Improving the Sound Absorption Properties of Flexible Polyurethane (PU) Foam using Nanofibers and Nanoparticles

Polyurethane foam as the most well-known absorbent materials has a suitable absorption coefficient only within a limited frequency range. The aim of this study was to improve the sound absorption coefficient of flexible polyurethane (PU) foam within the range of various frequencies using clay nanoparticles, polyacrylonitrile nanofibers, and polyvinylidene fluoride nanofibers. The response surface method was used to determine the effect of addition of nanofi- bers of PAN and PVDF, addition of clay nanoparticles, absorbent thickness, and air gap on the sound absorption coefficient of flexible polyurethane foam (PU) across different frequency ranges. The absorption coefficient of the samples was measured using Impedance Tubes device. Nano clay at low thicknesses as well as polyacrylonitrile nanofibers and polyvinyl fluoride nanofibers at higher thicknesses had a greater positive effect on absorption coefficient. The mean sound absorption coefficient in the composite with the highest absorption coeffi- cient at middle and high frequencies was 0.798 and 0.75, respectively. In comparison with pure polyurethane foam with the same thickness and air gap, these values were 2.22 times at the middle frequencies and 1.47 times at high frequencies, respectively. Surface porosity rose with increasing nano clay, but decreased with increasing polyacrylonitrile nanofibers and polyvinyl fluoride nanofibers. The results indicated that the absorption coefficient was elevated with increasing the thickness and air gap. This study suggests that the use of a combination of nanoparticles and nanofibers can enhance the acoustic properties of flexible polyurethane foam.     

Preavalanche instabilities in a granular pile

We investigate numerically the transition between static equilibrium and dynamic surface flow of a 2D cohesionless granular system driven by a continuous gravity loading. This transition is characterized by intermittent local dynamic rearrangements and can be described by an order parameter defined as the density of critical contacts, i.e., contacts where the friction is fully mobilized. Analysis of the spatial correlations of critical contacts shows the occurrence of "fluidized" clusters which exhibit a power-law divergence in size at the approach of the stability limit. The results are compatible with recent models that describe the granular system during the static/dynamic transition as a multiphase system.     

4E analysis and tri-objective optimization of a triple-pressure combined cycle power plant with combustion chamber steam injection to control NOx emission

This article presents a novel triple-pressure combined cycle power plant (CCPP) with a heat recovery steam generator (HRSG) configured with heat exchangers of multiple pressure levels, same as the real case. In addition, combustion chamber steam injection is added to the top cycle in order to reduce hazardous emissions. The research investigates energy, exergy, economic, and environmental aspects of the system to initiate sustainable development in said areas. A thorough parametric study is carried out to evaluate the effects of steam injection and other decision variable on emissions and system performance. Then, the total cost rate and the CO₂ index are minimized while maximizing the second law efficiency via a tri-objective optimization using the genetic algorithm. The outcome of the economic analysis is that the HRSG has the maximum total cost rate among all the components, namely 0.1673 $/s. The environmental impact  assessments indicate that the CO₂ and NO emission has considerable molar fractions of 0.035 and 6.88?×?10^?4, respectively. As a result of the tri-objective optimization, a 3D Pareto Frontier is presented, which pointed out the maximum attainable exergy efficiency is 50.32%, as well as the minimum total cost rates of 8.04 $/s and CO₂ index of 0.34 kg/kWh. Finally, the scatter distribution of major decision variables revealed the optimum range of decision variables in which the optimum points of the Pareto Frontier are obtained. Accordingly, the scatter distribution showed that 46 kg s^?1 is the optimum value for steam injection flow rate in terms of efficiency, cost and emission optimization.

     

In-plane vibration analysis of curved carbon nanotubes conveying fluid embedded in viscoelastic medium

The effect of the induced vibrations in the carbon nanotubes (CNTs) arising from the internal fluid flow is a critical issue in the design of CNT-based fluidic devices. In this study, in-plane vibration analysis of curved CNTs conveying fluid embedded in viscoelastic medium is investigated. The CNT is modeled as a linear elastic cylindrical tube where the internal moving fluid is characterized by steady flow velocity and mass density of fluid. A modified-inextensible theory is used in formulation and the steady-state initial forces due to the centrifugal and pressure forces of the internal fluid are also taken into account. The finite element method is used to discretize the equation of motion and the frequencies are obtained by solving a quadratic eigenvalue problem. The effects of CNT opening angle, the elastic modulus and the damping factor of the viscoelastic surrounded medium and fluid velocity on the resonance frequencies are elucidated. It is shown that curved CNTs are unconditionally stable even for a system with sufficiently high flow velocity. The most results presented in this investigation have been absent from the literature for fluid-induced vibration of curved CNTs embedded in viscoelastic foundations.     

Boundary stabilization of vibration of nonlocal micropolar elastic media

In this paper, we study the stabilization problem of vibration of linearized three-dimensional nonlocal micropolar elasticity. For this purpose, we need to demonstrate the well-posedness of the system of equations governing the vibration of three-dimensional nonlocal micropolar media for both forced (i.e. with boundary feedback) and unforced cases. We assume the non-homogeneous system of equations for the unforced (uncontrolled) case to establish the well-posedness. It should be pointed out that the well-posedness of the evolution equations in micropolar case has been studied by many authors; but, the well-posedness in the nonlocal micropolar is an open problem. Our tools in well-posedness analysis are the semigroup techniques. Afterwards, we pursue the stabilization problem and show that the vibration of the nonlocal micropolar elastic media will be eventually dissipated under boundary feedback actions consisting of stress and couple stress feedback laws. These control laws are simple, linear and can be easily implemented in practical applications. The stabilization proof is accomplished using Lyapunov stability and LaSalle’s invariant set theorems.