Simulation Of Phase Change Research Articles

Benchmarks and numerical methods for the simulation of boiling flows

Comparisons of different numerical methods suited to the simulations of phase changes are presented in the framework of interface capturing computations on structured fixed computational grids. Due to analytical solutions, we define some reference test-cases that every numerical technique devoted to phase change should succeed. Realistic physical properties imply some drastic interface jump conditions on the normal velocity or on the thermal flux. The efficiencies of Ghost Fluid and Delta Function Methods are compared to compute the normal velocity jump condition. Next, we demonstrate that high order extrapolation methods on the thermal field allow performing accurate and robust simulations for a thermally controlled bubble growth. Finally, some simulations of the growth of a rising bubble are presented, both for a spherical bubble and a deformed bubble.

Prediction of Mass Transfer Time Relaxation Parameter for Boiling Simulation on the Shell-Side of LNG Spiral Wound Heat Exchanger

The objective of this present study is to propose an approach to predict mass transfer time relaxation parameter for boiling simulation on the shell-side of LNG spiral wound heat exchanger (SWHE). The numerical model for the shell-side of LNG SWHE was established. For propane and ethane, a predicted value of mass transfer time relaxation parameter was presented through the equivalent evaporation simulations and was validated by the Chisholm void fraction correlation recommended under various testing conditions. In addition, heat transfer deviations between simulations using the predicted value of mass transfer time relaxation parameter and experiments from Aunan were investigated. The boiling characteristics of SWHE shell-side were also visualized based on the simulations with VOF model. The method of predicting mass transfer time relaxation parameter may be well applicable to various phase change simulations.

Lattice-Boltzmann-based two-phase thermal model for simulating phase change

A lattice Boltzmann (LB) method is presented for solving the energy conservation equation in two phases when the phase change effects are included in the model. This approach employs multiple distribution functions, one for a pseudotemperature scalar variable and the rest for the various species. A nonideal equation of state (EOS) is introduced by using a pseudopotential LB model. The evolution equation for the pseudotemperature variable is constructed in such a manner that in the continuum limit one recovers the well known macroscopic energy conservation equation for the mixtures. Heats of reaction, the enthalpy change associated with the phase change, and the diffusive transport of enthalpy are all taken into account; but the dependence of enthalpy on pressure, which is usually a small effect in most nonisothermal flows encountered in chemical reaction systems, is ignored. The energy equation is coupled to the LB equations for species transport and pseudopotential interaction forces through the EOS by using the filtered local pseudotemperature field. The proposed scheme is validated against simple test problems for which analytical solutions can readily be obtained.

Finite Element Simulative Analysis on the Influence of Solid-State Transformation on Welding Residual Stress

This paper takes the Q345 steel as an example, adopting finite element simulative analysis to study the influence of solid-state transformation on welding residual stress. By setting the value of the thermal strain in different temperature, the change in volume caused by the phase changes is equivalent to the thermal strain. Simulation includes two cases which are consideration of phase transformations and not consideration. The results showed that the distribution trend of the longitudinal stress of the weld zone is substantially the same in the two simulations. In the case of not consider the simulation of phase change, there is a lot of stress in the weld zone and the heat affected zone and the maximum value could be 427 MPa. In regard to transverse stress, phase change not only affects the value of the stress, but also changes the direction of the stress of the weld middle portion. Welding residual stress is also measured by X-ray. Phase change simulation and experimental results are in good agreement, it can be concluded that phase change in the welding process will result in a significant impact on the distribution of the residual stress, which could not be ignored in the finite element simulation of welding process.

Extended lattice Boltzmann method for numerical simulation of thermal phase change in two-phase fluid flow

In this article, a method based on the multiphase lattice Boltzmann framework is presented which is applicable to liquid-vapor phase-change phenomena. Both liquid and vapor phases are assumed to be incompressible. For phase changes occurring at the phase interface, the divergence-free condition of the velocity field is no longer satisfied due to the gas volume generated by vaporization or fluid volume generated by condensation. Thus, we extend a previous model by a suitable equation to account for the finite divergence of the velocity field within the interface region. Furthermore, the convective Cahn-Hilliard equation is extended to take into account vaporization effects. In a first step, a D1Q3 LB model is constructed and validated against the analytical solution of a one-dimensional Stefan problem for different density ratios. Finally the model is extended to two dimensions (D2Q9) to simulate droplet evaporation. We demonstrate that the results obtained by this approach are in good agreement with theory.

211 相変化を考慮した固体高分子型燃料電池内の水輸送解析

211 相変化を考慮した固体高分子型燃料電池内の水輸送解析

Coupled Level-Set/Volume-of-Fluid Method for Simulation of Injector Atomization

This paper presents results of a multiphase computational fluid dynamics code using a coupled level-set/volume-of-fluid method to simulate liquid atomization. This interface-capturing approach combines the mass conservation properties of the volume-of-fluid method with the accurate surface reconstruction properties of the level-set method, and it includes surface tension as a volume force calculated with second-order accuracy. Developed by one of the authors, the multiphase code builds upon the combined level-set/volume-of-fluid methodology to enable bubbly flow, liquid breakup, and phase-change simulations. The extension presented in this paper couples a Lagrangian dispersed phase model for postbreakup tracking of droplets with block-structured adaptive mesh refinement on the Eulerian grid. Under an appropriate set of criteria, the transfer of droplets representation from the Eulerian to the Lagrangian discretization enables the simulation of sprays on larger domains and for longer physical times without degrading the overall accuracy of the calculation. A demonstration for like-on-like jet impingement of relevance to cold startup of liquid rocket engines is presented here at low, moderate, and high injection velocities, including validation with experimental data.

Modelling and simulation of liquid-vapor phase transition in compressible flows based on thermodynamical equilibrium

In the present work we investigate the numerical simulation of liquid-vapor phase change in compressible flows. Each phase is modeled as a compressible fluid equipped with its own equation of state (EOS). We suppose that inter-phase equilibrium processes in the medium operate at a short time-scale compared to the other physical phenomena such as convection or thermal diffusion. This assumption provides an implicit definition of an equilibrium EOS for the two-phase medium. Within this framework, mass transfer is the result of local and instantaneous equilibria between both phases. The overall model is strictly hyperbolic. We examine properties of the equilibrium EOS and we propose a discretization strategy based on a finite-volume relaxation method. This method allows to cope with the implicit definition of the equilibrium EOS, even when the model involves complex EOS’s for the pure phases. We present two-dimensional numerical simulations that shows that the model is able to reproduce mechanism such as phase disappearance and nucleation.

Computer animation challenges for computational fluid dynamics

Computer animation requirements differ from those of traditional computational fluid dynamics (CFD) investigations in that visual plausibility and rapid frame update rates trump physical accuracy. We present an overview of the main techniques for fluid simulation in computer animation, starting with Eulerian grid approaches, the Lattice Boltzmann method, Fourier transform techniques and Lagrangian particle introduction. Adaptive grid methods, precomputation of results for model reduction, parallelisation and computation on graphical processing units (GPUs) are reviewed in the context of accelerating simulation computations for animation. A survey of current specific approaches for the application of these techniques to the simulation of smoke, fire, water, bubbles, mixing, phase change and solid–fluid coupling is also included. Adding plausibility to results through particle introduction, turbulence detail and concentration on regions of interest by level set techniques has elevated the degree of accuracy and realism of recent animations. Basic approaches are described here. Techniques to control the simulation to produce a desired visual effect are also discussed. Finally, some references to rendering techniques and haptic applications are mentioned to provide the reader with a complete picture of the challenges of simulating fluids in computer animation.

Implementation of PISO algorithm for simulating unsteady cavitating flows

The present study focuses on the implementation of PISO algorithm to simulate cavitating flows. For simulation of unsteady behaviors of cavitation which have practical applications, the development of unsteady PISO algorithm based on the non-conservative approach is investigated. The effects of mixture compressibility are considered to improve accuracy of simulations. For multi-phase simulation, single-fluid Navier–Stokes equations, along with the volume fraction transport equation, are employed. The bubble dynamics model is utilized to simulate phase change. To prove capabilities of the developed PISO algorithm to simulate cavitating flows, unsteady simulation of cavitation around NACA0015 hydrofoil, a two dimensional flat plat, and a three dimensional circular disk are performed. The frequency of flow, pressure distributions, cavitating vortex shedding, and cavity characteristics are analyzed to discern results accuracy. To investigate accuracy of results, comparisons with available published experimental data are made and good agreement is achieved.

An approach to numerical simulation and analysis of molten corium coolability in a boiling water reactor lower head

This paper discusses an approach for application of the computational fluid dynamics (CFD) method to support development and validation of computationally effective methods for safety analysis, on the example of molten corium coolability in a BWR lower head. The approach consists of five steps designed to ensure physical soundness of the effective method simulation results: (i) analysis and decomposition of a severe accident problem into a set of separate-effect phenomena, (ii) validation of the CFD models on relevant separate-effect experiments for the reactor prototypical ranges of governing parameters, (iii) development of effective models and closures on the base of physical insights gained from relevant experiments and CFD simulations, (iv) using data from the integral experiments and CFD simulations performed under reactor prototypic conditions for validation of the effective model with quantification of uncertainty in the prediction results and (v) application of the computationally effective model to simulate and analyze the severe accident transient under consideration, including sensitivity and uncertainty analysis. Implementation of the approach is illustrated on a so-called effective convectivity model for simulation of turbulent natural convection heat transfer and phase changes in a decay-heated corium pool. It is shown that detailed information obtained from the CFD simulations are instrumental to ensure the effective models capture safety-significant local phenomena, e.g. the enhanced downward heat flux in the vicinity of a cooled control rod guide tube.

The effective convectivity model for simulation of melt pool heat transfer in a light water reactor pressure vessel lower head. Part I: Physical processes, modeling and model implementation

This paper, and its companion paper [Tran C.T., Dinh, T.N. The effective convectivity model for simulation of melt pool heat transfer in a light water reactor pressure vessel lower head. Part II: Model assessment and application. Progress in Nuclear Energy (companion paper), in preparation] document the development, validation and applications of a simulation platform for computationally-effective, sufficiently-accurate numerical predictions of core melt-structure-water interactions in the light water reactor lower head during a postulated severe core-melting accident. The centerpiece of this work is the Effective Convectivity Model (ECM) for description of energy splitting in a core melt pool. Built on the concept of characteristic velocities in Effective Convectivity Conductivity Model and supported by the key findings obtained from Computational Fluid Dynamics (CFD) simulations of turbulent natural convection, heat transfer and phase changes in volumetrically heated liquid pools, the ECM is refined and extended to three-dimensions and phase changes to enable simulations of melt pool formation and corium coolability in complex geometry such as a Boiling Water Reactor (BWR) lower plenum.

Drag force on a flat plate in cavitating flows

The present study focuses on the simulation of two dimensional unsteady cavitating flows. For simulation of unsteady behaviors of cavitation which have practical applications, the development of unsteady PISO algorithm based on the non-conservative approach is utilized. For multi-phase simulation, single-fluid Navier–Stokes equations, along with the volume fraction transport equation, are employed. The bubble dynamics model is utilized to simulate phase change between vapor and liquid phases of the water. Unsteady simulation of cavitation around NACA66(MOD) and supercavitation around a flat plate normal to flow direction are performed to clarify accuracy of presented model. Numerical results and comparisons with experimental data are provided. The accuracy is good, and it is possible to apply this method to more complex shapes.

CFD Model of Apple Atmospheric Freeze Drying at Low Temperature

An Internet-controlled atmospheric freeze dryer was designed, built, and tested in the Department of Heat and Mass Transfer, Technical University of Łódź. A film sublimation-based CFD model was developed and verified using Fluent 6.1 commercial CFD software. The model enables the simulation of phase change and water vapor diffusion process within porous media. Transport of non-condensable species can be calculated with species transport inbuilt model of Fluent 6.1 and used to predict sublimation rate under given conditions. Results were compared with AFD experimental data of 10-mm Idared apple cubes. The viability of applying the film sublimation model to atmospheric freeze-drying process was demonstrated. Higher mass flux was found on the leading edge, relatively uniform mass flux within the porous zone illustrates that vapor diffusion dominates atmospheric freeze drying process at low temperature (below 0°C). CFD results for apple cubes show a predomination of diffusional resistance of porous tissue.

Numerical Simulation of Phase Change at High Hydrostatic Pressure under Variable-Gravity Environment

We present a numerical investigation of phase change of water at high hydrostatic pressure conditions occurring under a gravitational environment and a low-gravity environment. A computational model consists of conservation equations of mass, momentum, and energy. Phase-transition phenomena are modeled with an enthalpy–porosity approach. Numerical solutions are obtained for process stages dominated by free convection as well as by forced convection. A high pressure-assisted freezing / thawing is carried out in the pressure range from the ambient pressure up to 150 MPa. Differences in the thermofluid dynamics observed in the high-pressure chamber operated under gravity and low-gravity conditions are discussed. The results reveal significant influence of gravity on the distribution of temperature and velocity and consequently on the different ice growth characteristics.

Scenarios of heterogeneous nucleation and growth studied by cell dynamics simulation

The dynamics of phase transformation due to homogeneous nucleation has long been analyzed using the classic Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory. However, the dynamics of phase transformation due to heterogeneous nucleation has not been studied systematically even though it is vitally important technologically. In this report, the author studies the dynamics of heterogeneous nucleation theoretically and systematically using the phenomenological time-dependent Ginzburg-Landau (TDGL)-type model combined with the cell dynamics method. In this study the author focuses on the dynamics of phase transformation when the material is sandwiched by two supporting substrates. This model is supposed to simulate phase change storage media. Since both homogeneous and heterogeneous nucleations can occur simultaneously, the author predicts a few scenarios of phase transformation including homogeneous nucleation regime, heterogeneous nucleation regime, and the homogeneous-heterogeneous coexistence regime. These predictions are directly confirmed by numerical simulation using the TDGL model. The outcome of the study was that the KJMA formula has limited use when heterogeneous nucleation exists, but it could still give some information about the microscopic mechanism of phase transformation at various stages during phase transformation.

A numerical model for heat sinks with phase change materials and thermal conductivity enhancers

The effectiveness of thermal conductivity enhancers (TCEs) in improving the overall thermal conductance of phase change materials (PCMs) used in cooling of electronics is investigated numerically. With respect to the distribution of TCE and PCM materials, the heat sink designs are classified into two types. The first type of heat sink has the PCM distributed uniformly in a porous TCE matrix, and the second kind has PCM with fins made of TCE material. A transient finite volume method is used to model the heat transfer; phase change and fluid flow in both cases. A generalized enthalpy based formulation and numerical model are used for simulating phase change processes in the two cases. The performance of heat sinks with various volume fractions of TCE for different configurations is studied with respect to the variation of heat source (or chip) temperature with time; melt fraction and dimensionless temperature difference within the PCM. Results illustrate significant effect of the thermal conductivity enhancer on the performance of heat sinks.

Thermal Analysis and Structural Design of Phase Change Random Access Memory

AbstractA thermal modeling based on Finite Element Method (FEM) was used to simulate Phase Change Random Access Memory (PCRAM) cells. The factors affecting temperature distribution of a PCRAM cell structure such as geometry, device structure and the properties of the individual materials were investigated. The results of the analysis provided the fundamental design of a novel cell structure which has a better performance and reliability.

Simulated microgravity affects passive incremental stiffness and spindle efficacy of the rat soleus muscle

Recent developments in scientific computing now allow to consider realistic applications of numerical modelling to medicine. In this work, a numerical method is presented for the simulation of phase change occurring in cryosurgery applications. The ultimate goal of these simulations is to accurately predict the freezing front position and the thermal history inside the ice ball which is essential to determine if cancerous cells have been completely destroyed. A semi-phase field formulation including blood flow considerations is employed for the simulations. Numerical results are enhanced by the introduction of an anisotropic remeshing strategy. The numerical procedure is validated by comparing the predictions of the model with experimental results.

Numerical prediction of freezing fronts in cryosurgery: Comparison with experimental results

Recent developments in scientific computing now allow to consider realistic applications of numerical modelling to medicine. In this work, a numerical method is presented for the simulation of phase change occurring in cryosurgery applications. The ultimate goal of these simulations is to accurately predict the freezing front position and the thermal history inside the ice ball which is essential to determine if cancerous cells have been completely destroyed. A semi-phase field formulation including blood flow considerations is employed for the simulations. Numerical results are enhanced by the introduction of an anisotropic remeshing strategy. The numerical procedure is validated by comparing the predictions of the model with experimental results.