MINIATURE PULSE TUBE FOR THE COOLING OF ELECTRONIC DEVICES: FUNCTIONING PRINCIPLES AND PRACTICAL MODELING

The miniaturization of refrigerating systems represents a very current scientific and technical challenge to improve the performances of numerous electronic components. This work presents a global approach to the problem and suggests studying the cooling by means of small channels filled with an oscillating gas: the double inlet pulse tube refrigerator (DIPTR). A great level of miniaturization based on the technology of carving silicon is exposed. This study proposes to apply an electric analogy for modeling both hydrodynamic and thermal phenomena. Considering the complexity of the theoretical problem including mechanical, thermal, thermodynamical, and acoustic considerations, the authors take care to summarize the main governing equations in a particular form so any scientific engineer could understand the DIPTR principle.     

Effets thermoacoustiques dans un régénérateur cylindrique contenant un empilement de billesThermoacoustical effects in a cylindrical regenerator filled with a stack of beads

Linear thermoacoustic theory is applied to study the regenerator of a Stirling machine or a Pulsed gas refrigerator regenerator filled with a stack of beds in a cylindrical envelope. Pressure, velocity and temperature amplitude are calculated, and a formulation of the thermal time constant of the regenerator is proposed. A global equivalent electrical scheme of the regenerator is also deduced. The total axial energy flux expression is demonstrated and the influence of the thermal time constant and of the various impedance is discussed. Axial variations of the energy flux, which correspond to thermal exchanges with the surroundings are expressed through consideration of thermoacoustical phenomena: thermal relaxation of the acoustical energy, viscous losses, thermoacoustical source/sink. Theoretical results are analysed to comment on temperature profiles along the regenerator in a periodical regime.     

Comparison of two models of a double inlet miniature pulse tube refrigerator: Part B electrical analogy

The design of a double inlet pulse tube refrigerator is investigated by means of an analogy with an electric circuit. The results obtained are compared with both those of the thermodynamic model (Part A) and experiments. The basic formulation of equivalent electronic components is discussed and a few improvements are proposed for adjusting the theoretical expressions of the electric impedance concerning the capillaries and the regenerator. Then additional effects such as pressure drops due to geometrical singularities are taken into account considering the different internal flow regimes that may occur. Besides a simplified formulation for the regenerator efficiency is deduced from considerations on its harmonic functioning. In this analysis, the emphasis concerns principally the design of miniature cryocoolers dedicated to electronic applications. Those models are applied to a commercial miniature refrigerator. A discussion of their relevance is achieved and a few suggestions on the refrigerator design are proposed in order to improve the cooling production.     

Solids flow pattern in gas-flowing solids-fixed bed contactors. Part I: Experimental

An experimental investigation of the solids flow pattern in gas-flowing solids-fixed bed contactors is presented. The apparatus and procedures for determining the dynamic and static solids holdups, solids residence time distribution and the extent and rate of the exchange between particles in the static and dynamic solids holdup are described in detail.Experiments were performed in a bench scale system, containing a column (diameter ) packed with glass beads of 16 mm in diameter packed up to the height of 0.8 m. Tracer experiments with a step input in flowing solids phase were used for determining the residence time distribution and exchange between particles. Fine solids (spheres with mean diameter of ) of two different colors (all other properties being the same) were used in the tracer experiments to determine the residence time distribution and the exchange between static and dynamic solids holdup. In both types of experiments, the response curves have been obtained via color analysis of digital photos. All experiments have been repeated at different operating conditions, with a broad variation of solids mass flux and gas velocity, and reproducibility at set conditions was checked.The obtained experimental results are discussed and the observed important characteristics of the solids flow pattern are outlined. The effects of the solids flux and gas velocity on the solids flow pattern are presented and analyzed.     

Understanding and Predicting Data Hotspots in Cellular Networks

The unprecedented growth in mobile data usage is posing significant challenges to cellular operators. One key challenge is how to provide quality of service to subscribers when their residing cell is experiencing a significant amount of traffic, i.e. becoming a traffic hotspot. In this paper, we perform an empirical study on data hotspots in today’s cellular networks using a 9-week cellular dataset with 734K+ users and 5327 cell sites. Our analysis examines in details static and dynamic characteristics, predictability, and causes of data hotspots, and their correlation with call hotspots. We show that using standard machine learning methods, future hotspots can be accurately predicted from past observations. We believe the understanding of these key issues will lead to more efficient and responsive resource management and thus better QoS provision in cellular networks. To the best of our knowledge, our work is the first to empirically characterize traffic hotspots in today’s cellular networks.     

Neutrophils Mediate Pulmonary Artery Thrombosis In Situ

Pulmonary embolism is a life-threatening condition, which can result in respiratory insufficiency and death. Blood clots occluding branches of the pulmonary artery (PA) are traditionally considered to originate from thrombi in deep veins (usually in legs). However, growing evidence suggests that occlusion of the vessels in the lungs can develop without preceding deep vein thrombosis (DVT). In this work, we used an inferior vena cava (IVC) complete ligation model of DVT in Wistar rats to explore the possibility and mechanisms of PA thrombosis under the conditions where all routes of thrombotic mass migration from peripheral veins are blocked. We demonstrate that rats both with normal and reduced neutrophil counts developed thrombi in the IVC, although, neutropenia caused a substantial decrease in thrombus size and a shift from fresh fibrin toward mature fibrin and connective tissue inside the thrombus. Massive fibrin deposition was found in the PA branches in the majority of DVT rats with normal neutrophil counts, but in none of the neutropenic animals. Neutrophil ablation also abolished macroscopic signs of lung damage. Altogether, the results demonstrate that thrombi in the lung vasculature can form in situ by mechanisms that require local neutrophil recruitment taking place in the DVT setting.     

Cobalt Ferrite Magnetic Nanoparticles for Tracing Mesenchymal Stem Cells in Tissue: A Preliminary Study

Therapy with mesenchymal stem cells (MSCs) is promising in many diseases. Evaluation of their efficacy depends on adequate follow-up of MSCs after transplantation. Several studies have shown that MSCs can be labeled and subsequently visualized with magnetic nanoparticles (NPs). We investigated the homing of MSCs labeled with magnetic cobalt ferrite NPs in experimentally induced acute kidney injury in mice. To explore the homing of MSCs after systemic infusion into mice, we developed a pre-infusion strategy for optimal tracing and identification of MSCs with polyacrylic acid-coated cobalt ferrite (CoFe₂O₄) NPs by light and transmission electron microscopy (TEM) in various organs of mice with cisplatin-induced acute kidney injury and control mice. By correlative microscopy, we detected MSCs labeled with NPs in the lungs, spleen, kidney, and intestine of cisplatin-treated mice and in the lungs and spleen of control mice. Our results confirm that labeling MSCs with metal NPs did not affect the ultrastructure of MSCs and their ability to settle in various organs. This study demonstrates the usefulness of cobalt ferrite NPs in ex vivo visualization of MSCs and offers correlative microscopy as a useful method in routine histopathology laboratories for tracing MSCs in paraffin-embedded tissue.     

The effect of surface modification on the corrosion protection ability of the passive films of sensitized UNS S31803 duplex stainless steel

ABSTRACT

The influence of sensitisation heat treatment, surface roughness and repetitive cyclic voltammetry (RCV) procedure (that was used to create passive films on the surface of sensitised samples) on the corrosion behaviour of UNS S31803 duplex stainless steel was evaluated using sodium hydroxide etching, double-loop electrochemical potentiodynamic reactivation (DLEPR), potentiodynamic polarisation, Mott–Schottky analysis, X-ray diffraction and optical microscopy techniques. The results showed that prolonged sensitisation time leads to the formation of defective passive films on the surface. In addition, as the substrate surface roughness decreases, the defect concentration in the resulting passive film decreases. Moreover, thick passive films that were created at a high number of RCV cycles contain fewer defects than the thin ones. Finally, the specimens with smooth surfaces, thick passive films and low degree of sensitisation exhibit high corrosion resistance due to their intact passive layers formed on their surfaces.