| 1. |
- Abbood, Sahar, et al.
(author)
-
Numerical study of turbulent forced convection in a finned tube with and without CuO nano fluid
- 2016
-
In: International Journal of Numerical Methods for Heat & Fluid Flow. - 0961-5539. ; 26:7, s. 2252-2270
-
Journal article (peer-reviewed)abstract
- Purpose - The purpose of this paper is to carry out a numerical investigation to study forced convection for a tube with fins on the inner and outer surfaces with water as the base fluid and adding particles of CuO to have a nanofluid. Design/methodology/approach - Three geometries are designed by using Solid Works and the number of inside fins is 20 with height 6 mm and thickness 1mm. The number of outside fins is 20 with height 12mm and thickness 1 mm. The length of the tube is 1,000mmand the inner and outer diameter is 26 and 28mm, respectively. The annular diameter is 60mm. The geometries are imported to Gambit where the mesh is chosen and the boundary conditions are specified. The commercial software package Fluent version 14.0 has been used to numerically solve the governing equations for the three cases. Findings - The temperature contours for the three models at different air velocities, i.e., 5, 7.5 and 10 m/s and water velocities 0.8, 1 and 3 m/s have been investigated. The enhancement of heat transfer by using CuO nanoparticles has been investigated at different nanofluid concentrations. Originality/value - In this paper, a numerical study is presented to analyze internal and external longitudinal finned tubes with water inside and air outside, with and without nano CuO particles.
|
|
| 2. |
- Amberg, Gustav, et al.
(author)
-
Thermocapillary convection and phase change in welding
- 2008
-
In: International journal of numerical methods for heat & fluid flow. - : Emerald. - 0961-5539 .- 1758-6585. ; 18:3-4, s. 378-386
-
Journal article (peer-reviewed)abstract
- Purpose - In welding there is an intricate coupling between the composition of the material and the shape and depth of the weld pool. In certain materials, the weld pool may not penetrate the material easily, so that it is difficult or impossible to weld, while other seemingly quite similar materials may be well suited for welding. This is due to the convective heat transfer in the melt where the flow is driven primarily by surface tension gradients. This paper aims to study how surface active agents affect the flow and thus the welding properties by surveying some recent 3D simulations of weld pools. Design/methodology/approach - Some basic concepts in the modelling of flow in a weld pool are reviewed. The mathematical models for a convecting melt, with a detailed model for the surface tension and the Marangoni stress in the presence of surfactants, are presented. The effect of the sign of the Marangoni coefficient on the flow pattern, and thus, via melting and freezing, on the shape of the weld pool, is discussed. Findings - It is seen that it is beneficial to have surfactants present at the pool surface, in order to have good penetration. Results from a refined surface tension model that accounts for non-equilibrium redistribution of surfactants are presented. It is seen that the surfactant concentration is significantly modified by the fluid flow. Thereby, the effective surface tension and the Marangoni stresses are altered, and the redistribution of surfactants will affect the penetration depth of the weld pool. Originality/value - The importance of surfactants for weld pool shapes, and in particular the convective redistribution of surfactants, is clarified.
|
|
| 3. |
- Andersson, Magnus, et al.
(author)
-
Numerical model for vacuum infusion manufacturing of polymer composites
- 2003
-
In: International journal of numerical methods for heat & fluid flow. - : Emerald. - 0961-5539 .- 1758-6585. ; 13:3, s. 383-394
-
Journal article (peer-reviewed)abstract
- The focus is set on the development and evaluation of a numerical mgodel describing the impregnation stage of a method to manufacture fibre reinforced polymer composites, namely the vacuum infusion process. Examples of items made with this process are hulls to sailing yachts and containers for the transportation industry. The impregnation is characterised by a full 3D flow in a porous medium having an anisotropic, spatial- and time-dependent permeability. The numerical model has been implemented in a general and commercial computational fluid dynamic software through custom written subroutines that: couple the flow equations to the equations describing the stiffness of the fibre reinforcement; modify the momentum equations to account for the porous medium flow; remesh the computational domain in each time step to account for the deformation by pressure change. The verification of the code showed excellent agreement with analytical solutions and very good agreement with experiments. The numerical model can easily be extended to more complex geometry and to other constitutive equations for the permeability and the compressibility of the reinforcement.
|
|
| 4. |
- Arshad, Salman, 1987, et al.
(author)
-
A strategy for large-scale scalar advection in large eddy simulations that use the linear eddy sub-grid mixing model
- 2018
-
In: International Journal of Numerical Methods for Heat and Fluid Flow. - 0961-5539. ; 28:10, s. 2463-2479
-
Journal article (peer-reviewed)abstract
- Purpose The purpose of this numerical work is to present and test a new approach for large-scale scalar advection (splicing) in large eddy simulations (LES) that use the linear eddy sub-grid mixing model (LEM) called the LES-LEM. Design/methodology/approach The new splicing strategy is based on an ordered flux of spliced LEM segments. The principle is that low-flux segments have less momentum than high-flux segments and, therefore, are displaced less than high-flux segments. This strategy affects the order of both inflowing and outflowing LEM segments of an LES cell. The new splicing approach is implemented in a pressure-based fluid solver and tested by simulation of passive scalar transport in a co-flowing turbulent rectangular jet, instead of combustion simulation, to perform an isolated investigation of splicing. Comparison of the new splicing with a previous splicing approach is also done. Findings The simulation results show that the velocity statistics and passive scalar mixing are correctly predicted using the new splicing approach for the LES-LEM. It is argued that modeling of large-scale advection in the LES-LEM via splicing is reasonable, and the new splicing approach potentially captures the physics better than the old approach. The standard LES sub-grid mixing models do not represent turbulent mixing in a proper way because they do not adequately represent molecular diffusion processes and counter gradient effects. Scalar mixing in turbulent flow consists of two different processes, i.e. turbulent mixing that increases the interface between unmixed species and molecular diffusion. It is crucial to model these two processes individually at their respective time scales. The LEM explicitly includes both of these processes and has been used successfully as a sub-grid scalar mixing model (McMurtry et al., 1992; Sone and Menon, 2003). Here, the turbulent mixing capabilities of the LES-LEM with a modified splicing treatment are examined. Originality/value The splicing strategy proposed for the LES-LEM is original and has not been investigated before. Also, it is the first LES-LEM implementation using unstructured grids.
|
|
| 5. |
- Bai, Xue-Song, et al.
(author)
-
A Fast Multi-Grid Method for 3-D Turbulent Incompressible Flows
- 1992
-
In: International Journal of Numerical Methods for Heat & Fluid Flow. - : Emerald. - 1758-6585 .- 0961-5539. ; 2:2, s. 127-137
-
Journal article (peer-reviewed)abstract
- The averaged Navier‐Stokes and the k‐e turbulence model equations are used to simulate turbulent flows in some internal flow cases. The discrete equations are solved by different variations of Multigrid methods. These include both steady state as well as time dependent solvers. Locally refined grids can be added dynamically in all cases. The Multigrid schemes result in fast convergence rates, whereas local grid refinements allow improved accuracy with rational increase in problem size. The applications of the solver to a 3‐D (cold) furnace model and to the simulation of the flow in a wind tunnel past an object prove the efficiency of the Multigrid scheme with local grid refinement.
|
|
| 6. |
- Bergström, John, et al.
(author)
-
Estimation of numerical accuracy for the flow field in a draft tube
- 1999
-
In: International journal of numerical methods for heat & fluid flow. - : Emerald. - 0961-5539 .- 1758-6585. ; 9:4, s. 472-486
-
Journal article (peer-reviewed)abstract
- The potential for overall efficiency improvements of modern hydro power turbines is a few percent. A significant part of the losses occurs in the draft tube. To improve the efficiency by analysing the flow in the draft tube, it is therefore necessary to do this accurately, i.e. one must know how large the iterative and the grid errors are. This was done by comparing three different methods to estimate errors. Four grids (122,976 to 4,592 cells) and two numerical schemes (hybrid differencing and CCCT) were used in the comparison. To assess the iterative error, the convergence history and the final value of the residuals were used. The grid error estimates were based on Richardson extrapolation and least square curve fitting. Using these methods we could, apart from estimate the error, also calculate the apparent order of the numerical schemes. The effects of using double or single precision and changing the under relaxation factors were also investigated. To check the grid error the pressure recovery factor was used. The iterative error based on the pressure recovery factor was very small for all grids (of the order 10-4 percent for the CCCT scheme and 10-10percent for the hybrid scheme). The grid error was about 10 percent for the finest grid and the apparent order of the numerical schemes were 1.6 for CCCT (formally second order) and 1.4 for hybrid differencing (formally first order). The conclusion is that there are several methods available that can be used in practical simulations to estimate numerical errors and that in this particular case, the errors were too large. The methods for estimating the errors also allowed us to compute the necessary grid size for a target value of the grid error. For a target value of 1 percent, the necessary grid size for this case was computed to 2 million cells.
|
|
| 7. |
- Burman, Jörgen, et al.
(author)
-
Influence from numerical noise in the objective function for flow design optimisation
- 2001
-
In: International journal of numerical methods for heat & fluid flow. - : Emerald. - 0961-5539 .- 1758-6585. ; 11:1, s. 6-19
-
Journal article (peer-reviewed)abstract
- The overall pressure drop in an axisymmetric contraction is minimised using two different grid sizes. The transition region was parameterised with only two design variables to make it possible to create surface plots of the objective function in the design space, which were based on 121 CFD calculations for each grid. The coarse grid showed to have significant numerical noise in the objective function while the finer grid had less numerical noise. The optimisation was performed with two methods, a Response Surface Model (RSM) and a gradient-based method (the Method of Feasible Directions) to study the influence from numerical noise. Both optimisation methods were able to find the global optimum with the two different grid sizes (the search path for the gradient-based method on the coarse grid was able to avoid the region in the design space containing local minima). However, the RSM needed fewer iterations in reaching the optimum. From a grid convergence study at two points in the design space the level of noise appeared to be sufficiently low, when the relative step size is 10-4 for the finite difference calculations, to not influence the convergence if the errors are below 5 per cent for this contraction geometry.
|
|
| 8. |
- De Boer, Gregory Nicholas, et al.
(author)
-
Three computational methods for analysing thermal airflow distributions in the cooling of data centres
- 2018
-
In: International journal of numerical methods for heat & fluid flow. - 0961-5539 .- 1758-6585. ; 28:2, s. 271-288
-
Journal article (peer-reviewed)abstract
- Purpose – This aim of this work is to investigate different modelling approaches for air-cooled data centres. The study employs three computational methods, which are based on finite element, finite volume and lattice Boltzmann methods and which are respectively implemented via commercial Multiphysics software, opensource computational fluid dynamics code and graphical processing unit-based code developed by the authors. The results focus on comparison of the three methods, all of which include models for turbulence, when applied to two rows of datacom racks with cool air supplied via an underfloor plenum. Design/methodology/approach – This paper studies thermal airflows in a data centre by applying different numerical simulation techniques that are able to analyse the thermal airflow distribution for a simplified layout of datacom racks in the presence of a computer room air conditioner. Findings – Good quantitative agreement between the three methods is seen in terms of the inlet temperatures to the datacom equipment. The computational methods are contrasted in terms of application to thermal management of data centres. Originality/value – The work demonstrates how the different simulation techniques applied to thermal management of airflow in a data centre can provide valuable design and operational understanding. Basing the analysis on three very different computational approaches is new and would offer an informed understanding of their potential for a class of problems.
|
|
| 9. |
- Diószegi, Attila, 1962-, et al.
(author)
-
Modelling and simulation of heat conduction in 1-D polar spherical coordinates using control volume-based finite difference method
- 2016
-
In: International journal of numerical methods for heat & fluid flow. - 0961-5539 .- 1758-6585. ; 26:1, s. 2-17
-
Journal article (peer-reviewed)abstract
- Purpose - The purpose of this paper is to obtain a finite difference method (FDM) solution using control volume for heat transport by conduction and the heat absorption by the enthalpy model in the sand mixture used in casting manufacturing processes. A mixture of sand and different chemicals (binders) is used as moulding materials in the casting processes. The presence of various compounds in the system improve the complexity of the heat transport due to the heat absorption as the binders are decomposing and transformed into gaseous products due to significant heat shock. Design/methodology/approach - The geometrical domain were defined in a 1D polar coordinate system and adapted for numerical simulation according to the control volume-based FDM. The simulation results were validated by comparison to the temperature measurements under laboratory conditions as the sand mould mixture was heated by interacting with a liquid alloy. Findings - Results of validation and simulation methods were about high correspondence, the numerical method presented in this paper is accurate and has significant potential in the simulation of casting processes. Originality/value - Both numerical solution (definition of geometrical domain in 1D polar coordinate system) and verification method presented in this paper are state-of-the-art in their kinds and present high scientific value especially regarding to the topic of numerical modelling of heat flow and foundry technology.
|
|
| 10. |
- Du, Wei, et al.
(author)
-
Enhanced heat transfer in a labyrinth channels with ribs of different shape
- 2020
-
In: International Journal of Numerical Methods for Heat and Fluid Flow. - 0961-5539. ; 30:2, s. 724-741
-
Journal article (peer-reviewed)abstract
- Purpose: The purpose of this study is to enhance the thermal performance in the labyrinth channel by different ribs shape. The labyrinth channel is a relatively new cooling structure to decrease the temperature near the trailing region of gas turbine. Design/methodology/approach: Based on the geometric similarity, a simplified geometric model is used. The k − ω turbulence model is used to close the Navier–Stokes equations. Five rib shapes (one rectangular rib, two arched ribs and two trapezoid ribs) and five Reynolds numbers (10,000 to 50,000) are considered. The Nusselt number, flow structure and friction factor are analyzed. Findings: Nusselt number is tightly related to the rib shape in the labyrinth channel. The different shapes of the ribs result in different horseshoe vortex and wake region. In general, the arched rib brings the highest Nusselt number and friction factor. The Nusselt number is increased by 15.8 per cent compared to that of trapezoidal ribs. High Nusselt number is accompanied by the high friction factor in a labyrinth channels. The friction factor is increased by 64.6 per cent compared to rectangular ribs. However, the rib shape has a minor effect on the overall thermal performance. Practical implications: This study is useful to protect the trailing region of advanced gas turbine. Originality/value: This paper presents the flow structure and heat transfer characteristics in a labyrinth channel with different rib shapes.
|
|
