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Oxide ceramic matrix composites are currently being developed for aerospace applications such as the exhaust, where the parts are subject to moderately high temperatures (≈ 700 ∘C) and oxidation. These composite materials are normally formed by, among other steps, impregnating a ceramic fabric with a slurry of ceramic particles. This impregnation process can be complex, with voids possibly forming in the fabric depending on the process parameters and material properties. Unwanted voids or macroporosity within the fabric can decrease the mechanical properties of the parts. In order to design an efficient manufacturing process able to impregnate the fabric well, numerical simulations may be used to design the process as well as the slurry. In this context, a tool is created for modeling different processes. Thetis, which solves the Navier-Stokes-Darcy-Brinkman equation using finite volumes, is expanded to take into account capillary pressures on the mesoscale. This formulation allows for more representativity than for Darcy’s law (homogeneous preform) simulations while avoiding the prohibitive simulation times of a full discretization for the composing fibers at the representative elementary volume scale. The resulting tool is first used to investigate the effect of varying the slurry parameters on impregnation evolution. Two different processes, open bath impregnation and wet lay-up, are then studied with emphasis on varying their input parameters (e.g. inlet velocity).

This study presents the dynamic behavior of a liquid volume (bridge, film), submitted to different physical phenomena. Effects of surface phenomena such as wetting, surface tension and mechanical phenomena such as strong body force (centrifugation) in various configurations are explored. A numerical modeling approach was undertaken, using Thetis software, developed by I2M (Institute of Mechanics and Engineering of Bordeaux), to study the influence of the mentioned physical phenomena on the displacement of a viscous liquid. At first, in order to illustrate the ability of the numerical model of wettability and to assess its validity, two different test cases are presented. Post-treatment of the results show that static and dynamic contact angles, were satisfactorily simulated. Subsequently, numerical description of the formation of a liquid bridge, due to the growth and coalescence of two microdrops growing at constant flow rate is studied. Then, the evolution of the liquid bridge under the action of a body force is investigated. This case is of practical importance for self-healing matrix composites, whose lifetime depends on the formation and residence time of a viscous plug in micrometric cracks.

In this study, a technique is proposed to improve the filling process of the injection molding and minimize the solidification during the filling to achieve a complete filling of the mold cavity. Two methods are proposed: stopping the flow rate of the cooling fluid or passing cooling fluid inside the cooling channels at higher temperature during a period of the injection molding cycle. The configuration studied consists of the mold with cuboids-shape cavity having two different thicknesses. A validation of the numerical model used by an experimental work is presented. The results show that, stopping of cooling fluid on a period of injection cycle has not great effect on the improvement of injection cycle. The results indicate that passing coolant fluid at higher temperature during the ejection stage decreases the solidification of the polymer during the filling stage by about 40%. © POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers.

Injection molding is one of the most exploited industrial processes in the production of plastic parts. Shrinkage behavior of a molded plastic part plays an important role in determining final dimensions of the part. In this paper, the effect of the cooling system on the shrinkage rate of a polystyrene product during injection molding is carried out. A compressible fluid model for the physical system is presented. A finite volume numerical solution is used for the solution of the physical model. A validation of the numerical model in case of square cavity filled with polymer material is presented. The compressibility behavior of polymer material observed by the pressure, volume, and temperature for the state equation (P–v–T equation) is represented by Tait's equation. A Cross type rheological model depending on the temperature and pressure is assumed for the polymer material. The studied configurations consist of a mold having T-shaped plastic part and four cooling channels. Different cooling channels' positions are assumed and the effect of their positions on the cooling process is studied. The results indicate a good agreement between the numerical solution and those of the literatures. They also, show that the position of the cooling channels has a great effect on final product temperature and the shrinkage rate distribution throughout the product.

Particle transport and heat transfer between particles and ambient fluid are observed in many industrial and natural processes (metal forming, heat storage, etc.). To optimize the manufacturing processes, it is necessary to understand the relationships between global flow, particle transport and heat transfer. The present work deals with the numerical simulation of a cold particle falling (sedimentation) in a hot fluid (of the same material). For a single cold particle in a hot fluid with a low Prandtl number, the results prove that the velocity field is influenced by the thermal behaviour, as natural convection can increase or decrease Archimedes’ forces. The aim is to link the fluid and particles’ properties to the flow and heat transfer to predict occurrence or disappearance of aggregates, numerical methods overcoming limitations previously published.

Impact and solidification of droplets onto a dry solid substrate generates thin or thick substrates depending on size, velocity and physical properties of the fluid particles (during spray or deposition, for example). Numerous and complex phenomena occur in such processes. Difficulties are due to a lack of control of the resulting film, since fluid dynamics (due to droplets impact, for example) heat transfer and solidification interact and influence the final shape and properties of the film. This paper proposes the numerical study of the influence of some material properties upon the manufacturing of a thick coating. For this purpose, a simplified 1D model is used to simulate “deposition” and solidification of successive metal layers upon an initially cold substrate, assuming that there is no fluid flow. Then, the influence of several parameters is studied: layers thickness, initial substrate and layers temperatures, thermal contact resistance between two elements (substrate or layer) and impact frequency.

The present article reports on the fluid flow and heat transfer characteristics associated with cooling an in-line array of discrete protruding heated blocks in a channel by using a single laminar slot air jet. Numerical experiments have been carried out for different values of jet Reynolds number, channel height, slot width, spacing between blocks, block height, and block thermal conductivity. The effects of variation of these parameters are detailed to illustrate important fundamental and practical results that are relevant to the thermal management of electronic packages. In general, the effective cooling of blocks has been observed to increase with the increase of Reynolds number and the decrease of channel height. Heat transfer rates are enhanced for shorter and widely spaced heated blocks. Circulation cells that may appear on the top surface of the downstream blocks have been shown to decrease the value of Nusselt number for these blocks. The values of surface averaged Nusselt number attain their maximum at the block just underneath the impinging air jet, decrease for the downstream blocks, and approximately reach a constant value after the third block. Useful design correlations have been obtained for the mean Nusselt number for the heated blocks underneath and downstream the impinging jet.

The present work deals with heat transfer and fluid flow characteristics during laminar multiple impinging slot jets cooling of a flat plate with symmetrical exhaust ports in the confinement surface. Numerical experiments have been carried out for different values of jet Reynolds number, jet-to-plate separation distance, and exhaust port location. Results have been compared with the base case of single slot jet without cross flow. Reduction in the heat transfer rates induced by spent air removal through the symmetric exhaust ports has been found to be significant for a range of geometrical parameters of practical interest. The introduction of confinement surface protrusions, proposed in the present study, has been shown to be effective in reducing the interaction between jet inlet and exhaust flows and enhancing the heat transfer rates. The corresponding increase in pressure drop is practically not significant. The present results and the effectiveness of the configuration proposed in enhancing heat transfer rates are useful for application to practical devices.

The present article reports on the formulation, numerical implementation, and application of a single-domain coupled micro/macroscopic model for simulation of dendritic alloy solidification. Microscopic solutal non-equilibrium effects have been included in the macroscopic modeling of solidification by using a fixed grid dual scale numerical approach. Salient features of the present approach include a continuum model for conservation of mass, momentum, energy and species on the macroscopic scale, a microscopic solute redistribution model, and the solution procedure and auxiliary equations necessary for coupling between the two models. The coupling between macro and micro scale models is practically made possible by introducing an iterative micro/macro time step scheme. The local solidification rate is calculated by implicit iterations of macroscopic conservation equations and the microscopic solute redistribution model. The present model is capable to simulate eutectic reaction, local re-melting, and account for the inter-linkage between micro and macroscopic solute redistribution (micro and macrosegregation).

PurposeTo study heat transfer kinetics at the fiber scale in order to describe injection of liquid metal through a fibrous perform initially situated in a preheated mould, which is one of the various methods used in order to produce metal matrix composite materials (MMCs).Design/methodology/approachThe first part presents a preliminary study in a static case to describe heat transfer kinetics between a fiber and the matrix in the case of a sudden contact of both components initially set up at different temperatures. This model enables to study the influence of the various parameters of the problem on heat transfer kinetics with phase change. In the second part, we present a modeling which takes into account the metal convection within the pores of the preform.FindingsThe numerical results of these two models justify the instantaneous thermal equilibrium assumption classically admitted to describe MMCs manufacturing methods. The results of this dynamic microscopic model are compared with the results issued from a single temperature macroscopic model to justify the methodological approach and the choice of the microscopic domain geometry representative of the MMCs manufacturing process.Research limitations/implicationsThis first numerical model at the microscopic scale deals with the study of heat transfer between fibers and a pure metal. Next step will be the extension of this study to the preform infiltration by a metal alloy. Injection of matrix alloy implies the appearance of phenomena generated by segregation during phase changes.Originality/valueThe results of simulation tests, making use of the usual conditions of MMCs processing, show pretty good agreement with those of macroscopic models describing the anisothermal flow of a pure metal through a porous medium. From this coherence and from the results of the microscopic models as well, the hypothesis of instantaneous thermal equilibrium between fibers and metal (widely used in the literature to study the production of MMCs by infiltration of the liquid metal through the fibrous reinforcement) is justified. Moreover, it will be possible to extend it to the study of infiltration by an alloy, taking then into account thermal and solutal coupled transfers inside the study domain defined in the present work.

In the present study, two numerical approaches for single-domain modelling of microsegregation during solidification of binary alloys are presented. In the first approach, the concentration jump at the moving solid/liquid interface is formulated using a volumetric term and a Boolean function. The governing solute redistribution equation, valid for the whole domain comprising the solid and liquid regions, is derived in terms of the liquid phase composition. The effects of microstructure coarsening on microsegregation has been described and included in the model. In the second approach, the continuum mixture theory is utilized to derive a single domain solute redistribution equation in terms of the mixture composition. The solidification front motion and dendrite arm coarsening effects are accommodated by considering the representative elementary volume to consist of solid, interdendritic, and extradendritic liquid phases. Numerical solutions have been obtained using a control-volume based finite-difference method with a fixed grid. Good agreement has been observed between the predictions of the present fixed-domain models and the exact analytical and experimental results.

The present article reports on a numerical study of heat transfer and fluid flow characteristics during the impact and solidification of liquid droplets on a cold substrate. The impact dynamics and the associated thermal phenomena have been modelled by adopting a fixed grid (Eulerian) approach with a single set of mass, momentum and energy conservation equations. The VOF advection method with TVD front capturing approach has been utilized to track the time dependent droplets location and the interface between the solid, liquid and gas phases within the domain. Numerical experiments have been carried out for both simultaneous and sequential impact of indium liquid droplets on a cold substrate. Findings of the present study are deemed useful for better understanding and control of related surface coating and thin film manufacturing processes. The fluid behaviour and morphology of the resulting thin film have been found to be dependent upon the droplet impact velocity and frequency, and the fluid particles and substrate characteristics.

Numerical experiments have been carried out to investigate natural convection heat transfer and fluid flow characteristics from a horizontal fluid layer with finned bottom surface. The effects of fin height and fin spacing have been investigated for a sufficiently wide range of Rayleigh number. Quantitative comparisons of heat transfer rates and finned surface effectiveness have been reported. The insertion of heat conducting fins has been found to induce an upward fluid motion along the fin walls. For a given value of fin spacing, the number of convection cells between two adjacent fins is function of the values of fin height and Rayleigh number. In comparison with a bare plate, the heat transfer rates for low values of fin height may be decreased by the insertion of fins. For high values of fin height, the finned surface effectiveness is greater than one for a wide range of fin spacing. For low values of Rayleigh number and high values of fin height, the finned surface effectiveness increases linearly with the decrease of fin spacing. Useful guidelines have been suggested to enhance the heat transfer rates from the finned surface.

A fixed-domain numerical model for microsegregation during alloy solidification is developed. The phenomena of solute partitioning at the moving solid/liquid interface and subsequent redistribution by diffusion in the solid and liquid phases have been formulated using volumetric terms. A solute balance equation valid for the whole domain comprising the solid and liquid phases has been obtained in terms of the liquid concentration. The effects of microstructure coarsening on microsegregation has been described and included in the present model. Numerical experiments and comparisons have been carried out between the present fixed-domain model, previous deforming-domain models, and the exact analytical solutions available in the literature. Good agreement has been observed between the predictions of the present fixed-domain model and the exact analytical solutions. Further extensions of the present model for the analysis of two-dimensional microsegregation have been also reported.

Numerical simulations on confined bubble trains formed by cross-flowing streams are carried out with the numerical code THETIS which is based on the Volume of Fluid (VOF) method and has been developed for two phase flow studies and especially for a gas-liquid system. The surface tension force, which needs particular attention in order to determine the shape of the interface accurately, is computed using the Continuum Surface Force model (CSF). Through the coupling of a VOF-PLIC technique (Piecewise-Linear Interface Calculation) and a smoothing function of adjustable thickness, the Smooth Volume of Fluid technique (SVOF) is intended to capture accurately strong interface distortion, rupture or reconnection with large density and viscosity contrasts between phases. This approach is extended by using the regular VOF-PLIC technique, while applying a smoothing procedure affecting both physical characteristics averaging and surface tension modeling. The front-capturing strategy is extended to gas injection. We begin by introducing the main physical phenomena occurring during bubble formation in microfluidic systems. Then, an experimental study performed in a cylindrical T-junction for different wetting behaviors is presented. For the wetting configuration, Cartesian 2D numerical simulations concerning the gas-liquid bubble production performed in a T-junction with rectangular, planar cross sections are shown and compared with experimental measurements. Finally, the results obtained of bubble break-up mechanism, shape, transport and pressure drop along the channel will be presented, discussed and compared to some experimental and numerical outcomes given in the literature.

International audience; Recent experimental developments showed that the use of the radiation pressure, induced by a continuous laser wave, to control fluid-fluid interface deformations at the microscale, represents a very promising alternative to electric or magnetic actuation. In this article, we solve numerically the dynamics and steady state of the fluid interface under the effects of buoyancy, capillarity, optical radiation pressure and viscous stress. A precise quantitative validation is shown by comparison with experimental data. New results due to the nonlinear dependence of the optical pressure on the angle of incidence are presented, showing different morphologies of the deformed interface going from needle-like to finger-like shapes, depending on the refractive index contrast. In the transient regime, we show that the viscosity ratio influences the time taken for the deformation to reach steady state.

Natural convection in a square cavity in which differentially heated vertical walls are covered with thin porous layers is studied by using a control volume formulation and a SIMPLER algorithm for pressure-velocity coupling. Comparisons with benchmark solutions for natural convection in fluid-filled cavities are first presented for Rayleigh numbers up to 108. The problem of the square cavity with thin porous layers on its vertical walls is then studied by using a modified form of the Navier-Stokes equations by addition of a Darcy term. It is shown that the main effect of the introduction of porous layers is to produce a large decrease of the overall Nusselt number when the permeability is reduced. The higher the Rayleigh number is, the stronger is the decrease, and obviously the decrease also increases with the layer thickness. Moreover, porous layers having a thickness of the order of the boundary layer thickness are sufficient, and taking thicker ones only induces a small decrease of the heat transfer. The main effect of porous layers is to reduce the upwind flow and then to decrease the convective heat transfer.

International audience; We study numerically the deformation of sessile dielectric drops immersed in a second fluid when submitted to the optical radiation pressure of a continuous Gaussian laser wave. Both drop stretching and drop squeezing are investigated at steady state where capillary effects balance the optical radiation pressure. A boundary integral method is implemented to solve the axisymmetric Stokes flow in the two fluids. In the stretching case, we find that the drop shape goes from prolate to near-conical for increasing optical radiation pressure whatever the drop to beam radius ratio and the refractive index contrast between the two fluids. The semi-angle of the cone at equilibrium decreases with the drop to beam radius ratio and is weakly influenced by the index contrast. Above a threshold value of the radiation pressure, these ``optical cones'' become unstable and a disruption is observed. Conversely, when optically squeezed, the drop shifts from an oblate to a concave shape leading to the formation of a stable ``optical torus''. These findings extend the electrohydrodynamics approach of drop deformation to the much less investigated "optical domain" and reveal the openings offered by laser waves to actively manipulate droplets at the micrometer scale.

This paper represents a synthesis of results, both experimental and theoretical, that were obtained from a study of natural convection in fibrous insulat ing materials presenting a permeable interface to the adjacent fluid layer(s)

The purpose of this paper is to simulate numerically the impact of droplets onto a substrate to give a better knowledge of coating manufacturing. A 2D model is proposed, with symmetry conditions to simulate the impact of an infinity of identical droplets. The dynamic behavior is studied, and a thermal study of superimposed splats is added separately to give a better understanding of each phenomenon.Copyright © 2002 by ASME

International audience; During the solidification of metal alloys, chemical heterogeneities at the product scale (macrosegregation) develop. Numerical simulation tools are beginning to appear in the industry, however their predictive capabilities are still limited. We present a numerical benchmark exercise treating the performance of models in the prediction of macrosegregation. In a first stage we defined a "minimal" (i.e. maximally simplified) solidification model, describing the coupling of the solidification of a binary alloy and of the transport phenomena (heat, solute transport and fluid flow) that lead to macrosegregation in a fully columnar ingot with a fixed solid phase. This model is solved by four different numerical codes, employing different numerical methods (FVM and FEM) and various solution schemes. We compare the predictions of the evolution of macrosegregation in a small (10 x 6 cm) ingot of Sn-10wt%Pb alloys. Further, we present the sensitivities concerning the prediction of instabilities leading to banded channel mesosegregations