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Methods for Reducing the Complexity of Geometrical Structures Based on CFD Programming: Time Efficient Simulations Based on Volume Forces Coupled with Single and Two-phase Flow
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Throughout recent years, computer based programs have been applied to solve and analyse industrial problems encountered global fields such as automobile design for reduction of CO2-gas, designing wind parks aimed at increasing power output etc. One of these developed programs is Computational Fluid Dynamics (CFD) which numerically solves complex flow behaviour based on computer power.

As there is an ongoing expansion of CFD usage in industry, certain issues need to be addressed as they are becoming more frequently encountered. The general demand for the simulation of larger control volumes and more advanced flow processes result in an extensive requirement of computer resources. Moreover, the implementation of commercial CFD codes in small-scaled industrial companies seems to generally be utilised as a black box based on the knowledge of fluid mechanic theory. Increased partnerships between industry and the academic world involving various CFD based design processes generally yield to a verbal communication interface, which is a crucial step in the process given the level of dependency between both sides.

Based on these notions, a method for establishing time efficient CFD-models with implementation of volume forces as sink terms in the momentum equation is presented. The internal structure, or parts of the structure, in the simulation domain is removed which reduces the geometrical complexity and along with it, computational demand.  These models are the basis of assessing the benefits of utilizing a numerical based design process in industry in which the CFD code is used as a communication tool for knowledge sharing with counterparts in different fields.

Abstract [en]

As there is an ongoing expansion of CFD usage in industry, certain issues need to be addressed as they are becoming more frequently encountered. The general demand for the simulation of larger control volumes and more advanced flow processes result in an extensive requirement of computer resources. Moreover, the implementation of commercial CFD codes in small-scaled industrial companies seems to generally be utilised as a black box based on the knowledge of fluid mechanic theory. Increased partnerships between industry and the academic world involving various CFD based design processes generally yield to a verbal communication interface, which is a crucial step in the process given the level of dependency between both sides.

Based on these notions, a method for establishing time efficient CFD-models with implementation of volume forces as sink terms in the momentum equation is presented. The internal structure, or parts of the structure, in the simulation domain is removed which reduces the geometrical complexity and along with it, computational demand.  These models are the basis of assessing the benefits of utilizing a numerical based design process in industry in which the CFD code is used as a communication tool for knowledge sharing with counterparts in different fields.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2014. , 104 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2014:32
Keyword [en]
Numerical design cycle, CFD, porous media, volume forces, heat exchanger, vacuum dewatering, time efficient simulations
National Category
Fluid Mechanics and Acoustics
Research subject
Environmental and Energy Systems
Identifiers
URN: urn:nbn:se:kau:diva-31983ISBN: 978-91-7063-565-6 (print)OAI: oai:DiVA.org:kau-31983DiVA: diva2:715080
Public defence
2014-06-09, 9C 204, Universitetsgatan 2, Karlstad, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2014-05-19 Created: 2014-04-25 Last updated: 2016-10-06Bibliographically approved
List of papers
1. Geometry development of the internal duct system of a heat pump tumble dryer based on fluid mechanic parameters from a CFD software
Open this publication in new window or tab >>Geometry development of the internal duct system of a heat pump tumble dryer based on fluid mechanic parameters from a CFD software
2011 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 88, no 5, 1596-1605 p.Article in journal (Refereed) Published
Abstract [en]

One aspect of reducing the energy consumption of a household tumble dryer is to reduce the pressure drop of the circulating air in the internal duct system. It is, however, costly and time consuming to design several prototypes for airflow measurements. In this paper, several fluid mechanic parameters in a partial model of the internal duct system of a tumble dryer have been studied in the CFD software Comsol MultiPhysics. The purpose was to establish a numerically based design process, where the design is conducted based on visual analysis of air velocity and vorticity, and two design criteria. The geometry design was conducted by a CAD-engineer, which was the counterpart of this project. In order to enable a successful design process, it was essential to establish a strong relation between fluid parameters and design criteria in order to share knowledge effectively with the CAD engineer. Two geometry modifications, based on a standard model, were conducted on the duct. Based on the design criteria, the pressure drop and the non-uniformity coefficient of the outlet airflow, the second modification (Modification 2) represents an improvement as the pressure drop is reduced by 23% and the uniformity at the outflow section is increased by 3%.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
CFD, Tumble dryer, Fluid mechanics, Design process, Geometry design
National Category
Fluid Mechanics and Acoustics
Research subject
Environmental and Energy Systems
Identifiers
urn:nbn:se:kau:diva-6673 (URN)10.1016/j.apenergy.2010.10.047 (DOI)000288360500015 ()
Available from: 2010-12-09 Created: 2010-12-09 Last updated: 2017-12-11Bibliographically approved
2. A fast running numerical model based on the implementation of volume forces for prediction of pressure drop in a fin-tube heat exchanger
Open this publication in new window or tab >>A fast running numerical model based on the implementation of volume forces for prediction of pressure drop in a fin-tube heat exchanger
2014 (English)In: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, no 24, 5822-5835 p.Article in journal (Refereed) Published
Abstract [en]

Numerical based design of geometrical structures is common whenstudying systems involving heat exchangers, a central component inseveral fields, such as industrial, vehicle and household systems. Thegeometrical structure of heat exchangers is generally comprised byclosely placed fins and tube bundles. The creation of a mesh grid for ageometrically compact heat exchanger will result in a dense structure,which is not feasible for personal computer usage. Hence, volume forceswere created based on Direct Numerical Simulations (DNS) on a FlowRepresentative Volume (FRV) of a tube fin heat exchanger in an internalduct system of a heat pump tumble dryer. A relation of the volumeaveraged velocity and the volume averaged force was established in twodifferent FRV-models with a finite element simulation in COMSOL. Thisrelation was subsequently used to create flow resistance coefficientsbased on volume averaged expressions of fluid velocity and volume forces.These flow resistance coefficients were implemented in two respectiveporous models, which represent the entire heat exchanger except theinterior arrangements of fins and tube bundles. Hence, the computationtime was reduced thanks to the absence of a dense mesh grid. Experimentalresults of the entire heat exchanger showed good agreement with thesecond porous model in terms of pressure drop and volume flow rate.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
CFD, Heat exchanger, Tumble dryer, Comsol MultiPhysics, Volume forces
National Category
Fluid Mechanics and Acoustics
Research subject
Environmental and Energy Systems
Identifiers
urn:nbn:se:kau:diva-31951 (URN)10.1016/j.apm.2014.04.051 (DOI)000346214000005 ()
Available from: 2014-04-22 Created: 2014-04-22 Last updated: 2017-12-05Bibliographically approved
3. Modelling of water removal during a paper vacuum dewatering process using a Level-Set method
Open this publication in new window or tab >>Modelling of water removal during a paper vacuum dewatering process using a Level-Set method
2013 (English)In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 101, 543-553 p.Article in journal (Refereed) Published
Abstract [en]

Water removal in paper manufacturing is an energy-intensive process. The dewatering process generally consists of four stages of which the first three stages include mechanical water removal through gravity filtration, vacuum dewatering and wet pressing. In the fourth stage, water is removed thermally, which is the most expensive stage in terms of energy use.

In order to analyse water removal during a vacuum dewatering process, a numerical model was created by using a Level-Set method. Various different 2D structures of the paper model were created in MATLAB code with randomly positioned circular fibres with identical orientation. The model considers the influence of the forming fabric which supports the paper sheet during the dewatering process, by using volume forces to represent flow resistance in the momentum equation.

The models were used to estimate the dry content of the porous structure for various dwell times. The relation between dry content and dwell time was compared to laboratory data for paper sheets with basis weights of 20 and 50 g/m2 exposed to vacuum levels between 20 kPa and 60 kPa. The comparison showed reasonable results for dewatering and air flow rates. The random positioning of the fibres influences the dewatering rate slightly. In order to achieve more accurate comparisons, the random orientation of the fibres needs to be considered, as well as the deformation and displacement of the fibres during the dewatering process.

Keyword
Simulation, Multiphase flow, Level-Set method, Porous media, Microstructure, Vacuum dewatering
National Category
Energy Engineering
Research subject
Environmental and Energy Systems
Identifiers
urn:nbn:se:kau:diva-29705 (URN)10.1016/j.ces.2013.07.005 (DOI)000323603700054 ()
Available from: 2013-10-18 Created: 2013-10-18 Last updated: 2017-12-06Bibliographically approved
4. Simulation of water removal in paper based on a 2D Level-Set model coupled with volume forces representing fluid resistance in 3D fiber distribution
Open this publication in new window or tab >>Simulation of water removal in paper based on a 2D Level-Set model coupled with volume forces representing fluid resistance in 3D fiber distribution
2015 (English)In: Drying Technology, ISSN 0737-3937, E-ISSN 1532-2300, Vol. 33, no 5, 605-615 p.Article in journal (Refereed) Published
Abstract [en]

A numerical model of a vacuum dewatering process was established with a Level-Set method to simulate two-phase flow in a 2-dimensional paper sheet model with constructed volume forces representing flow resistance in a 3-dimensional environment. Nine cases of various volume force representations were compared to numerical and experimental data. Based on the dry content and dwell time relation, the case with the influence of the paper/wire interface showed the most pleasing result compared to experimental data. Compared to the other numerical cases, considering the blockage of the pore space at the top layer of the forming fabric plays an essential role in determining the flow resistance during the vacuum process. Also, the dynamics of the dewatering rate is captured well with the influence of the blockage of fibers on the top layer of the forming fabric. The peak of the dewatering rate could be investigated further with a higher frequency of sample points on new experimental data.

The computational time for the two-phase flow models in this study is extensively reduced due to the removal of the internal structure. This distinction enables time efficient simulations of vacuum dewatering process in which several dewatering parameters such as level of vacuum, influence of moving vacuum pulse and higher basis weights could be investigated.

Place, publisher, year, edition, pages
Taylor & Francis, 2015
Keyword
Volume force, Volume-averaging, Two-phase flow, Level-Set method, Porous media, Vacuum dewatering
National Category
Fluid Mechanics and Acoustics
Research subject
Environmental and Energy Systems
Identifiers
urn:nbn:se:kau:diva-31962 (URN)10.1080/07373937.2014.967401 (DOI)000350361600010 ()
Available from: 2014-04-23 Created: 2014-04-23 Last updated: 2017-10-24Bibliographically approved
5. Characterizing flow resistance in 3-dimensional disordered fibrous structures based on Forchheimer coefficients for a wide range of Reynolds numbers
Open this publication in new window or tab >>Characterizing flow resistance in 3-dimensional disordered fibrous structures based on Forchheimer coefficients for a wide range of Reynolds numbers
2016 (English)In: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 40, no 21-22, 8898-8911 p.Article in journal (Refereed) Published
Abstract [en]

The flow resistance in 3-dimensional fibrous structures are investigated in particle Reynolds number representing flow characteristics with strong inertia. The resistance coefficients are established based on steady state simulations of single-phase processes of water numerically. An automatized simulation process in COMSOL is developed with a MATLAB algorithm in which production runs could be carried for various 3-dimensional fibrous structures. Simulation of flow processes ranging from Reynolds numbers at creeping flow levels to high Reynolds number at approximately 1000 are calculated and a numerical data set is established in order to estimate Forchheimer coefficients which are used to correlate a dimensionless friction factor to a modified Reynolds expression for porous media.

The friction factor and dimensionless permeability are calculated for fibrous structures with (i) disordered unidirectional fibers (ii) an isotropic fiber orientation in-plane perpendicular to the flow, and (iii) an isotropic fiber structure in a the 3-dimensional space. Empirical correlations of the friction factor and Reynolds number are used to compare our simulation data in order to assess the validity of our models and flow resistance estimations. The dimensionless permeability is moreover compared to other numerical simulations of flow through fibrous structures in order to assess flow resistance at low Reynolds number.

It is concluded that flow resistance in the isotropic fiber arrangement in space is lower than the in-plane isotropic orientation and disordered unidirectional fiber arrangements at creeping flow conditions, however, all friction actors converges towards the same value at higher Reynolds numbers indicating that fiber orientation is independent at high inertia flow regimes. Overall, our numerical simulations agree well to classical empirical formulations for a wide range of Reynolds number. However, the comparison differs considerably depending on the porosity level.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Flow resistance; Volume-averaging; Porous media; Forchheimer coefficients; Friction factor; Fibrous structure
National Category
Fluid Mechanics and Acoustics
Research subject
Environmental and Energy Systems
Identifiers
urn:nbn:se:kau:diva-46411 (URN)10.1016/j.apm.2016.05.036 (DOI)000384853900005 ()
Note

This article was published as manuscript in Kamal Rezk thesis.

Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2017-10-26Bibliographically approved

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Citation style
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  • ieee
  • modern-language-association-8th-edition
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  • en-GB
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Output format
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