Numerical modeling of a human stented trachea under different stent designs

Endotracheal stenting is a common treatment for tracheal disorders as stenosis, cronic cough or dispnoea episodes. However, medical treatment and surgery are still challenging due to the difficulties in overcoming potential prosthesis complications. In this work we analyze the response of the tracheal wall during breathing and coughing conditions under different stent implantations. A finite element model of a human trachea was developed and used to analyze tracheal deformability after prosthesis implantation under normal breathing and coughing using a fluid-structure interaction approach (FSI). The geometry of the trachea is obtained from computed tomography (CT) images of a healthy patient. A structured hexahedral-based grid for the tracheal wall and an unstructured tetrahedral-based mesh with coincident nodes for the fluid were used to perform the simulations with a finite element-based commercial software code. Tracheal wall is modeled as a fiber reinforced hyperelastic solid material in which the anisotropy due to the orientation of the fibers is taken into account. Deformations of the tracheal cartilage rings and of the muscle membrane, as well as the maximum principal stresses in the wall, are analyzed and compared with those of the healthy trachea in absence of prosthesis. The results showed that, the presence of the stent prevents tracheal muscle deflections especially during coughing. In addition, we proposed a methodology to evaluate, through numerical simulations, the predisposition of the stent to migrate.     

Modeling of the fluid structure interaction of a human trachea under different ventilation conditions

This work is focused on the analysis of the response of the tracheal wall to different ventilation conditions. Thus, a finite element model of a human trachea is developed and used to analyze its deformability under normal breathing and mechanical ventilation. The geometry of the trachea is obtained from computed tomography (CT) images of a healthy man. A fluid structure interaction approach is used to analyze the deformation of the wall when the fluid (in this case, air) moves inside the trachea. A structured hexahedral-based grid for the tracheal walls and an unstructured tetrahedral-based mesh with coincident nodes for the fluid are used to perform the simulations with the finite element-based commercial software code (ADINA R & D Inc.). The tracheal wall is modeled as a fiber reinforced hyperelastic solid material in which the anisotropy due to the orientation of the fibers is introduced. Deformation of the tracheal walls is analyzed under different conditions. Normal breathing is performed assuming a sinus shape of the pressure at the inlet and air speed at the outlet based on real data which represent the inspiration and the expiration processes respectively. Mechanical ventilation is simulated as smooth square shape velocity airflow considering positive values of pressure using data from a mechanical ventilation machine. Deformations of the tracheal cartilage rings and of the muscle membrane, as well as the maximum principal stresses in the wall, are analyzed. The results show that, although the deformation and stresses are quite small for both conditions, forced ventilation does not exactly imitate the physiological response of the trachea, since with always positive pressure values the trachea does not collapse during mechanical breathing.     

Unsteady blood flow and mass transfer of a human left coronary artery bifurcation: FSI vs. CFD

In this study, a Fluid Solid Interaction analysis (FSI) of a computerized tomography (CT) scan reconstructed left coronary artery was performed. The arterial wall was modeled as an isotropic hyperelastic material. The arterial wall shear stress (WSS) was computed in order to investigate a correlation between flow-induced wall shear stress and geometry of the artery. An unsteady state FSI analysis with a commercial finite element software was performed in order to evaluate the maximum and the minimum wall shear stress as a function of the flow regime and the arterial wall compliance in the left coronary. As boundary conditions, physiological pressure waveforms were applied. Comparison of the computational results between the FSI and rigid-wall models showed that the wall shear stress (WSS) distributions were substantially affected by the arterial wall compliance. In particular, the minimum and maximum WSS values significantly vary.     

Enhanced heat transfer in free convection-dominated melting in a rectangular cavity with an isothermal vertical wall

Free convection-dominated melting of a phase change material in a rectangular cavity with an isothermally heated vertical wall is simulated using the streamline upwind/Petrov–Galerkin finite element technique in combination with a fixed-grid primitive variable method. The enthalpy–porosity model is employed to account for the physics of the evolution of the flow at the solid/liquid interface. A penalty formulation is used to treat the incompressibility constraint in the momentum equations. Inverting of the container at an appropriate stage during the melting process is proposed as a simple but effective technique for enhancement of free convection-controlled heat transfer in the phase change material. The technique results in more than 50% increase of the energy charge rate during the melting process for some specific cases.     

Geometric optimization of a thermoacoustic regenerator

This work illustrates the optimization of thermoacoustic systems, while taking into account thermal losses to the surroundings that are typically disregarded. A simple thermoacoustic engine is used as an example for the methodology. Its driving component, the thermoacoustic regenerator (also referred to as the stack), is modeled with a finite element method and its dimensions are varied to find an optimal design with regard to thermal losses. Thermoacoustic phenomena are included by considering acoustic power, and viscous and capacitive losses that are characteristic for the regenerator. The optimization considers four weighted objectives and is conducted with the Nelder–Mead Simplex method. When trying to minimize thermal losses, the presented results show that the regenerator should be designed to be as short as possible. It was found that there is an optimal regenerator diameter for a given length. The results are presented for a variety of materials and weights for each objective.     

A parametric study of forced convection in an enclosure with stationary heated cylinders

This work performs an analysis of forced convection in an enclosure with a tube bank composed of 18 stationary cylinders. One wall is allowed to transfer heat while the remaining ones are insulated. The flow is induced by one fan placed near the upper horizontal wall. Numerical and experimental comparisons are also carried out to validate the code. Temperature and velocity distributions are presented showing their effect on the Nusselt number for various Reynolds numbers. Some recirculations worked as isolation layers that made heat transfer more difficult. Some tubes were seen to change negligible heat transfer which may be taken out of the tube set. A future work on the optimization is recommended by the authors.     

The interaction of pressure, in-plane moment and torque loadings on piping elbows

This study concerns the load interaction behaviour of 90° smooth piping elbows with circular cross-section and long straight tangent pipes. The finite element method is used for stress analysis of elbows having a wide range of bend and pipe factors. The main aim of the study is to establish the first yield interaction behaviour when an elbow is subjected to a combination loading of in-plane bending, torsion and internal pressure. The study shows that load interaction is influenced by pipe factor, bend radius and load coupling effect, with thinner elbows being affected to a larger degree.     

A finite element procedure for prediction of tube hole distortion due to welding of large perforated plates

Large tubesheets are critical components of huge heat exchangers and have to be fabricated by welding together two or more pieces. These parts are previously drilled in order to get reasonable productivity, but are affected by welding-induced residual stress and distortion. The tube holes are important, as several thousand tubes will be fastened to them, so it is crucial to evaluate their welding distortion to properly design the manufacturing process.     

Finite element simulation of MHD combined convection through a triangular wavy channel

The present paper implements the analysis of magnetohydrodynamic (MHD) combined convective flow and heat transfer characteristics through a triangular wavy vertical channel using the Galerkin weighted residual finite element method. The flow enters at the bottom and exits from the top surface. The wavy vertical walls are at constant temperature and the cold flow enters the channel from the inlet. The numerical model is based on a 2D Navier–Stokes incompressible flow and energy equation. The effects of Grashof number, Reynolds number and Prandtl number on flow and thermal fields are investigated. The variation of local Nusselt number along the vertical walls for the mentioned parameters is also presented. The study reveals that the flow as well as thermal field strongly depends on the aforesaid parameters.     

Experimental and numerical investigation of a flat-plate solar collector

In the present paper we present an experimental analysis and a thermal and hydrodynamic modelling of a newly designed flat-plate solar collector characterized by its corrugated channel and by the high surface area directly in contact with the heat transport fluid. The thermal and hydrodynamic modelling of the collector has been performed by means of the Finite Element Method (FEM), validated with analytical results for a well-known fin-and-tube type solar collector. The thermodynamic efficiency of the collector is analyzed by means of its experimental heating curves. The yield of the new collector has been compared to a previously existing commercial collector of related geometry but with less area in direct contact with the heat transport fluid. The experimental results are seen to adequately fit the simulation predictions, and a methodology to use in order to compute the parameters characterizing the thermal behavior of the collector is introduced.     

后向台阶圆柱绕流的区域分解有限元数值模拟

研究钝体绕流这一燃烧、传热领域的基本流动现象 ,针对圆柱绕流问题 ,给出了求解粘性不可压缩定常 Navier Stokes方程的区域分解算法 ,证明了其收敛性 ,并用区域分解有限元方法对后向台阶圆柱绕流问题进行了数值模拟。     

Modelling the thermal behaviour of a lithium-ion battery during charge

A method for modelling the thermal behaviour of a lithium-ion battery (LIB) during charge is presented. The effect of charge conditions on the thermal behaviour is examined by means of the finite element method. A comparison of the experimental charge curves with the modelling results validates the two-dimensional modelling of the potential and current density distribution on the electrodes of an LIB as a function of charge time during constant-current charge followed by constant-voltage charge. The heat generation rates as a function of the charge time and the position on the electrodes are calculated to predict the temperature distributions of the LIB based on the modelling results for potential and current density distributions. The temperature distributions obtained from the modelling are in good agreement with the experimental measurements.     

Modeling of the capacity loss of a 12 V automotive lead-acid battery due to ageing and comparison with measurement data

One-dimensional modeling was carried-out to predict the capacity loss of a 12 V automotive lead-acid battery due to ageing. The model not only accounted for electrochemical kinetics and ionic mass transfer in a battery cell, but also considered the anodic corrosion of lead in sulfuric acid. In order to validate the modeling, modeling results were compared with the measurement data of the cycling behaviors of the lead-acid batteries having nominal capacity of 68 Ah that are mounted on the automobiles manufactured by Hyundai Motor Company. The cycling was performed under the protocol of the constant-current discharge and the constant-voltage charge. The discharge rate of C/3 was used. The range of state of charge was between 1 and 0.85. The voltage was kept constant at the gassing voltage until the charge current tapered to 10 mA. The retention capacity of the battery was measured with C/3 discharge rate before the beginning of cycling and after every 40 cycles of cycling. The modeling results were in good agreement with the measurement data.     

The creeping flow of a polymeric fluid through bent square ducts with heat dissipation

The 3D non-isothermal creeping flow of nylon-6 in a bent square duct with uniform temperature is studied numerically. The non-Newtonian characteristics of this fluid polymer are represented by a differential-type non-isothermal White-Metzner model. Computational results are obtained by the elastic-viscous split-stress (EVSS) finite element method, incorporating the streamline-upwind Petrov-Galerkin (SUPG) scheme. The generated thermal field is entirely due to viscous heating. Essential flow characteristics, including temperature distribution in the flow field, are predicted. The resulting average Nusselt numbers along the walls are obtained. Subsequently, the effects of flow-rate and geometry are investigated.     

Effect of electrode configuration on the thermal behavior of a lithium-polymer battery

A thermal modeling was performed to study the effect of the electrode configuration on the thermal behavior of a lithium-polymer battery. It was examined the effect of the configuration of the electrodes such as the aspect ratio of the electrodes and the placing of current collecting tabs as well as the discharge rates on the thermal behavior of the battery. The potential and current density distribution on the electrodes of a lithium-polymer battery were predicted as a function of discharge time by using the finite element method. Then, based on the results of the modeling of potential and current density distributions, the temperature distributions of the lithium-polymer battery were calculated. The temperature distributions from the modeling were in good agreement with those from the experimental measurement for the batteries with three different types of electrodes at the discharge rates of 1C, 3C, and 5C.     

Finite difference thermal model of a latent heat storage system coupled with a photovoltaic device: Description and experimental validation

The use of photovoltaic (PV) systems has been showing a significant growth trend but for a more effective development of this technology it is essential to have higher energy conversion performances. Producers of PV often declare an higher efficiency respect to real conditions and this deviation is mainly due to the difference between nominal and real temperature conditions of the PV. To improve the solar cell energy conversion efficiency many authors have proposed a methodology to keep lower the temperature of a PV system: a modified PV system built with a normal PV panel coupled with a Phase Change Material (PCM) heat storage device. In this paper is described a thermal model analysis of the PV–PCM system based on a theoretical study using finite difference approach. The authors developed an algorithm based on an explicit finite difference formulation of energy balance of the PV–PCM system. To this aim, a forward difference at time t and a first-order central difference for the space derivative at position x was used. Two sets of recursive equations were developed for two types of spatial domains: a boundary domain and an internal domain .The reliability of the developed model is tested by a comparison with data coming from a test facility. Results of this experience confirm the performed numerical simulations and show that the proposed model is valid and can be used to determine the thermal behaviour of a solar cell coupled with a PCM heat storage device.     

Stress intensity factors in a finite cracked cylinder made of functionally graded materials

In this article, elastic analysis for a cracked cylinder made of functionally graded materials (FGMs) is carried out. The crack is internal fully circumferential or external fully circumferential. The cylinder is subject to a longitudinal tension. The property of the FGMs is assumed to be exponential function in form. The finite element method is developed to solve the boundary value problem. An indirect method, the energy release method, is developed to evaluate the stress intensity factors (SIFs). The results obtained show that the property of FGMs has a significant influence on the value of SIF. Numerical results are given to assess the safety of the cracked cylinder made of FGMs.     

Finite element solution of radiative transfer across a slab with variable spatial refractive index

To avoid the complicated computation of ray trajectories, a finite element formulation is developed to solve the radiative transfer problem in a one-dimensional absorbing-emitting-scattering semitransparent slab with variable spatial refractive index. A problem of radiative equilibrium is taken as an example to verify this finite element formulation. The predicted temperature distributions are determined by the proposed method and compared with the data in references. The results show that the finite element formulation presented in this paper has good accuracy in solving the radiative transfer in one-dimensional absorbing-emitting-scattering semitransparent medium with variable spatial refractive index.     

Residual stresses in the welding joint of the nozzle-to-head area of a layered high-pressure hydrogen storage tank

In this study, a sequentially coupling finite element analysis (FEA) program was utilized to simulate the welding temperature and predict the residual stress in the weld and Heat Affect Zone (HAZ) between the nozzle and the head. The results of a numerical calculation indicated that complex residual stresses were generated and concentrated in the weld and HAZ. Due to the existence of the interlayer gap between the two layers of the head, discontinuous stress distributions occurred. There were several sections where there are stress concentrations because of the discontinuous structure. The influences of welding heat input and preheating temperature were also investigated in this study. When the preheating temperature increased, the peak residual stresses in the structure decreased. The welding heat input had little effect on the residual stresses.