Commercial CFD Tool Research Articles

Experimental and numerical investigation of a free rising droplet

A toluene droplet rising in a continuous aqueous phase is studied both experimentally and numerically. The experiments have been performed in a small scale, high-speed measuring unit with a high magnification device to image the process in all details. Numerical simulations are performed with the aid of the level set method implemented in the commercial CFD tool COMSOL Multiphysics 3.3a by COMSOL AB. A new technique for quantitative comparison of experimental and numerical results with respect to the droplet shape is presented. Both terminal velocity and droplet shape in experiments and simulations are found to be in excellent agreement. Based on the experimental and numerical data obtained, an improvement of an existing terminal velocity correlation is suggested. The adjusted correlation can be used for the design and optimisation of liquid–liquid extraction units.

Study of multiple injections in (Homogeneous charge compression ignition) HCCI engine using computational fluid dynamics

Due to the stringent emission norms, the research in the field of internal combustion engines in general and diesel engines in particular gathered huge importance and also increasing demand on fuel consumption the importance of detailed simulation of fuel injection, mixing and combustion have been increased in the recent years. In this research a CFD simulation is carried out on direct injection diesel engine to study the flow, combustion and emissions. In a diesel engine the flow pattern, that is, the turbulence inside the engine will control the combustion and thereby the emission mainly NOx, SOx and Soot. So it is very important to study the flow phenomenon first before the emission study is carried out. In this work an attempt is made to study the flow patterns for a cylinder design using split injection. Commercial CFD tool FLUENT is used for numerical simulation. It solves the basic governing equations of fluid flow that is continuity, momentum, species transport and energy equation. Using finite volume method Turbulence is modeled by using standard k-e model. Injection is modeled using Lagrangian system. The reaction is modeled using non–premixed combustion which considers the effects of turbulence and detailed chemical mechanism into account to model the reaction rates. The specific heats for all the species is approximated by using piecewise polynomials. In this research simulation has been carried out for triple injection and compared with experimental results and found that simulation results of triple injections have shown good agreement. Key words: Multi injection, pilot injection, triple injection, duration of injection, CFD.

Modelling and simulation of the near-wall velocity of a turbulent ceiling attached plane jet after its impingement with the corner

At present, ceiling-mounted diffusers are very popular for indoor air distribution, particularly in offices, owing to greater efficiency in the distribution of the air supply and a more comfortable indoor environment. The objective of this study is to construct an effective model to design the indoor airflow of an attached plane jet after its impingement with the corner in a room. In this study, a full-scale test facility was set up to obtain detailed experimental data. One commercial CFD tool, CFX 11.0, was used to simulate the air velocity distribution of an attached plane air jet bounded by the ceiling and an insulated wall. One semi-empirical model was also constructed to predict the impingement jet velocity. The results show that bout the semi-empirical model and CFX 11.0 were able to predict the maximum velocity of an impinging jet at low Reynolds numbers, 1000 and 2000, with an inaccuracy of ±11%. However, the semi-empirical model could be more conveniently used to predict the maximum jet velocity decay after its impingement the corner in a room than CFD simulation in terms of accuracy and the time required to design the indoor airflow pattern.

Studies on numerical site calibration over complex terrain for wind turbines

The estimation of wind turbine performance over complex terrain is very difficult because of the document of standard IEC61400-12 is adapted for flat or slightly complex topography. And the cost of constructing a meteorological mast is higher with scaling wind turbine up. We have proposed a numerical site calibration (NSC) technique in order to estimate the inflow velocity at the position of wind turbine by using CFD tool to calculate the flow field around the site. The present paper shows the problems for the procedure of NSC in which a commercial nonlinear CFD tool and the improvement method are used to gain a more accurate result. It is clarified that the wind turbine performance which is estimated by using the wind speed on the meteorological mast has a good result for annual energy production.

모세관 인력으로 작동되는 열공압형 마이크로 펌프의 형상이 성능에 미치는 영향

Recently, we developed a simple thermopneumatic micropump having neither a membrane nor a valve. This micropump discharges liquid by a thermopneumatic pressure and refill by a capillary attraction. In case of the micropump driven by the capillary attraction, the flow characteristic depends mainly on the geometry of the micropump. In this paper, we investigated the influence of the geometry of the micropump on the performance of the micropump to illustrate the properness of the micropump shape. We analyzed the micropump characteristics of six types having different geometries by FVM simulation with a commercial CFD tool. Also we fabricated the micropumps with PDMS and glass by micromachining, and tested the performances. The simulation and the test results illustrate that the discharge volume and the discharge time depend on the chamber volume. The expansion angle of the inlet channel location has influence on the refill time, while the front air channel direction has influence on the backward flow loss.

Numerical investigation on the combustion behaviour of pre-dried Greek lignite

Dry coal firing is expected to play an increasingly important role in future lignite power plants. The planned evolution from the conventional lignite drying concept with hot recirculated flue gas to the “fluidized bed drying with internal heat utilization, WTA”, technology in the next generation of lignite power plants is estimated to bring an additional efficiency increase of 2–4% points compared to the today’s state of the art. In this framework NTUA/LSB and CERTH/ISFTA has performed experimental investigations at a semi industrial scale 1 MWth facility on the characterization of Greek pre-dried lignite’s combustion behaviour in terms of temperature fields, heat transfer, emissions, slagging and fouling tendency and residues quality. The present work focuses on the numerical investigation of Greek dry lignite combustion firstly by post-processing and evaluation of available experimental data and secondly by combustion simulations. Q–T plots describing the heat transfer in the experimental facility are derived and specific cases of the performed tests are simulated with a commercial CFD tool, in order to estimate flow, temperature fields, NOx emissions and compare with the available experimental data. A good agreement between simulated and experimental results will support the further work on large scale boiler simulations in raw and dry coal co-firing mode, where the possibility of validation with experimental data is limited. The obtained Q–T diagrams are used to evaluate the influence of co-firing on the heat transfer in the facility and to further extrapolate the conclusions of the performed semi industrial tests on the large scale. The overall results of the CFD simulations, including predictions of temperature and NOx profiles, are in good agreement with the available experimental data at the reference case, while at the dry coal co-firing cases succeed on reproducing the basic trends of the performed experiments.

A coupled finite volume method for the computational modelling of mould filling in very complex geometries

Simulation of shape casting processes involves the simultaneous capture of the rapidly developing metal–air free surface and the liquid–solid moving boundary in the metal phase. Although a wide range of techniques have been developed to address this problem, there are occasions where existing techniques are limited, especially in very complex geometries where the mesh quality may inevitably be relatively poor at some key locations. Cell-centred finite volume methods, typical of most commercial CFD tools, are computationally efficient, but can lead to convergence problems on meshes that feature cells with highly non-orthogonal shapes. The control volume–finite element method uses a vertex-based approach and handles distorted meshes with relative ease, but is computationally expensive. A combined vertex-based–cell-centre technique, described in this paper, solves the flow field at the cell or element vertex, and all other variables are solved at the cell centre (as in conventional CFD tools). This flow solver strategy is much more tolerant of meshes with poor quality elements, allowing solutions on distorted meshes where purely cell-centred solutions procedures fail. The combined method enables the solution of simultaneous free surface flows coupled with solidification in very complex meshes, thus enabling the capture of such behavior in arbitrarily complex geometrical structures. It is relatively straightforward to embed within generic CC based CFD tools allowing it to be employed in a wide variety of processing applications.

Detailed CFX-5 study of the coolant mixing within the reactor pressure vessel of a VVER-1000 reactor during a non-symmetrical heat-up test

As part of the reactor dynamics activities of FZK/IRS, the qualification of a detailed 3D CFD model of a reactor pressure vessel is a key step in safety evaluations for improving predictive capabilities and acceptability of commercial CFD tools in reactor physics. The VVER-1000 Coolant Transient Benchmark, initiated by OECD, represents an excellent opportunity for validation. In this work a CFD model for the complete VVER-1000 reactor pressure vessel is presented. Due to computational limits simplifications of the core and of some other geometrical details are introduced. The simulated scenario is the heat-up of one coolant loop in case of the isolation of a steam generator while the reactor is operating at a low power level. Two transient runs with a first and second order approximation of the spatial discretization are performed. Unexpectedly, the first order method reveals better agreement with measured data.

Three-dimensional heat and mass transfer analysis in an air-breathing proton exchange membrane fuel cell

There is strong interaction between heat/mass transfer and electrode dynamics in an air-breathing proton exchange membrane fuel cell (PEMFC). To investigate the heat/mass transfer characteristics of PEMFC, their effect on oxygen transport and then on the performance of the cell, a coupled three-dimensional (3D) mathematical model has been developed for an air-breathing PEMFC using non-dimensional heat/mass transfer coefficients, and a detailed interpretation of heat/mass transfer parameters in it has been done. The full elliptic Navier Stokes and energy equations are simultaneously solved in the composite domain with commercial CFD tool STAR-CD based on the finite volume numerical method. And the model has been confirmed by experimental results. From the numerical simulation results obtained, a simple equation has been suggested on the function of the sum of Grashof number and diffusional Grashof number for the calculation of dimensionless mass transfer coefficient Sherwood number. The equation is believed to be useful for a profound understanding of the nature of oxygen transport limitation coupled with the heat/mass transfer in the air-breathing PEMFC, and further for the optimum design of the air-breathing PEMFCs. The concentration over-potential as a function of limited oxygen mass transfer rate, as well as profiles of velocity, temperature, reactant and water concentration, current density are presented and discussed to analyze the coupling problems between heat/mass transfer and electrode dynamics in an air-breathing PEMFC.

A coupled finite volume method for the solution of flow processes on complex geometries

AbstractCFD modelling of ‘real‐life’ thermo‐fluid processes often requires solutions in complex three‐dimensional geometries, which can result in meshes containing aspects that are badly distorted. Cell‐centred finite volume methods (CC‐FV), typical of most commercial CFD tools, are computationally efficient, but can lead to convergence problems on meshes that feature cells with highly non‐orthogonal shapes. The control volume‐finite element method (CVFE) uses a vertex‐based approach and handles distorted meshes with relative ease, but is computationally expensive. A combined vertex‐based—cell‐centre technique (CFVM), detailed in this paper, allows solutions on distorted meshes where purely cell‐centred solutions procedures fail. The method utilizes the ability of the vertex‐based approach to resolve the flow field on a distorted mesh, enabling well established cell‐centred physical models to be employed in the solution of other transported quantities. The vertex‐based flow code is verified against a manufactured 3D solution and error norms are compared on meshes with various degrees of distortion. The CFVM method is validated with benchmark solutions for thermally driven flow and turbulent flow. Finally, the method is illustrated on three‐dimensional turbulent flow over an aircraft wing on a distorted mesh where purely cell‐centred techniques fail. The CFVM is relatively straightforward to embed within generic CC based CFD tools allowing it to be employed in a wide variety of processing applications. Copyright © 2006 John Wiley & Sons, Ltd.

Electric fields coupled with ion space charge. Part 2: computation

The coupled computation of the electric field distribution together with ion space charge is considered for the case of a point-plane arrangement, where ions are induced by a high-voltage needle. The general physical problem and its numerical implementation are focused as well as the usage of Peek's Law. The computations, which are destined for coupling with flow field simulations, are performed with a modified commercial CFD-Tool (FLUENT), based on the finite volume method. The numerical results are compared with experimental ones performed by means of the set-up presented in Meyer et al. (Electric fields coupled with ion-space-charge: I. Measurements, Electrostatics, 2005, accepted for publication). The results obtained are problematic, and therefore this method is limited to coupled simulation, if only a rough estimation of the electric field is needed or if a complex geometry avoids measurements. Additionally, the influence of ion wind is tested as well as different meshing techniques, which are all found to be of minor importance for the simulation of this arrangement.

CFD simulation tool for solid oxide fuel cells

A 3D simulation tool for solid oxide fuel cells is presented. The aim of this work is to predict current density, flow, temperature and concentration fields in order to compare and optimize repeat element geometry for a whole stack. A commercial CFD tool was used, solving mass, momentum and energy equations; whereas chemical kinetic equations are computed from external sub-routines. A steady-state case is presented, fed with hydrogen. The flow is laminar for both air and fuel. Radiative heat transfer is taken into account between inner surfaces. On boundaries, convective and radiative heat transfers are assumed at external surfaces between repeat element and oven. Due to the large range of dimensions (cells: 300 mum thick, gas channels: 1 mm height, whole cell: 80 mm x 80 mm) a fine mesh was needed. Data for conductivities and kinetics were estimated from experiments performed in- house. Simulation results are presented and compared to real repeat element test measurements for the current-potential characteristics.