Waqar Khan - Academia.edu (original) (raw)

Papers by Waqar Khan

Research paper thumbnail of Mutation Analysis of the HBB Gene in Selected Bangladeshi β-Thalassemic Individuals: Presence of Rare Mutations

Genetic Testing and Molecular Biomarkers, 2010

Research paper thumbnail of Combination of two rare mutations causes -thalassaemia in a Bangladeshi patient

Screening of mutations that cause b-thalassaemia in the Bangladeshi population led to the identif... more Screening of mutations that cause b-thalassaemia in the Bangladeshi population led to the identification of a patient with a combination of two rare mutations, Hb Monroe and HBB: -92 C > G. The b-thalassaemia major male individual was transfusion-dependent and had an atypical b-globin gene cluster haplotype. Of the two mutations, Hb Monroe has been characterized in detail. Clinical effects of the other mutation, HBB: -92 C > G, are unknown so far. Bioinformatics analyses were carried out to predict the possible effect of this mutation. These analyses revealed the presence of a putative binding site for Egr1, a transcription factor, within the HBB:-92 region. Our literature survey suggests a close relationship between different phenotypic manifestations of b-thalassaemia and Egr1 expression.

Research paper thumbnail of Fluid Flow Around and Heat Transfer From Elliptical Cylinders: Analytical Approach

Journal of Thermophysics and Heat Transfer, 2005

An integral method of boundary-layer analysis is employed to derive closed-form expressions for t... more An integral method of boundary-layer analysis is employed to derive closed-form expressions for the calculation of total drag and average heat transfer for flow across an elliptical cylinder under isothermal and isoflux thermal boundary conditions. The Von Kármán-Pohlhausen integral method is used to solve the momentum and energy equations for both thermal boundary conditions. A fourth-order velocity profile in the hydrodynamic boundary layer and a third-order temperature profile in the thermal boundary layer are used. The present results are in good agreement with existing experimental/numerical data and, in the limiting cases, can be used for circular cylinders and finite plates.

Research paper thumbnail of Optimal Design of Tube Banks in Crossflow Using Entropy Generation Minimization Method

Journal of Thermophysics and Heat Transfer, 2007

An entropy generation minimization method is applied as a unique measure to study the thermodynam... more An entropy generation minimization method is applied as a unique measure to study the thermodynamic losses caused by heat transfer and pressure drop for a fluid in cross flow with tube banks. The use of entropy generation minimization allows the combined effect of heat transfer and pressure drop to be assessed through simultaneous interaction with the tube bank. A general dimensionless expression for the entropy generation rate is obtained by considering a control volume around a tube bank and applying conservation equations for mass and energy with entropy balance. Analytical/empirical correlations for heat transfer coefficients and friction factors are used, where the characteristic length is used as the diameter of the tubes and reference velocity used in Reynolds number and pressure drop is based on the minimum free area available for the fluid flow. Both inline and staggered arrangements are studied and their relative performance is compared for the same thermal and hydraulic conditions. A parametric study is also performed to show the effects of different design variables on the overall performance of tube banks. It is shown that all relevant design parameters for tube banks, including geometric parameters and flow conditions, can be simultaneously optimized.

Research paper thumbnail of Analytical Modeling of Fluid Flow and Heat Transfer in Microchannel/Nanochannel Heat Sinks

Journal of Thermophysics and Heat Transfer, 2008

Laminar forced convection in two-dimensional rectangular microchannels and nanochannels under hyd... more Laminar forced convection in two-dimensional rectangular microchannels and nanochannels under hydrodynamically and thermally fully developed conditions is investigated analytically in the slip-flow regime. Closed-form solutions for fluid friction and Nusselt numbers are obtained by solving the continuum momentum and energy equations with the first-order velocity slip and temperature jump boundary conditions at the channel walls. An isoflux thermal boundary condition is applied on the heat sink base. The results of the present analysis are presented in terms of the channel aspect ratio, hydraulic diameter, momentum and thermal accommodation coefficients, Knudsen number, slip velocity, Reynolds number, and Prandtl number. It is found that fluid friction decreases and heat transfer increases compared with no-slip flow conditions, depending on the aspect ratios and Knudsen numbers that include the effects of the channel size or rarefaction and the fluid/wall interaction.

Research paper thumbnail of Analytical Model for Convection Heat Transfer From Tube Banks

Journal of Thermophysics and Heat Transfer, 2006

The main objective of this study is to investigate heat transfer from tube banks in crossflow und... more The main objective of this study is to investigate heat transfer from tube banks in crossflow under isothermal boundary conditions. Because of the complex nature of fluid flow and heat transfer in a tube bank, the heat transfer from a tube in the first row of an in-line or staggered bank is determined first. For this purpose, a control volume is selected from the leading row of a tube bank and an integral method of boundary layer analysis is employed to determine the average heat transfer from the front stagnation point to the separation point, whereas the heat transfer from the separation point to the rear stagnation point is determined by an empirical correlation. To include the effect of the remaining rows, an empirical correlation is employed. The models for in-line and staggered arrangements are applicable for use over a wide range of Reynolds and Prandtl numbers as well as longitudinal and transverse pitch ratios.

Research paper thumbnail of Performance of Shrouded Pin-Fin Heat Sinks for Electronic Cooling

Journal of Thermophysics and Heat Transfer, 2006

The objective of this study is to analyze the performance of a cylindrical pin-fin heat sink in l... more The objective of this study is to analyze the performance of a cylindrical pin-fin heat sink in laminar forced convection. The mathematical models are presented for predicting thermal and hydraulic resistances for both in-line and staggered arrangements. Analytical/empirical correlations of friction and heat transfer coefficients are used in the analysis. The analyses are performed by using parametric variation of resistances. The effects of thermal joint, spreading, and contact resistances as well as the thermal conductivity on the overall thermal performance are examined. For a given size and heat load, it is observed that the overall performance of a pin-fin heat sink depends on a number of parameters including the dimensions of the pin-fins, pin density, longitudinal and transverse spacings, interface material, location and size of heat sources, method of manufacturing, type of heat-sink material, approach velocity, and arrangement of pins. It is also observed that the thermal resistance decreases whereas pressure drop increases with an increase in approach velocity, pin diameter, and pin density.

Research paper thumbnail of The Influence of Material Properties and Spreading Resistance in the Thermal Design of Plate Fin Heat Sinks

Journal of Electronic Packaging, 2007

The thermal design of plate fin heat sinks can benefit from optimization procedures where all des... more The thermal design of plate fin heat sinks can benefit from optimization procedures where all design variables are simultaneously prescribed, ensuring the best thermodynamic and air flow characteristic possible. While a cursory review of the thermal network established between heat sources and sinks in typical plate fin heat sinks would indicate that the film resistance at the fluid-solid boundary dominates, it is shown that the effects of other resistance elements, such as the spreading resistance and the material resistance, although of lesser magnitude, play an important role in the optimization and selection of heat sink design conditions. An analytical model is presented for calculating the best possible design parameters for plate fin heat sinks using an entropy generation minimization procedure with constrained variable optimization. The method characterizes the contribution to entropy production of all relevant thermal resistances in the path between source and sink as well as the contribution to viscous dissipation associated with fluid flow at the boundaries of the heat sink. The minimization procedure provides a fast, convenient method for establishing the "best case" design characteristics of plate fin heat sinks given a set of prescribed boundary conditions. It is shown that heat sinks made of composite materials containing nonmetallic constituents, with a thermal conductivity as much as an order of magnitude less that typical metallic heat sinks, can provide an effective alternative where performance, cost, and manufacturability are of importance. It is also shown that the spreading resistance encountered when heat flows from a heat source to the base plate of a heat sink, while significant, can be compensated for by making appropriate design modifications to the heat sink.

Research paper thumbnail of Fluid Flow Around and Heat Transfer From Elliptical Cylinders: Analytical Approach

Journal of Thermophysics and Heat Transfer, 2005

An integral method of boundary-layer analysis is employed to derive closed-form expressions for t... more An integral method of boundary-layer analysis is employed to derive closed-form expressions for the calculation of total drag and average heat transfer for flow across an elliptical cylinder under isothermal and isoflux thermal boundary conditions. The Von Kármán-Pohlhausen integral method is used to solve the momentum and energy equations for both thermal boundary conditions. A fourth-order velocity profile in the hydrodynamic boundary layer and a third-order temperature profile in the thermal boundary layer are used. The present results are in good agreement with existing experimental/numerical data and, in the limiting cases, can be used for circular cylinders and finite plates.

Research paper thumbnail of Optimization of Cylindrical Pin-Fin Heat Sinks Using Genetic Algorithms

IEEE Transactions on Components and Packaging Technologies, 2009

... Ayesha M. Sheikh received the MS degree in com-puter science from COMSATS Institute of Inform... more ... Ayesha M. Sheikh received the MS degree in com-puter science from COMSATS Institute of Informa-tion Technology, Abbottabad ... Waqar Ahmed Khan is an Associate Professor of mechanical engineering at the National University of Sciences and Technology, Karachi, Pakistan ...

Research paper thumbnail of Optimal Design of Tube Banks in Crossflow Using Entropy Generation Minimization Method

Journal of Thermophysics and Heat Transfer, 2007

An entropy generation minimization method is applied as a unique measure to study the thermodynam... more An entropy generation minimization method is applied as a unique measure to study the thermodynamic losses caused by heat transfer and pressure drop for a fluid in cross flow with tube banks. The use of entropy generation minimization allows the combined effect of heat transfer and pressure drop to be assessed through simultaneous interaction with the tube bank. A general dimensionless expression for the entropy generation rate is obtained by considering a control volume around a tube bank and applying conservation equations for mass and energy with entropy balance. Analytical/empirical correlations for heat transfer coefficients and friction factors are used, where the characteristic length is used as the diameter of the tubes and reference velocity used in Reynolds number and pressure drop is based on the minimum free area available for the fluid flow. Both inline and staggered arrangements are studied and their relative performance is compared for the same thermal and hydraulic conditions. A parametric study is also performed to show the effects of different design variables on the overall performance of tube banks. It is shown that all relevant design parameters for tube banks, including geometric parameters and flow conditions, can be simultaneously optimized.

Research paper thumbnail of Effect of Bypass on Overall Performance of Pin-Fin Heat Sinks

Journal of Thermophysics and Heat Transfer, 2007

One of the most important factors influencing the overall performance of cylindrical pin-fin heat... more One of the most important factors influencing the overall performance of cylindrical pin-fin heat sinks is the bypass phenomenon. Depending upon the total bypass area in comparison to the flow area between pin fins, a significant portion of the approaching airflow bypasses the heat sink. In this study, the effects of side and top bypass on the hydraulic and thermal performances of a cylindrical pin-fin heat sink will be investigated in laminar forced convection. Theoretical models, based on laws of conservation of mass, momentum, and energy, are developed to predict flow velocity, pressure drop, and heat transfer from the heat sink and the bypass regions. These models will help in determining hydraulic and thermal resistances in each region. Both in-line and staggered arrangements are analyzed in this study. Analytical and empirical correlations are used to determine friction factors and heat transfer coefficients in both arrangements. The effects of thermal spreading and joint resistances are neglected in this study.

Research paper thumbnail of Analytical Modeling of Fluid Flow and Heat Transfer in Microchannel/Nanochannel Heat Sinks

Journal of Thermophysics and Heat Transfer, 2008

Laminar forced convection in two-dimensional rectangular microchannels and nanochannels under hyd... more Laminar forced convection in two-dimensional rectangular microchannels and nanochannels under hydrodynamically and thermally fully developed conditions is investigated analytically in the slip-flow regime. Closed-form solutions for fluid friction and Nusselt numbers are obtained by solving the continuum momentum and energy equations with the first-order velocity slip and temperature jump boundary conditions at the channel walls. An isoflux thermal boundary condition is applied on the heat sink base. The results of the present analysis are presented in terms of the channel aspect ratio, hydraulic diameter, momentum and thermal accommodation coefficients, Knudsen number, slip velocity, Reynolds number, and Prandtl number. It is found that fluid friction decreases and heat transfer increases compared with no-slip flow conditions, depending on the aspect ratios and Knudsen numbers that include the effects of the channel size or rarefaction and the fluid/wall interaction.

Research paper thumbnail of Optimization of Microchannel Heat Sinks Using Entropy Generation Minimization Method

IEEE Transactions on Components and Packaging Technologies, 2009

In this study, an entropy generation minimization (EGM) procedure is employed to optimize the ove... more In this study, an entropy generation minimization (EGM) procedure is employed to optimize the overall performance of microchannel heat sinks. This allows the combined effects of thermal resistance and pressure drop to be assessed simultaneously as the heat sink interacts with the surrounding flow field. New general expressions for the entropy generation rate are developed by considering an appropriate control volume and applying mass, energy, and entropy balances. The effect of channel aspect ratio, fin spacing ratio, heat sink material, Knudsen numbers and accommodation coefficients on the entropy generation rate is investigated in the slip flow region. Analytical/empirical correlations are used for heat transfer and friction coefficients, where the characteristic length is used as the hydraulic diameter of the channel. A parametric study is also performed to show the effects of different design variables on the overall performance of microchannel heat sinks.

Research paper thumbnail of Analytical Model for Convection Heat Transfer From Tube Banks

Journal of Thermophysics and Heat Transfer, 2006

The main objective of this study is to investigate heat transfer from tube banks in crossflow und... more The main objective of this study is to investigate heat transfer from tube banks in crossflow under isothermal boundary conditions. Because of the complex nature of fluid flow and heat transfer in a tube bank, the heat transfer from a tube in the first row of an in-line or staggered bank is determined first. For this purpose, a control volume is selected from the leading row of a tube bank and an integral method of boundary layer analysis is employed to determine the average heat transfer from the front stagnation point to the separation point, whereas the heat transfer from the separation point to the rear stagnation point is determined by an empirical correlation. To include the effect of the remaining rows, an empirical correlation is employed. The models for in-line and staggered arrangements are applicable for use over a wide range of Reynolds and Prandtl numbers as well as longitudinal and transverse pitch ratios.

Research paper thumbnail of Modeling of Cylindrical Pin-Fin Heat Sinks for Electronic Packaging

IEEE Transactions on Components and Packaging Technologies, 2008

Analytical models are developed for determining heat transfer from in-line and staggered pin-fin ... more Analytical models are developed for determining heat transfer from in-line and staggered pin-fin heat sinks used in electronic packaging applications. The heat transfer coefficient for the heat sink and the average temperature of the fluid inside the heat sink are obtained from an energy balance over a control volume. In addition, friction coefficient models for both arrangements are developed from published data. The effects of thermal conductivity on the thermal performance are also examined. All models can be applied over a wide range of heat sink parameters and are suitable for use in the design of pin-fin heat sinks. The present models are in good agreement for high Reynolds numbers with existing experimental/numerical data.

Research paper thumbnail of Performance of Shrouded Pin-Fin Heat Sinks for Electronic Cooling

Journal of Thermophysics and Heat Transfer, 2006

The objective of this study is to analyze the performance of a cylindrical pin-fin heat sink in l... more The objective of this study is to analyze the performance of a cylindrical pin-fin heat sink in laminar forced convection. The mathematical models are presented for predicting thermal and hydraulic resistances for both in-line and staggered arrangements. Analytical/empirical correlations of friction and heat transfer coefficients are used in the analysis. The analyses are performed by using parametric variation of resistances. The effects of thermal joint, spreading, and contact resistances as well as the thermal conductivity on the overall thermal performance are examined. For a given size and heat load, it is observed that the overall performance of a pin-fin heat sink depends on a number of parameters including the dimensions of the pin-fins, pin density, longitudinal and transverse spacings, interface material, location and size of heat sources, method of manufacturing, type of heat-sink material, approach velocity, and arrangement of pins. It is also observed that the thermal resistance decreases whereas pressure drop increases with an increase in approach velocity, pin diameter, and pin density.

Research paper thumbnail of Fluid Flow And Heat Transfer from a Cylinder Between Parallel Planes

Journal of Thermophysics and Heat Transfer, 2004

An integral approach is employed to investigate the effects of blockage on fluid flow and heat tr... more An integral approach is employed to investigate the effects of blockage on fluid flow and heat transfer from a circular cylinder confined between parallel planes. The integral form of the boundary-layer momentum equation is solved using the modified von Kármán-Pohlhausen method, which uses a fourth-order velocity profile inside the hydrodynamic boundary layer. The potential flow velocity, outside the boundary layer, is obtained by the method of images. A third-order temperature profile is used in the thermal boundary layer to solve the energy integral equation for isothermal and isoflux boundary conditions. Closed-form solutions are obtained for the fluid flow and heat transfer from the cylinder with blockage ratio and Reynolds and Prandtl numbers as parameters. It is shown that the blockage ratio controls the fluid flow and the transfer of heat from the cylinder and delays the separation. The results for both thermal boundary conditions are found to be in a good agreement with experimental/numerical data for a single circular cylinder in a channel.

Research paper thumbnail of The Influence of Material Properties and Spreading Resistance in the Thermal Design of Plate Fin Heat Sinks

Journal of Electronic Packaging, 2007

The thermal design of plate fin heat sinks can benefit from optimization procedures where all des... more The thermal design of plate fin heat sinks can benefit from optimization procedures where all design variables are simultaneously prescribed, ensuring the best thermodynamic and air flow characteristic possible. While a cursory review of the thermal network established between heat sources and sinks in typical plate fin heat sinks would indicate that the film resistance at the fluid-solid boundary dominates, it is shown that the effects of other resistance elements, such as the spreading resistance and the material resistance, although of lesser magnitude, play an important role in the optimization and selection of heat sink design conditions. An analytical model is presented for calculating the best possible design parameters for plate fin heat sinks using an entropy generation minimization procedure with constrained variable optimization. The method characterizes the contribution to entropy production of all relevant thermal resistances in the path between source and sink as well as the contribution to viscous dissipation associated with fluid flow at the boundaries of the heat sink. The minimization procedure provides a fast, convenient method for establishing the "best case" design characteristics of plate fin heat sinks given a set of prescribed boundary conditions. It is shown that heat sinks made of composite materials containing nonmetallic constituents, with a thermal conductivity as much as an order of magnitude less that typical metallic heat sinks, can provide an effective alternative where performance, cost, and manufacturability are of importance. It is also shown that the spreading resistance encountered when heat flows from a heat source to the base plate of a heat sink, while significant, can be compensated for by making appropriate design modifications to the heat sink.

Research paper thumbnail of Optimization of pin-fin heat sinks using entropy generation minimization

IEEE Transactions on Components and Packaging Technologies, 2005

In this study, an entropy generation minimization, EGM, technique is applied as a unique measure ... more In this study, an entropy generation minimization, EGM, technique is applied as a unique measure to study the thermodynamic losses caused by heat transfer and pressure drop in cylindrical pin-fin heat sinks. The use of EGM allows the combined effect of thermal resistance and pressure drop to be assessed through the simultaneous interaction with the heat sink. A general expression for the entropy generation rate is obtained by considering the whole heat sink as a control volume and applying the conservation equations for mass and energy with the entropy balance. Analytical/empirical correlations for heat transfer coefficients and friction factors are used in the optimization model, where the characteristic length is used as the diameter of the pin and reference velocity used in Reynolds number and pressure drop is based on the minimum free area available for the fluid flow. Both in-line and staggered arrangements are studied and their relative performance is compared on the basis of equal overall volume of heat sinks. It is shown that all relevant design parameters for pin-fin heat sinks, including geometric parameters, material properties and flow conditions can be simultaneously optimized.

Research paper thumbnail of Mutation Analysis of the HBB Gene in Selected Bangladeshi β-Thalassemic Individuals: Presence of Rare Mutations

Genetic Testing and Molecular Biomarkers, 2010

Research paper thumbnail of Combination of two rare mutations causes -thalassaemia in a Bangladeshi patient

Screening of mutations that cause b-thalassaemia in the Bangladeshi population led to the identif... more Screening of mutations that cause b-thalassaemia in the Bangladeshi population led to the identification of a patient with a combination of two rare mutations, Hb Monroe and HBB: -92 C > G. The b-thalassaemia major male individual was transfusion-dependent and had an atypical b-globin gene cluster haplotype. Of the two mutations, Hb Monroe has been characterized in detail. Clinical effects of the other mutation, HBB: -92 C > G, are unknown so far. Bioinformatics analyses were carried out to predict the possible effect of this mutation. These analyses revealed the presence of a putative binding site for Egr1, a transcription factor, within the HBB:-92 region. Our literature survey suggests a close relationship between different phenotypic manifestations of b-thalassaemia and Egr1 expression.

Research paper thumbnail of Fluid Flow Around and Heat Transfer From Elliptical Cylinders: Analytical Approach

Journal of Thermophysics and Heat Transfer, 2005

An integral method of boundary-layer analysis is employed to derive closed-form expressions for t... more An integral method of boundary-layer analysis is employed to derive closed-form expressions for the calculation of total drag and average heat transfer for flow across an elliptical cylinder under isothermal and isoflux thermal boundary conditions. The Von Kármán-Pohlhausen integral method is used to solve the momentum and energy equations for both thermal boundary conditions. A fourth-order velocity profile in the hydrodynamic boundary layer and a third-order temperature profile in the thermal boundary layer are used. The present results are in good agreement with existing experimental/numerical data and, in the limiting cases, can be used for circular cylinders and finite plates.

Research paper thumbnail of Optimal Design of Tube Banks in Crossflow Using Entropy Generation Minimization Method

Journal of Thermophysics and Heat Transfer, 2007

An entropy generation minimization method is applied as a unique measure to study the thermodynam... more An entropy generation minimization method is applied as a unique measure to study the thermodynamic losses caused by heat transfer and pressure drop for a fluid in cross flow with tube banks. The use of entropy generation minimization allows the combined effect of heat transfer and pressure drop to be assessed through simultaneous interaction with the tube bank. A general dimensionless expression for the entropy generation rate is obtained by considering a control volume around a tube bank and applying conservation equations for mass and energy with entropy balance. Analytical/empirical correlations for heat transfer coefficients and friction factors are used, where the characteristic length is used as the diameter of the tubes and reference velocity used in Reynolds number and pressure drop is based on the minimum free area available for the fluid flow. Both inline and staggered arrangements are studied and their relative performance is compared for the same thermal and hydraulic conditions. A parametric study is also performed to show the effects of different design variables on the overall performance of tube banks. It is shown that all relevant design parameters for tube banks, including geometric parameters and flow conditions, can be simultaneously optimized.

Research paper thumbnail of Analytical Modeling of Fluid Flow and Heat Transfer in Microchannel/Nanochannel Heat Sinks

Journal of Thermophysics and Heat Transfer, 2008

Laminar forced convection in two-dimensional rectangular microchannels and nanochannels under hyd... more Laminar forced convection in two-dimensional rectangular microchannels and nanochannels under hydrodynamically and thermally fully developed conditions is investigated analytically in the slip-flow regime. Closed-form solutions for fluid friction and Nusselt numbers are obtained by solving the continuum momentum and energy equations with the first-order velocity slip and temperature jump boundary conditions at the channel walls. An isoflux thermal boundary condition is applied on the heat sink base. The results of the present analysis are presented in terms of the channel aspect ratio, hydraulic diameter, momentum and thermal accommodation coefficients, Knudsen number, slip velocity, Reynolds number, and Prandtl number. It is found that fluid friction decreases and heat transfer increases compared with no-slip flow conditions, depending on the aspect ratios and Knudsen numbers that include the effects of the channel size or rarefaction and the fluid/wall interaction.

Research paper thumbnail of Analytical Model for Convection Heat Transfer From Tube Banks

Journal of Thermophysics and Heat Transfer, 2006

The main objective of this study is to investigate heat transfer from tube banks in crossflow und... more The main objective of this study is to investigate heat transfer from tube banks in crossflow under isothermal boundary conditions. Because of the complex nature of fluid flow and heat transfer in a tube bank, the heat transfer from a tube in the first row of an in-line or staggered bank is determined first. For this purpose, a control volume is selected from the leading row of a tube bank and an integral method of boundary layer analysis is employed to determine the average heat transfer from the front stagnation point to the separation point, whereas the heat transfer from the separation point to the rear stagnation point is determined by an empirical correlation. To include the effect of the remaining rows, an empirical correlation is employed. The models for in-line and staggered arrangements are applicable for use over a wide range of Reynolds and Prandtl numbers as well as longitudinal and transverse pitch ratios.

Research paper thumbnail of Performance of Shrouded Pin-Fin Heat Sinks for Electronic Cooling

Journal of Thermophysics and Heat Transfer, 2006

The objective of this study is to analyze the performance of a cylindrical pin-fin heat sink in l... more The objective of this study is to analyze the performance of a cylindrical pin-fin heat sink in laminar forced convection. The mathematical models are presented for predicting thermal and hydraulic resistances for both in-line and staggered arrangements. Analytical/empirical correlations of friction and heat transfer coefficients are used in the analysis. The analyses are performed by using parametric variation of resistances. The effects of thermal joint, spreading, and contact resistances as well as the thermal conductivity on the overall thermal performance are examined. For a given size and heat load, it is observed that the overall performance of a pin-fin heat sink depends on a number of parameters including the dimensions of the pin-fins, pin density, longitudinal and transverse spacings, interface material, location and size of heat sources, method of manufacturing, type of heat-sink material, approach velocity, and arrangement of pins. It is also observed that the thermal resistance decreases whereas pressure drop increases with an increase in approach velocity, pin diameter, and pin density.

Research paper thumbnail of The Influence of Material Properties and Spreading Resistance in the Thermal Design of Plate Fin Heat Sinks

Journal of Electronic Packaging, 2007

The thermal design of plate fin heat sinks can benefit from optimization procedures where all des... more The thermal design of plate fin heat sinks can benefit from optimization procedures where all design variables are simultaneously prescribed, ensuring the best thermodynamic and air flow characteristic possible. While a cursory review of the thermal network established between heat sources and sinks in typical plate fin heat sinks would indicate that the film resistance at the fluid-solid boundary dominates, it is shown that the effects of other resistance elements, such as the spreading resistance and the material resistance, although of lesser magnitude, play an important role in the optimization and selection of heat sink design conditions. An analytical model is presented for calculating the best possible design parameters for plate fin heat sinks using an entropy generation minimization procedure with constrained variable optimization. The method characterizes the contribution to entropy production of all relevant thermal resistances in the path between source and sink as well as the contribution to viscous dissipation associated with fluid flow at the boundaries of the heat sink. The minimization procedure provides a fast, convenient method for establishing the "best case" design characteristics of plate fin heat sinks given a set of prescribed boundary conditions. It is shown that heat sinks made of composite materials containing nonmetallic constituents, with a thermal conductivity as much as an order of magnitude less that typical metallic heat sinks, can provide an effective alternative where performance, cost, and manufacturability are of importance. It is also shown that the spreading resistance encountered when heat flows from a heat source to the base plate of a heat sink, while significant, can be compensated for by making appropriate design modifications to the heat sink.

Research paper thumbnail of Fluid Flow Around and Heat Transfer From Elliptical Cylinders: Analytical Approach

Journal of Thermophysics and Heat Transfer, 2005

An integral method of boundary-layer analysis is employed to derive closed-form expressions for t... more An integral method of boundary-layer analysis is employed to derive closed-form expressions for the calculation of total drag and average heat transfer for flow across an elliptical cylinder under isothermal and isoflux thermal boundary conditions. The Von Kármán-Pohlhausen integral method is used to solve the momentum and energy equations for both thermal boundary conditions. A fourth-order velocity profile in the hydrodynamic boundary layer and a third-order temperature profile in the thermal boundary layer are used. The present results are in good agreement with existing experimental/numerical data and, in the limiting cases, can be used for circular cylinders and finite plates.

Research paper thumbnail of Optimization of Cylindrical Pin-Fin Heat Sinks Using Genetic Algorithms

IEEE Transactions on Components and Packaging Technologies, 2009

... Ayesha M. Sheikh received the MS degree in com-puter science from COMSATS Institute of Inform... more ... Ayesha M. Sheikh received the MS degree in com-puter science from COMSATS Institute of Informa-tion Technology, Abbottabad ... Waqar Ahmed Khan is an Associate Professor of mechanical engineering at the National University of Sciences and Technology, Karachi, Pakistan ...

Research paper thumbnail of Optimal Design of Tube Banks in Crossflow Using Entropy Generation Minimization Method

Journal of Thermophysics and Heat Transfer, 2007

An entropy generation minimization method is applied as a unique measure to study the thermodynam... more An entropy generation minimization method is applied as a unique measure to study the thermodynamic losses caused by heat transfer and pressure drop for a fluid in cross flow with tube banks. The use of entropy generation minimization allows the combined effect of heat transfer and pressure drop to be assessed through simultaneous interaction with the tube bank. A general dimensionless expression for the entropy generation rate is obtained by considering a control volume around a tube bank and applying conservation equations for mass and energy with entropy balance. Analytical/empirical correlations for heat transfer coefficients and friction factors are used, where the characteristic length is used as the diameter of the tubes and reference velocity used in Reynolds number and pressure drop is based on the minimum free area available for the fluid flow. Both inline and staggered arrangements are studied and their relative performance is compared for the same thermal and hydraulic conditions. A parametric study is also performed to show the effects of different design variables on the overall performance of tube banks. It is shown that all relevant design parameters for tube banks, including geometric parameters and flow conditions, can be simultaneously optimized.

Research paper thumbnail of Effect of Bypass on Overall Performance of Pin-Fin Heat Sinks

Journal of Thermophysics and Heat Transfer, 2007

One of the most important factors influencing the overall performance of cylindrical pin-fin heat... more One of the most important factors influencing the overall performance of cylindrical pin-fin heat sinks is the bypass phenomenon. Depending upon the total bypass area in comparison to the flow area between pin fins, a significant portion of the approaching airflow bypasses the heat sink. In this study, the effects of side and top bypass on the hydraulic and thermal performances of a cylindrical pin-fin heat sink will be investigated in laminar forced convection. Theoretical models, based on laws of conservation of mass, momentum, and energy, are developed to predict flow velocity, pressure drop, and heat transfer from the heat sink and the bypass regions. These models will help in determining hydraulic and thermal resistances in each region. Both in-line and staggered arrangements are analyzed in this study. Analytical and empirical correlations are used to determine friction factors and heat transfer coefficients in both arrangements. The effects of thermal spreading and joint resistances are neglected in this study.

Research paper thumbnail of Analytical Modeling of Fluid Flow and Heat Transfer in Microchannel/Nanochannel Heat Sinks

Journal of Thermophysics and Heat Transfer, 2008

Laminar forced convection in two-dimensional rectangular microchannels and nanochannels under hyd... more Laminar forced convection in two-dimensional rectangular microchannels and nanochannels under hydrodynamically and thermally fully developed conditions is investigated analytically in the slip-flow regime. Closed-form solutions for fluid friction and Nusselt numbers are obtained by solving the continuum momentum and energy equations with the first-order velocity slip and temperature jump boundary conditions at the channel walls. An isoflux thermal boundary condition is applied on the heat sink base. The results of the present analysis are presented in terms of the channel aspect ratio, hydraulic diameter, momentum and thermal accommodation coefficients, Knudsen number, slip velocity, Reynolds number, and Prandtl number. It is found that fluid friction decreases and heat transfer increases compared with no-slip flow conditions, depending on the aspect ratios and Knudsen numbers that include the effects of the channel size or rarefaction and the fluid/wall interaction.

Research paper thumbnail of Optimization of Microchannel Heat Sinks Using Entropy Generation Minimization Method

IEEE Transactions on Components and Packaging Technologies, 2009

In this study, an entropy generation minimization (EGM) procedure is employed to optimize the ove... more In this study, an entropy generation minimization (EGM) procedure is employed to optimize the overall performance of microchannel heat sinks. This allows the combined effects of thermal resistance and pressure drop to be assessed simultaneously as the heat sink interacts with the surrounding flow field. New general expressions for the entropy generation rate are developed by considering an appropriate control volume and applying mass, energy, and entropy balances. The effect of channel aspect ratio, fin spacing ratio, heat sink material, Knudsen numbers and accommodation coefficients on the entropy generation rate is investigated in the slip flow region. Analytical/empirical correlations are used for heat transfer and friction coefficients, where the characteristic length is used as the hydraulic diameter of the channel. A parametric study is also performed to show the effects of different design variables on the overall performance of microchannel heat sinks.

Research paper thumbnail of Analytical Model for Convection Heat Transfer From Tube Banks

Journal of Thermophysics and Heat Transfer, 2006

The main objective of this study is to investigate heat transfer from tube banks in crossflow und... more The main objective of this study is to investigate heat transfer from tube banks in crossflow under isothermal boundary conditions. Because of the complex nature of fluid flow and heat transfer in a tube bank, the heat transfer from a tube in the first row of an in-line or staggered bank is determined first. For this purpose, a control volume is selected from the leading row of a tube bank and an integral method of boundary layer analysis is employed to determine the average heat transfer from the front stagnation point to the separation point, whereas the heat transfer from the separation point to the rear stagnation point is determined by an empirical correlation. To include the effect of the remaining rows, an empirical correlation is employed. The models for in-line and staggered arrangements are applicable for use over a wide range of Reynolds and Prandtl numbers as well as longitudinal and transverse pitch ratios.

Research paper thumbnail of Modeling of Cylindrical Pin-Fin Heat Sinks for Electronic Packaging

IEEE Transactions on Components and Packaging Technologies, 2008

Analytical models are developed for determining heat transfer from in-line and staggered pin-fin ... more Analytical models are developed for determining heat transfer from in-line and staggered pin-fin heat sinks used in electronic packaging applications. The heat transfer coefficient for the heat sink and the average temperature of the fluid inside the heat sink are obtained from an energy balance over a control volume. In addition, friction coefficient models for both arrangements are developed from published data. The effects of thermal conductivity on the thermal performance are also examined. All models can be applied over a wide range of heat sink parameters and are suitable for use in the design of pin-fin heat sinks. The present models are in good agreement for high Reynolds numbers with existing experimental/numerical data.

Research paper thumbnail of Performance of Shrouded Pin-Fin Heat Sinks for Electronic Cooling

Journal of Thermophysics and Heat Transfer, 2006

The objective of this study is to analyze the performance of a cylindrical pin-fin heat sink in l... more The objective of this study is to analyze the performance of a cylindrical pin-fin heat sink in laminar forced convection. The mathematical models are presented for predicting thermal and hydraulic resistances for both in-line and staggered arrangements. Analytical/empirical correlations of friction and heat transfer coefficients are used in the analysis. The analyses are performed by using parametric variation of resistances. The effects of thermal joint, spreading, and contact resistances as well as the thermal conductivity on the overall thermal performance are examined. For a given size and heat load, it is observed that the overall performance of a pin-fin heat sink depends on a number of parameters including the dimensions of the pin-fins, pin density, longitudinal and transverse spacings, interface material, location and size of heat sources, method of manufacturing, type of heat-sink material, approach velocity, and arrangement of pins. It is also observed that the thermal resistance decreases whereas pressure drop increases with an increase in approach velocity, pin diameter, and pin density.

Research paper thumbnail of Fluid Flow And Heat Transfer from a Cylinder Between Parallel Planes

Journal of Thermophysics and Heat Transfer, 2004

An integral approach is employed to investigate the effects of blockage on fluid flow and heat tr... more An integral approach is employed to investigate the effects of blockage on fluid flow and heat transfer from a circular cylinder confined between parallel planes. The integral form of the boundary-layer momentum equation is solved using the modified von Kármán-Pohlhausen method, which uses a fourth-order velocity profile inside the hydrodynamic boundary layer. The potential flow velocity, outside the boundary layer, is obtained by the method of images. A third-order temperature profile is used in the thermal boundary layer to solve the energy integral equation for isothermal and isoflux boundary conditions. Closed-form solutions are obtained for the fluid flow and heat transfer from the cylinder with blockage ratio and Reynolds and Prandtl numbers as parameters. It is shown that the blockage ratio controls the fluid flow and the transfer of heat from the cylinder and delays the separation. The results for both thermal boundary conditions are found to be in a good agreement with experimental/numerical data for a single circular cylinder in a channel.

Research paper thumbnail of The Influence of Material Properties and Spreading Resistance in the Thermal Design of Plate Fin Heat Sinks

Journal of Electronic Packaging, 2007

The thermal design of plate fin heat sinks can benefit from optimization procedures where all des... more The thermal design of plate fin heat sinks can benefit from optimization procedures where all design variables are simultaneously prescribed, ensuring the best thermodynamic and air flow characteristic possible. While a cursory review of the thermal network established between heat sources and sinks in typical plate fin heat sinks would indicate that the film resistance at the fluid-solid boundary dominates, it is shown that the effects of other resistance elements, such as the spreading resistance and the material resistance, although of lesser magnitude, play an important role in the optimization and selection of heat sink design conditions. An analytical model is presented for calculating the best possible design parameters for plate fin heat sinks using an entropy generation minimization procedure with constrained variable optimization. The method characterizes the contribution to entropy production of all relevant thermal resistances in the path between source and sink as well as the contribution to viscous dissipation associated with fluid flow at the boundaries of the heat sink. The minimization procedure provides a fast, convenient method for establishing the "best case" design characteristics of plate fin heat sinks given a set of prescribed boundary conditions. It is shown that heat sinks made of composite materials containing nonmetallic constituents, with a thermal conductivity as much as an order of magnitude less that typical metallic heat sinks, can provide an effective alternative where performance, cost, and manufacturability are of importance. It is also shown that the spreading resistance encountered when heat flows from a heat source to the base plate of a heat sink, while significant, can be compensated for by making appropriate design modifications to the heat sink.

Research paper thumbnail of Optimization of pin-fin heat sinks using entropy generation minimization

IEEE Transactions on Components and Packaging Technologies, 2005

In this study, an entropy generation minimization, EGM, technique is applied as a unique measure ... more In this study, an entropy generation minimization, EGM, technique is applied as a unique measure to study the thermodynamic losses caused by heat transfer and pressure drop in cylindrical pin-fin heat sinks. The use of EGM allows the combined effect of thermal resistance and pressure drop to be assessed through the simultaneous interaction with the heat sink. A general expression for the entropy generation rate is obtained by considering the whole heat sink as a control volume and applying the conservation equations for mass and energy with the entropy balance. Analytical/empirical correlations for heat transfer coefficients and friction factors are used in the optimization model, where the characteristic length is used as the diameter of the pin and reference velocity used in Reynolds number and pressure drop is based on the minimum free area available for the fluid flow. Both in-line and staggered arrangements are studied and their relative performance is compared on the basis of equal overall volume of heat sinks. It is shown that all relevant design parameters for pin-fin heat sinks, including geometric parameters, material properties and flow conditions can be simultaneously optimized.