Double-diffusive and Soret-induced convection of a micropolar fluid in a vertical channel

This paper reports an investigation of the fully developed natural convection heat and mass transfer of a micropolar fluid in a vertical channel. Asymmetric temperature and concentration boundary conditions are applied to the walls of the channel. The cases of double diffusion and Soret-induced convection are both considered. The governing parameters for the problem are the buoyancy ratio and the various material parameters of the micropolar fluid. The resulting non-dimensional boundary value problem is solved analytically in closed form using MAPLE software. A numerical solution of the time dependent governing equations is demonstrated to be in good agreement with the analytical model. The influence of the governing parameters on the fluid flow as well as heat and solute transfers is demonstrated to be significant.     

A multi-objective optimization problem in mixed and natural convection for a vertical channel asymmetrically heated

Structural and Multidisciplinary Optimization - This paper deals with a multi-objective topology optimization problem in an asymmetrically heated channel, based on both pressure drop minimization...     

Mixed convective heat and mass transfer in a vertical wavy channel with traveling thermal waves and porous medium

We investigate the problem of mixed convection heat and mass transfer through a vertical wavy channel with porous medium. The flow is generated by the periodic thermal waves prescribed at the wavy walls of the channel. The equations of momentum energy and concentration are solved subject to a set of appropriate boundary conditions by assuming that the solution consists of a mean part and a perturbed part. The effects of various pertinent parameters on flow, heat and mass transfer characteristics are discussed numerically and explained graphically.     

具有非均匀内热源的多孔介质中自然对流传热的数值研究

采用数值方法分析了填充多孔介质的竖直同心套管内,非均匀分布的内热源和内外壁面温差对自然对流的影响。考察了高宽比A、内热源分布系数m以及内外Rayleigh数之比Rai/Ra对流场、温度场和内外壁面Nusselt数的影响。结果表明：Rai/Ra较大时流场中部形成逆向环流,并出现θ＞1的高温区;内壁面Nusselt数呈现先增大后减小的趋势,大约在Z=0．8处出现转折。外壁面Nusselt数在Z＞0．8处变化加剧,表明外壁面对流传热主要集中在管上部区域。m增大流体中心逆向环流随之减小并最终消失。     

Particle transport in flow through a ratchet-like channel

We present a fluidic device that shows ratchet-like characteristics for particle transport at low Reynolds. The ratchet consists of a two-dimensional saw-tooth channel, within which a laminar flow is generated by imposing a longitudinal pressure gradient. Particle trajectories are calculated by solving the continuity and Navier–Stokes equations for the fluid flow and the equations for particle transport in both flow directions. The ratchet-like effect is connected with a large asymmetry in the mean transit time, with regard to flow direction, due to particle motion within zones of low flow velocity near the asymmetric wall profile. We show how to obtain ratchet of particles with select Stokes under given flow conditions by adjusting the geometry of the ratchet channel.     

Product diversity in a vertical distribution channel under monopolistic competition

In Russia, chain stores have achieved considerable market power. In this work, we combine a Dixit-Stiglitz industry model with a monopolistic retailer in order to address the following questions: does the retailer always impair prices, variety of goods, and ultimately welfare? Which market structure is worse: Nash or Stackelberg? What should be the public policy in this area?     

Effect of aspect ratio on convection in a porous enclosure with partially active thermal walls

The aim of the present numerical investigation is to understand the effect of aspect ratio and partially thermally active zones on convective flow and heat transfer in a rectangular porous enclosure. Five different heating and cooling zones are considered along the vertical walls while the remaining portions of the sidewalls and top and bottom of the enclosure are adiabatic. The Brinkman-Forchheimer extended Darcy model is used in the study. The governing equations are solved by the finite volume method with the SIMPLE algorithm. The computations are carried out for a wide range of parameters and the results are presented graphically. The results reveal that the location of heating and cooling zones has a significant influence on the flow pattern and the corresponding heat transfer in the enclosure. The rate of heat transfer approaches to a constant value for very low values of the Darcy number. The heat transfer rate is decreased on increasing the aspect ratio.     

A high-efficiency planar micromixer with convection and diffusion mixing over a wide Reynolds number range

Over a wide Reynolds number range (0.1 ≤ Re ≤ 40), the new planar obstacle micromixer has been demonstrated over 85% mixing efficiency covering the mixing improvement in both convection-enhanced (higher Re flow) and diffusion-enhanced (lower Re flow) mechanisms. Mixing behavior between two operation windows was investigated by numerical simulations and experiments. For the adaptive design, numerical simulations and Taguchi method were used to study the effect of four geometrical factors on sensitivity of mixing. The factors are gap ratio (H/W), number of mixing units, baffle width (W _b) and chamber ratio (W _m /W). The degree of sensitivity using the Taguchi method can be ranked as: Gap ratio > Number of mixing units > Baffle width > Chamber ratio. Micromixer performance is greatly influenced by the gap ratio and Reynolds number. Beside the wide Reynolds number range, good mixing efficiency can be obtained at short distance of a mixing channel and relatively low-pressure drop. This micromixer had improved both complex fabrication process of multi-layer or 3D micromixers and low mixing efficiency of planar micromixer at Re < 100. The trend of the verified experimental results is in agreement with the simulate results.     

A lattice Boltzmann modeling and analysis of the thermal convection in a lithium-ion battery

Thermal convection is a critical problem in the design of thermal management system, and is widely encountered in electric and hybrid electric vehicles. In the present work, the lattice Boltzmann method is adopted to investigate the thermal convection in the $\mathrm{LiN}{\mathrm{i}}_{\mathrm{x}}\mathrm{C}{\mathrm{o}}_{\mathrm{y}}\mathrm{M}{\mathrm{n}}_{\mathrm{z}}{\mathrm{O}}_{\mathrm{2}}$ (NCM) lithium-ion battery. The numerical results reveal that the thermal convection model considered in the current study can clearly depict the temperature evolution in the case of the thermal runaway. Additionally, it is found that as the adiabatic boundary condition is adopted, the maximum temperature inside the battery can reach 320°C at 240s, which in turn affects the surrounding batteries. To prevent the thermal runaway propagation in such a case, we also analyzed the forced convective heat transfer in this situation, and the numerical results indicate that thermal runaway can be effectively decreased if the value of the surface heat transfer coefficient for battery cell increases up to 200Wm^?2K^?1. Moreover, it is noted that when the temperature inside the battery reaches 110°C, the subsequent temperature distributions inside the battery have little influence on the surrounding batteries, which suggests that the thermal management of battery pack in both normal charge and discharge process should be considered.     

Natural convection boundary layer of a non-Newtonian fluid about a permeable vertical cone embedded in a porous medium saturated with a nanofluid

An analysis was performed to study the effect of uniform transpiration velocity on free convection boundary-layer flow of a non-Newtonian fluid over a permeable vertical cone embedded in a porous medium saturated with a nanofluid. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. The governing partial differential equations are transformed into a set of non-similar equations and solved numerically by an efficient implicit, iterative, finite-difference method. Comparisons with previously published work are performed and excellent agreement is obtained. A parametric study of the physical parameters is conducted and a representative set of numerical results for the velocity, temperature, and volume fraction profiles as well as the local Nusselt and Sherwood numbers is illustrated graphically to show interesting features of the solutions.     

A direct numerical simulation of transition phenomena in a mixed convection channel flow

This study intends to provide an increased understanding of the laminar-turbulent transition phenomena for the buoyancy-assisted heated vertical channel flow during the early transient stage. The spectral method with weak formulation is applied in the direct numerical simulation. Initial disturbances consist of the finite-amplitude two-dimensional TS wave and a pair of three-dimensional oblique waves for the K-type disturbances. The results from the harmonic energy competitions of different wave modes show that for the buoyancy-assisted heated flow, the (k_x=1, k_z=1) or (1,1) and (1,0) modes would gain energy immediately and start to rise at almost the same rate. This phenomenon is different from that of the buoyancy-opposed flow, where the (1,1) mode decays slowly in the beginning until other modes gain enough energy and then it begins to grow quickly and overtakes the (1,0) mode after a short time period. These different transition patterns match with the experimental results that the flow transition is supercritical and subcritical for the buoyancy-assisted and -opposed flows, respectively. Buoyancy-assisted heated flow transition follows the general trend of an isothermal flow in the beginning, but the thermal-buoyant force is crucial in accelerating the instability and also causing notable differences during the subsequent transition process. All of the results for the vortex structures development, kinetic energy budget of the disturbances, flow visualization by tagged fluid particles, and the local temperature fluctuations are consistent in pointing to a clear pattern for the buoyancy-assisted heated flow transition.     

On the behavior of upwind schemes in the low Mach number limit: II. Godunov type schemes

This paper presents an analysis of Godunov scheme in the low Mach number regime. We study the Riemann problem and show that the interface pressure contains acoustic waves of order where M_* is the reference Mach number even if the initial data are well-prepared and contain only pressure fluctuations of order . We then propose to modify the fluxes computed by Godunov type schemes by solving a preconditioned Riemann problem instead of the original one. We show that this strategy allows to recover a correct scaling of the pressure fluctuations. Numerical experiments confirm these theoretical results.     

Numerical prediction of sound generated from flows with a low Mach number

Numerical computations of sound generated from flows with a low Mach number are presented based on Lighthill’s acoustic analogy with an assumption that sound does not alter the flow field from which it is generated. The source fluctuations of the flow field are computed by a large-eddy simulation (LES) with Dynamic Smagorinsky Model (DSM) and they are fed to the following acoustical computation as input data. An explicit/implicit finite element method with second order accuracy both in time and space is used for flow field discretization. The method is applied to the prediction of sound in three different classes of problems: far-field sound generated from flow around a bluff body, sound resulting from blade-stator interaction of turbomachinery and sound due to a turbulent boundary layer on an aerofoil. The computed frequency spectra of the sound show a fairly good agreement with the measured spectra for all the cases.     

Double-diffusive convection with variable viscosity from a vertical truncated cone in porous media in the presence of magnetic field and radiation effects

This work is focused on the study of combined heat and mass transfer or double-diffusive convection near a vertical truncated cone embedded in a fluid-saturated porous medium in the presence of thermal radiation, magnetic field and variable viscosity effects. The viscosity of the fluid is assumed to be an inverse linear function of the fluid temperature. A boundary-layer analysis is employed to derive the non-dimensional governing equations. The governing equations for this investigation are transformed into a set of non-similar equations and solved numerically using the fourth-order Runge–Kutta integration scheme with the Newton–Raphson shooting technique. Comparisons with previously published work on special cases of the problem are performed and the results are found to be in excellent agreement. A parametric study illustrating the influence of the radiation parameter, magnetic field parameter, viscosity-variation parameter, buoyancy ratio and the Lewis number on the fluid velocity, temperature and solute concentration profiles as well as the Nusselt number and Sherwood number is conducted. The results of this parametric study are shown graphically and the physical aspects of the problem are highlighted and discussed.     

The thermal regime of urban parks in two cities with different summer climates

Differences between the temperature of vegetated urban parks and that of their surrounding built environment are reported. The study is based on observations of surface and air temperature in Vancouver, BC and Sacramento, CA. during summer conditions. A combination of remotely sensed surface temperature and air temperature from fixed station and mobile (car and bicycle) traverses is used to characterize the magnitude of park-induced coolness the 'park cool island' (PCI) effect. Relatively large surface PCI are present by day and at night, although for different reasons. Air temperature effects are smaller. In Vancouver, parks are typically 1-2 C, but in ideal conditions can be almost 5 C cooler than their surroundings. Larger PCI are possible in Sacramento where irrigated greenspace can be 5-7 C cooler. Park type, especially the extent of irrigation and the presence of trees, is important in PCI development. During the day trees may play an important role in establishing a cool park effect, perhaps through a combination of shade and evaporative cooling. At night it appears that the surface geometry and moisture status of the park are important controls on surface cooling. Open parks (with higher sky view factors) that have dry soils (and hence lower thermal admittance) cool the most. Nocturnal cooling in open grass parks is often similar to that at rural sites. The influence of parks on air temperatures appears to be restricted to a distance of about one park width.     

Computing with proteins in a dynamic regime

Dynamic local search [1] has been applied to the evolution of interactions between protein-like structures. These are composed of a randomly selected sequence of amino acids that are linked together to form linear polymers in three dimensions. The objective function chosen for optimisation is the potential energy given by a Toy protein model. Proteins fold, move and interact with other chains to minimise their objective function at a given rate, F_rate, depending on the sum of the rates for re-organisation of their structures. The interaction between different proteins gives a whole range of local attraction/repulsion regimes that result in new structures with new bonds, broken bonds and recursive loops.     

Simulations of subsonic vortex-shedding flow past a 2D vertical plate in the near-continuum regime by the parallelized DSMC code

A general-purpose Parallel Direct Simulation Monte Carlo Code, named PDSC, is used to simulate near-continuum subsonic flow past a 2D vertical plate for studying the vortex-shedding phenomena. An unsteady time-averaging sampling method and a post-processing procedure called DREAM (DSMC Rapid Ensemble Averaging Method) have also been implemented, reducing the overall computational expense and improving the sampling quality of time-dependent flow problems in the rarefied flow regime. Parametric studies, including the temporal variable time step (TVTS) factor, the number of particles per cell, the domain size, and the Reynolds number, have been conducted, obtaining the Strouhal number and various aerodynamic coefficients of the flow. Results are compared to experimental data in the continuum regime available in the literature, demonstrating the capacity of PDSC and DREAM to simulate near-continuum vortex-shedding problems within acceptable computational time.     

Efficient mixing of viscoelastic fluids in a microchannel at low Reynolds number

In this paper, we examined mixing of various two-fluid flows in a silicon/glass microchannel based on the competition of dominant forces in a flow field, namely viscous/elastic, viscous/viscous and viscous/inertial. Experiments were performed over a range of Deborah and Reynolds numbers (0.36 < De < 278, 0.005 < Re < 24.2). Fluorescent dye and microshperes were used to characterize the flow kinematics. Employing abrupt convergent/divergent channel geometry, we achieved efficient mixing of two-dissimilar viscoelastic fluids at very low Reynolds number. Enhanced mixing was achieved through elastically induced flow instability at negligible diffusion and inertial effects (i.e. enormous Peclet and Elasticity numbers). This viscoelastic mixing was achieved over a short effective mixing length and relatively fast flow velocities.