Symmetric Melts Research Articles

Evaluation of melting front kinetics in cylindrical phase change materials module using infrared thermography and digital imaging

An experimental study examined buoyancy-driven convection in the unconstrained melting of PCM within a cylindrical module. Melting progress and PCM fraction were analyzed using IRT and digital imaging. Initial symmetric melting led to accelerated phase change at the bottom and a rippled PCM surface. Thermal stratification in the lower module was evident, with molten liquid displacing colder fluid. Observations underestimated bottom PCM waviness and melting. As the melt zone expanded, buoyancy-driven convection intensified, speeding up melting at the top while the bottom relied more on conduction. The study details transient front positions, temperature profiles, solid PCM amounts, and melting times. Unstable fluid layers near the bottom caused waviness due to chaotic fluctuations. Nucleation rates critically impacted PCM crystallization kinetics and properties.

Effect of Finned-Tube on the Enhancement of Melting Process of PCM in Eccentric Cylinders

Abstract The process of melting of a phase-change material (PCM) in eccentric horizontal cylinders geometry is studied numerically. Numerical simulations are performed for symmetric melting of phase change material between the two cylinders using the finite volume method. The inner cylinder is a finned-tube to enhance the heat transfer between the inner cylinder and the PCM. Inner cylindrical is considered as hot wall while outer is insulated. These simulations show the melting process from the beginning to the end. As result, it is found that the use of fins on the inner tube increases the melting process by decreasing the time of melting by 72.72 %.

A multiphysics model of the electroslag rapid remelting (ESRR) process

This paper presents a numerical model (3D) incorporating multiphysics for an electroslag rapid remelting (ESRR) process of industrial scale. The electromagnetic field is calculated in the whole system including the electrode, molten slag, ingot, graphite ring, and mold; the interaction between the turbulent flow and electromagnetic field is calculated for all fluid domains (molten slag and melt pool); the thermal field is calculated in the molten slag, ingot and mold. The solidification of the billet ingot and the formation of solid slag skin layer along the T-mold are considered as well. The formation of the skin layer adjacent to the T-mold can remarkably impact the electric current path in the whole system. The modeling result indicates that no skin layer would form on the graphite ring, as the local electric current density is very high. In contrast, a thick slag skin layer forms along the inclined part of the T-mold, blocks the electric current path there. Those modeling results are verified by experiments. A typical non-axis symmetry flow/thermal field in the slag region, which has been observed in-situ from the slag surface during operation, is predicted. Detailed analyses of the quasi-steady state results of flow/thermal fields are presented. A symmetric melt pool (profile of the solidifying mushy zone) of the ingot is predicted, which agrees with the experiments.

Phase diagrams of ABC linear triblock copolymers under nanopore confinements

The phase diagrams of triblock copolymers in cylindrical nanopores are investigated using the real-space self-consistent field theory in a two-dimensional space. We concentrate on pores with neutral surfaces and invariable diameters, whose rich variety of phases are considered to originate from pure geometric frustration. A series of triangular phase diagrams are constructed by varying the volume fractions for several sets of interaction parameters. These diagrams are sorted into three categories according to their interaction parameters. The confined phase diagrams exhibit several characteristic features that differ from those observed in the bulk. First, a rich variety of geometric frustration phases with strong symmetries, such as cylindrical and square, are observed in the triangular phase space because of the symmetry constraint in the geometric boundary. Second, the frustrated phases present some novel and complex features compared with those in the bulk, demonstrating that the confined morphologies much more sensitively depend on the subtle variation in the binary interaction parameters than those in the bulk. Meanwhile, the entropic energies of the symmetric melts with equal end block volume fractions are investigated to further understand the geometric frustration phase behaviors in the triangular phase diagrams. The reasonable formation mechanisms of the frustration phases are also discussed.

Numerical study of melting inside concentric and eccentric horizontal annulus

This paper presents numerical investigations on melting of phase change material using N-eicosane inside a cylindrical container. Numerical simulations are performed for symmetric melting of phase change material between two cylinders in concentric and eccentric arrays using the FLUENT software which is sub-cooled initially to 1°C. Inner cylindrical tube is considered hot wall while outer tube is insulated. Predicted result shows that melting rate is the same approximately for concentric and eccentric array before time of 15min. After this time, melting rate decreases in concentric array. It is due to the pure conduction between hot tube and cold solid phase change material.

Experimental and computational study of constrained melting of phase change materials (PCM) inside a spherical capsule

An experimental and computational investigation directed at understanding the role of buoyancy-driven convection during constrained melting of phase change materials (PCM) inside a spherical capsule is reported. The computations are based on an iterative, finite-volume numerical procedure that incorporates a single-domain enthalpy formulation for simulation of the phase change phenomenon. A Darcy’s Law-type porous media treatment links the effect of phase change on convection. Paraffin wax n-octadecane was constrained during melting inside a transparent glass sphere through the use of thermocouples installed inside the sphere. The melting phase front and melting fraction of the PCM are analyzed and compared with numerical solution obtained from the CFD code Fluent. Following a short period of symmetric melting due to prominence of diffusion, expedited phase change in the top region of the sphere and a wavy surface at the bottom of the PCM are observed. The computational predictions point to the strong thermal stratification in the upper half of the sphere that results from rising of the molten liquid along the inner surface of the sphere thus displacing the colder fluid. The waviness and excessive melting of the bottom of the PCM is shown to be underestimated by the experimental observation. This discrepancy is linked to the use of a support structure to hold the sphere. Measured temperature data and computational results near the bottom indicate the establishment of an unstable fluid layer that promotes chaotic fluctuations and is responsible for waviness of the bottom of the PCM. On the other hand, the comparison between the measured and computed temperatures in the top half of the sphere show the stable nature of the molten liquid layer. The computational results start to deviate from the thermocouple readings as one moves lower from the top of the sphere. This delay in predicting the melting instant is linked to the thermal stratification within the “constant temperature bath” that encloses the capsule.

Application of power expansion in the thermodynamics of compressible Markovian copolymers

The problem of constructing phase diagrams for a compressible melt of a binary Markovian copolymer is reduced to a set of nonlinear differential equations in partial derivatives with transcendental relationships. Using power expansions, the closed set of nonlinear differential equations is derived. This set allows its further analytical study. Eigenvalues of a linearized system are analyzed, and the boundaries of the thermodynamic stability of melts are defined. Nonlinear equations in normal coordinates are obtained; for symmetric melts, these equations are reduced to a single equation by adiabatic elimination of small-scale variables. Binodal curves are calculated for such solutions of this equation, which correspond to the free energy minimum of melts. Corrections reflecting the effect of melt nonsymmetry are found. The results are applied for copolymers, whose composition is similar to that of homopolymers, diblock copolymers, and random and regularly alternating copolymers. Spinodals and binodals corresponding to microphase separation are constructed.

Numerical Modeling of Electrothermal Effects in Silicon Nanowires

AbstractAsymmetric melting was observed in electrically pulsed n-type (phosphorus) nanocrystalline silicon (nc-Si) wires fabricated lithographically. Scanning electron microscope (SEM) images taken from the pulsed wires showed that melting initiates from the ground terminal end of the wires instead of the center as initially expected. Asymmetry in the temperature profile is caused by heat exchanged between charge carriers and phonons when an electrical current is passed along a temperature gradient. This effect is known as Thomson effect, a thermoelectric heat transfer mechanism. One dimensional (1D) time dependent heat diffusion equation including Thomson heat term was solved to model the temperature profile on our structures. The modeling results show that Thomson effect introduces significant shifts in the temperature distribution. The effect of Thomson heat is modeled for various electrical pulse conditions and wires dimensions. Our results indicate that Thomson effect is significant in small scale electronic devices operating under high current densities.

Coherence of thermal transitions in poly(N-vinyl pyrrolidone)–poly(ethylene glycol) compatible blends2. The temperature of maximum cold crystallization rate versus glass transition

Differential scanning calorimeteric (DSC) heating thermograms of amorphous poly(N-vinyl pyrollidone) (PVP) blends with short-chain poly(ethylene glycol) (PEG) feature exotherms of cold crystallization coupled with symmetric melting endotherms, which relate to the state of the crystalline component, PEG, while PVP–PEG hydrogen-bonded complex reveals itself as an amorphous phase. As PEG content in blends exceeds a characteristic level, PEG cold crystallization occurs upon heating of the cool-quenched samples through their glass transition temperatures (Tg). The contributions of both thermodynamic and kinetic factors to the occurrence of non-crystallizable PEG have been analysed by considering the dependence of the PEG cold crystallization temperature, Tc, on blend Tg and composition along with the compositional dependence of the heat of melting of PEG. The stoichiometry of the PVP–PEG H-complex was evaluated from DSC thermograms as the amount of non-crystallizable PEG in PVP-underloaded blends.

Single chain conformations in the system of symmetric and asymmetric diblock copolymers

A theory which describes a local structure and global properties of a diblock copolymer melt has been developed in the framework of the one-loop self-consistent approximation. We have derived expressions for the sizes of a single diblock macromolecule and its parts. Two different behaviors of single macromolecule conformations in the disordered melt have been obtained depending on the asymmetry of chains and morphologies occurring in ordered states after the order-disorder transition (ODT). In the nearly symmetric melt, 0.35 < f ≤ 0.5 (f is a composition), the blocks of both types shrink a little initially as the temperature decreases and then, at some temperature, they begin to swell. In strongly asymmetric melts, f < 0.35, the block of a macromolecule which consists of the monomers of minority type shrinks monotonically, while the other block monotonically swells. We have found nearly Gaussian behavior of the individual blocks and stretching near the chemical bond joining the blocks. Near the ODT the chains are stretched with a magnitude which is of the order of a few percent of their Gaussian sizes. We have calculated the peak position in the scattering curve as a function of the Flory-Huggins interaction parameter, composition and degree of polymerization. Less then 5% change in the size of copolymer molecules lead to a 25% shift of the scattering peak in comparison to the Gaussian limit. We have found a good quantitative agreement of our theoretical results with the experimental neutron scattering data.

Diblock Copolymer Thin Films on Heterogeneous Striped Surfaces: Commensurate, Incommensurate and Inverted Lamellae

We study a symmetric melt $\mathrm{AB}$ diblock copolymer thin film which is placed on a periodically striped $\mathrm{CD}$ surface. Because of different surface energies each stripe has a different preference for each of the blocks. There is thus a competition between the preferred bulk lamellar spacing, ${L}_{b}$, and the width of the stripes, $\ensuremath{\lambda}$. This system is similar to the Frenkel-Kontorowa model of solid state physics. We consider the case ${L}_{b}\ensuremath{\ge}\ensuremath{\lambda}$, and assume perpendicular alignment. The periodic striped surface will induce lamellae with unequal spacings. We also show that the striped potential can induce inverted bilayers, i.e., an $AB,AB$ pattern rather than the more usual $AB,BA$.

Comparison of Experimental with Theoretical Melting of the Yeast Genome and Individual Yeast Chromosome Denaturation Mapping Using the Program Meltsim

AbstractThe yeast S. cereviseae represents the first eukaryotic organism whose genome has been entirely sequenced as a result of the Human Genome Project(1). In this report we demonstrate the good agreement between an experimental high resolution melting curve of total nuclear S. cereviseae DNA and the theoretical melting calculated for the complete yeast DNA genome (12,067,277 bp: Saccharomyces Genome Database) by the statistical thermodynamics program MELTSIM, parameterized for long DNA sequences(2,3). The experimental and theoretical melting curves are both fairly symmetrical and possess nearly identical Tmvalues. Calculated melting of coding and flanking DNA regions indicates that flanking DNAs are more (A+T)-rich than coding sequences and account for the earliest melting DNA. Calculated melting curves of the 16 individual yeast chromosomes are very similar and with few exceptions exhibit symmetric melting curves. MELTSIM was also used to calculate a theoretical denaturation map of Chromosome III DNA. The agreement between MELTSIM calculated and experimental melting data demonstrates our ability to accurately simulate long DNA sequence melting in complex eukaryotic genomes, whose sequences are becoming increasingly available for study in public databases. This has important consequences for the understanding of sequence dependent energetic properties of DNA in their biological sequence context and also for their potential use in biomaterials applications.

Phase separation in a grafted polymer layer.

We examine equilibrium phase separation in a grafted polymer layer composed of mutually immiscible chains. Under symmetric melt conditions, we predict that there is a transition to a ``rippled'' phase--a density wave in composition running along the surface. This transition is expected for a molecular weight 2.27 times that for the same species in a simple blend at its demixing threshold. The ripple wavelength is 1.97 times the chain rms end-to-end distance. The lateral rippling is accompanied by a composition oscillation with depth.

Caustic Patterns Associated With Melt Zones in Solidified Glass Samples-Part II: Asymmetric Cases

Ray-tracing algorithms have been developed to follow the propagation of a collimated beam of light traveling along and refracting out of a glass rod in a region of monotonically decreasing cross section. These algorithms have been used to study the formation and distribution of caustics as a function of the changing cross-section area. Axial profile data taken from the melt, or drawdown, zone of a solidified fiber-drawing sample provide the geometrical information needed to predict the loci of two major and two minor families of caustics. General principles for relating the observable far-field caustic patterns to the actual shapes of symmetric melt zones in glass samples are discussed.

Caustic Patterns Associated With Melt Zones in Solidified Glass Samples-Part I: Symmetric Cases

Ray-tracing algorithms have been developed to follow the propagation of a collimated beam of light traveling along and refracting out of a glass rod in a region of monotonically decreasing cross section. These algorithms have been used to study the formation and distribution of caustics as a function of the changing cross-section area. Axial profile data taken from the melt, or drawdown, zone of a solidified fiber-drawing sample provide the geometrical information needed to predict the loci of two major and two minor families of caustics. General principles for relating the observable far-field caustic patterns to the actual shapes of symmetric melt zones in glass samples are discussed.

Thermal denaturation of calf thymus DNA: existence of a GC-richer fraction.

In 2.5 x 10(-4)M EDTA buffer, the derivative melting curve of calf thymus DNA shows a major band at 47 degrees with a shoulder at about 54 degrees . The fraction of melting area of this shoulder is about 13%. For reconstituted polylysine-calf thymus DNA complexes, in addition to the melting of free DNA regions at about 50 degrees (T(m)) there is another melting at about 106 degrees (T(m)) of polylysine-bound regions. The melting band of the complex at T(m) is not symmetrical. As more polylysine is bound to DNA the melting amplitude is diminished greatly on the major band at 47 degrees but only slightly on the shoulder at 54 degrees . The insensitivity of this shoulder appears to result from the existence of a 13% fraction of calf thymus DNA containing 55% GC. It is not favorably bound by polylysine. It remains in the supernatant after centrifugation and melts at about 54-56 degrees . This conclusion is further supported by two facts: the reconstitution method provides a condition for selective binding of polylysine to AT-rich DNA, and it yields a fully symmetric melting band at T(m) for complexes of polylysine with homogeneous bacterial DNA such as the one from M. luteus.

Scanning electron microscope study of laser-damaged beryllium thin films

Laser damage in Be films vacuum vapor deposited onto polished glass substrates was investigated in the scanning electron microscope. Surface scratches were observed to raise thermal gradients and promote premature melting and film degradation in Be films 0.1–0.3 μ thick for beam intensities of 40–55 MW. The damage zones were characterized by radially symmetric melting and evaporation consistent with a cross-sectional distribution of laser-beam energy. The results indicate that vaporization contributes prominently to Be film degradation by laser-beam irradiation.