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Article
Publication date: 9 August 2011

Omar A. Melhem, Bekir S. Yilbas and S.Z. Shuja

The purpose of this paper is to investigate the heat transfer rates from the kerf surfaces and skin friction at the kerf wall due to the jet impingement in relation to…

Abstract

Purpose

The purpose of this paper is to investigate the heat transfer rates from the kerf surfaces and skin friction at the kerf wall due to the jet impingement in relation to laser cutting process.

Design/methodology/approach

Three‐dimensional modeling for the flow and heat transfer analysis is considered. The numerical scheme using the control volume approach is introduced to solve the governing equations of flow and heat transfer. The k‐w turbulence model is incorporated to account for the turbulence.

Findings

It is found that the Nusselt number and the skin friction remains high in the region next to the kerf inlet and it decreases towards the kerf exit for all kerf thicknesses considered. The flow acceleration in the kerf also results in the second peak of the Nusselt number and the skin friction.

Research limitations/implications

The melting at the kerf surface was omitted and the constant temperature boundary representing the melt surface is incorporated in the analysis. However, care was taken during the mesh generation to avoid grid dependent solutions.

Practical implications

The findings and discussions provide the useful information on the practical laser cutting process, in particular, physical insight into the effect of the kerf thickness on the heat transfer and skin friction.

Originality/value

No previous work has been carried out in three‐dimensional space to predict the heat transfer and skin friction, which are important for practical laser cutting applications. Therefore, the work reported is original.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 21 no. 6
Type: Research Article
ISSN: 0961-5539

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Article
Publication date: 22 March 2013

Awad B.S. Alquaity, Salem A. Al‐Dini and Bekir S. Yilbas

Heat capacity enhancement is important for variety of applications, including microchannel cooling and solar thermal energy conversion. A promising method to enhance heat…

Abstract

Purpose

Heat capacity enhancement is important for variety of applications, including microchannel cooling and solar thermal energy conversion. A promising method to enhance heat capacity of a fluid is by introducing phase change particles in a flow system. The purpose of this paper is to investigate heat capacity enhancement in a microchannel flow with the presence of phase change material (PCM) particles.

Design/methodology/approach

Discrete phase model (DPM) and homogeneous model have been compared in this study. Water is used as the carrier fluid and lauric acid as the PCM particles with different volume concentrations, ranging from 0 to 10%. Both the models neglect the particle‐particle interaction effects of PCM particles.

Findings

The DPM indicates that presence of 10% volume concentration of PCM particles does not cause an increase in the pressure drop along the channel length. However, prediction from the homogeneous model shows an increase in the pressure drop due to the addition of nanoparticles in such a way that 10% volume concentration of particles causes 34.4% increase in pressure drop.

Research limitations/implications

The study covers only 10% volume concentration of PCM particles; however, the model may be modified to include higher volume concentrations. The laminar flow is considered; it may be extended to study the turbulence effects.

Practical implications

This work provides a starting framework for the practical use of different PCM particles, carrier fluid properties, and different particle volume concentrations in electronic cooling applications.

Originality/value

The work presented is original and the findings will be very useful for researchers and engineers working in microchannel flow in cooling and thermal storage applications.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 23 no. 2
Type: Research Article
ISSN: 0961-5539

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Article
Publication date: 21 June 2011

Bekir Sami Yilbas, Cihan Karataş, Bahadır Ersu and Sevil Gurgan

The purpose of this paper is to investigate the morphological and metallurgical changes of laser gas‐assisted nitriding of titanium implants.

Abstract

Purpose

The purpose of this paper is to investigate the morphological and metallurgical changes of laser gas‐assisted nitriding of titanium implants.

Design/methodology/approach

Laser gas‐assisted nitriding of titanium implant is carried out and the metallurgical as well as the morphological changes in the nitride layer are examined using optical microscopy, SEM, XRD, and X‐ray photoelectron spectroscopy. Temperature and thermal stress fields are computed during the laser heating process adopting the finite element method. The residual stress formed in the nitride layer is measured using the XRD technique while micro‐indentation tests are carried out to determine the fracture toughness of the surface after the laser treatment process.

Findings

It is found that nitride depth layer extends to 40 μm below the surface and it is free from the cracks and micro‐voids. The residual stress formed on the surface region is higher than at some depth below the surface in the nitride layer, provided that the maximum residual stress is less than the elastic limit of the substrate material.

Originality/value

The paper contains original findings and the findings are not submitted any other journal for publication.

Details

Industrial Lubrication and Tribology, vol. 63 no. 4
Type: Research Article
ISSN: 0036-8792

Keywords

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Article
Publication date: 27 May 2014

Shahzada Zaman Shuja and Bekir Yilbas

The heat transfer rates from the body to the working fluid can be improved through altering geometric configurations of the body and its arrangement in the flow system…

Abstract

Purpose

The heat transfer rates from the body to the working fluid can be improved through altering geometric configurations of the body and its arrangement in the flow system. One of the arrangements for this purpose is to locate the body at the channel inlet while the convection current opposes it. Since the flow field in the channel inlet influences the heat transfer rates, changing the aspect ratio and inclination of the body is expected to modify the flow field while enhancing the heat transfer rates. Consequently, investigation into the influence of the aspect ratios and tilting angles of the body on the heat transfer rates in the channel flow becomes essential. The paper aims to discuss these issues.

Design/methodology/approach

Numerical simulation of flow in a channel with the presence of solid block is carried out. The block aspect ratio is changed while keeping the area of the block constant for all aspect ratios. The tilting angle is also incorporated analysis to examine its effect on the Nusselt number.

Findings

The throttling effect of the block at channel inlet accelerates the flow between the channel wall and the block faces. This, in turn modifies the thermal boundary layer around the block. In this case, heat transfer rates increase considerably at the block faces where the flow acceleration suppresses the thermal boundary layer thickness. This is more pronounced for large block tilting angles. The Nusselt number attains low values for the block face opposing to the flow at the channel inlet and the back face of the block. This is attributed to the mixing of the thermal current emanating from the side faces of the block in the region close to the back surface. In this case, thermal boundary layer thickens and the heat transfer rates from the block reduce significantly. The Nusselt number improves with reducing the block aspect ratio, which is particularly true along the side faces of the block. In addition, the influence of the block tilting angle on the Nusselt number is considerable for the low block aspect ratios.

Research limitations/implications

The model study is validated with the previous studies for the drag coefficient. The study covers all the aspects of the flow situations and discusses the resulting fluid field and the heat transfer rates from the block.

Practical implications

It is an interesting work for cooling applications. The block aspect ratio and its tilting angle in the channel influence considerably the flow field and the Nusselt number variation around the block faces.

Social implications

The cooling technology may be improved through implementing the findings of the current work.

Originality/value

It is an original work and it has never been submitted to other journals.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 24 no. 5
Type: Research Article
ISSN: 0961-5539

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Article
Publication date: 22 June 2010

Bekir Yilbas, A.F.M. Arif, Cihan Karatas, B.J. Abdul Aleem and Nouer Tabet

The laser nitriding process is involved with high temperature heating and high cooling rates. This, in turn, results in high levels of thermal stresses in the heated…

Abstract

Purpose

The laser nitriding process is involved with high temperature heating and high cooling rates. This, in turn, results in high levels of thermal stresses in the heated region. Moreover, the residual stress in the heated region remains high after the completion of the heating process, which limits the application of the laser nitriding process. The purpose of this paper is to investigate thermal stresses development and residual stress levels in the nitrided region.

Design/methodology/approach

The microstructural changes and residual stress development in the laser gas‐assisted nitrided zone are examined. Finite element modeling is carried out to predict temperature and stress fields in the laser nitrided layer. The indentation tests and X‐ray diffraction (XRD) technique are used to determine the residual stress levels while previously derived analytical formula is used to predict the residual stress levels in the nitrided region.

Findings

The residual stress predicted attains values within 230 MPa, which remains almost uniform in the nitrided layer, except in the surface region. In this case, residual stress reduces slightly due to the low temperature gradient developed in this region and the unconstrained expansion of the free surface. When comparing the residual stress predicted with the measurement results as obtained from the XRD technique as well as the indentation tests, all the results are in reasonably good agreement. The small discrepancies between the experimental data and predictions are attributed to the assumptions made in the model study and the measurement errors.

Research limitations/implications

The depth of nitrided layer is limited 60 μm. This limits the applicability of the coating for high wearing rates.

Practical implications

The nitrided surface improves the surface properties of steel, which can be used in industry more efficiently.

Originality/value

The paper describes an original model study on stress formation, an experiment for surface characterization and estimation of residual stress formation and contains new findings.

Details

Industrial Lubrication and Tribology, vol. 62 no. 4
Type: Research Article
ISSN: 0036-8792

Keywords

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Article
Publication date: 27 May 2014

Shahzada Zaman Shuja and Bekir Yilbas

In laser drilling applications, hole wall remains almost the melting temperature of the substrate material and the thermodynamic pressure developed at high temperature…

Abstract

Purpose

In laser drilling applications, hole wall remains almost the melting temperature of the substrate material and the thermodynamic pressure developed at high temperature molten surface vicinity influences the heat transfer rates and the skin friction at the surface of the hole wall. This effect becomes complicated for the holes drilled in coated substrates. In this case, melting temperatures of the coating and base materials are different, which in turn modifies the flow field in the hole due to jet impingement. Consequently, investigation of the heat transfer rates from the hole wall surfaces and the skin friction at the hole surface becomes essential. The paper aims to discuss these issues.

Design/methodology/approach

Numerical solution for jet impingement onto a hole with high wall temperature is introduced. Heat transfer rates and skin friction from the hole wall is predicted. The numerical model is validated with the experimental data reported in the open literature.

Findings

The Nusselt number attains high values across the coating thickness and it drops sharply at the interface between the coating and the base material in the hole. Since fluid temperature in the vicinity of the substrate surface is higher than that of the wall temperature, heat transfer occurs from the fluid to the substrate material while modifying the Nusselt number along the hole wall. This results in discontinuity in the Nusselt variation across the coating-base material interface. The Raighly line effect enhances the flow acceleration toward the hole exit while increasing the rate of fluid strain. Consequently, skin friction increases toward the hole exit. The influence of average jet velocity on the Nusselt number and the skin friction is significant.

Research limitations/implications

The findings are very useful to analyze the flow field in the hole at different wall temperature. In the simulations hole diameter is fixed in line with the practical applications. However, it may be changed to examine the influence of hole diameter on the flow field and heat transfer. However, this extension be more toward academic study than the practical significance.

Practical implications

The complete modeling of turbulent flow jet flow impinging onto a hole is introduced and boundary conditions are well defined for the numerical solutions. The method of handing the physical problem will be useful for those working in the area of heat transfer and fluid flow. In addition, the importance of heat transfer rates and skin friction at the hole wall is established, which will benefit the practical engineers and the academicians working in the specific area of laser machining.

Social implications

The findings are useful for those working to improve the laser technology in the machining area.

Originality/value

The work presented is original and never being published anywhere else. The findings are reported in detail such that academicians and engineers are expected to benefit from this original contribution.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 24 no. 5
Type: Research Article
ISSN: 0961-5539

Keywords

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Article
Publication date: 1 March 2013

Bekir Yilbas, A.F.M. Arif and Sohail Akhtar

It has been observed from the history of failed dies used in local extrusion industry that after certain press cycles, severe die damage occurs by using more number of…

Abstract

Purpose

It has been observed from the history of failed dies used in local extrusion industry that after certain press cycles, severe die damage occurs by using more number of in‐house recycled billets. The purpose of this paper is to focus on the effect of billet quality on the extrusion die service life, based on using microstructural and finite element analyses.

Design/methodology/approach

Numerical solution of stress distribution in extrusion die using microstructural and finite element analyses.

Findings

Simulation results demonstrate that extrusion die experiences high stresses and strains at critical locations by running secondary billets. Billet deformation behavior also shows that secondary billet has more resistance to flow during extrusion cycle, which results in such high stresses and strains in the die.

Research limitations/implications

The study includes a particular die used to extrude the aluminum alloy billets. It may need to generalized including materials other than aluminum alloy.

Practical implications

The findings are original and believed to be useful for engineers working in the extrusion dies. Since it is shown that secondary billets (recycled billets) have more resistance to flow in the dye, a care should be taken when estimating the die life for the practical applications.

Originality/value

It is an original work. It deals with the comparison of new and recycled billets's performance in terms of stress formation in the die during the extrusion cycle.

Details

Industrial Lubrication and Tribology, vol. 65 no. 2
Type: Research Article
ISSN: 0036-8792

Keywords

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Article
Publication date: 16 August 2011

Bekir Sami Yilbas, Mazen Khaled, Sohail Akhtar and Cihan Karatas

Laser bending is a good candidate to replace the flame bending process. The electrochemical response of laser bending region changes due to the microstructural…

Abstract

Purpose

Laser bending is a good candidate to replace the flame bending process. The electrochemical response of laser bending region changes due to the microstructural modifications and high level of residual stress developed in the laser‐irradiated region after the bending process. Consequently, investigation into laser bending and microstructural changes in the irradiated region as well as the electrochemical response of bending section becomes essential. This paper aims to focus on the laser bending process.

Design/methodology/approach

The laser bending of steel sheets was carried out. The microstructural changes in the bending region are examined using the scanning electron microscopy and X‐ray diffraction. The electrochemical response of the bended sections is investigated through potentiodynamic tests.

Findings

It is found that laser‐irradiated surface is free from cracks and cavitations. However, deep pit sites due to secondary pitting are observed in the bending sections.

Research limitations/implications

The experiment is limited to certain thickness of the steel sheets. Increasing workpiece thickness reduces the bend angle. However, introducing high‐intensity laser beams improves the bend angle on the expense of high surface roughness in the bend section.

Practical implications

Laser bending process is involved with non‐mechanical tooling with low cost and precision of operation. Moreover, laser bending is a good candidate to replace the flame bending process. Consequently, laser bending finds application in industry. However, under the corrosive environment care should be taken.

Originality/value

The work presented is original and has not been published anywhere before. The findings will be useful for researchers and engineers working in the sheet metal forming area.

Details

Industrial Lubrication and Tribology, vol. 63 no. 5
Type: Research Article
ISSN: 0036-8792

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Article
Publication date: 1 April 2005

Suat Canbazoğlu and Bekir Sami Yilbaş

A stall model to predict the performance of a blade row operating under rotating stall conditions, is proposed.

Abstract

Purpose

A stall model to predict the performance of a blade row operating under rotating stall conditions, is proposed.

Design/methodology/approach

The experiments were carried out on an isolated rotor row of an axial flow compressor of a radius ratio of 0.66 hub/tip. Wall static pressure tappings were used for measurement of blade row pressure rise. The mass flow rate through the machine was determined from the pressure drop at the intake. Detailed flow measurements were made using a hot wire “V” probe and transducers. An online data acquisition system was used in which data sampling was phase‐locked with respect to stall cell trailing edge.

Findings

Measurements indicate that a pressure depression occurs in the stalled region. The assumption of uniform static pressure at the exit of a stalled blade row is not supported by the present work. The assumption of uniform static pressure at the exit of a stalled row together with the assumption that flow in unstalled regions operates at fixed point on the unstalled characteristic leads to the conclusion that total‐to‐static pressure rise during stalled operation is independent of blockage. This view is not supported by the experiments carried out on an isolated rotor.

Research limitations/implications

Additional experimental studies for axial compressors having different rotor and blade geometries and rotor speeds, are required.

Practical implications

The results can be used in the design and operation of axial compressor rotors.

Originality/value

A new stall model is presented in which the behavior during stalled operation with large blockage is different from that during, low blockage.

Details

Aircraft Engineering and Aerospace Technology, vol. 77 no. 2
Type: Research Article
ISSN: 0002-2667

Keywords

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