A novel linear motor for a linear refrigeration compressor: modelling, measurement and sensor-less stroke detection

With the increasing global awareness of the environmental conservation, linear compressors have attracted growing attention with their applications in domestic and cryogenic refrigeration systems. A linear compressor is driven directly by a linear motor and the free-piston design allows piston stroke to be variable. An active control of stroke prevents piston-cylinder collision and enables efficient cooling capacity modulation. This thesis introduces the performance of a novel moving magnet type linear compressor/motor and investigates the approaches to sensor-less stroke detection. An experimental test facility incorporating the linear compressors into a vapour compression refrigeration system was introduced, in which piston displacement was measured with a displacement sensor. The piston stroke and offset were controlled with PID controllers implemented in LabVIEW. To investigate the characteristics of the moving magnet linear motor, a finite element analysis (FEA) model was built in ANSYS Maxwell 19.2. Simulations were validated through static force measurements. Force constant was given by the static shaft force against current. Saturation can be observed with the increase of current. A smaller saturation current was shown for a larger armature displacement. For the purpose of increasing cooling capacity of the linear compressor, operations with small axial clearance volumes were considered. Refrigeration performance using R1234yf as refrigerant with various clearance volumes and with an offset of 0 mm were experimentally compared. The cooling capacity for a pressure ratio of 2.5 and a stroke of 13 mm increases by 12% as the clearance decreases from 1.07 mm to 0.4 mm. Piston stroke detection without a displacement sensor reduces the cost and facilitates the stroke control especially in miniature linear compressors. An artificial neural network (ANN) based stroke detection was presented. Fast Fourier transform (FFT) analysis was performed on current and voltage signals to extract harmonic terms as inputs of the neural network model to predict the stroke. The ANN technique can achieve a good accuracy for most of the cases, but reliability remains a problem. A more reliable sensor-less stroke detection technique based on flux linkage variation using inductive coils was proposed. The technique requires resonant operation. A 1D (One-Dimensional) electromagnetic model and a 3D (Three-Dimensional) FEA model were built to compute the flux linkage variations. The open-circuit flux linkage in each core produced by NdFebB magnets varies linearly with the piston displacement. Flux linkage difference at two zero-crossing points of current was used to infer stroke. The proposed low-cost sensor-less stroke detection technique can achieve error of only 4%. The adoption of this novel technique is crucial to the commercialization of free-piston machines for high efficiency

Comparisons between heat pipe, thermoelectric system, and vapour compression refrigeration system for electronics cooling

Passive systems such as air for electronics cooling have now effectively reached their limits. This paper evaluated three comparable systems for electronics cooling, including heat pipe (HP, passive system), thermoelectric (TE) and vapour compression refrigeration (VCR) systems (active systems). Mathematical model has been built for the heat pipe and the thermoelectric system respectively. Measurements have been conducted to validate the model and to compare the performance among a HP, a single stage TE system and a two-stage TE system, a combination of the HP and the TE system, and a VCR system using an oil-free linear compressor. Close agreements between the modelling and measurements have been achieved in terms of electric power input and cooling capacity at various temperatures. The HP improved the cooling capacity and the coefficient of performance (COP) of the TE system by 53% and 42% respectively at a cold end temperature of 10 °C. Heat pipe is more attractive for cooling large devices at higher temperatures. Two-stage TE system can be used for cooling devices at lower temperatures. VCR system is capable of dissipating much higher heat flux (200 W/cm2 ) at lower temperature than all other technologies

Perpendicular magnetic anisotropy in as-deposited CoFeB/MgO thin films

Fabrication of perpendicularly magnetized ferromagnetic films on various buffer layers, especially on numerous newly discovered spin-orbit torque (SOT) materials to construct energy-efficient spin-orbitronic devices, is a long-standing challenge. Even for the widely used CoFeB/MgO structures, perpendicular magnetic anisotropy (PMA) can only be established on limited buffer layers through post-annealing above 300 {\deg}C. Here, we report that the PMA of CoFeB/MgO films can be established reliably on various buffer layers in the absence of post-annealing. Further results show that precise control of MgO thickness, which determines oxygen diffusion in the underneath CoFeB layer, is the key to obtaining the as-deposited PMA. Interestingly, contrary to previous understanding, post-annealing does not influence the well-established as-deposited PMA significantly but indeed enhances unsaturated PMA with a thick MgO layer by modulating oxygen distributions, rather than crystallinity or Co- and Fe-O bonding. Moreover, our results indicate that oxygen diffusion also plays a critical role in the PMA degradation at high temperature. These results provide a practical approach to build spin-orbitronic devices based on various high-efficient SOT materials.Comment: 15 pages, 4 figure

Micelle-like particles formed by carboxylic acid-terminated polystyrene and poly(4-vinyl pyridine) in chloroform/methanol mixed solution

Abstract Preparation and characterization of isolated particles and clusters of carboxylic acid-terminated polystyrene (CPS) and poly(4-vinyl pyridine) (P4VP) blends in chloroform/methanol (9/1, v/v) solution are reported. In chloroform CPS-P4VP blends form graft-like structure due to the interaction between terminal carboxylic acid and 4VP units. Upon addition of methanol, isolated particles and clusters with micelle-like structure were prepared. Dynamic light scattering (DLS) results and transmission electron microscopy (TEM) images both suggest the existence of isolated particles and clusters with micelle-like structure in the mixed solution. The effects of weight ratio of CPS to P4VP and addition of copper ion on the size of isolated particles and clusters were also investigated.

Experimental study of R1234yf as a drop-in replacement for R134a in an oil-free refrigeration system

R1234yf is a synthetic refrigerant with global warming potential (GWP) of 4 and similar thermodynamic properties to R134a. This paper experimentally compares the performance of R1234yf with R134a in an oil-free vapour compression refrigeration (VCR) system. The oil-free VCR system consisting of oil-free linear compressors, an off-the-shelf condenser and an evaporator with an electric heater avoids the impact of oil lubricant on the heat transfer so that the two refrigerants can be appropriately compared with each other. Experiments for two refrigerants were carried out for compressor strokes of 9-13 mm, operating frequency of 32-38 Hz, pressure ratios of 2- 4, and condenser temperatures of 40-50 °C with refrigerant charge of 250 g. The experimental results show that the coefficient of performance (CoP) of R1234yf is 20% lower than R134a with condenser temperature of 40 °C and evaporator temperature of 0 °C. The volumetric efficiency of R1234yf is 5% lower than R134a with condenser temperature of 40 °C and evaporator temperature of -1.5 °C. Results of evaporator pressure drop, superheat, power input, and cooling capacity are also reported

Moving from contractor to owner operator: Impact on safety culture; a case study

Purpose – The purpose of this paper is to investigate whether a change in staffing contractual arrangements, specific training in hazard identification, mentoring of supervisors and the introduction of a robust safety system could improve an organisation\u27s safety culture. How safety conditions change under contracted out labour compared to direct labour and the influence that contracting out has on organisational safety culture is explored. Design/methodology/approach – The study used a case study methodology to detail how the change occurred over a six month period in 2011. As part of the analysis a model of the change process and push-pull factors is offered. Findings – As a result of the change, all areas saw some improvement. Work-related injury statistics dropped significantly, supervisors were clear of their roles, actively monitoring their crews to ensure they worked in a safer manner than before, and staff were actively addressing work-place hazards. With the safety system in place the organisation should be deemed compliant and diligent by the state auditing authorities. This study has also shown that using contractor workers together with in-house workers that are managed under different safety regimes is problematic. The problems don’t occur due to the contractor\u27s safety systems being less robust than the parent company\u27s or that contract workers are themselves less safe; it is the added complexity of managing multiple safety regimes and the lack of trust of the robustness of each system that create conflict. Research limitations/implications – The paper reports on the change process of one mining organisation in Western Australia as a case study from a managerial sample and is thereby limited. Practical implications – This study demonstrates the difficulties in changing safety culture in an underground mining organisation. The paper argues the need for specialised training in identifying hazards by the staff, the mentoring of supervisory staff and the adoption of a robust safety system to support improved safety culture. Originality/value – There is little research conducted in the resources sector researching changes in human resource supply and OHS management, in particular moving from contracted labour to hiring in-house. This case provides an insight into how a change in staffing hiring arrangements, together with specific safety initiatives, has a positive impact on safety performance

Does Your Zip Code Decide Your Vote? Moving to Opportunity and Its Impact on Long-term Political Participation

Economic inequality is distinctly linked to political inequality. The American political system is meant to provide an avenue to hold elected officials accountable, but this assumption is undermined by the fact that poor communities are less likely to participate in the political process. The literature suggests that exposure to disadvantaged neighborhoods tend to decrease political participation. What happens when people are given the chance to move out of disadvantaged neighborhoods? Drawing on data from the Moving to Opportunity for Fair Housing Demonstration (MTO)—a federal program that provided housing vouchers for economically marginalized families in five cities between 1994 and 1998—I test whether moving to low-poverty neighborhoods affected the long-term political behavior of participants, with a focus on youth voter turnout. Contrary to my expectation, I found no evidence that moving to low-poverty neighborhoods improved the voter turnout of those who were children at random assignment. Additional tests reveal that the null effect may be explained by the lack of substantial improvements in long-term outcomes that correlate with higher political engagement. My research illustrates how moving into lower-poverty neighborhoods affects long-term political engagement, a topic that is understudied within the field of political science. In addition, my findings have practical implications on the implementation of housing voucher programs

Characteristics of a novel moving magnet linear motor for linear compressor

Linear compressors consisting of linear motor and suspension spring have been widely used for cryogenic and refrigeration systems. This paper characterises a novel moving magnet linear motor for linear compressor including magnet spring, motor force, resonant frequency and overall performance. First, the static force of the linear motor was modelled using FEA (Finite Element Analysis) method for various excitation current and armature positions. A specific test rig was built to measure the motor force and to verify the results from FEA simulation. The magnet spring was found to be 1.3 N/mm for peak stroke. The magnet spring effect was taken account into the calculation of the resonant frequency, which mainly depends on mechanical spring and gas spring. The linear compressor using the proposed linear motor and flexure springs was instrumented for measurements of the resonant frequency at different pressure ratios and strokes. Good agreement was shown between the calculated and measured resonant frequencies for various conditions. The high efficiency novel magnet linear motor is very attractive for linear compressor

Single well injection withdraw (SWIW) - based tracer test approach for in-situ permeability estimation in an enhanced geothermal system

Efficient estimation of near-wellbore permeability is critical for evaluating fracturing effect and updating fracturing plan. However, due to technical or cost constraints, there is still a lack of methods for in-situ testing and estimating near-wellbore permeability in deep geothermal reservoirs. Considering that the hydraulic fracturing is often associated with injection breaking and back drainage to control the reservoir pressure, this study proposes a single-well-injection-withdraw-based tracer test approach and two permeability interpretation methods based on numerical and analytical solutions, which allow in-situ permeability estimation at low cost. Implementation of the proposed method at a realistic enhanced geothermal system indicates that the numerical interpretation method can still reasonably estimate near-wellbore permeability under the condition of incomplete tracer breakthrough curve in the single well injection and withdraw test, but the computational efficiency is low. Once the tracer breakout curve is relatively complete (i.e. the peak tracer concentration is monitored), the analytical method can be used to quickly estimate the permeability. However, the analytical method cannot accurately consider the long-distance tracer migration process inside wellbore and the influence of dispersion on the tracer breakthrough curve, hence the accuracy is relatively low. The numerical and analytical permeability estimations are at the same order of magnitude. The results suggest that in addition to the numerical method, the analytical method can still be used as an effective method for in-situ rapid permeability estimation. The proposed methodology may provide a new tool for in-situ permeability estimation in deep geothermal reservoirs

Multiple positive solutions for fourth-order three-point p-Laplacian boundary-value problems

In this paper, we study the three-point boundary-value problem for a fourth-order one-dimensional $p$-Laplacian differential equation $$ig(phi_p(u''(t))ig)''+ a(t)fig(u(t)ig)=0, quad tin (0,1),$$ subject to the nonlinear boundary conditions: $$displaylines{ u(0)=xi u(1),quad u'(1)=eta u'(0),cr (phi _{p}(u''(0))' =alpha _{1}(phi _{p}(u''(delta))', quad u''(1)=sqrt[p-1]{eta _{1}}u''(delta), }$$ where $phi_{p}(s)=|s|^{p-2}s$, $p>1$. Using the five functional fixed point theorem due to Avery, we obtain sufficient conditions for the existence of at least three positive solutions