Simplified Axisymmetric Models Research Articles

Comparison of PTS analyses of RPVs based on 3D-CFD and RELAP5

The integrity of a reactor pressure vessel related to pressurized thermal shocks is one of the most important issues for the assessment of life time extension of a nuclear power plant. The most critical scenario occurs during cold water injection through the cold leg due to a Loss-Of-Coolant Accident (LOCA). Due to the difficulties associated with the crack modeling with the three-dimensional finite element method (FEM), simple geometries and crack configurations are usually employed.In the present study, a hypothetical medium break LOCA is assumed in one of the hot legs for an adopted reference design of a two-loop pressurized water reactor. The boundary conditions obtained from RELAP5 calculations are used as input for the three-dimensional computational fluid dynamics simulations in order to provide three-dimensional temperature distribution for the structural mechanics analysis in which submodeling and eXtended FEM (XFEM) are applied. The results from these three-dimensional computations are compared with those from simplified axisymmetric models based on reference data temperatures.

Two-Phase Flow Modeling and Measurements in Low-Pressure Turbines—Part I: Numerical Validation of Wet Steam Models and Turbine Modeling

In this publication, an overview of the current state of wetness modeling at the Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM) is given. For the modeling, an Euler–Euler method implemented in the commercial flow solver Ansys CFX is used. This method is able to take into account the nonequilibrium state of the steam and models the interactions between the gaseous and liquid phases. This paper is the first part of a two-part publication and deals with the numerical validation of wet steam models by means of condensing nozzle and cascade flows. A number of issues with regard to the quality of the computational fluid dynamics (CFD) code and the applied condensation models are addressed comparing the results to measurements. It can be concluded that a calibration of the models is necessary to achieve a satisfying agreement with the experimental results. Moreover, the modeling of the low pressure model steam turbine operated at the ITSM is described focusing on the asymmetric flow field in the last stage caused by the axial–radial diffuser. Different simplified axisymmetric diffuser models are investigated in steady state simulations, and the results and the arising issues for part-load, design-load, and over-load conditions are discussed. Thereafter, a comparison between the equilibrium and nonequilibrium steam modeling approaches is performed and the advantage of the nonequilibrium model is highlighted. The second part of the publication focuses on experimental investigations and compares the numerical results to wetness measurement data. For this purpose, different loads are also considered.

Comparison of finite element models of natural hip joint

A three dimensional anatomic model of a human hip joint was created from CT scans and also two and three dimensional simple axisymmetric models of head-and-neck region of a natural hip joint were created by using finite element method. A comparison of the contact mechanics between articular cartilages of femur and pelvis, and stress distribution on bone were investigated for all models considered in this study. Finite element analyses of different models showed while the contact mechanics studies of simplified axisymmetric models have similar results compared to realistic anatomic model, the stress distribution studies have significant differences for different finite element models. Therefore, it is concluded that using of three dimensional anatomic finite element models to investigate the stress distribution of bone will give more realistic results than simple axisymmetric models. However, for the contact mechanics studies, all considered models showed good agreement in this study. Therefore it can be concluded that the axisymmetric finite element models can be used for contact mechanics studies and especially for comparative parametric studies because of their simple geometry and short computation time.

Investigation of solute concentrations in a 3D model of intervertebral disc

As the disc is the largest avascular structure in the body, disc cells depend for their normal function on an adequate supply of nutrients (oxygen and glucose) and the removal of metabolic by-products (lactic acid) via blood vessels at the cartilaginous endplates and annulus periphery. Concentration gradients develop depending on the balance between the rates of transport and rates of cellular activity. Since consumption and production rates are coupled via extracellular pH, the gradients are interdependent. This is a novel model study which takes into account the realistic 3D geometry of a L5-S1 lumbar disc in solving the nonlinear coupled diffusion equations. Effects of perturbations (calcification, sclerosis) in endplates, increases in cell metabolic rates following growth factor injection and changes in lumbar posture (kyphotic or lordotic) on extreme values of nutrient and metabolite concentrations and their spatial locations are investigated. Solute concentrations, particularly those of glucose, substantially diminish as a consequence of disturbances in supply at the endplates, increases in cell metabolic rate and more lordotic postures. Results, when compared to those from simplified axisymmetric models, demonstrate the importance of consideration of realistic 3D disc geometry.

Simulation of Pendant Droplets and Falling Films in Horizontal Tube Absorbers

Recent literature suggests that the droplets that form in horizontal-tube, falling-film absorbers play a major role in the absorption process. The performance of such absorbers is critical to the performance of many absorption heat pump systems. The simulation of droplets of aqueous Lithium Bromide pendant from horizontal tubes was performed by numerically solving the equations of motion on a fixed three-dimensional (3D) grid. The so-called volume of fluid method was used to handle the interface between the liquid and vapor phase. Results are compared with simplified axisymmetric models and with high speed video taken during flow visualization experiments. The results show that simplified axisymmetric models do not satisfactorily represent the evolution of the droplets under horizontal tubes, and that the 3D numerical model appears to accurately match the important characteristics of droplet formation, detachment, and impact observed in the experiments.

Magnetically Linked Star‐Disk Systems. I. Force‐free Magnetospheres and Effects of Disk Resistivity

We consider the interaction between a magnetic star and its circumstellar disk under the assumption that the stellar magnetic field permeates the disk and that the system's magnetosphere is force-free. Using simplified axisymmetric models (both semianalytic and numerical), we study the time evolution of the magnetic field configuration induced by the relative rotation between the disk and the star. We show that if both the star and the magnetosphere are perfectly conducting, then there is a maximum disk surface conductivity Σmax for which a steady state field configuration can be established. For larger values of conductivity, no steady state is possible, and the field lines inflate and effectively open up when a critical twist angle (which for an initially dipolar field is on the order of a few radian) is attained. We argue that for thin astrophysical disks, surface conductivities are likely to exceed the local Σmax except in the immediate vicinity of the corotation radius in a Keplerian disk. If the disk conductivity is high enough, then the radial magnetic field at the disk surface will become large and induce radial migration of the field lines across the disk. We find, however, that the radial diffusion in the disk is generally much slower than the field-line expansion in the magnetosphere, which suggests that the opening of the magnetosphere is achieved before the diffusive outward expulsion of the field lines from the disk can occur. The effects of magnetospheric inertial effects and of field-line reconnection are considered in the companion paper.

Comparison of the creep and damage failure prediction of the new, service-aged and repaired thick-walled circumferential CrMoV pipe welds using material properties at 640°C

Finite element creep and damage analyses were performed for a series of new, service-aged, fully repaired and partially repaired circumferential welds in CrMoV main steam pipes, under an internal pressure and a uniform axial stress, using simplified axisymmetric models. The material properties used were those of a 1/2Cr1/2Mo1/4V: 2 1/4Cr1Mo weldment at 640°C. Failure lives and failure positions were obtained using damage modelling and these were compared with corresponding results obtained from steady-state analyses. The effect of the weld width, in the practical range, and the effect of the axial (system) loading, on the failure life and position, of the various weld situations were also investigated. Comparison of the various failure predictions allows the effects of the differences in the relative properties of the constituents of the weld, the geometry and system loading, to be identified. The information is useful for assessing the performance of practical service-aged/repaired welds in power plants pipework.

Effects Of End Loading On The Creep FailureBehaviour Of CrMoV Welds In Main SteamPipelines

This paper describes an investigation into the effects of end (system) loading on the creep failure behaviour of internally pressurised thick walled pipe welds, using the FE method with simplified axisymmetric models. Steady-state and creep damage analyses were performed for a series of new, service-aged, and repaired welds in main steam pipes in power generation plants. The material properties used were related to CrMoV weldment materials at 640° C. Stress distributions were obtained and failure lives were predicted for each weld situation, under a closed-end condition and with additional axial loading. The rupture stresses showed very little variation across the heat-affected zone (HAZ), in the dominant stress regions, for closed-end loading. However, the variation is more significant when additional axial loading is applied. The results of damage variations across the HAZ in these critical regions confirmed the effect of end loading indicated by steady-state results. This suggests that, in addition to the pressure, for the welds investigated, there is a high probability of type IV cracking occurring in pressurised pipes, as the axial loading is increased, which is consistent with laboratory and power plant experience.

Dynamic stiffness and implications for assisting the operation of the left ventricle

The distribution of bending strain and stiffness in the wall of the left ventricle (LV) is relevant to the augmentation of its function by a modified skeletal-muscle wrap in the new surgical procedure of cardiomyoplasty. A novel approach to ventricular mechanics is presented which blends some finite-element results in engineering with new data available on ventricular geometry. Two simplified axisymmetric strip-element models of the LV are used to illustrate aspects of myocardial stiffness in the bending-strain-energy distribution and the effect on wrap synchronization of a change in cross-fibre stiffness when the heart has nonuniform or ectopic beats. The nonlinear and time-dependent nature of both camping and wall stiffness is derived from differential equations governing the dynamic paths from systole to diastole of finite wall elements around the periphery of an oblique LV slice using magnetic resonance imaging (MRI) data. This leads to a geometric method for determining these parameters. Results for time-dependent stiffnesses of elements in their trajectories are presented for a normal heart.