Heterogeneous reaction induced by the piston effect in supercritical binary mixtures

It is now well established that the large compressibility of supercritical fluids is responsible for the strong enhancement of the thermo-acoustic heating, leading to the speeding up of the heat transport thanks to the piston effect instead of the expected slowing down. We show in this paper, through numerical simulations, that the hydrodynamics behavior of supercritical fluids also couples with the critical behavior of the solubility of solids to cause the release of a heterogeneous reaction at solid surfaces in dilute binary supercritical mixtures.     

Cooling enhancement of planar-balanced magnetron cathode

In physical vapor deposition on a magnetron cathode, temperature of sensitive components must be kept under threshold limit, so as to ensure the cathode reliability,the process reproducibility, and the best quality of thin films.This can be achieved by an adequate design to enhance the dissipation of heat generated at the cathode. In this paper,temperature distribution and streamlines velocity of the cathode coolant inside a cathode magnetron are analyzed by using CFD solver ANSYS FLUENT in the single-phase method in combination with k–e standards turbulent model.The results show that the design is appropriate under the calculation parameters, and for high heat densities some improvements are necessary to enhance heat dissipation and keep temperature under the threshold limit.     

Effect of printing temperature on microstructure,thermal behavior and tensile properties of 3D printed nylon using fused deposition modeling

The present investigation aims at the thermal conditions for the printability of nylon using fused deposition modeling (FDM). Dog-bone like specimens are manufactured under two printing temperatures to measure the tensile performance of 3D printed nylon with respect to the feedstock material properties. Both Scanning Electron Microscopy (SEM) and X-ray micro-tomography analysis are conducted to shed more light on the microstructural arrangement of nylon filaments. Finite element computation based on microstructural implementation is considered to study the main deformation mechanisms associated with the nylon filament arrangement and the process-induced porosity. The results show a narrow temperature range for printability of nylon, and a significant influence of the printing temperature on the thermal cycling, porosity content and mechanical performance. With the support of both numerical and experimental results, complex deformation mechanisms are revealed involving shearing related to the filament sequencing, compression at the junction points and tension within the raster and the frame. All these mechanisms are associated with the particular and regular arrangement of nylon filaments.     

Cement-based mixes: Shearing properties and pore pressure

This study presents original results on the rheological measurement of concrete mixes. It focuses on how to determine their mechanical and physical behavior under shearing stress. More specifically, the influence of aggregate content on shearing properties is studied. A vane rheometer was developed to characterize fresh cement-based materials. In addition to the conventional concrete rheometer, a special hydraulic pressure transducer was fitted to the container to monitor the pore water pressure variation while shearing the material. Experiments on cement paste, mortar, and concrete bring a new approach to help us understand the behavior of fresh-state mixes. The results show 1) a correlation between water pore pressure and torque applied on the vane; 2) a critical sand volume fraction, ?_c, as a limit between colloidal interaction behavior and frictional behavior in mortars; beyond this critical fraction, a leap in yield stress and a drop in pore pressure due to granular dilatancy are noticed; 3) the granular content clearly influences the increase in yield stress of the cement mixes: above ?_c, this increase becomes negligible.     

Extension of Einstein's Law for Power‐Law Fluid to Describe a Suspension of Spherical Particles: Application to Recycled Polymer Flow

In this work, Einstein's equation is extended considering a power‐law suspending fluid without any Newtonian approximation. To validate the developed equation, an experimental setup is carried out. Polypropylene (PP) and polyethylene (PE) are injected at different volume fractions. The pressure drops measured in a cylindrical die are analyzed. The results show that the developed relationship allows better prediction of the viscosity of PP/PE blends compared to existing laws. During the recycling of PP, some pollutants are likely to be present in the polymer, mostly PE which tends to form a heterogeneous melt with PP. At low volume fractions, PE disperses mostly as solid spheres in PP due to its higher viscosity, but the viscosity of the PP/PE mixtures is hard to predict. Several studies have derived equivalent viscosity equations for dispersed spherical suspensions in shear‐thinning polymers. Nevertheless, these equations mainly refer to Einstein's equation for suspended spheres in Newtonian fluids. POLYM. ENG. SCI., 59:E387–E396, 2019. © 2019 Society of Plastics Engineers     

Internal melting and coarsening of liquid droplets in an Al–Cu alloy: a 4-D experimental study

In conventional melting experiments of pure monocrystalline metals, the phase transformation starts at the sample surface and progresses inwards according to thermal gradients. In solutionized alloys, traces of internal melting are usually observed after reheating and quenching from the semi-solid state. The formation and development of these liquid pockets are not fully understood despite their significance in semi-solid processing, where the formability is greatly influenced by the distribution of liquid within the feedstock material. In situ X-ray microtomography was performed in this study to shed light on this phenomenon. We report in detail the melting and isothermal holding of a model binary alloy where a remarkable number of liquid droplets were observed to develop and coalesce. Various computational tools have been used to study their statistical evolution as well as the local ripening mechanisms involved. We analysed an interesting case of particle coarsening which differs from classical case studies by the fact that the fast-diffusing liquid phase is entrapped within the slow-diffusing solid medium.     

WRE‐OLSR,a new scheme for enhancing the lifetime within ad hoc and wireless sensor networks

Within ad hoc and wireless sensor networks, communications are accomplished in dynamic environments with a random movement of mobile devices. Thus, routing protocols over these networks are an important concern to offer efficient network scalability, manage topology information, and prolong the network lifetime. Optimized link state routing (OLSR) is one of those routing protocols implemented in ad hoc and wireless sensor networks. Because of its proactive technique, routes between two nodes are established in a very short time, but it can spend a lot of resources for selecting the multipoint relays (MPRs: nodes responsible for routing data) and exchanging topology control information. Thus, nodes playing for a long time a role of MPR within networks implementing such protocol can rapidly exhaust their batteries, which create route failures and affect the network lifetime. Our main approach relies on analyzing this concern by introducing a new criterion that implements a combination between the residual energy of a node and its reachability in order to determine the optimal number of MPRs and sustain the network lifetime. Simulations performed illustrate obviously that our approach is more significant compared with the basic heuristic used by original OLSR to compute the MPR set of a node.     

Near-room-temperature mid-infrared quantum well photodetector

We demonstrate InGaAs mid-infrared quantum well infrared photodetectors (MIR PV-QWIPs) that enable cost-effective mature GaAs-based detection and imaging technologies, with exceptional material uniformity, reproducibility, and yield, over a large area, with high spectral selectivity, innate polarization sensitivity, radiation hardness, high detectivity, and high speed operation at TEC temperatures without bias.     

Evaluation of wall boundary condition parameters for gas–solids fluidized bed simulations

Wall boundary conditions for the solids phase have significant effects on numerical predictions of various gas–solids fluidized beds. Several models for the granular flow wall boundary condition are available in the open literature for numerical modeling of gas–solids flow. A model for specularity coefficient used in Johnson and Jackson boundary conditions by Li and Benyahia (Li and Benyahia, AIChE J. 2012;58:2058–2068) is implemented in the open‐source CFD code‐MFIX. The variable specularity coefficient model provides a physical way to calculate the specularity coefficient needed by the partial‐slip boundary conditions for the solids phase. Through a series of two‐dimensional numerical simulations of bubbling fluidized bed and circulating fluidized bed riser, the model predicts qualitatively consistent trends to the previous studies. Furthermore, a quantitative comparison is conducted between numerical results of variable and constant specularity coefficients to investigate the effect of spatial and temporal variations in specularity coefficient. Published 2013 American Institute of Chemical Engineers AIChE J, 59: 3624–3632, 2013