Direction Of Temperature Gradient Research Articles

Computation of casting solidification feed-paths using gradient vector method with various boundary conditions

In the present work, a computationally efficient numerical approach called gradient vector method (GVM) is proposed to visualise feed-paths and to evaluate the design of feeding systems with various thermal boundary conditions. It involves the computation of liquid–solid interfacial heat flux vector using an analytically derived solution to phase change-heat transport equation. The resultant of flux vectors gives the direction of the highest temperature gradient, which is continuously tracked to generate complete feed-paths. The originating points of the feed-paths indicate hot-spots that manifest as shrinkage defects. Mathematical models are proposed to handle the effect of various boundary conditions like insulation or exothermic sleeves around feeders and chill blocks in mould. The accuracy of GVM in predicting the location of shrinkage porosity defect has been validated by pouring and sectioning benchmark castings. Computation of feed-paths using this method was found to be an order of magnitude faster than level-set method for casting solidification simulation. The implementation of the method in three dimensions and its application to an industrial casting are also presented.

Detailed measurement of the magnitude and orientation of thermal gradients in lined boreholes for characterizing groundwater flow in fractured rock

• TVP measures the detail variations in the thermal gradient within a lined borehole. • Components of the thermal gradient vector characterize fracture flow through rock. • Complex thermal patterns exist in a lined borehole adjacent to fractures. • Data are reproducible and consistent with three nearby multilevel installations. • Variations in “thermal stratigraphy” are similar to hydro-stratigraphic layers. Recent developments have led to revitalization of the use of temperature logging for characterizing flow through fractured rock. The sealing of boreholes using water-filled, flexible impermeable liners prevents vertical cross connection between fractures intersecting the hole and establishes a static water column with a temperature stratification that mimics that in the surrounding formation. Measurement of the temperature profile of the lined-hole, water column (using a high sensitivity single-point probe achieving resolution on the order of 0.001 °C) has identified fractures with active flow under ambient groundwater conditions (without cross connecting flow along the borehole). Detection of flow in fractures was further improved with the use of a heater to create thermal disequilibrium in the active line source (ALS) technique and eliminate normal depth limitations in the process. This paper presents another advancement; detailed measurement of the magnitude and direction of the thermal gradient to characterize flow through fractured rock. The temperature within the water column is measured along the length of the lined hole using a temperature vector probe (TVP): four high sensitivity sensors arranged in a tetrahedral pattern oriented using three directional magnetometers. Based on these data, the horizontal and vertical components of the thermal field, as well as the direction of temperature gradient are determined, typically at depth intervals of less than 0.01 m. This probe was assessed and refined by trials in over 30 lined boreholes; the results from two holes through a fractured dolostone aquifer in Guelph, Ontario are used as exampled. Since no other device exists for measuring flow magnitude and direction under the ambient flow condition created by lined holes, the performance of the TVP is assessed by examining the reproducibility of the temperature measurements through an ALS test, and by the consistency of the results relative to other types of larger-scale information from the study area. Temperature profiles were measured in lined holes under both ambient thermal conditions and subject to ALS heating of the entire length of the holes to demonstrate resolution and reproducibility. The hydraulic gradient in three-dimensional space, based on pressure measurements from three depth discrete, multilevel monitoring systems in nearby holes, was used to independently estimate variations in groundwater flow directions. The characteristics of the hydraulic and thermal regimes are compared to assess response to changes in flow in a fractured rock system. When used in the lined holes, the level of detail provided by this multi-sensor probe is much greater than that provided by a single-sensor probe and this detail strongly supports inferences concerning the relative magnitude and direction of the flow. The results of this study indicate that the details of the thermal gradient can be measured and provides superior results compared to a conventional one dimensional temperature profile, thereby substantially enhancing the characterization of groundwater flow in fractured rock.

Orientational phase-separated domains in a polyolefin blend under a temperature gradient field

We investigate the effect of a temperature gradient on the orientation of phase-separated structures in a polyolefin blend system. Phase contrast optical microscopy (PCOM) has been used to measure the morphology of phase separation via spinodal decomposition as a function of phase separation time and temperature gradient. The bicontinuous and interconnected tubelike structure, the characteristic morphology of the spinodal decomposition process, exhibits a preferential alignment along the direction of temperature gradient after phase separation. The orientation of the bicontinuous and interconnected tubelike structures gradually increases with phase separation time and temperature gradients. Also the orientation of phase-separated domains can respond really quickly to the change in the direction of external temperature gradient field. The results suggest that “thermal force” induced by the temperature inhomogeneity might play an important role in aligning phase-separated domains preferentially along the temperature gradient direction.

Influence of Position on the Microstructure of Carbon Black/Polyvinyl Alcohol Composite Obtained by the Directional Freeze-drying Process

Carbon black (CB)/polyvinyl alcohol (PVA) composites with porous structure were obtained by a directional freeze-drying process of their solution in water. The microstructures of this composite at different positions through the thickness were investigated. The study shows that the composite formed a lamellar structure at the lower surface of the specimen because of the basic crystallographic and crystal growth characteristics of ice. The composite near the top surface of the specimen showed an alignment direction of the ice vertical to the container's wall and the direction of solution lowering into the liquid nitrogen. The alignment direction deflects gradually and finally converges with the aligned direction grown from the bottom position near the middle part of the CB/PVA composite. The ice crystals grow very fast along the direction of temperature gradients, which results in that a small fraction of PVA solute was entrapped within the ice crystals, leading to the formation of trans-bridge lamellar structure near the top surface of the specimen. This result shows that the position in the sample has a great influence on the structure of the porous CB/PVA composite obtained by the directional freeze-drying process.

Macro-micro coupled simulation of competitive dendrite growth in different areas of the welding pool

A macro-micro coupled model is developed to simulate the competitive dendrite growths in different areas of the welding pool in the solidification process. The transient solidification conditions in welding pool are obtained by the three-dimensional (3D) macro-scale FEM model. The thermal conditions used in the micro-scale cellular automata model is obtained from the macro-scale FEM model by using the interpolation algorithm. The simulation results indicate that the micro-scale cellular automata model developed in this paper can simulate the morphologies of dendrites with various growth directions accurately. The solidification conditions in welding pool have obvious effects on the competitive dendrite growth. The dendrites with their preferential orientations parallel to the direction of the highest temperature gradient are more competitive. The morphology of grain structure is determined by the competition among different dendritic arrays. The dendritic arrays with more favorable growth direction can gradually crowd out other dendritic arrays and occupy more space through dendrite branching. The area near the central line of welding pool has a lower temperature gradient, a higher solidification rate, and a higher cooling rate in the solidification process, and such solidification conditions lead to the finer microstructure. The simulation results of the secondary dendrite arm spacing are in agreement with the experimental results under the corresponding solidification conditions.

Mechanism of silk fibroin scaffolds with oriented multichannels and its cytocompatibility

As a biomaterial, besides excellent biocompatibility and biodegradability, suitable macropores and pores structure should be provided to guide cell extension and migration. In present study, the silk fibroin (SF) scaffold with uniaxial channels was prepared by directional temperature field freezing technique. The average pore diameter, pore density and porosity of the scaffold with oriented channels are ∼128.7 µm, ∼158 mm−2 and ∼91.4 %, respectively. By controlling of the temperature gradient direction, the oriented multichannels of the scaffolds were formed in longitudinal easily. In process of the scaffolds fabrication, the directional growth of ice crystal could shear and draft to the silk fibroin molecule segments, which resulted in the new crystal nucleus formation in new zone and increase of β-sheet components in the scaffolds. In vitro, L929 cells were seeded on the scaffolds with oriented channels to evaluate the effect on cell behavior. Cell viability, adhesion and morphology were determined by methyl thiazolyl tetrazolium, confocal microscope and scanning electron microscope. The results showed that the cells anchored on the oriented channels, spread along the direction of the channels and hold a higher viability on the scaffolds with oriented channels. These new oriented multichannel scaffold could guide the adhesion and proliferation of L929 cells, which hold a potential in tissue engineering.

A Contribution of Thermoelectric Properties of the Quaternary Chalcogenide Compound Tl_2GaInSe_4 Crystal

Thermoelectric transport measurements were made on single crystal samples of Tl2GaInSe4. The crystal was prepared by a special design based on the Bridgman technique. Measurements of thermoelectric power were carried out in a special high vacuum-tight calorimeter when the direction of temperature gradient is perpendicular to the cleavage plane. The measurements covered a temperature range extending from 300 to 725 K. The results indicate P -type conductivity for our investigated samples. At room temperature the value of thermoelectric power was 735 μV/deg. The electron to hole mobility ratio was found to be 1.35. The e ective mass of holes at room temperature was evaluated as 4.635×10−29 kg, while for electron was equal to 8.468×10−31 kg. The relaxation time of majority and minority carriers was estimated as τp = 2.968×10−10 s and τn = 7.326×10−12 s, respectively. Also, the di usion coe cient of holes and electrons at room temperature was calculated and found to be 265.132 cm/s and 358.139 cm/s, respectively. The di usion length of holes and electrons are found to be Lp = 2.805× 10−4 cm and Ln = 5.122 × 10−5 cm. In addition to these pronounced parameters, the e ciency of thermoelectric element ( gure of merit) was evaluated which leads to better applications in many elds.

Control of the molecular order and cracks of the 6,13-bis(triisopropylsilylethynyl)-pentacene on a polymeric insulator by anisotropic solvent drying

We describe a control scheme of the molecular order and cracks in a solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) film on a polymeric insulator by the anisotropic temperature-gradient in the process of solvent drying. The solvent for TIPS-pentacene sequentially evaporates from the high temperature region to the low temperature region. As a result, the elongated domains of TIPS-pentacene molecules are developed along the temperature-gradient direction. As increasing of temperature-gradient, TIPS-pentacene molecules tend to be more ordered, which facilitates intramolecular charge transport. Consequently, it is found that the mobility increases and reaches about 0.4cm2/Vs in an organic thin-film transistor fabricated with the TIPS-pentacene film prepared at the condition of room temperature −90°C. Comparing with 0.1cm2/Vs for the case of a conventional solvent drying method, the mobility is enhanced about four-times. On further increasing of temperature-gradient, however, the crack begins to develop at the higher temperature region toward the lower temperature region, and the corresponding mobility decreases drastically. This indicates that there is a trade-off between the enhancement of the molecular order and the generation of the cracks of a solution-processed organic semiconductor prepared by anisotropic solvent drying.

Spin-relaxation modulation and spin-pumping control by transverse spin-wave spin current in Y3Fe5O12

Heat-current-induced manipulation of spin relaxation in Y3Fe5O12 under an in-plane temperature gradient is investigated. We show that the linewidth of the ferromagnetic resonance spectrum, i.e., the spin relaxation, in an Y3Fe5O12 film increases or decreases depending on the temperature-gradient direction and that this modulation is attributed to the spin-transfer torque caused by a thermally induced transverse spin-wave spin current in the Y3Fe5O12 film. The experimental results also show that the spin-current magnitude generated by spin pumping in an attached Pt film is inversely proportional to the square of the modulated Gilbert damping constant, consistent with a phenomenological spin-pumping model.

Stability of narrow-gap Taylor–Dean flow with radial heating: Stationary critical modes

A linear stability analysis for the Taylor–Dean flow, a viscous flow between concentric horizontal cylinders with a constant azimuthal pressure gradient, keeping the cylinders at different temperatures, when the inner cylinder is rotating and outer one is stationary has been implemented. The analysis is made under the assumption that the gap spacing between the cylinders is small compared to the mean radius. A parametric study covering wide ranges of β, a parameter characterizing the ratio of representative pumping and rotation velocities and N, the parameter characterizing the direction of temperature gradient is conducted. The eigenvalue problem is solved by differential transform method using unit disturbance scheme along with shooting technique. Emphasis is given to the occurrence of critical stability for the onset of instability by finding the intersection of the two neutral curves for the inner and outer part in a range of values of the radial temperature gradient parameter N,−1.25<N<0.75. We find that the existence of such critical point of stability is realizable near N=−0.995. It is found that with the introduction of radial heating, the discontinuity in critical Taylor number is not seen for N (=−0.75,−1.25). The most stable state occurs for β=βmax (=−3.62,−3.66,−3.95,−4.2) at which maximum of critical Taylor number occurs corresponding to N (=0.25,0,−0.75,−1.25) and can be concluded that for N<0, the most stable state occurs, when βmax<−3.666 (the value of βmax near which the maximum Taylor number occurs, when N=0).

Fully HOC Scheme for Mixed Convection Flow in a Lid-Driven Cavity Filled with a Nanofluid

AbstractA fully higher-order compact (HOC) finite difference scheme on the 9-point two-dimensional (2D) stencil is formulated for solving the steady-state laminar mixed convection flow in a lid-driven inclined square enclosure filled with water-Al2O3 nanofluid. Two cases are considered depending on the direction of temperature gradient imposed (Case I, top and bottom; Case II, left and right). The developed equations are given in terms of the stream function-vorticity formulation and are non-dimensionalized and then solved numerically by a fourth-order accurate compact finite difference method. Unlike other compact solution procedure in literature for this physical configuration, the present method is fully compact and fully higher-order accurate. The fluid flow, heat transfer and heat transport characteristics were illustrated by streamlines, isotherms and averaged Nusselt number. Comparisons with previously published work are performed and found to be in excellent agreement. A parametric study is conducted and a set of graphical results is presented and discussed to elucidate that significant heat transfer enhancement can be obtained due to the presence of nanoparticles and that this is accentuated by inclination of the enclosure at moderate and large Richardson numbers.

High Altitude Bird Migration at Temperate Latitudes: A Synoptic Perspective on Wind Assistance

At temperate latitudes the synoptic patterns of bird migration are strongly structured by the presence of cyclones and anticyclones, both in the horizontal and altitudinal dimensions. In certain synoptic conditions, birds may efficiently cross regions with opposing surface wind by choosing a higher flight altitude with more favourable wind. We observed migratory passerines at mid-latitudes that selected high altitude wind optima on particular nights, leading to the formation of structured migration layers at varying altitude up to 3 km. Using long-term vertical profiling of bird migration by C-band Doppler radar in the Netherlands, we find that such migration layers occur nearly exclusively during spring migration in the presence of a high-pressure system. A conceptual analytic framework providing insight into the synoptic patterns of wind assistance for migrants that includes the altitudinal dimension has so far been lacking. We present a simple model for a baroclinic atmosphere that relates vertical profiles of wind assistance to the pressure and temperature patterns occurring at temperate latitudes. We show how the magnitude and direction of the large scale horizontal temperature gradient affects the relative gain in wind assistance that migrants obtain through ascending. Temperature gradients typical for northerly high-pressure systems in spring are shown to cause high altitude wind optima in the easterly sectors of anticyclones, thereby explaining the frequent observations of high altitude migration in these synoptic conditions. Given the recurring synoptic arrangements of pressure systems across temperate continents, the opportunities for exploiting high altitude wind will differ between flyways, for example between easterly and westerly oceanic coasts.

Synthesis of Fe based ZrB2 composite coating by gas tungsten arc welding

ZrB2/Fe composite coating was in situ synthesised by gas tungsten arc welding cladding process on AISI 1020 steel. Zr, B4C and Fe–B alloy powders were used as precursor powders. The phase composition and microstructure were investigated by X-ray diffraction analysis, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Microhardness of ZrB2/Fe composite coating at room temperature was examined. Main phases obtained from Zr and B4C precursor are ZrB2 and α-Fe, and those obtained from Zr and Fe–B precursor are ZrB2 and FeB. In the upper part of these composite coatings, ZrB2 phase mainly grows along temperature gradient direction. The middle part of these composite coatings has the highest ZrB2 content and highest microhardness. Gradient dispersions of ZrB2 reinforcements appeared in the composite coating from the middle to the bottom, leading to gradient dispersions of microhardness. With decreasing dilution rate, ZrB2 content and microhardeness increase.

Influence of pulling velocity on microstructure and morphologies of SCN-DC eutectic alloy

Eutectic solidification is very important in the development of new materials in which the periodic multiphase structures may have a remarkable or enhanced functionality. The morphology evolution during eutectic solidification is investigated experimentally using slab-geometry slides of succinonitrile-(D)camphor (SCN-DC) transparent organic eutectic material. By specifically focusing on the effect of pulling velocity on microstructure in directional growth, the temperature gradient and the thickness are kept the same in all the experiments. It is found that eutectic seeds first occur in the grain boundary channel or the specimen side-wall groove. And the growth of eutectic seeds is both parallel to the direction of temperature gradient and along the liquid/solid interface at the same time. At a low pulling velocity (0.064–0.44 μm/s), the macroscopic growth morphology is flat, and the inner microstructure is rod-shaped, which is parallel to the growth direction. It is obvious that the eutectic spacing becomes smaller with the increase of pulling velocity. At a high pulling velocity (0.67–1.56 μm/s), the macroscopic growth morphology becomes cellular. However, the inner microstructure is still rod-shaped, but its distribution is radially outward. And the eutectic spacing decreases as pulling velocity increases.

Quantitative Characterization of 2-D Porous Channels by Using Fractal Random Walk Method

An improved random walk mode is proposed based on fractal geometry and random walk theory to characterize the features of porous channels quantitatively. Three different types of fractal Sierpinski carpet structures are obtained by iterative method. The effective thermal conductivities and fractal dimensions are calculated by finite volume method and statistical fractal algorithm respectively. The influences of porosity and pore distribution on effective thermal conductivity are investigated. Results indicate that, the effective thermal conductivity may be different for various structures with the same porosity, and the direction of porous channels formed by connected pores is the critical factor. The spectral fractal dimension of porous channels dD in the direction of temperature gradient coincides with the effective thermal conductivity of Sierpinski carpet.

Casting Solidification Feed-Path Computation and Industrial Application

Metal casting solidification is accompanied by volumetric shrinkage, compensated by microscopic flow of liquid from adjacent hotter regions along the direction of the highest temperature gradients, referred to as feed-paths. The feed-paths converge to local hot spots, which lead to shrinkage porosity defects. Ideally, hot spots should be inside feeders, implying shrinkage defect-free castings. Visualization of 3D feed-paths facilitates analyzing and optimizing the design of feeders to obtain the desired quality at the highest possible yield. We present our work in this area, spanning the last two decades starting from Modulus Vector Method in 2D (1988) and 3D (1996), Vector Element Method (2006), and Gradient Vector Method (2012). The melt is assumed to be stationary, and convection effects are neglected. It is mainly applicable to thick-walled castings poured under gravity in sand molds. The feed-path computation has been incorporated in an integrated casting design software, and validated on several industrial case studies.

Magnetostriction and structural characterization of Fe–Ga bulk alloy prepared by copper mold casting

Rapidly solidified Fe100–xGax (x=16–20) alloy rods were prepared by induction melting and copper mold casting under the protection of inert gases. The optical microscopy observation shows that the large and elongated columnar grains grow along the radial direction, which is parallel to the temperature gradient direction. The <110> preferred orientation texture along the axial direction of the rod was detected by XRD. With the increase of Ga content, the saturation magnetization (Ms) of the alloys decreases distinctly and the dynamic response in low magnetic field increases drastically, the maximum longitudinal saturation magnetostriction for as-cast Fe82Ga18 alloy rods is 92×10−6 under an applied magnetic field strength of 30 kA/m. The large magnetostriction of Fe100–xGax alloys is attributed to the rapidly solidified disordered A2 phase and the high concentration of short range order of Ga atom clusters, which are arranged in the <100> direction and finally trigger the formation of modified-DO3 structure, just as shown by the split of the (200) diffraction peak. Ordered DO3 phase is not conducive to the magnetostriction.

Generating and reversing spin accumulation by temperature gradient in a quantum dot attached to ferromagnetic leads

We propose to generate and reverse the spin accumulation in a quantum dot (QD) by using the temperature difference between the two ferromagnetic leads connected to the dot. The electrons are driven purely by the temperature gradient in the absence of an electric bias and a magnetic field. In the Coulomb blockade regime, we find two ways to reverse the spin accumulation. One is by adjusting the QD energy level with a fixed temperature gradient, and the other is by reversing the temperature gradient direction for a fixed value of the dot level. The spin accumulation in the QD can be enhanced by the magnitudes of both the leads' spin polarization and the asymmetry of the dot—lead coupling strengths. The present device is quite simple, and the obtained results may have practical usage in spintronics or quantum information processing.

Test Simulations of the Kinetic Model for the Thermal Force based on the Monte Carlo Binary Collision Model

AbstractWe have developed a numerical model of the thermal force based on the Monte Carlo Binary Collision model, where a distorted Maxwellian velocity distribution is used for a background plasma. Our numerical model can be applied to transport simulation of test ions in a high‐collisionality background plasma without magnetic field. In this study, by a series of systematic test simulations, we have confirmed the consistency of our numerical modeling approach with the form of the thermal force analytically derived from the distorted Maxwllian velocity distribution. The test of our model has been done with respect to its dependence on parameters such as: (1) direction of the background temperature gradient, (2) test particle mass, (3) background plasma ion species, and (4) background flow velocity (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Mechanics of thermophoretic and thermally induced edge forces in carbon nanotube nanodevices

A double walled carbon nanotube thermal actuator consisting of a short outer tube sliding along a long inner tube under a temperature gradient is used as a model system to investigate the mechanics of thermophoretic and thermally induced edge forces in nanoscale contact based on the theory of lattice dynamics. It is shown that the total thermophoretic force has two components: a gradient force due to the change in van der Waals energy in the direction of temperature gradient and an unbalanced edge force due to the temperature difference between the two tube ends. Closed-form analytical expressions are derived for the gradient and unbalanced edge forces, with results in excellent agreement with molecular dynamics simulations. This study represents a first analytical study of thermophoretic and thermally induced edge forces between two solid bodies, and may have far reaching implications on thermomechanical nanodevices and nanoscale contact.