Browsing by Author "Yilbas B.S."
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Item Entropy generation in non-Newtonian fluid flow in a slider bearing(Indian Academy of Sciences, 2004) Pakdemirli M.; Yilbas B.S.; Yurusoy M.In the present study, entropy production in flow fields due to slider bearings is formulated. The rate of entropy generation is computed for different fluid properties and geometric configurations of the slider bearing. In order to account for the non-Newtonian effect, a special type of third-grade fluid is considered. It is found that the rate of entropy generation is influenced significantly by the height of the bearing clearance and the non-Newtonian parameter of the fluid.Item Analytical solution for temperature field in thin film initially heated by a short-pulse laser source(2005) Yilbas B.S.; Pakdemirli M.; Mansoor S.B.Analytical solution for electron and lattice temperature distribution in the solid initially heated by a laser short-pulse is presented. Strained parameters method is introduced when formulating electron and lattice temperature distributions. Laser short pulse heating of gold film is simulated numerically and temperature data at the end of the heating pulse are adopted as initial condition to the governing equations of energy transport for analytical solutions. This enables to solve the governing equations of energy analytically in the cooling period. It is found that electron temperature decays sharply while lattice site temperature increases gradually in the surface regions during the cooling cycle. As the depth from the surface increases change in both temperatures become gradual. © Springer-Verlag 2005.Item Entropy generation due to the flow of a non-newtonian fluid with variable viscosity in a circular pipe(2005) Yilbas B.S.; Pakdemirli M.The non-Newtonian fluid can be considered as a third-grade fluid with variable viscosity. In this case, the rate of fluid strain can be formulated using the third-grade fluid analogy. In the present study, entropy generation due to non-Newtonian fluid flow in a pipe is investigated. A third-grade fluid with variable viscosity is accommodated in the analysis. Analytical solutions for velocity and temperature distributions are presented, and an entropy generation number is computed for different non-Newtonian parameters, viscosity parameters, and Brinkman numbers. It is found that increasing the non-Newtonian parameter lowers the entropy generation number. This is more pronounced in the region close to the pipe wall. Increasing the viscosity parameter and Brinkman number enhances the entropy generation number, particularly in the vicinity of the pipe wall. Copyright © Taylor and Francis Inc.Item Entropy generation in a pipe due to non-Newtonian fluid flow: Constant viscosity case(Indian Academy of Sciences, 2006) Pakdemirli M.; Yilbas B.S.Non-Newtonian fluid flow in a pipe system is considered and a third grade non-Newtonian fluid is employed in the analysis. The velocity and temperature distributions across the pipe are presented. Entropy generation number due to heat transfer and fluid friction is formulated. The influences of non-Newtonian parameter and Brinkman number on entropy generation number are examined. It is found that increasing the non-Newtonian parameter reduces the fluid friction in the region close to the pipe wall. This in turn results in low entropy generation with increasing non-Newtonian parameter. Increasing Brinkman number enhances the fluid friction and heat transfer rates; in which case, entropy number increases with increasing Brinkman number.Item Entropy generation for pipe flow of a third grade fluid with Vogel model viscosity(2006) Pakdemirli M.; Yilbas B.S.The flow of fluid-solid mixtures in a pipe can be treated as non-Newtonian fluids of third grade. Depending upon the fluid viscosity, entropy generation in the flow system varies. In the present study, flow of third grade fluid in a pipe is considered. The Vogel model is introduced to account for the temperature-dependent viscosity. Entropy generation due to fluid friction and heat transfer in the flow system is formulated. The influence of viscosity parameters A and B on the entropy generation number is investigated. It is found that increasing viscosity parameter A reduces the entropy generation number and opposite is true for increasing viscosity parameter B. © 2005 Elsevier Ltd. All rights reserved.Item Analytical solution for temperature field in electron and lattice sub-systems during heating of solid film(2006) Yilbas B.S.; Pakdemirli M.The analytical solution for non-equilibrium temperature field in solid substrate is presented. Closed form solutions for electron and lattice site temperature rise are obtained for a solid layer heated at the surface with a time-decaying intensity pulse. In the analytical solutions, a perturbation method of strained parameters is introduced. Temperature simulations are carried out for a gold layer with different thicknesses. It is found that increasing layer thickness lowers electron and lattice site temperatures at the surface. Electron temperature at the surface decays sharply with progressing heating period, which is more pronounced for thin layer. Moreover, lattice site temperature continues to rise despite reducing electron temperature in the surface region. The results obtained from the analytical solution for the lattice site temperature agrees well with the numerical predictions. © 2006 Elsevier B.V. All rights reserved.Item Analytical solution of laser short-pulse heating of gold films(2007) Yilbas B.S.; Pakdemirli M.Laser short-pulse heating of metallic surfaces results in the thermal separation of electron and lattice sub-systems. In this case, the electron temperature well in excess of the lattice site temperature results. The process is concerned in the thermal coupling of two sub-systems and the resultant energy transport equation becomes hyperbolic for the timescales of time and space considered. In the present study, analytical solutions for electron and lattice site temperatures are obtained using a perturbation method. In the analysis, a gold film is used and the laser pulse intensity is varied exponentially. The absorption mechanism is assumed to follow an exponential decay within the absorption depth of the substrate material. It is found, that temporal behaviour of the electron temperature is similar to that corresponding to the laser pulse intensity, provided that both curves have different temporal gradients. The electron temperature decays wheas the lattice site temperature increases continuously with time. © 2007 Old City Publishing. Inc.Item Analytical solution for non-equilibrium energy transfer in gold: Influence of ballistic contribution of electrons on energy transfer(2009) Yilbas B.S.; Dolapçi I.T.; Pakdemirli M.The analytical solution for non-equilibrium energy transfers in gold substrate and ballistic contribution of electrons to the energy transfer mechanism is examined. The non-equilibrium energy equation including the ballistic contribution of electrons is obtained using the electron kinetic theory approach. The analytical solution using the perturbation method is introduced to formulate lattice and electron temperature distributions in the film. A numerical method using the finite difference scheme is employed to predict and compare electron and lattice site temperatures to those obtained from the analytical solution. It is found that temperature remains high in the surface region of the gold film due to the cases: (i) accounting for the ballistic contribution of electrons to non-equilibrium energy transfer, and (ii) excluding the ballistic contribution to the non-equilibrium energy transfer. This is true for electron and lattice temperatures. Crown Copyright © 2008.Item Characterization of microplastic deformation produced in 6061-T6 by using laser shock processing(2014) Gencalp Irizalp S.; Saklakoglu N.; Yilbas B.S.High dislocation densities are formed in the irradiated region of the workpiece during the laser shock processing; in which case, surface hardening is resulted. The process involves with recoil pressure loading at the workpiece surface with the minimum heating effects in the irradiated region. This favors the process to be a good candidate for the surface treatment of metallic materials. Therefore, in the present study, laser shock processing of 6061-T6 aluminum alloy is carried out and the influence of a number of laser pulses and irradiated spot diameter on the treated layer characteristics, including morphology and hardness, are investigated. It is found that the number of laser pulses has significant influence on the resulting surface characteristics such as surface roughness, crystallite size, micro-strain, and microhardness of the alloy. In this case, surface roughness is deteriorated by increasing number of laser pulses and pulse intensity. In addition, fine crystallite structure takes place in the laser-treated region. © 2013 Springer-Verlag London.Item Laser shock processing of 6061-T6 aluminium alloy: Thermal modelling and analysis(Old City Publishing, 2016) Zafar H.; Saklakoglu N.; Irizalp S.G.; Khan S.; Shuja S.Z.; Boran K.; Yilbas B.S.Laser shock processing of 6061-T6 aluminium alloy is carried out. Temperature and stress fields are simulated in line with the experimental conditions. Metallurgical changes due to the laser shock and microhardness in the laser treated region are examined using analytical tools that include scanning and transmission electron microscopes and microhardness tester. The depth of shock affected region, plastic strain, and dislocation density are determined numerically and experimentally in the laser treated region. It is found that the temperature attains high values at the centre of the irradiated spot resulting in high rate of evaporation at the surface. The recoil pressure formed, due to high evaporation rate, at the laser treated surface results in plastic deformation of about 500 mm below the surface. A dislocation density of the order 2 × 1013 to 4 × 1013 cm-2 occurs in the surface region. Although high temperature gradients result in high stress levels in the region below the surface vicinity, high recoil pressure results in crack free surface with compressive stress. © 2016 Old City Publishing, Inc.