Mathematical modeling of the evaporator of two-phase heat transfer devices

This study focuses on the mathematical modeling of the evaporator section of the two-phase heat transfer devices: heat pipes, loop heat pipes and capillary pumped loops. Although the heat pipe technology made its first public appearance in the early forties, some operational aspects of two-phase systems are still not well understood, and research in this area continues. The evaporation and condensation process, taking place in these systems is among the most complex phenomena encountered in engineering applications. In this study, full three-dimensional incompressible energy, momentum and mass conservation equations are solved by using the finite element method to predict thermal operational characteristics of the two-phase heat transfer devices. The main focus of the study is the modeling of the phase transition region in the evaporator section. Copyrigh

Cellular simulation of the dendrite growth in Al-Si alloys

A cellular model for a computer simulation of dendrite growth in alloys is described. In this model temperature and concentration at the interphase boundary are not prescribed as boundary conditions but their evolution is calculated with the use of transport equations and a kinetic equation which relates the local solidification rate in each cell containing the interface with thermo-chemical conditions and an interface curvature averaged through the cell. The simulation was carried out for Al-Si alloys. The dependence of the growth velocity and tip radius on the supercooling are computed and compared with analytical model data

Mathematical modeling of the two-phase capillary-pumped heat transfer devices

The main objective of this study is to develop a mathematical model for the simulation of the thermal characteristics of two-phase capillary pumped devices. The mathematical model presented in this paper is an extension of the earlier mathematical model developed for a conventional heat pipe. The three-dimensional incompressible energy, momentum and mass conservation equations are solved by using the finite element method. Except in the wick region, the viscous terms in the governing equations are neglected. However, the pressure drops due to frictional losses are introduced. The interface between vapor and liquid phases is assumed static and only converged steady-state solutions are retained. The reservoir dynamic is not modeled. The energy, momentum and mass jump conditions are written across the interface. The resulting set of equations is solved iteratively until the overall mass conservation is satisfied between the evaporator and condenser. Copyrigh

Femoral fracture load and fracture pattern is accurately predicted using a gradient-enhanced quasi-brittle finite element model

Nonlinear finite element (FE) modeling can be a powerful tool for studying femoral fracture. However, there remains little consensus in the literature regarding the choice of material model and failure criterion. Quasi-brittle models recently have been used with some success, but spurious mesh sensitivity remains a concern. The purpose of this study was to implement and validate a new model using a custom finite element designed to mitigate mesh sensitivity problems. Six specimen-specific FE models of the proximal femur were generated from quantitative tomographic (qCT) scans of cadaveric specimens. Material properties were assigned a-priori based on average qCT intensities at element locations. Specimens were experimentally tested to failure in a stumbling load configuration, and the results were compared to FE model predictions. There was a strong linear relationship between FE predicted and experimentally measured fracture load (R2= 0.79), and error was less than 14% over all cases. In all six specimens, surface damage was observed at sites predicted by the FE model. Comparison of qCT scans before and after experimental failure showed damage to underlying trabecular bone, also consistent with FE predictions. In summary, the model accurately predicted fracture load and pattern, and may be a powerful tool in future studies

Qualitative characterization of fatigue damage propagation in laminated carbon fibre reinforced polymers by using micro-computed tomography

Whilst quality standards and damage tolerance techniques are unique for each industry, within the aircraft industry there is an overarching need to maintain flight safety. As the aircraft industry continues to strive for improved fuel efficiency and reduced airframe weight there is growing use of advanced laminated carbon fibre reinforced polymers (CFRP) in all types of structures. Therefore, it is essential to predict the future mechanical behaviour and service life of "flight critical structures" made from CFRP in such applications. Experimentally validated failure simulation models are not abundant in the literature for modern CFRP laminates and complex structures. Neither is there abundant quantitative data capable of defining the characteristics of initial microscopic damage evolution in such orthotropic laminates. Early prediction and modelling of damage nucleation and subsequent evolution is thus precluded due to insufficient experimental data. In this work, it is proposed to analyze the 3D microscopic damage behaviour observed in CFRPs subjected to service-replicated (e.g. spectrum) fatigue loading. Probing the evolution of the cracks and damage was performed using X-ray micro-computed tomography (XR-microCT) which proved to be a reliable non-destructive testing (NDT) tool for qualitatively visualizing and monitoring damage mechanisms relative to the fatigue life. Findings may be potentially employed to develop and validate enrichment functions developed to span the asymptotic fields at the crack-tip for implementation in X-FEM to simulate combined damage mechanisms. The work described in this paper is the first step in a larger project to quantitatively examine fatigue damage progression in advanced aerospace composites. Copyright 2014 by Naglaa ElAgamy

A macro-micro model of fusion zone microstructure evolution in Mn-C low-alloy steel coupled with thermal stress analysis

A macro-micro-model for microstructure evolution in the fusion zone of a l.2 Mn and 0.11 C low-alloy steel is described. The macro-model is a 3D transient thermal analysis of a welded structure that resolves the weld pool with element size greater than 1 mm and time steps greater than 1 second. The micromodel has cell size of about 1 micron and time step size of about 10 micro-seconds with a grid of about 80×80×500 cells. The micro model is positioned on the liquid-solid interface of the weld pool in the macro-model. The boundary conditions for the micro-model are mapped from the macro-model. The micro-model solves the 3D transient solute diffusion equations for Mn and C. The micro-model computes the liquid-solid interface movement with local velocities determined by local temperature, compositions of solid and liquid phases and interface curvature to predict columnar or dendritic solidification structures. As the solid cools from the melting point to room temperature, the evolution of austenite, ferrite, pearlite, bainite and martensite phases are computed. The 3D transient stress due to temperature and phase changes is computed in the micro-model as it cools from the melting temperature to room temperature. At room temperature a micro-model tensile test is run to 4% strain. The macro-stress and strain is compared to the micro-stress and strain distributions. The model is intended to be used to initialize models of fracture, fatigue and creep in weld fusion zones

Computational weld mechanics of hot crack nucleation in nickel-based welds

Computational weld mechanics (CWM) is used to estimate the likelihood of hot crack nucleation in a weld joint. A hot crack nucleates when the evolution of the local state of stress, strain, temperature and microstructure in the hot cracking temperature region reaches a critical value. The local evolution of state is determined by a high-resolution 3D transient CWM analysis and compared to experimental data characterizing the material resistance for each type of hot cracking. This paper evaluates the susceptibility to ductility dip cracking (DDC) and solidification cracking, separately, for single bead-on-plate welds of nickel-based alloys (FM82 and Inconel 600). An algorithm determines the hot cracking risk based on the temperature, temperature profile, strain increment, and rate of strain in the hot cracking temperature region. The critical values are obtained from the existing experimental data. The objective is to demonstrate that CWM can be used in the design stage to choose weld parameters, such as weld speed, to reduce the risk of hot cracking for a given material, weld joint and weld structure

The L2 norm of the deviation between the measured and computed transient displacement field in a test weld

The transient displacement field caused by an arc weld was measured experimentally with an Aramis stereo-camera. The transient displacement field was also predicted using a computational weld mechanics program called VrWeld for ten different sets of model parameters. The objective was to identify the effect of model parameters on the deviation between experimental data and the data computed by computational weld mechanics. For each set of model parameters, the L2 normof the difference between the experimental and computed displacement fields was computed for each of the 800 time steps. The structure being welded was a 50 × 600 × 10 mm stringer fillet welded to a 300 × 600 × 10 mm low alloy steel plate. The displacement field was measured on the 'back' surface of the plate. A robot made the metal-inert-gas weld. The parameters varied in the ten models included welding current, width of the power density d