exception of the mechanical springs and the linear motor. The model results showed good agreement when validated against the experimental results. The piston stroke is predicted within 1.3% MAE. The volumetric and overall isentropic effciencies are predicted within 24% and 31%, MAE respectively

Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity

Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of "medium purity" Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800 - 1400 C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2x1010 at 2.0 K and peak surface magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of (5+-1)1010 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 C. This is the highest value ever reported at this temperature, frequency and field. Samples heat treated with the cavity at 1400 C were analyzed by secondary ion mass spectrometry, secondary electron microscopy, energy dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a complex surface composition which includes titanium oxide, increased carbon and nitrogen content but reduced hydrogen concentration compared to a non heat-treated sample

Loss Analysis of Rotating Spool Compressor Based on High-Speed Pressure Measurements

A rotating spool compressor is a novel compressor technology that was recently introduced by Kemp et al. (2008). To accelerate the development of the technology, a breakdown of the key losses within the device is presented. The losses include flow losses associated with leakage and over/under compression due to valves and porting. Additionally, frictional losses associated with the key sealing elements and moving components are calculated. All of these losses are combined into Pareto of losses for the spool compressor. This Pareto identifies the dynamic sealing elements as key components to continue development on to achieve the best improvement in efficiency

Updated Performance and Operating Characteristics of a Novel Rotating Spool Compressor

The basic mechanism of the novel rotary spool compressor has been described previously by Kemp et al. (2008, 2010). The device combines various aspects of rotary and reciprocating devices that are currently well understood.  Due to increasing pressure in the global market for refrigerants with very low GWP levels extensive modeling was conducted to explore a spool compressor design for operation on medium pressure low GWP refrigerants in sizes applicable to the commercial air conditioning marker, specifically for application on air and water cooled chillers. The basis for a compressor design in this space is operation on R134a realizing that most low-GWP medium pressure gases are similar in nature to R134a from a compressor design point of view. A new compressor design with specific aspect ratio optimized for operation on R134a has been constructed and tested. The compressor was tested at a range of conditions suitable for evaluation in this application. The current compressor performance is comparable with today’s screw compressors operating in this size range

Influence of Volumetric Displacement and Aspect Ratio on the Performance Metrics of the Rotating Spool Compressor

A theoretical study of the influence of scaling is presented for the rotating spool compressor. This study uses the previously developed comprehensive compressor model for the rotating spool compressor developed by Bradshaw and Groll (2013). Using this model a study of the influence of the compressor volumetric displacement is performed. This study relies on a set of scaling rules to determine the size of the compressor features as the compressor displacement changes. The study finds that as the volumetric displacement increases, the volumetric efficiency asymptotically increases. It is also found that there is an optimum in overall isentropic efficiency as the volumetric displacement increases which suggests a trade-off between sealing and port restrictions. The compressor aspect ratio (axial length to bore diameter) is varied between roughly 0.2 and 3.5 at eccentricity ratios (rotor diameter to bore diameter) of 0.825, 0.85, 0.893, and 0.92. It is found that for a given eccentricity ratio, there exists an optimum aspect ratio that maximizes sealing. Additionally, the eccentricity ratio shows a high level of sensitivity to the overall performance of the compressor. As the eccentricity ratio decreases, the overall isentropic efficiency of the compressor increases until an eccentricity ratio of 0.85. Below an eccentricity ratio of 0.85 the overall isentropic efficiency does not increase despite an increase in volumetric efficiency