Iranian Journal of Science and Technology Transactions of Mechanical Engineering
Volume:39 Issue: 1, 2015

A nonlocal beam model based on the Bernoulli–Euler beam theory is presented to investigate the dynamic stability of embedded single-walled carbon nanotubes (SWCNTs) in thermal environment under combined static and periodic axial loads. The dynamic stability analysis is carried out by including the effects of small-scale parameter, temperature change and elastic medium. The equation of motion is reduced to the extended Mathieu-Hill equation, the stability of which is analyzed through the Floquet-Lyapunov theory as well as bounded and unbounded solution theory. The instability regions obtained from both theories are examined and compared with each other. Also, the effects of the small-scale parameter, temperature change, elastic medium, compressive static axial load and excitation frequency on the dynamic stability of SWCNTs are discussed in detail. The prediction of dynamic instability of carbon nanotubes enables one to eliminate this phenomenon in cases that may fall within the range of practical significance.

Multi frequency vibrations of a system of two isotropic circular plates interconnected by a rolling visco-elastic layer that has nonlinear characteristics are considered. The system with two circular plates and interconnecting discrete continually distributed rolling visco nonlinear elastic rheological elements presents the model of hybrid nonlinear system. The analytical solutions of first asymptotic approximation describing stationary behavior, in the regions around the resonances, are the principal results of the author. On the basis of those results the influence on the system dynamics of rolling coupling element was numerically analyzed. A series of the amplitude-frequency and phase-frequency curves of the two-frequency like vibration regimes were numerically obtained and presented. These curves present the evolution of the first asymptotic approximation of solutions for different nonlinear harmonics obtained by changing external excitation frequencies through discrete as well as continuous values for different values of rolling elements masses. Such an analysis proves that the presence of rolling elements in the interconnected layer of two plates causes frequency overlap of resonant regions of nonlinear modes, which at the same time causes the enlargement of the mode mutual interactions.

Differential steering of in-wheel electric vehicle provides the functions of both active steering and power assisted steering with the coupling control of force and displacement transfer characteristic of system. A collaborative optimization model of the differential power-assisted steering system of in-wheel electric vehicle is built, with steering economy as the main system optimization goal, steering road feel, steering sensitivity and torque sensor performance as the subsystem optimization goals. Considering the coupled relationship of each discipline, the main system is optimized by the particle swarm algorithm, and the subsystems are optimized by the directional heuristic search algorithm which is good on local optimization. The simulation results show that the collaborative optimization based on particle swarm algorithm has more optimal solution sets and fast convergence by considering the coupling relationship between different disciplines, and the comprehensive performance of in-wheel electric vehicle is improved.

Aluminum foams are a novel branch of advanced materials with superior properties. Sandwich structures with aluminum foam core are good energy absorbers. In this paper, mechanical properties and energy absorption of aluminum foam sandwich panels subjected to quasi-static perforation tests with conical-nosed indenter were investigated experimentally. For this purpose sandwich panels consisting of two aluminum face-sheets and a closed cell aluminum foam core were fabricated. Quasi-static perforation tests on fully fixed sandwich panels were carried out by a universal testing machine at a cross head speed of 0.02 mm/s. Force-displacement curves were recorded and peak piercing force and absorbed energy of sandwich panels were calculated accordingly. Effects of foam core density (0.5, 0.6 and 0.7 g/cm3) and thickness of face-sheets (0.6, 1 and 2 mm) and foam core (10, 20 and 30 mm) on the mechanical properties and energy absorption of samples were discussed. The results showed that increasing foam core and face-sheet thickness and foam core density led to more total absorbed energy being achieved and higher piercing force.

Fridges are used in various industries, and heat exchanger is one of the main components of these systems; therefore, efficiency analysis of these systems is very important. In many fridges, frost over the heat exchanger tubes or pipes affects the system operation and reduces heat transfer between the refrigerant and ambient, so defrosting systems are usually used to enhance heat transfer. Designing defrosting systems requires sufficient information about frost formation processes. Therefore, the objective of this study is experimental investigation of frost formation over a heat exchanger pipe in a pilot fridge. To do this, some steps are considered. For distinct cases, experiments are done to collect database for frost parameters under desired conditions. Then, based on experimental measurements some correlations are proposed for practical applications of frost growth and defrosting process over heat exchanger tubes.

Two continuously regenerating traps CRT-A and CRT-B were evaluated under the controlled carbonyl emissions from a turbocharged diesel engine run on an AC electrical dynamometer. The gas-phase carbonyls were taken in silica gel cartridges coated with 2,4-dinitrophenylhydrazine both upstream (baseline) and downstream of the traps, and a total of fourteen carbonyls were identified and then quantified using the high performance liquid chromatography (HPLC) with ultraviolet (UV) detector. Carbonyls were predominant during higher load modes as well as at maximum speed modes. The conversion/reduction rate (CR) of carbonyls was directly affected by the physical characteristics of the traps. The CR revealed a strong correlation with NO2/NOx ratios of CRT units, and the performance of retrofits was adversely affected by the increasing upstream back pressure (P) due to the clogging phenomenon. Carbonyls such as formaldehyde, acetaldehyde, acrolein + acetone and propionaldehyde were in abundance, and abated up to 75.4, 83, 60, and 69.3% respectively with CRT-A, and 81.2, 81.5, 57.3, and 65.2% respectively with CRT-B. Nevertheless, formaldehyde and propionaldehyde exhibited reluctance to CR during lower load modes, which was higher with CRT-B relative to CRT-A due to the different features of the traps. A positive correlation was observed between the conversion of acetaldehyde and propionaldehyde within the cycle, and between the mean relative contributions of formaldehyde and acetaldehyde within the CRT units. Moreover, total carbonyls including aromatic aldehydes, and their specific reactivity (SR) both were reduced with CRT technologies showing the order of magnitude as Baseline > CRT-A > CRT-B.

In the present work, the simultaneous effects of fuel injection rate and injection of air jet into the combustion chamber on exhaust emissions, combustion process and performance parameters in a direct injection diesel engine were investigated. In order to create an air jet, a design was presented in which a secondary chamber (air cell) was created inside the cylinder and was joined to the main chamber by throats. The obtained results showed that creating the air cell had no major effect on the power and specific fuel consumption, and brought about the reduction of emitted smoke particles from the combustion chamber in all four conditions of 100% load, 75% load, 50% load and 25% load. In comparison with the base engine, the rates of oxides of nitrogen (NOx) emissions decreased at 100% and 75% loads, yet increased at 50% and 25% loads. The outcomes of the current study were compared with those existing in the relevant literature and displayed acceptable behavior.

Nuclear cross sections that determine core multiplication strongly depend on core temperature (e.g., the Doppler, moderator density effects etc). On the other hand, since this heat is generated by the neutron flux in the reactor core, the temperature distribution in the core will depend heavily on its neutronic behavior. Fuel centerline temperature could be the limiting constraint on reactor power because of the concern for fuel melting. Likewise, high clad temperature is also a possible limiting factor on reactor power because of the potential degradation of clad material or on-set of critical heat flux phenomenon.An assessment of the steady state and transient thermal hydraulic capabilities of the computer code COBRA 3C/RERTR was made using model for a PHWRs reactor core. The temperature distributions determined for fuel, clad and coolant are compared with analytical results and with the results quoted in safety report. It was found that when the code was run for full power at reduced flow of 70% the bulk coolant temperature remained below the saturation temperature, so there is an adequate design margin is available for safety related scenarios.

The vehicle braking system is considered to be one of the most fundamental safety-critical systems in modern vehicles, as its main purpose is to stop or decelerate the vehicle. The frictional heat generated during braking application can cause numerous negative effects on the brake assembly, such as brake fade, premature wear, thermal cracks and disc thickness variation. In the past, surface roughness and wear at the pad interface have rarely been considered in studies of the thermal analysis of a disc brake assembly using the finite element method. The ventilated pad-disc brake assembly is built by a three-dimensional model with a thermomechanical coupling boundary condition and multi-body model technique. The numerical simulation for the coupled transient thermal field and stress field is carried out sequentially with the thermal-structural coupled method, based on ANSYS software, to evaluate the stress fields of deformations that are established in the disc with the pressure of the pads and in the conditions of tightening of the disc; thus, the contact pressure distributions field in the pads is obtained, which is another significant aspect in this research. The results obtained by the simulation are satisfactorily compared with those of the specialized literature.

This paper presents the results of the assembly process of a gasketed bolted flanged pipe joint for two different bolt tightening strategies, i.e. ASME and Industrial using torque control of preload method. The final clamping force is achieved in four and five passes in ASME and Industrial strategy respectively. Axial bolt stress variation, at the end of each pass, individual bolt bending behavior, gasket stress and flange stress variations for both strategies are discussed.

Nowadays, textile and woven fabric composites are taken into consideration for applications in high mechanical properties and every in-plane direction. However, developments in modelling and characterisation of the fabric reinforced composite materials are considered more in the effects of element types used in the mesh generation of the crack tip. The type of element selected for the crack tip, a critical point for evaluation of crack growth in a double cantilever beam (DCB) sample, is extremely important. In this study, results of strain energy release rate (SERR) meshed with singular and or brick elements with the experimental data were compared to select better element type. The plotted results of the crack tips meshed with the brick elements and the diagrams revealing the SERR in contrast to the crack length was evaluated. In addition, the aforementioned operation has been repeated for singular elements. The theory of the failure mechanics has been used to calculate the amounts of SERR for several crack lengths. It is concluded that, numerical results from SERR of the crack tip when meshed with singular elements were closer to the experimental results compared with data of SERR when meshed with brick elements.

In this study, He’s Variational Approach (VA) is used to solve the non-natural vibrations and oscillations. The method works very well for the whole range of initial amplitudes and does not demand small perturbation. It is also sufficiently accurate to both linear and nonlinear physics and engineering problems. We consider some examples to illustrate the effectiveness and convenience of the method. Runge-Kutta’s [RK] algorithm was also implemented to show the examples through a numerical method. Finally, to show the accuracy of the VA, the results have been shown graphically and compared with numerical and exact solution.