Abstract: Mixed convection of Cu-water nanofluid in an enclosure
with thick wavy bottom wall has been investigated numerically.
A co-ordinate transformation method is used to transform the
computational domain into an orthogonal co-ordinate system. The
governing equations in the computational domain are solved through
a pressure correction based iterative algorithm. The fluid flow
and heat transfer characteristics are analyzed for a wide range
of Richardson number (0.1 ≤ Ri ≤ 5), nanoparticle volume
concentration (0.0 ≤ ϕ ≤ 0.2), amplitude (0.0 ≤ α ≤ 0.1) of
the wavy thick- bottom wall and the wave number (ω) at a fixed
Reynolds number. Obtained results showed that heat transfer rate
increases remarkably by adding the nanoparticles. Heat transfer rate
is dependent on the wavy wall amplitude and wave number and
decreases with increasing Richardson number for fixed amplitude
and wave number. The Bejan number and the entropy generation are
determined to analyze the thermodynamic optimization of the mixed
convection.
Abstract: In this study, we have analyzed the transport of analytes
under a two dimensional steady incompressible flow of power-law
fluids through rectangular nanochannel. A mathematical model
based on the Cauchy momentum-Nernst-Planck-Poisson equations is
considered to study the combined effect of mixed electroosmotic
(EO) and pressure driven (PD) flow. The coupled governing
equations are solved numerically by finite volume method. We
have studied extensively the effect of key parameters, e.g., flow
behavior index, concentration of the electrolyte, surface potential,
imposed pressure gradient and imposed electric field strength on
the net average flow across the channel. In addition to study
the effect of mixed EOF and PD on the analyte distribution
across the channel, we consider a nonlinear model based on
general convective-diffusion-electromigration equation. We have also
presented the retention factor for various values of electrolyte
concentration and flow behavior index.
Abstract: Migration of a core-shell soft particle under the
influence of an external electric field in an electrolyte solution is
studied numerically. The soft particle is coated with a positively
charged polyelectrolyte layer (PEL) and the rigid core is having
a uniform surface charge density. The Darcy-Brinkman extended
Navier-Stokes equations are solved for the motion of the ionized
fluid, the non-linear Nernst-Planck equations for the ion transport and
the Poisson equation for the electric potential. A pressure correction
based iterative algorithm is adopted for numerical computations. The
effects of convection on double layer polarization (DLP) and diffusion
dominated counter ions penetration are investigated for a wide range
of Debye layer thickness, PEL fixed surface charge density, and
permeability of the PEL. Our results show that when the Debye
layer is in order of the particle size, the DLP effect is significant
and produces a reduction in electrophoretic mobility. However, the
double layer polarization effect is negligible for a thin Debye layer
or low permeable cases. The point of zero mobility and the existence
of mobility reversal depending on the electrolyte concentration are
also presented.
Abstract: Natural circulation loops (NCLs) are buoyancy driven flow systems without any moving components. NCLs have vast applications in geothermal, solar and nuclear power industry where reliability and safety are of foremost concern. Due to certain favorable thermophysical properties, especially near supercritical regions, carbon dioxide can be considered as an ideal loop fluid in many applications. In the present work, a high temperature NCL that uses supercritical carbon dioxide as loop fluid is analysed. The effects of relevant design and operating variables on loop performance are studied. The system operating under steady state is modelled taking into account the axial conduction through loop fluid and loop wall, and heat transfer with surroundings. The heat source is considered to be a heater with controlled heat flux and heat sink is modelled as an end heat exchanger with water as the external cold fluid. The governing equations for mass, momentum and energy conservation are normalized and are solved numerically using finite volume method. Results are obtained for a loop pressure of 90 bar with the power input varying from 0.5 kW to 6.0 kW. The numerical results are validated against the experimental results reported in the literature in terms of the modified Grashof number (Grm) and Reynolds number (Re). Based on the results, buoyancy and friction dominated regions are identified for a given loop. Parametric analysis has been done to show the effect of loop diameter, loop height, ambient temperature and insulation. The results show that for the high temperature loop, heat loss to surroundings affects the loop performance significantly. Hence this conjugate heat transfer between the loop and surroundings has to be considered in the analysis of high temperature NCLs.
Abstract: We have studied the migration of a charged permeable aggregate in electrolyte under the influence of an axial electric field and pressure gradient. The migration of the positively charged aggregate leads to a deformation of the anionic cloud around it. The hydrodynamics of the aggregate is governed by the interaction of electroosmotic flow in and around the particle, hydrodynamic friction and electric force experienced by the aggregate. We have computed the non-linear Nernest-Planck equations coupled with the Dracy- Brinkman extended Navier-Stokes equations and Poisson equation for electric field through a finite volume method. The permeability of the aggregate enable the counterion penetration. The penetration of counterions depends on the volume charge density of the aggregate and ionic concentration of electrolytes at a fixed field strength. The retardation effect due to the double layer polarization increases the drag force compared to an uncharged aggregate. Increase in migration sped from the electrophretic velocity of the aggregate produces further asymmetry in charge cloud and reduces the electric body force exerted on the particle. The permeability of the particle have relatively little influence on the electric body force when Double layer is relatively thin. The impact of the key parameters of electrokinetics on the hydrodynamics of the aggregate is analyzed.
Abstract: A numerical study on the influence of electroosmotic flow on analyte preconcentration by isotachophoresis ( ITP) is made. We consider that the double layer induced electroosmotic flow ( EOF) counterbalance the electrophoretic velocity and a stationary ITP stacked zones results. We solve the Navier-Stokes equations coupled with the Nernst-Planck equations to determine the local convective velocity and the preconcentration dynamics of ions. Our numerical algorithm is based on a finite volume method along with a secondorder upwind scheme. The present numerical algorithm can capture the the sharp boundaries of step-changes ( plateau mode) or zones of steep gradients ( peak mode) accurately. The convection of ions due to EOF reduces the resolution of the ITP transition zones and produces a dispersion in analyte zones. The role of the electrokinetic parameters which induces dispersion is analyzed. A one-dimensional model for the area-averaged concentrations based on the Taylor-Aristype effective diffusivity is found to be in good agreement with the computed solutions.