Kunt Atalık

Non-linear temporal stability analysis of viscoelastic plane channel flows using a fully-spectral method

A non-linear analysis of the temporal evolution of finite, two-dimensional disturbances is conducted for plane Poiseuille and Couette flows of viscoelastic fluids. A fully-spectral method of solution is used with a stream-function formulation of the problem. The upper-convected Maxwell (UCM), Oldroyd-B and Giesekus models are considered. The bifurcation of solutions for increasing elasticity is investigated both in the high and low Reynolds number regimes. The transition mechanism is discussed in terms of both the transient linear growth of misfit disturbances due to non-normality, and their possible saturation into finite-amplitude periodic solutions due to non-linear effects

Prediction of Thermal Conductivity and Shear Viscosity of Water-Cu Nanofluids Using Equilibrium Molecular Dynamics

Nanofluids are new class of fluids which can be used for many engineering applications due to their enhanced thermal properties. The macroscopic modeling tools used for flow simulations usually rely on effective thermal and rheological properties of the nanofluids that can be predicted through various effective medium theories. As these theories significantly under-predict, using correlations based on experimental data is considered as the only reliable means for prediction of these effective properties. However, the behavior might change significantly once the particle material or base fluid change due to different particle fluid interactions in the molecular level. One of the most promising means of modeling effective properties of the nanofluids is the molecular dynamics simulations where all the intermolecular effects can be modeled. This study investigates equilibrium molecular dynamics simulation of the water-Cu nanofluids to predict the thermal and rheological properties. The molecular dynamics simulation is carried out to achieve a thermodynamic equilibrium, based on a state that is defined by targeted thermodynamic properties of the system. The Green-Kubo method is used to predict the thermal conductivity and viscosity of the system. The study considers the use of different combining rules such as Lorentz-Berthelot and sixth-power rules for defining the inter-atomic potentials for water modeled by SPC/E and nanoparticles modeled by Lennard-Jones potential. The predicted effective properties that are thermal conductivity and shear viscosity are then compared with experimental data from literature. The predicted transport properties at different temperatures and particle concentrations are compared to experimental data from literature for model validation.Copyright

Synthesis and Experimental Investigation of Rheological Behavior of EG and Water Based hBN Nanofluids

Hexagonal boron nitride (hBN) is a ceramic material with high thermal conductivity and superior chemical stability that makes it a suitable candidate for nanofluid synthesis. The studies that focus on hBN nanofluids are mostly limited to hBN-oil dielectric nanofluids. However, using hBN with other base fluids has also significant potential applications. This study focuses on stability and rheological behavior of hBN-water and hBN-ethylene glycol nanofluids that have not been investigated in detail. Nanofluids with hBN nanoparticles of an effective diameter of 70 nm are synthesized using the two-step method. The main problem in this method is sedimentation as a result of disproportionate amount of clustering of nanoparticles within the nanofluids. In the preparation procedure of nanofluids, sonication and pH level control are used. The stability of the hBN-water nanofluids is determined either by quantitative methods such as zeta potential measurements and/or effective particle size measurements via dynamic light scattering, or qualitative methods such as SEM (Scanning Electron Microscopy) and STEM (Scanning Transient Electron Microscopy). Following the investigation of the effect of several parameters, such as sonication time, pH level of the suspension on the stability of the nanofluids, rheological behavior is investigated experimentally. Rheology experiments are conducted with a cone–plate rheometer. In these experiments different volume concentrated (0.5–1%) nanofluids are considered. Validation studies for preparation and characterization methods are carried out with alumina (Al2O3), nanofluids which are synthesized by dispersing γ-Al2O3 nanoparticles of 15 nm into deionized water.© 2013 ASME

Assessment of Single and Two-Phase Models for Nanofluid Flow at the Entrance Region of a Uniformly Heated Tube

Hydrodynamic and thermal characteristics of Al2O3 – water nanofluid flow at entry region of a uniformly heated pipe are studied applying finite control volume method (FCV). Single phase and Eulerian-Eulerian two-phase models were used in modelling of nanofluid flow and heat transfer. The two methods are evaluated by comparing predicted convective heat transfer coefficients and friction factor with experimental results from literature. Solutions with two different velocity pressure coupling algorithms, Full Multiphase Coupled, and Phase Coupled Semi-Implicit Method for Pressure Linked Equations are also compared in terms of accuracy and computational cost. Two-phase model predicts convective heat transfer coefficient and friction factor more accurately at the entry region. Moreover, computational cost can be reduced by implementing Full Multiphase Coupled scheme.Copyright

Investigation of Single Phase Models for Predicting Pressure Drop in Nanofluid Flow in Circular Pipes

Macroscopic modeling of hydrodynamic behavior of nanofluid flow in a uniformly heated circular pipe is considered. Single-phase models with Brownian and dispersion viscosity models are evaluated by comparing predicted pressure drop and apparent friction factor with experimental and two-phase Eulerian-Eulerian model results from literature. Single-phase models are capable of predicting heat transfer of nanofluids better when dispersion models are used. However, they fail to accurately predict pressure drop when used with standard viscosity models. Two-phase models on the other hand, can accurately predict both thermodynamic and hydrodynamic field at the expense of computational time. A new viscosity model, which is based on dispersion viscosity, is proposed to increase accuracy of single-phase models in predicting hydrodynamic field of nanofluid flow. Results suggest that single-phase dispersion viscosity model is the most accurate single-phase model.Copyright

Isothermal and non-isothermal viscoelastic flow of PTT fluid in lid-driven polar cavity

The isothermal and non-isothermal viscoelastic flow of Phan-Thien-Tanner (PTT) fluids is considered in liddriven polar cavity geometry, using a numerical solution method with parameter continuation technique. Thermoelastic effects, in terms of elastic/elongational effects and viscous dissipation, are demonstrated by the changes in vortical structure, temperature/stress distributions and heat transfer characteristics in the curved cavity. Central vortex/maximum temperature location shifts are observed under elastic and elongational (strain hardening and strain softening/shear thinning) effects for isothermal and non-isothermal conditions. The growth in size and strength of a secondary vortex is denoted in the downstream stationary corner of the cavity for the viscoelastic fluid under strain hardening, which also introduces an increase in stress gradients. Viscous heating is observed with elongational effects near the central vortex in the cavity. Stress components and their gradients decrease under viscous dissipation. The changes in temperature field and heat transfer properties in the cavity are revealed.

Computational parametric analysis of rotating surface flow

Abstract The flow due to rotating surfaces in a cylindrical enclosure is commonly used in many applications such as rheometry, electronic cooling, and turbomachinery, and has drawn scientists’ attention for many years. The main objective of this study is to solve the problem with a robust and efficient code. It is achieved by using the ‘Portable, Extensible Toolkit for Scientific computation’ (PETSc), which is a computational tool for the parallel solution of scientific problems. By keeping the Newtons method as the non-linear solver, different linear solvers, preconditioning techniques, and numbers of processors are tested for the performance. In addition to computational parameters such as the performance of the linear solver, performance of the preconditioner, parallel performance, and grid dependence, the effects of some physical parameters such as the Reynolds number, aspect ratio, and altering of the rotating surface are investigated. The results indicate that the core of the circulation moves towards the stationary boundaries and the boundary layers become thinner with the increasing Reynolds number. Furthermore, decreasing the aspect ratio makes diffusion harder and lowers the maximum velocities. Some of the results are compared with data in the literature.