Modeling the onset of thermosolutal convective instability in a non-Newtonian nanofluid-saturated porous medium layer

The onset of double-diffusive (thermosolutal) convection in horizontal porous layer saturated with an incompressible couple stress nanofluid saturated is studied with thermal conductivity and viscosity dependent on the nanoparticle volume fraction. To represent the momentum equation for porous media, a modified Darcy-Maxwell nanofluid model incorporating the effects of Brownian motion and thermophoresis has been used. The thermal energy equation includes regular diffusion and cross diffusion (Soret thermo-diffusion and Dufour diffuso-thermal) terms. A linear stability analysis depends on the normal mode technique and the onset criterion for stationary and oscillatory convection is derived analytically. The nonlinear theory based on the representation of the Fourier series method is applied to capture the behavior of heat and mass transfer. It is found that the couple stress parameter enhances the stability of the system in both the stationary and oscillatory convection modes. The viscosity ratio and conductivity ratio both enhance heat and mass transfer. Transient Nusselt number is found to be oscillatory when time is small. However, when time becomes very large, all the three transient Nusselt number values approach to their steady state values.     

考虑Soret和Dufour效应的方腔内双扩散自然对流格子Boltzmann模拟

采用格子Boltzmann方法,考虑Soret和Dufour效应,对内置高浓度发热圆的方腔内部双扩散自然对流现象进行数值模拟.高浓度发热圆位于方腔中心,四周壁面均为低温低浓度.在该模型中,用三个独立的LBGK方程分别模拟速度场、温度场和浓度场,并通过Boussinesq近似将它们耦合起来.分析Soret数和Dufour数对方腔内部双扩散自然对流的影响,得到流线图、等温线图、等浓度线图、发热圆表面平均Nusselt数和平均Sherwood数.结果表明：Soret和Dufour效应对方腔内双扩散自然对流影响明显,不能忽略.     

The onset of Darcy-Brinkman thermosolutal convection in a horizontal porous media

On the basis of Brinkman model, the onset of double-diffusive (thermosolutal) convection with a reaction term in a horizontal sparsely packed porous media is studied using the normal mode analysis. The effects of Brinkman term, the reaction term, the normalized porosity ε and Lewis number Le on the Rayleigh number for stationary and oscillatory case are presented graphically. The Darcy number destabilize the system in case of both stationary and oscillatory mode. The effects of Lewis number and reaction term depend on the value of the values of solutal Rayleigh number. Furthermore, some results of Darcy model can be recovered in limit cases.     

多孔介质传热传质中耦合扩散效应的研究

本文采用加罚函数有限元方法数值模拟在温度梯度和浓度梯度同时存在时双浮升力自然对流及其伴随的传热传质,着重探讨了多孔介质中传热传质的交叉耦合扩散Soret效应和Dufour效应的基本影响规律,展示了流场、温度场和浓度场随热附加扩散准则数和扩散附加热准则数的变化状况。     

Mixed Convective Viscoelastic Nanofluid Flow Past a Porous Media with Soret-Dufour Effects

The present study is carried out to see the thermal-diffusion (Dufour) and diffusion-thermo (Soret) effects on the mixed convection boundary layer flow of viscoelastic nanofluid flow over a vertical stretching surface in a porous medium. Optimal homotopy analysis method (OHAM) is best candidate to handle highly nonlinear system of differ-ential equations obtained from boundary layer partial differential equations via appropriate transformations. Graphical illustrations depicting different physical arising parameters against velocity, temperature and concentration distributions with required discussion have also been added. Numerically calculated values of skin friction coefficient, local Nusselt and Sherwood numbers are given in the form of table and well argued. It is found that nanofluid velocity increases with increase in mixed convective and viscoelastic parameters but it decreases with the increasing values of porosity parameter. Also, it is observed that Dufour number has opposite behavior on temperature and concentration profiles.     

Influence of magnetic field by free convection flow past an impulsively started vertical cylinder in the presence of Dufour and Soret effects

Numerical modeling of two-dimensional, unsteady, laminar, isothermal heat and mass transfer by MHD free convection flow past an impulsively started vertical cylinder in the presence of Dufour and Soret effects is considered. The governing equations of the flow problems, valid in the free convection regime, are transformed into a nondimensional form using appropriate nondimensional parameters. These equations are solved numerically by an efficient, implicit, iterative, finite-difference scheme of Crank-Nicolson type. A parametric study illustrating the influence of the magnetic field, Grashof number, and the Dufour and Soret numbers on the fluid velocity, temperature, and concentration, as well as the local and average skin friction, Nusselt and Sherwood numbers is conducted. The obtained results are shown graphically and the physical aspects of the problem are discussed.     

On couple-stress fluid heated from below in porous medium in hydromagnetics

A layer of electrically conducting couple-stress fluid heated from below in porous medium in presence of magnetic field is considered. For stationary convection, the couplestress and magnetic field postpone the onset of convection whereas the medium permeability hastens the onset of convection. The magnetic field introduces oscillatory modes in the system which were non-existent in its absence. A sufficient condition for the non-existence of overstability is obtained.     

On micropolar fluids heated from below in hydromagnetics in porous medium

The thermal instability of electrically conducting micropolar fluids heated from below in the presence of uniform vertical magnetic field in porous medium has been considered. It is found that the presence of coupling between thermal and micropolar effects, magnetic field and permeability may introduce oscillatory motions in the system. The increase in Rayleigh number for stationary convection and decrease in Rayleigh number for overstability with the increase in magnetic field is depicted graphically. Also the Rayleigh number is found to decrease with the increase in permeability.     

Analytical and numerical exploration of oscillatory convection in porous media

This study is part of the research about the influence of the thermal gradient on the composition and the stability of fluids in geological environments. This paper presents modelling results of oscillatory convection in a porous medium using the METSOR code. The model solves the heat and mass transport equations in a porous medium and takes into account the Soret effect (mass transport under thermal gradient). Oscillatory convection may occur in pure fluids and in binary mixtures (as a consequence of the Soret effect). Experimental data confirm the existence of this phenomenon in porous media. Here, the outputs of the METSOR code are verified against existing analytical solutions. A first modelling attempt of oscillatory convection in mixtures is presented (salt aquifer).     

Linear stability analysis on the onset of Soret-driven motion in a nanoparticles suspension

The onset of Soret-driven instability in binary mixture heated from above is analysed using the linear stability theory. The horizontal fluid layer placed between two plates is in a thermally stable state but the Soret diffusion can induce buoyancy-driven convection in the case of a negative Soret coefficient. It is well known that convective motion sets in from both boundaries if the Rayleigh number exceeds its critical value. For the case of highly unstable density stratification the buoyancy-driven motion sets in during the transient diffusion stage. The new stability equations are derived and are solved analytically and numerically. Here the stability limits which are related to the onset time of instabilities and wave number are presented as a function of the Rayleigh number, Lewis number and the separation ratio. The present stability criteria are compared with the existing experimental data.     

浮力对环形液池内热毛细对流的影响

为了了解浮力的影响,对水平温度梯度作用时环形液池内的热毛细对流进行了非稳态三维数值模拟,环形液池外壁被加热,半径为40 mm,内壁被冷却,半径为20 mm,液池深度为(1-17)mm,流体为0．65cSt的硅油,其Pτ数为6．7．模拟结果表明,当水平温度梯度较小时,流动为轴对称稳态流动,随着温度梯度的增加,流动将会失去其稳定性,在浅液池内,转化为热流体波,浮力对失稳后的流型无影响,但会使热流体波的振幅下降;在深液池内,在常重力条件下,转化成三维稳定流动,在微重力和小重力条件下,转化为三维振荡流动．     

Thermosolutal convection in a Maxwellian viscoelastic fluid in porous medium

The thermosolutal convection in a layer of Maxwellian viscoelastic fluid heated and soluted from below in porous medium is considered. The effects of uniform magnetic field and uniform rotation on the thermosolutal convection are also considered. For stationary convection, the Maxwellian viscoelastic fluid behaves like a Newtonian fluid. The sufficient conditions for the nonexistence of overstability are obtained. The critical Rayleigh number is found to increase with the increase in magnetic field, rotation and stable solute gradient.     

Stability of thermal convection of an Oldroyd-B fluid in a porous medium with Newtonian heating

A linear stability analysis determining the onset of oscillatory convection of an Oldroyd-B fluid in a bounded two-dimensional rectangular porous medium generated by Newtonian heating is conducted. Influences of viscoelastic parameters and Biot number on the onset of oscillatory convection, preferred modes and patterns of disturbed temperature contours are discussed.     

Mixed Convection Heat and Mass Transfer in a Micropolar Fluid with Soret and Dufour Effects

A mathematical model for the steady, mixed convection heat and mass transfer along a semi-infinite vertical plate embedded in a micropolar fluid in the presence of Soret and Dufour effects is presented. The non-linear governing equations and their associated boundary conditions are initially cast into dimensionless forms using local similarity transformations. The resulting system of equations is then solved numerically using the Keller-box method. The numerical results are compared and found to be in good agreement with previously published results as special cases of the present investigation. The non-dimensional velocity, microrotation, temperature and concentration profiles are displayed graphically for different values of coupling number, Soret and Dufour numbers. In addition, the skin-friction coefficient, the Nusselt number and Sherwood number are shown in a tabular form.     

分离比对混合流体Rayleigh-Bénard对流解的影响

混合流体Rayleigh-Bénard对流是研究非平衡对流的非线性动力学特性的典型模型之一.基于流体力学方程组的数值模拟,首先探讨了矩形腔体中具有强Soret效应(分离比Ψ=-0.60)的混合流体行波对流的分叉特性及斑图演化,沿着分叉曲线的上部分支,随着相对瑞利数的增加,此系统依次出现了局部行波对流、具有缺陷的行波对流、行波对流、摆动行波对流及定常对流5种行波对流解.然后,研究了分离比Ψ对对流解的影响,与弱Soret效应(Ψ=-0.11)时的对流解相比较,强Soret效应(Ψ=-0.60)时出现的对流解更丰富.由于有强Soret效应的对流的复杂性,Ψ=-0.60时的对流解与Ψ=-0.20,-0.4时的对流解不同.     

Slip boundary conditions and through flow effects on double-diffusive convection in internally heated heterogeneous Brinkman porous media

A model for double-diffusive convection in a heterogeneous porous layer with a constant throughflow is explored, with penetrative convection being simulated via an internal heat source using the Brinkman model. In particular, we analyse the effect of slip boundary conditions on the stability of the model. Because of the many applications in micro-electro-mechanical systems (MEMS) and other microfluidic devices, a study of this problem is necessary. Both linear instability analysis and nonlinear stability analysis are employed. We accurately analyse when stability and instability will commence and determine the critical Rayleigh number as a function of the slip coefficient.     

Numerical simulations of binary fluid convection in large aspect ratio annular containers

The patterns arising in large aspect ratio annular containers heated from below are analyzed for water-ethanol mixtures with negative Soret coupling. The subcritical Hopf bifurcation at the onset of convection leads to a very rich dynamics. Using high resolution numerical tools based on spectral methods to solve the hydrodynamic equations we obtain and review the properties of the different regimes arising in early stages of convection: spatially extended stationary and travelling wave convective roll structures, dispersive chaotic states, and several types of localized convection. The dynamics triggered by the Eckhaus–Benjamin–Feir instability is presented for a mixture with a moderate negative value of the separation ratio.     

方腔内双扩散混合对流非线性格子Boltzmann研究

采用格子Boltzmann方法对方腔内双扩散混合对流的非线性特性进行数值模拟.发热圆位于方腔中心,流体从方腔左侧底部流进,流出位置分别为顶部右侧、中间和左侧三种情况.结果表明,不同参数下的双扩散混合对流存在稳态定常解、周期性振荡解和非周期性振荡解,监测点速度相图分别表现为最终到达一个点、一个封闭环和没有规律的曲线.     

Soret效应对具有自由表面的圆柱形浅池内双组分溶液热对流影响的实验研究

为了探究Soret效应对具有自由表面的圆柱形浅液池内双组分溶液热对流过程的影响, 通过实验观察了质量分数为50%的正癸烷/正己烷混合溶液在不同深宽比的液池内流动失稳后的自由表面耗散结构及液池内的温度波动. 结果表明, 双组分溶液流动失稳的临界热毛细Reynolds数小于纯工质的值, 且其随液层深宽比的变化规律与纯工质相同. 当深宽比小于0.0848时, 流动失稳后在自由表面观察到热流体波, 监测点处温度波动主频随热毛细Reynolds数增大而增加; 当深宽比大于0.0848时, 随热毛细Reynolds数的增大, 流动失稳后自由表面依次呈现轮辐状、花苞状、分离-合并-分离交替变化的条纹状结构.     

Significance of Stefan blowing effect on flow and heat transfer of Casson nanofluid over a moving thin needle

The current mathematical model explains the influence of non-linear thermal radiation on the Casson liquid flow over a moving thin needle by considering Buongiorno's nanofluid model.The influences of Stefan blowing, Dufour and Soret effects are also considered in the model. The equations which represent the described flow pattern are reduced to ordinary differential equations(ODEs) by using apt similarity transformations and then they are numerically solved with Runge–Kutta-Fehlberg's fourth fifth-order method(RKF-45) with shooting process. The impacts of pertinent parameters on thermal, mass and velocity curves are deliberated graphically.Skin friction, rate of heat and mass transfer are also discussed graphically. Results reveal that, the increase in values of Brownian motion, thermophoresis, Dufour number, heating and radiative parameters improves the heat transfer. The increasing values of the Schmidt number deteriorates the mass transfer but a converse trend is seen for increasing values of the Soret number. Finally,the escalating values of the radiative parameter decays the rate of heat transfer.