Bubble formation and motion in non-crimp fabrics with perturbed bundle geometry

The motion of the liquid front during impregnation of non-crimp fabrics has been considered by using Sethian’s level set method. Particular attention is put on the creation of bubbles at the liquid front and a virtual 3D model mimicking biaxial fabrics has been built for this purpose. The saturated fluid flow is governed by the Navier–Stokes Equations and Darcy law, while capillary pressure has been accounted for at the liquid flow front and continuity maintained. The influence of perturbation in the bundle geometry has been investigated. Local correlations of the dimensions of neighbouring gaps formed between the bundles are of paramount importance. Focus is on inter-bundle bubbles. An existing model for bubble dynamics is used based on a probabilistic approach for bubbles moving, splitting, merging, and dissolving. The same approach was used for intra-bundle bubbles, the difference being that their motion appears to be much slower. The obtained void fractions of inter-bundle bubbles at different vacuum levels applied at the liquid flow front are compared to those from real mouldings with a high degree of conformity.     

Mechanism of transition from columnar to equiaxed zone in ingots

Abstract

A new theory for the transition from columnar to equiaxed crystals has been developed. It is proposed that free crystals are formed in the liquid ahead of the growing front and that these crystals grow in the liquid as a result of natural convection. When the free crystals are sufficiently large or sufficiently numerous they physically block the growth of the columnar crystals by adhering to the solidification front. The growth rate of the free crystals is determined by the cooling rate of the mould and the kinetics of the solidification process. The theory indicates that the higher the cooling rate and the slower the solidification process the easier the transition. The calculations are compared with experimental observations on a large steel ingot.

MST/272     

An enthalpy formulation based on an arbitrarily deforming mesh for solution of the Stefan problem

Traditionally schemes for dealing with the Stefan phase change problem are separated into fixed grif or front tracking (deforming grid) schemes. A standard fixed grid scheme is to use an enthalpy formulation and track the movement of the phase front via a liquid fraction variable. In this paper, an enthalpy formulation is applied on a continuously deforming finite element grid. This approach results in a general numerical scheme that incorporates both front tracking and fixed grid schemes. It is shown how on appropriate setting of the grid velocity a fixed or deforming grid solution can be generated from the general scheme. In addition an approximate front tracking scheme is developed which can produce accurate non-oscillatory predictions at a computational cost close to an efficient fixed grid scheme. The versatility of the general scheme and the approximate front tracking scheme are demonstrated on solution of a number of Stefan problems in both one and two dimensions.     

Retardation of a Heat Front in a Porous Medium Containing an Evaporating Liquid

We have investigated heat transfer in a layer of silica gel impregnated with a liquid (water, aqueous solutions of calcium and magnesium chlorides, formic acid, and carbon tetrachloride). The layer was arranged on a substrate impenetrable for vapor and it was heated from above by a concentrated light flux. It has been found that the evaporation of the liquid contained in the pores of silica gel substantially slows down the propagation of the heat front into the layer so that the effective thermal conductivity of the layer can be reduced to 0.01 W/(m·K); this value is approximately 4–20 times smaller than the values typical of the majority of standard heat-insulating materials. The time of the front lag depends on the layer thickness, density of the incident heat flux, amount of liquid in the pores, and evaporation heat of the liquid. The observed trends in the motion of the front have been described by a simple one-dimensional model that takes into account phase transition (liquid evaporation) in the interior of the porous matrix.     

On the nature of the Vorob’evs effect in pulse breakdown of solid dielectrics

The Vorob’evs effect consists in certain features of the discharge observed when a solid dielectric in contact with two rodlike electrodes is placed in a liquid dielectric medium and a voltage pulse with increasing front is applied to the electrodes. When the pulse front slope is small, the discharge develops in the liquid over the solid dielectric surface; whereas the discharge at a sufficiently large slope of the pulse front penetrates into the solid and produces its fracture with cleavage of the surface fragments. In order to explain this phenomenon, it is suggested that, at a sufficiently high voltage buildup rate, a displacement current that is related to the motion of the surface discharge plasma passes through a microprotrusion occurring on the electrode surface at the contact site and causes the electric explosion of this microprotrusion. The metal plasma jet generated as a result of this explosion penetrates into the solid dielectric and forms a discharge channel in depth of this material. The surface discharge plasma formed at a small slope of the voltage pulse front closes the electrode circuit, thus preventing the discharge penetration in depth of the solid.     

Investigation of Waves Interaction in Annular Gas–Liquid Flow Using High-Speed Fluorescent Visualization Technique

High-speed fluorescent visualization complex has been developed for quantitative investigation of the process of interaction of waves on liquid film in annular two-phase flow. Evolution of ripples on disturbance waves surface and disturbance waves coalescence are investigated. It is shown that all the ripples in presence of disturbance waves appear at the base of the back front of disturbance waves and then either decelerate, travel on substrate and are overtaken by the following disturbance wave or accelerate, grow and then disappear at the front of disturbance wave. The disappearance happens due to entrainment of liquid into the core of gas stream. Several scenarios of coalescence of disturbance waves were identified. For disturbance waves with close velocities different types of remote interaction were observed.     

Modeling of the Microstructure Evolution in Continuously Cast Al—Pb Alloys

A numerical model is presented describing the microstructure evolution of an immiscible alloy under the continuous casting conditions.Calculations are carried out to investigate the microstructure evolution in a vertical strip cast sample of Al 5wt pct Pb alloy.The numerical results show that there exists a peak value for the supersaturation in front of the solid/liquid interface ,and the minority phase droplets are nucleated in a region around this peak.Under strip casting conditions the Marangoni migration dominates the motion of droplets.This leads to an accumulation of the minority phase droplets in front of the solid /liquid interface.     

Carbon alloy formation during graphite pulse laser melting in a medium with pressure of ∼10 MPa

The growth of metastable carbon (MC) from the melt formed by melting of high oriented pyrolitic graphite (HOPG) with a laser pulse in helium is discussed. It has been found that in the melt of the HOPG the basal face grows in the form of stepped hillocks nucleating on screw dislocations, which does not necessitate substantial overcooling of the liquid. On the contrary, the carbon alloy from MCs with different phase compositions (diamond included) is formed from the melt of the HOPG prism face. The only explanation of this fact is homogeneous nucleation in a high supercooled melt. The numerical simulation of the heating process showed that homogeneous nucleation is related to the second solidification front directed from the melt-helium interface toward the principle solidification front moving from the bottom of a liquid bath toward its surface. Correlation between the formation of the second front with laser-induced electromagnetic waves on the melt surface (SEWs) manifesting themselves as periodic surface structures in the solidified melt is demonstrated.     

On reducing the size of liquid separators for injector circulation plate freezers

Liquid separators for injector plate freezers are large because the liquid level rises towards the end of the freezing process. To calculate the volume of liquid being collected in the separator during freezing the freezing time and the heat removed must be evaluated. A simple method of freezing time estimation based on the progression of a phase change front is proposed. The size of separators can be reduced considerably by letting part of the liquid feed by-pass the injector during initial freezing. With this arrangement the injector dimensions are based upon a refrigerating capacity lower than the maximum.     

Three-dimensional surface profiling and optical characterization of liquid microlens using a Shack-Hartmann wave front sensor

We demonstrate three-dimensional (3D) surface profiling of the water-oil interface in a tunable liquid microlens using a Shack-Hartmann wave front sensor. The principles and the optical setup for achieving 3D surface measurements are presented and a hydrogel-actuated liquid lens was measured at different focal lengths. The 3D surface profiles are then used to study the optical properties of the liquid lens. Our method of 3D surface profiling could foster the improvement of liquid lens design and fabrication, including surface treatment and aberration reduction.     

Modeling of Liquid Metal Infiltration of Porous Fiber Preform During Squeeze Casting

The present work adopts a new approach to the analytical modeling of infiltration of porous fiber preforms by liquid metal in the squeeze casting of metal matrix composites, with the assumption that the process is adiabatic and that the flow is unidirectional. Fluid dynamics is described on the basis of Darcy's law, while separate equations are derived to explain the thermal behavior of the liquid metal and the fiber, assuming that the thermal interactions between the two are interfacial. Unlike earlier models, this approach does not consider the thermal behavior of a “composite,” but instead studies the behavior of the liquid metal and the fiber preform separately. In addition to the conventional application of heat balance techniques and development of partial differential equations involving temperatures, this work introduces supplementary conditions for temperature calculations, specifically at the entry and front points during infiltration. Differential equations are solved by a method of finite differences, and the problem of additional unknowns (preform temperature) at the infiltration front position is overcome using the “virtual point” concept. Simple expressions are derived for the calculation of process parameters like total time for complete infiltration and time for solidification, on the basis of which the occurrence of complete infiltration is predicted. A novel attempt in generating the profiles of the preform and liquid temperatures at specific instants during infiltration has also been made. The relative influence of the liquid superheat temperature, the preform preheat temperature, and the squeeze pressure on the infiltration mechanism is analyzed by studying the infiltration characteristics for various squeeze conditions.     

A mechanism of quasi-one-dimensional vapor phase growth of Si and GaP whiskers

Using vapor phase growth, we have studied the conditions and mechanisms of Si and GaP whisker growth on (111) and (100) substrates in the presence of Au, Cu, Ni, and Pt as metal solvents. Our experimental data demonstrate that the rate of whisker growth is unaffected by the nature and orientation of the substrate and that the liquid droplet wets the (111) singular face of the growth front. The three-phase line of contact acts as a source of steps on the singular face of the growth front under the droplet. Based on the present results, a physicochemical mechanism of quasi-one-dimensional vapor-droplet-solid whisker growth has been proposed and substantiated.     

Imbibition of Liquid Helium in Aerogels

We report optical measurements of the imbibition of liquid helium in a sample of silica aerogel with 90 % porosity. Both direct imaging and light scattering experiments were performed to determine the dynamics and the properties of the liquid-gas interface in both the normal and superfluid phases of liquid helium. In the normal phase, a classical Lucas Washburn behavior is observed for the rise of the imbibition front while the behavior in the superfluid phase is markedly different, as the fluid invades the sample from all sides with a constant speed. In both phases, the interface is rough, leading to light scattering. In addition, condensation ahead of the imbibition front is observed at low temperature in the superfluid phase.     

Photothermoacoustic conversion in porous matrix-liquid filler systems

Features of the photothermoacoustic conversion in systems of the porous silicon-liquid filler type on a single crystal silicon substrate have been experimentally studied. It is established that the presence of liquid in the pores leads to a significant increase in the photoacoustic signal amplitude and front slope, which is explained by a significant contribution of the liquid pressure to thermoinduced stresses in the material. The extrema observed in time series of the photoacoustic signal in the case of a low-viscosity liquid are related to the process of pressure relaxation in the pores.     

Wicking of liquid nitrogen into superheated porous structures

Evaporation in porous elements of liquid–vapor separation devices can affect the vapor-free cryogenic propellant delivery to spacecraft engines. On that account, the capillary transport of a cryogenic liquid subjected to evaporation needs to be understood and assessed. We investigate wicking of liquid nitrogen at saturation temperature into superheated porous media. A novel test facility was built to perform wicking experiments in a one-species system under non-isothermal conditions. A setup configuration enabled to define the sample superheat by its initial position in a stratified nitrogen vapor environment inside the cryostat. Simultaneous sample weight and temperature measurements indicated the wicking front velocity. The mass of the imbibed liquid nitrogen was determined varying the sample superheat, geometry and porous structure. To the author’s extent of knowledge, these are the first wicking experiments performed with cryogenic fluids subjected to evaporation using the weight–time measurement technique. A one-dimensional macroscopic model describes the process theoretically. Results revealed that the liquid loss due to evaporation at high sample superheats leads to only a slight imbibition rate decrease. However, the imbibition rate can be greatly affected by the vapor flow created due to evaporation that counteracts the wicking front propagation.     

Effect of thermocapillary perturbations on the wave motion in heated falling liquid film

The transformation of hydrodynamic perturbations into thermocapillary-wave structures in a locally heated water film flowing down a vertical plate has been experimentally studied using a high-speed IR imager for monitoring the temperature field and a fluorescent technique for determining the liquid film thickness. It is established that a three-dimensional (3D) front of the hydrodynamic wave acquires inhomogeneous temperature profile, which leads to a deformation of the liquid film under the action of thermocapillary forces and results in the formation of rivulets. Distances between the 3D waves and rivulets are determined as functions of the heatflux density. The experimental data are compared to the theoretical results.     

From liquid to solid bonding in cohesive granular media

We study the transition of a granular packing from liquid to solid bonding in the course of drying. The particles are initially wetted by a liquid brine and the cohesion of the packing is ensured by capillary forces, but the crystallization of the solute transforms the liquid bonds into partially cemented bonds. This transition is evidenced experimentally by measuring the compressive strength of the samples at regular intervals of times. Our experimental data reveal three regimes: (1) Up to a critical degree of saturation, no solid bonds are formed and the cohesion remains practically constant; (2) The onset of cementation occurs at the surface and a front spreads towards the center of the sample with a nonlinear increase of the cohesion; (3) All bonds are partially cemented when the cementation front reaches the center of the sample, but the cohesion increases rapidly due to the strengthening of cemented bonds. We introduce a model based on a parametric cohesion law at the bonds and a bond crystallization parameter. This model predicts correctly the phase transition and the relation between microscopic and macroscopic cohesion.     

Age hardening of EN AW 2014 alloy extruded in the semi-solid state

Response to T6 heat treatment of thixoextruded EN AW 2014 aluminium alloy was investigated in the present work. Extrusion of a 2014 slug heated to a liquid fraction of 15%, takes place in the semi-solid state until the liquid fraction in the final part of the slug is reduced via segregation to a level where semi-solid forming is no longer possible. Hence, the final part of the slug is extruded in the solid-state with a concurrent recrystallization process. This process has produced two distinctly different structures at the front and rear ends and an unexpected hardness profile in T6 temper along the length of the thixoextruded rod. The response to T6 heat treatment of the globular front has been age hardening as usual. The inferior age hardening potential with respect to the hot extruded counterpart is attributed to the grain boundary Al₂Cu phase which has grown too coarse via liquid segregation to be readily solutionized at typical solutionizing temperatures. The rear end of the extrudate on the other hand, has softened upon T6 heat treatment owing to Cu depletion and a fully recrystallized structure.     

Inverse thermal effect of displaced liquid in a porous medium

The inverse thermal effect of the liquid displaced in a porous medium is investigated, with the effect occurring because of the fact that, with the thermal conductivity being neglected, temperature jumps move with the speed of convective heat transfer. In piston displacement, the front moves with a true velocity which several times exceeds those of filtration and of convective heat transfer. Due to the faster advance of the displacement front, a special zone is formed, in which the process of inverse thermal effect of the displaced liquid on the displacing one is observed. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 4, pp. 139–145, July–August, 2006.     

Organic vapor sensing with ionic liquids entrapped in alumina nanopores on quartz crystal resonators

We report on a concept for vapor sensing with the quartz crystal microbalance where the vapor phase is absorbed into small droplets of an ionic liquid. The liquid is contained in the pores of a nanoporous alumina layer, created on the front electrode of the quartz crystal by anodization. Ionic liquids are attractive for vapor sensing because--being liquids--they equilibrate very fast, while at the same time having negligible vapor pressure. Containing the ionic liquids inside cylindrical cavities of a solid matrix removes all problems related to the liquid's softness as well as the possibility of dewetting and flow. The absence of viscoelastic effects is evidenced by the fact that the bandwidth of the resonance remains unchanged during the uptake of solvent vapor. The Henry constants for a number of solvents have been measured.