Free convection enhancement in an annulus between horizontal confocal elliptical cylinders using hybrid nanofluids

ABSTRACT

In the present paper, natural convection in an annulus between two confocal elliptic cylinders filled with a Cu-Al₂O₃/water hybrid nanofluid is investigated numerically. The inner cylinder is heated at a constant surface temperature while the outer wall is isothermally cooled. The basic equations are formulated in elliptic coordinates and developed in terms of the vorticity-stream function formulation using the dimensionless form for 2D, laminar and incompressible flow under steady-state condition. The governing equations are discretized using the finite volume method and solved by an in-house FORTRAN code. Numerical simulations are performed for various volume fractions of nanoparticles (0?≤???≤?0.12) and Rayleigh numbers (10³?≤?Ra?≤?3?×?10⁵). The eccentricity of the inner and outer ellipses and the angle of orientation are fixed at e₁?=?0.9, e₂?=?0.6 and γ?=?0° respectively. It is found that employing a Cu-Al₂O₃/water hybrid nanofluid is more efficient in heat transfer rate compared to the similar Al₂O₃/water nanofluid.     

Evaluation of the mechanical properties,in vitro biodegradability and cytocompatibility of natural chitosan/hydroxyapatite/nano-Fe3O4 composite

The main goal of this research was the preparation and evaluation of the mechanical properties, in vitro biodegradability and cytocompatibility, of natural chitosan/hydroxyapatite/nano magnetite (nano-Fe₃O₄) composite. Different ratios of these components were investigated, including chitosan/hydroxyapatite: 4/4 (S1), chitosan/hydroxyapatite: 4/6 (S2), and chitosan/hydroxyapatite: 6/4 (S3). Mechanical properties of fabricated composites were examined using bending and compression tests before immersion, and after 2 and 9 weeks of immersion in the Ringer's solution. Scanning electron microscope (SEM) was employed for observing the bending fracture surface and analyzing the degradation morphology. Human mesenchymal stem cells (hMSC) were also cultured on the samples in order to assess the cytocompatibility. The obtained results revealed that S1 had the highest bending strength before immersion, while S3 had the highest bending strength after 9 weeks immersion. Compressive strength of S2 was greater than that of S1 and S3 not only before immersion, but also after 9 weeks immersion. Although the bio-minerals were deposited on the surface of all samples during the immersion in Ringer's solution, S2 appeared to have the highest quantity of bio-minerals. According to the weight loss percentage (ΔW(%)), the biodegradation resistance of S1 was the lowest. Finally, the cytocompatibility of S1 was greater than that of S2 and S3.     

Trimethylolpropane trimethacrylate functionalized polypropylene/polyhexene-1 blend with enhanced melt strength

ABSTRACT

This study was aimed at investigating the variations in the melt strength of polypropylene as a result of blending with another polymer having long side branches in the presence of multi-functional agent and radical initiator. The formation of long chain branches was confirmed by strain hardening behavior in extensional viscometry and grafting efficiency data obtained from FTIR spectroscopy. strain hardening behavior, rheological parameters and grafting efficiency kept their ever-increasing trends by increasing the concentration of Trimethylol-propane-trimethacrylate (TMPTMA) until 3 phr, and the inefficiency of TMPTMA in branching reactions at concentrations higher than 3 phr is rooted in homo-polymerization of TMPTMA.     

Evaluation of mechanical and biocompatibility properties of hydroxyapatite/manganese dioxide nanocomposite scaffolds for bone tissue engineering application

The aim of this research was to evaluate the mechanical properties, biocompatibility, and degradation behavior of scaffolds made of pure hydroxyapatite (HA) and HA-modified by MnO₂ for bone tissue engineering applications. HA and MnO₂ were developed using sol-gel and precipitation methods, respectively. The scaffolds properties were characterized using X-ray diffraction (XRD), Fourier transform spectroscopy (FTIR), scanning electron microcopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The interaction of scaffold with cells was assessed using in vitro cell proliferation and alkaline phosphatase (ALP) assays. The obtained results indicate that the HA/MnO₂ scaffolds possess higher compressive strength, toughness, hardness, and density when compared to the pure HA scaffolds. After immersing the scaffold in the SBF solution, more deposited apatite appeared on the HA/MnO₂, which results in the rougher surface on this scaffold compared to the pure HA scaffold. Finally, the in vitro biological analysis using human osteoblast cells reveals that scaffolds are biocompatible with adequate ALP activity.     

A Comparative Study of Localization Methods in Indoor Environments

A comparative study, based on three different measurements (direction of ray arrival, time difference of arrival and received signal strength), to compute the unknown position of mobile stations in indoor environments is presented in this paper. The comparison is carried out considering the results of analyses in a real building in Madrid. To overcome the problems that arise in indoor areas due to the presence of non line of sight conditions, the fingerprinting technique is applied in each of the cases. Data for computations are provided by a simulation tool based on the uniform theory of diffraction and ray-tracing techniques. This information is stored in the fingerprinting database and contains information related to every mobile station, every reference node and every access point located inside the environment under analysis. Experimental results compare the mean error when localizing several mobile stations by using the three different approaches. The goal is to obtain high precision in the localization by means of alternative methods to the received signal strength classical measurement. These techniques will be useful in critical environments where high operational security requirement are demanded.     

Design of X‐band Moreno cross‐guide coupler based on superformula curves

This article deals with the design of Moreno cross‐guide couplers based on supershapes for X‐band applications. Crossed‐waveguide couplers are mainly used due to their compact structures. In these couplers, cross‐aperture structures are usually employed to offer flat coupling and high isolation. In the present article, the possible shapes for apertures and metal inserts that can be derived by the superfomula curves are explored and the effects of variations of superformula parameters are investigated on the performance of Moreno coupler. Finally, the proposed Moreno coupler is validated through fabrication and measurement. The experimental validation shows an excellent agreement with the simulated results. In the frequency range from 8 to 12.5 GHz, the measured coupling value changes from 18.8 to 20.8 dB and the directivity is better than 38 dB and 29 dB from 8 to 11 GHz and 11 to 12.5 GHz, respectively. The results are valuable for the design and evaluation of broadband high directive waveguide couplers.     

A Unified Adaptive Iterative Learning Control Framework for Uncertain Nonlinear Systems

In this note, we propose a unified framework for adaptive iterative learning control design for uncertain nonlinear systems. It is shown that if a Lyapunov based adaptive control law is available for the system under consideration and the Lyapunov function satisfies certain conditions, it is straightforward to extend the adaptive controller to handle repetitive systems operating over a finite time interval. According to the value of a certain parameter gamma, the parametric adaptation law can be a pure time-domain adaptation, a pure iteration-domain adaptation or a combination of both.A pure iteration-domain adaptation is described by a difference equation, a pure time-domain adaptation is described by a differential equation, and a combination of both is described by a differential-difference equation. The advantages and disadvantages of the three possible adaptation types are discussed and some illustrative examples are given. [All rights reserved Elsevier].     

Adaptive iterative learning control for robot manipulators

In this paper, we propose some adaptive iterative learning control (ILC) schemes for trajectory tracking of rigid robot manipulators, with unknown parameters, performing repetitive tasks. The proposed control schemes are based upon the use of a proportional-derivative (PD) feedback structure, for which an iterative term is added to cope with the unknown parameters and disturbances. The control design is very simple in the sense that the only requirement on the PD and learning gains is the positive definiteness condition and the bounds of the robot parameters are not needed. In contrast to classical ILC schemes where the number of iterative variables is generally equal to the number of control inputs, the second controller proposed in this paper uses just two iterative variables, which is an interesting fact from a practical point of view since it contributes considerably to memory space saving in real-time implementations. We also show that it is possible to use a single iterative variable in the control scheme if some bounds of the system parameters are known. Furthermore, the resetting condition is relaxed to a certain extent for a certain class of reference trajectories. Finally, simulation results are provided to illustrate the effectiveness of the proposed controllers.