Search

Research was conducted to examine the natural convection resulting from combined thermal and solutal buoyance forces in binary fluids. The porous layer was treated as a pseudo-fluid by the one-domain approach while the composite region was considered as a continuum. Results indicated the influence of thin porous layers on heat and mass transfer. They also showed that columnar growth formulation affects the transfers and dynamics of the processes.

The melting of an isothermal metal from a vertical wall for various Rayleigh (Ra) and Prandtl (Pr) numbers is analyzed. Results show that a scaling law based on the parameter RaPr can determine the transition from the initial conduction regime to the final convection limit. A general formulation for the time evolution of the average heat transfer at the melting front for a range of Ra and Pr numbers is also described.

The numerical analysis of transient heat and species transfer by natural convection in a binary fluid vertical layer is studied. The analysis concerns the average heat and species transfer in the system, before destabilization of the stagnant zone.

A conference paper about the impact and solidification of Yttria Stabilized Zirconia (YSZ) droplets plasma-sprayed onto a substrate is presented. It deals with numerical simulations of the impact of YSZ particles plasma sprayed onto smooth and rough substrates. It found that the distance between columns affects the geometry of the lamella resulting from the droplet impact and its solidification.

A stability analysis of thermal natural convection in superposed fluid and porous layers is carried out. The two-layer system is described using a one-domain formulation, and the eigenvalue problem resulting from the stability analysis is solved using the generalized integral transform technique (GITT). The numerical results confirm that the onset of convection can have a bimodal nature depending on the depth ratio. The influence of the dimensionless permeability and thermal diffusivity ratio are investigated. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we rely on the constructal law of maximization of flow access in order to construct a theory of geometry generation (selection, evolution) during molten droplet impact. We show that immediately after impact the liquid spreads inviscidly as a ring with a radial velocity that scales with the initial impact velocity. If the initial droplet is small and slow enough, the ‘splat’ comes to rest (dies) viscously, as a disc. If the droplet is large and fast enough, the ring splashes and is continued outward by needles that grow radially until they are arrested by viscous effects. We optimized the number of needles such that the total splash time is minimum. The theoretical dimensionless group that governs the selection of geometry (G) is the ratio of two lengths, the final radius of the disc that dies viscously, divided by the radius of the still inviscid ring that just wrinkles. Splats form when G ⩽O(1) and splashes are favored when G ⩾O(1). Experimental measurements reported in the literature confirm several of the features of the constructal development of splat vs. splash flow architecture. [Copyright &y& Elsevier]

Abstract: This paper deals with natural convection driven by combined thermal and solutal buoyancy forces in a binary fluid. The configuration under study is a confined enclosure partially filled with a vertical porous layer. The mathematical description of the problem is based on a one-domain formulation of the conservation equations. The set of numerical results presented here quantitatively shows the influence of the porous layer on the flow structure and on heat and species transfer in the enclosure. The paper is focused on the analysis of the influence of the characteristic parameters governing double diffusive convection, namely the ratios of solutal and thermal parameters: the diffusivities and the buoyancy forces. Heat and mass transfer is analyzed as a function of the permeability of the porous layer. It is shown that the coupling of the flow penetration in the porous layer with the combined buoyancy forces induces a specific behavior of the flow structure and average heat transfer in the enclosure. [Copyright &y& Elsevier]

This paper considers the onset of double diffusive natural convection in a vertical layer of a binary fluid submitted to horizontal thermal and compositional gradients. The analysis deals with the particular situation where the resulting buoyancy forces (the Grashof numbers corresponding to the thermal and solutal effects) are opposing and of equal intensity. The stability analysis for the infinite layer shows that the purely diffusive (motionless) solution prevails at moderate Grashof numbers, and an analytical expression of the critical Rayleigh number as a function of the Lewis number is obtained. These results are then compared to the critical values which are determined from numerical simulations in an enclosure. Numerical calculations in the transient regime are used to give an interpretation of the stability of the steady state diffusive regime when the buoyancy forces are below the critical value. [ABSTRACT FROM AUTHOR]

Presents an experimental and numerical analyses of thermohaline natural convection in an aqueous solution, driven by melting of a block of pure ice into the liquid mixture. Thermohaline convection overview; Governing equations; Flow structure; Fusion kinetics.

Abstract: Stability analyses of thermal and/or solutal natural convection in a configuration composed by a fluid layer overlying a homogeneous porous medium have been performed using different modeling approaches, especially for the treatment of the interfacial region. Comparisons between the one-domain approach and the two-domain formulation have shown important discrepancies of the marginal stability curves. This note shows that, according to Kataoka [I. Kataoka, Local instant formulation of two-phase flow, Int. J. Multiphase Flow 12(5) (1986) 745–758.], the differentiation of the macroscopic properties of the porous layer at the interface (porosity, permeability, thermal effective diffusivity) must be considered in the meaning of distributions. In that case, the one- and the two-domain approaches are shown to be equivalent and very good agreement is indeed found when comparing the results obtained with both approaches. [Copyright &y& Elsevier]

Abstract: This study deals with the onset of thermal natural convection in a system consisting of a fluid layer overlying a homogeneous porous medium. A linear stability analysis is carried out, using the so-called two-domain approach and including the Brinkman term in the porous region (2Ω DB). Results are systematically compared to those obtained using the one-domain approach (1Ω) and the classical Darcy formulation of the two-domain approach (2Ω D). A better agreement is found between the and 2Ω D neutral curves, than with the 1Ω curves, indicating that the inclusion of the Brinkman term plays a secondary role on the stability results. The different treatment of the interfacial region is discussed on the basis of these results. [Copyright &y& Elsevier]

Abstract: This paper deals with the macroscopic modeling and numerical simulation of columnar dendritic solidification of binary alloys. The macroscopic governing equations and associated effective transport properties were previously derived using a volume averaging technique with local closure. The macroscopic model takes into account the spatial variation of the pore-scale geometry within the mushy zone, which leads to additional terms involving porosity gradients. The second important feature concerns solute mass conservation, which is described by considering a macro-scale non-equilibrium accounting for chemical exchanges at the solid–liquid interface. A simplified version of the model is validated through a comparison of the numerical solution to three experiments available in the literature. Porosity extra terms are systematically estimated on the basis of these numerical simulations, and the influence on solidification of effective transport properties such as permeability and interfacial solute exchange coefficients is investigated. [Copyright &y& Elsevier]

This paper presents experimental results concerning the solidification of a binary aqueous solution flowing along a cold surface. Forced convection in the liquid phase is assumed to dominate over possible natural convection effects at the interface and the influence of the initial solute concentration is analyzed. Solute rejection at the interface is shown to influence the equilibrium temperature at the solidification front and the time evolution of the solid phase. Comparison with the numerical solution of a simple thermal phase change model shows that the solidification dynamics is compatible with a porous structure of the solid phase (solid fraction about 0.90), accounting for dendritic structure at the microscale. [Copyright &y& Elsevier]

The momentum balance at the interface between a liquid and a porous substrate is investigated for a configuration with forced flow parallel to the interface. An heterogeneous continuously varying transition layer between the two outer bulk regions is introduced. The stress jump coefficient earlier introduced in the jump interface condition is here derived as an explicit function of the variations of the velocity and effective properties of the transition layer. Agreement is found between our numerical results based on the single-domain approach and the existing ad hoc estimates in the literature. Further advantages of this non-homogeneous analysis are also provided. [Copyright &y& Elsevier]