Gas flows through micro-filters are simulated in the continuum and slip flow regimes as a functio... more Gas flows through micro-filters are simulated in the continuum and slip flow regimes as a function of the Knudsen, Reynolds and Mach numbers. The numerical simulations are based on the spectral element formulation of compressible Navier-Stokes equations, which utilize previously developed high-order velocity slip and temperature jump boundary conditions. Both slip and no-slip simulations are used to identify the rarefaction effects. The simulation results show skin friction and form-drag reduction with increased Knudsen number. Pressure drops across the filters are compared against several empirical scaling laws, available in the literature. Compressibility becomes important for high-speed flows, creating large density fluctuations across the micro-filter elements. For high Mach number flows, interactions between thermal and kinetic energies of the fluid are observed. It is also shown that viscous heating plays a significant role for highspeed gas flows, impacting heat transfer characteristics of micro-filters.
Direct current dielectrophoretic (DC-DEP) effects on the electrophoretic motion of charged polyst... more Direct current dielectrophoretic (DC-DEP) effects on the electrophoretic motion of charged polystyrene particles through an L-shaped microchannel were experimentally and numerically studied. In addition to the electrostatic and hydrodynamic forces, particles experience a negative DC-DEP force arising from the interaction between the dielectric particle and the induced spatially non-uniform electric field occurring around the corner of the L-shape microchannel. The latter force causes a cross-stream DEP motion so that the particle trajectory is shifted towards the outer corner of the turn. A two-dimensional (2D) Lagrangian particle tracking model taking into account the induced DC-DEP effect was used to predict the particle trajectory shift through the L-shaped channel, which achieves quantitative agreement with the experimental data.
Micro-Electro-Mechanical Systems (MEMS), Nov 15, 1998
Conceptual design of a reversible micro-pump system is demonstrated by numerical simulations. Uns... more Conceptual design of a reversible micro-pump system is demonstrated by numerical simulations. Unsteady, incompressible Navier-Stokes equations in a moving boundary system are solved by ΝεκΤαr, a spectral element (high-order) algorithm employing an Arbitrary Lagrangian Eulerian (ALE) formulation on unstructured meshes. The performance of the micro-pump is evaluated as a function of the Reynolds number and the geometric parameters. The volumetric flowrate is shown to increase as a function of the Reynolds number. However, the efficiency of the micro-pump decreases with increased Reynolds number, due to the increased leakage effects.
We demonstrate flow control concepts in a grooved micro-channel using selectively patterned, elec... more We demonstrate flow control concepts in a grooved micro-channel using selectively patterned, electroosmotically active surfaces and locally applied electric fields. This framework enables formation of rather complex flow patterns in simple micro-geometries. Ability to vary the electric field magnitude and its polarity also manifests time-dependent flow alterations, which results in flow and species transport control abilities. The results obtained in a single micro-groove constitute the proof of concept for flow and species transport ...
The boundary effects on DC‐electrokinetic behavior of colloidal cylinder(s) in the vicinity of a ... more The boundary effects on DC‐electrokinetic behavior of colloidal cylinder(s) in the vicinity of a conducting wall is investigated through a computational model. The contribution of the hydrodynamic drag, gravity, electrokinetic (i.e., electrophoretic and dielectrophoretic), and colloidal forces (i.e., forces due to the electrical double layer and van der Waals interactions) are incorporated in the model. The contribution of electrokinetic and colloidal forces are included by introducing the resulting forces as an external force acting on the particle(s). The colloidal forces are implemented with the prescribed expressions from the literature, and the electrokinetic force is obtained by integrating the corresponding Maxwell stress tensor over the particles' surfaces. The electrokinetic slip‐velocity together with the thin electrical double layer assumption is applied on the surfaces. The position and velocity of the particles and the resulting electric and flow fields are obtained...
A new micro heat spreader (MHS) concept for efficient dissipation of large, concentrated heat loa... more A new micro heat spreader (MHS) concept for efficient dissipation of large, concentrated heat loads is introduced. The MHS is a single-phase, closed micro-fluidic system, which utilize time-periodic forced convection cooling. We verified the MHS concept by numerically simulating its operation under various conditions. Our parametric studies have shown that, unlike the steady laminar forced convection, the Nusselt number for time-periodic forced convection laminar flows have strong dependence on the Reynolds and Prandtl numbers. The increase in the Nusselt number indicates enhanced cooling capability of the MHS device. Based on our parametric studies, we calculated the optimum operation conditions, device dimensions and the maximum heat-dissipation rates.
Miniaturization of microelectronic device components and the development of nano– electro– mechan... more Miniaturization of microelectronic device components and the development of nano– electro– mechanical systems require advanced understanding of thermal transport in nano-materials and devices, where the atomic nature of matter becomes important and the validity of well-known continuum approximations becomes questionable [1]. In the case of semiconductors and insulators, heat is carried primarily by vibrations in the crystal lattice known as phonons. Phonon transport is classically studied by lattice dynamics based on harmonic wave theory in the frequency space. However, the anharmonic behaviors forming in a crystal structure cannot be described with this theory. Alternatively, the coupled motions of the atoms in real space can be modeled by molecular dynamics (MD), which provides the natural formation and transport of phonons via vibrations in the crystal lattice. Hence, MD has been widely employed to model phonon transfer in nanostructures and channels [2,3]. The performance and reliability of aforementioned devices strongly depend on the removal of heat either to the ambient or to a coolant. In such cases, phonon transport observed at the interfaces of nanoscale device components and surrounding/confined fluid, or at the interfaces of suspended nanoparticles and fluid medium in nano-fluidic coolants plays a critical role. At such interfaces, heat transfer is interrupted with a temperature jump due to the deficiency in overlap between phonon dispersions of dissimilar materials. Classical theories considering specular or diffuse phonon scattering predict the upper or lower limits of interface thermal resistance (ITR), while a detailed investigation of intermolecular interactions is needed to resolve interface phonon scattering mechanisms. In this chapter, we present interface phonon transfer at the molecular level, and investigate the validity of continuum hypothesis and Fourier’s law in nano-channels. First, we focus on the conventional ways of using MD for heat transport problems. Most of the previous MD research sandwiched a liquid domain between two solid walls and induced heat flux by fixing the wall CONTENTS
Water desalination using positively and negatively charged single-layer nanoporous graphene membr... more Water desalination using positively and negatively charged single-layer nanoporous graphene membranes.
Gas flows through micro-filters are simulated in the continuum and slip flow regimes as a functio... more Gas flows through micro-filters are simulated in the continuum and slip flow regimes as a function of the Knudsen, Reynolds and Mach numbers. The numerical simulations are based on the spectral element formulation of compressible Navier-Stokes equations, which utilize previously developed high-order velocity slip and temperature jump boundary conditions. Both slip and no-slip simulations are used to identify the rarefaction effects. The simulation results show skin friction and form-drag reduction with increased Knudsen number. Pressure drops across the filters are compared against several empirical scaling laws, available in the literature. Compressibility becomes important for high-speed flows, creating large density fluctuations across the micro-filter elements. For high Mach number flows, interactions between thermal and kinetic energies of the fluid are observed. It is also shown that viscous heating plays a significant role for highspeed gas flows, impacting heat transfer characteristics of micro-filters.
Direct current dielectrophoretic (DC-DEP) effects on the electrophoretic motion of charged polyst... more Direct current dielectrophoretic (DC-DEP) effects on the electrophoretic motion of charged polystyrene particles through an L-shaped microchannel were experimentally and numerically studied. In addition to the electrostatic and hydrodynamic forces, particles experience a negative DC-DEP force arising from the interaction between the dielectric particle and the induced spatially non-uniform electric field occurring around the corner of the L-shape microchannel. The latter force causes a cross-stream DEP motion so that the particle trajectory is shifted towards the outer corner of the turn. A two-dimensional (2D) Lagrangian particle tracking model taking into account the induced DC-DEP effect was used to predict the particle trajectory shift through the L-shaped channel, which achieves quantitative agreement with the experimental data.
Micro-Electro-Mechanical Systems (MEMS), Nov 15, 1998
Conceptual design of a reversible micro-pump system is demonstrated by numerical simulations. Uns... more Conceptual design of a reversible micro-pump system is demonstrated by numerical simulations. Unsteady, incompressible Navier-Stokes equations in a moving boundary system are solved by ΝεκΤαr, a spectral element (high-order) algorithm employing an Arbitrary Lagrangian Eulerian (ALE) formulation on unstructured meshes. The performance of the micro-pump is evaluated as a function of the Reynolds number and the geometric parameters. The volumetric flowrate is shown to increase as a function of the Reynolds number. However, the efficiency of the micro-pump decreases with increased Reynolds number, due to the increased leakage effects.
We demonstrate flow control concepts in a grooved micro-channel using selectively patterned, elec... more We demonstrate flow control concepts in a grooved micro-channel using selectively patterned, electroosmotically active surfaces and locally applied electric fields. This framework enables formation of rather complex flow patterns in simple micro-geometries. Ability to vary the electric field magnitude and its polarity also manifests time-dependent flow alterations, which results in flow and species transport control abilities. The results obtained in a single micro-groove constitute the proof of concept for flow and species transport ...
The boundary effects on DC‐electrokinetic behavior of colloidal cylinder(s) in the vicinity of a ... more The boundary effects on DC‐electrokinetic behavior of colloidal cylinder(s) in the vicinity of a conducting wall is investigated through a computational model. The contribution of the hydrodynamic drag, gravity, electrokinetic (i.e., electrophoretic and dielectrophoretic), and colloidal forces (i.e., forces due to the electrical double layer and van der Waals interactions) are incorporated in the model. The contribution of electrokinetic and colloidal forces are included by introducing the resulting forces as an external force acting on the particle(s). The colloidal forces are implemented with the prescribed expressions from the literature, and the electrokinetic force is obtained by integrating the corresponding Maxwell stress tensor over the particles' surfaces. The electrokinetic slip‐velocity together with the thin electrical double layer assumption is applied on the surfaces. The position and velocity of the particles and the resulting electric and flow fields are obtained...
A new micro heat spreader (MHS) concept for efficient dissipation of large, concentrated heat loa... more A new micro heat spreader (MHS) concept for efficient dissipation of large, concentrated heat loads is introduced. The MHS is a single-phase, closed micro-fluidic system, which utilize time-periodic forced convection cooling. We verified the MHS concept by numerically simulating its operation under various conditions. Our parametric studies have shown that, unlike the steady laminar forced convection, the Nusselt number for time-periodic forced convection laminar flows have strong dependence on the Reynolds and Prandtl numbers. The increase in the Nusselt number indicates enhanced cooling capability of the MHS device. Based on our parametric studies, we calculated the optimum operation conditions, device dimensions and the maximum heat-dissipation rates.
Miniaturization of microelectronic device components and the development of nano– electro– mechan... more Miniaturization of microelectronic device components and the development of nano– electro– mechanical systems require advanced understanding of thermal transport in nano-materials and devices, where the atomic nature of matter becomes important and the validity of well-known continuum approximations becomes questionable [1]. In the case of semiconductors and insulators, heat is carried primarily by vibrations in the crystal lattice known as phonons. Phonon transport is classically studied by lattice dynamics based on harmonic wave theory in the frequency space. However, the anharmonic behaviors forming in a crystal structure cannot be described with this theory. Alternatively, the coupled motions of the atoms in real space can be modeled by molecular dynamics (MD), which provides the natural formation and transport of phonons via vibrations in the crystal lattice. Hence, MD has been widely employed to model phonon transfer in nanostructures and channels [2,3]. The performance and reliability of aforementioned devices strongly depend on the removal of heat either to the ambient or to a coolant. In such cases, phonon transport observed at the interfaces of nanoscale device components and surrounding/confined fluid, or at the interfaces of suspended nanoparticles and fluid medium in nano-fluidic coolants plays a critical role. At such interfaces, heat transfer is interrupted with a temperature jump due to the deficiency in overlap between phonon dispersions of dissimilar materials. Classical theories considering specular or diffuse phonon scattering predict the upper or lower limits of interface thermal resistance (ITR), while a detailed investigation of intermolecular interactions is needed to resolve interface phonon scattering mechanisms. In this chapter, we present interface phonon transfer at the molecular level, and investigate the validity of continuum hypothesis and Fourier’s law in nano-channels. First, we focus on the conventional ways of using MD for heat transport problems. Most of the previous MD research sandwiched a liquid domain between two solid walls and induced heat flux by fixing the wall CONTENTS
Water desalination using positively and negatively charged single-layer nanoporous graphene membr... more Water desalination using positively and negatively charged single-layer nanoporous graphene membranes.
Uploads
Papers by Ali Beskok