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Standard K (standard + k)

Distribution by Scientific Domains

Engineering	70%
Chemistry	30%

Selected Abstracts

Assessment of two-equation turbulence modelling for high Reynolds number hydrofoil flows

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2004
N. Mulvany
Abstract This paper presents an evaluation of the capability of turbulence models available in the commercial CFD code FLUENT 6.0 for their application to hydrofoil turbulent boundary layer separation flow at high Reynolds numbers. Four widely applied two-equation RANS turbulence models were assessed through comparison with experimental data at Reynolds numbers of 8.284×106 and 1.657×107. They were the standard k,,model, the realizable k,,model, the standard k,,model and the shear-stress-transport (SST) k,,model. It has found that the realizable k,,turbulence model used with enhanced wall functions and near-wall modelling techniques, consistently provides superior performance in predicting the flow characteristics around the hydrofoil. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Numerical simulation of turbulent free surface flow with two-equation k,, eddy-viscosity models

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 4 2004
V. G. Ferreira
Abstract This paper presents a finite difference technique for solving incompressible turbulent free surface fluid flow problems. The closure of the time-averaged Navier,Stokes equations is achieved by using the two-equation eddy-viscosity model: the high-Reynolds k,, (standard) model, with a time scale proposed by Durbin; and a low-Reynolds number form of the standard k,, model, similar to that proposed by Yang and Shih. In order to achieve an accurate discretization of the non-linear terms, a second/third-order upwinding technique is adopted. The computational method is validated by applying it to the flat plate boundary layer problem and to impinging jet flows. The method is then applied to a turbulent planar jet flow beneath and parallel to a free surface. Computations show that the high-Reynolds k,, model yields favourable predictions both of the zero-pressure-gradient turbulent boundary layer on a flat plate and jet impingement flows. However, the results using the low-Reynolds number form of the k,, model are somewhat unsatisfactory. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Verification testing in computational fluid dynamics: an example using Reynolds-averaged Navier,Stokes methods for two-dimensional flow in the near wake of a circular cylinder

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2003
Jennifer Richmond-Bryant
Abstract Verification testing was performed for various Reynolds-averaged Navier,Stokes methods for uniform flow past a circular cylinder at Re= 5232. The standard and renormalized group (RNG) versions of the k,, method were examined, along with the Boussinesq, Speziale and Launder constitutive relationships. Wind tunnel experiments for flow past a circular cylinder were also performed to obtain a comparative data set. Preliminary studies demonstrate poor convergence for the Speziale relationship. Verification testing with the standard and RNG k,, models suggests that the simulations exhibit global monotonic convergence for the Boussinesq models. However, the global order of accuracy of the methods was much lower than the expected order of accuracy of 2. For this reason, pointwise convergence ratios and orders of accuracy were computed to show that not all sampling locations had converged (standard k,, model: 19% failed to converge; RNG k,, model: 14% failed to converge). When the non-convergent points were removed from consideration, the average orders of accuracy are closer to the expected value (standard k,, model: 1.41; RNG k,, model: 1.27). Poor iterative and global grid convergence was found for the RNG k,,/Launder model. The standard and RNG k,, models with the Boussinesq relationship were compared with experimental data and yielded results significantly different from the experiments. Copyright © 2003 John Wiley & Sons, Ltd. [source]

URANS computations for an oscillatory non-isothermal triple-jet using the k,, and second moment closure turbulence models

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 9 2003
M. Nishimura
Abstract Low Reynolds number turbulence stress and heat flux equation models (LRSFM) have been developed to enhance predictive capabilities. A new method is proposed for providing the wall boundary condition for dissipation rate of turbulent kinetic energy, ,, to improve the model capability upon application of coarse meshes for practical use. The proposed method shows good agreement with accepted correlations and experimental data for flows with various Reynolds and Prandtl numbers including transitional regimes. Also, a mesh width about 5 times or larger than that used in existing models is applicable by using the present boundary condition. The present method thus enhanced computational efficiency in applying the complex turbulence model, LRSFM, to predictions of complicated flows. Unsteady Reynolds averaged Navier,Stokes (URANS) computations are conducted for an oscillatory non-isothermal quasi-planar triple-jet. Comparisons are made between an experiment and predictions with the LRSFM and the standard k,, model. A water test facility with three vertical jets, the cold in between two hot jets, simulates temperature fluctuations anticipated at the outlet of a liquid metal fast reactor core. The LRSFM shows good agreement with the experiment, with respect to mean profiles and the oscillatory motion of the flow, while the k,, model under-predicts the mixing due to the oscillation, such that a transverse mean temperature difference remains far downstream. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Numerical simulation of the vertical structure of discontinuous flows

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 1 2001
Guus S. Stelling
Abstract A numerical method to solve the Reynolds-averaged Navier,Stokes equations with the presence of discontinuities is outlined and discussed. The pressure is decomposed into the sum of a hydrostatic component and a hydrodynamic component. The numerical technique is based upon the classical staggered grids and semi-implicit finite difference methods applied for quasi- and non-hydrostatic flows. The advection terms in the momentum equations are approximated in order to conserve mass and momentum following the principles recently developed for the numerical simulation of shallow water flows with large gradients. Conservation of these properties is the most important aspect to represent near local discontinuities in the solution, following from sharp bottom gradients or hydraulic jumps. The model is applied to reproduce the flow over a step where a hydraulic jump forms downstream. The hydrostatic pressure assumption fails to represent this type of flow mainly because of the pressure deviation from the hydrostatic values downstream the step. Fairly accurate results are obtained from the numerical model compared with experimental data. Deviation from the data is found to be inherent to the standard k,, model implemented. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Hydrogen as burner fuel: modelling of hydrogen,hydrocarbon composite fuel combustion and NOx formation in a small burner

INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 11 2005
Mustafa Ilbas
Abstract The objective of this work is to investigate numerically the turbulent non-premixed hydrogen (H2) and hydrogen,hydrocarbon flames in a small burner. Numerical studies using Fluent code were carried out for air-staged and non-staged cases. The effects of fuel composition from pure hydrogen to natural gas (100%H2, 70%H2+30%CH4, 10%H2+90%CH4, and 100%CH4) were also investigated. The predictions are validated and compared against the experimental results previously obtained and results from the literature. Turbulent diffusion flames are investigated numerically using a finite volume method for the solution of the conservation equations and reaction equations governing the problem. Although, three different turbulence models were tested, the standard k,, model was used for the modelling of the turbulence phenomena in the burner. The temperature and major pollutant concentrations (CO and NOx) distributions are in good agreement with the existing experimental results. Air staging causes rich and lean combustion regions thus lower NOx emissions through the combustor exit. Blending hydrogen with methane causes considerable reduction in temperature levels and thus NO emissions. Increasing the mixture ratio from stoichiometric to leaner mixtures also decreases the temperature and thus NO emissions. Hydrogen may be considered a good alternative fuel for burners, as its use reduces the emission of pollutants, and as it is a renewable synthetic fuel. Copyright © 2005 John Wiley & Sons, Ltd. [source]

The performance of natural draft dry cooling towers under crosswind: CFD study

INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 2 2004
Rafat Al-Waked
Abstract The thermal performance of a natural draft dry cooling tower (NDDCT) under a crosswind has been investigated using a general-purpose CFD code. A three-dimensional study using the standard k,, turbulence model to simulate airflow in and around an NDDCT has been conducted. A parametric study has been carried out to examine the effect of crosswind velocity profile and air dry-bulb temperature on the thermal performance of an NDDCT. Two approaches have been considered in this study to quantify the crosswind effect. Firstly, simulations have been conducted at the nominal conditions and crosswind effect has been represented by thermal effectiveness parameter. Secondly, the ejected heat from the NDDCT has been maintained at a constant value (285 MW) and the crosswind effect has been represented by the change in the cooling tower approach parameter. After quantifying the effect of the crosswind on the thermal performance, windbreak walls have been introduced as a means of reducing this effect. The results in this paper show the importance of considering the crosswind velocity profile. Moreover, the introduction of windbreak walls has indicated an improvement in reducing the thermal performance losses due to the crosswind. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Computational and experimental fluid dynamics of jet loop reactor

AICHE JOURNAL, Issue 10 2009
Channamallikarjun S. Mathpati
Abstract A computational analysis using standard k-, model, RSM and LES has been carried out for jet loop reactors (JLR) to investigate the mean and turbulence quantities. These simulations have revealed that the flow in JLR was different from the self-similar round jets. RSM and LES showed better agreement with PIV measurements compared with standard k-, model. The modeled turbulence production and transport in k-, model overpredicted those estimated from LES data. To reduce the limitations, modified k-, models have been evaluated for JLR. Also, a hybrid k-, model has been suggested, which was found to perform better than other modified k-, models. This model was also found to hold for stirred tank reactors (STRs). Mixing time analysis has been carried out for JLR and STR at same power consumption. It has been shown that JLR can be inferior to STR if proper nozzle diameter is not selected. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

PDF simulations of ethylene decomposition in tubular LDPE reactors

AICHE JOURNAL, Issue 2 2005
Nitin H. Kolhapure
Abstract The present study deals with turbulent reacting flow simulation inside low-density polyethylene (LDPE) tubular reactors, based on a detailed computational fluid dynamics (CFD) technique,transported probability density function (PDF) methods. The ability of the PDF methods to provide an exact representation of chemical source terms is ideally suited for coupling complex LDPE chemistry with small-scale fluid dynamic fluctuations in turbulent flow. LDPE chemistry with a total of 16 scalars provides an ideal test case for illustrating the applicability of an efficient chemistry algorithm based on in-situ adaptive tabulation. A particle-based Monte Carlo algorithm is used to solve the joint-composition PDF equation, whereas a finite-volume code is used to obtain hydrodynamic fields from the standard k,, turbulence model. The influence of feed temperature, initiator concentration, and degree of premixing is investigated to gain detailed knowledge of micromixing effects on steady-state reactor performance. The computational approach provides a low-cost alternative to experimental and pilot-plant tests for exploring a variety of design options when making important design and operational decisions, or for investigating unstable reactor operating conditions. The ability of a simplified, but otherwise equivalent multi-environment-presumed PDF model to predict turbulence,chemistry interactions close to physical reality is validated using the detailed transported PDF simulations. The transported PDF method is shown to be an excellent tool for obtaining fundamental information on turbulent reacting flows, as well as for deriving simplified models for faster and easier interpretation of these flows when developing safe and efficient chemical processes. © 2005 American Institute of Chemical Engineers AIChE J, 51: 585,606, 2005 [source]

Sizing of Safety Valves Using ANSYS CFX-Flo®

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 2 2009
D. Moncalvo
Abstract This work discusses the effect of the degree of fineness of the flow volume discretization and that of the turbulence model on the accuracy of reproduction of air mass flow rates in two safety valves using the CFD software ANSYS Flo®. Calculations show that the degree of fineness of the discretization is the decisive factor affecting the exactness of the calculations and that the best reproduction is achieved with grids where at least two cells are built on the smallest edge. The selection of the turbulence model has by far in comparison a lower impact; however, the best accuracy is obtained using the standard k - , model and the SST modification of Menter. [source]