Uniform Temperature Gradient Research Articles

Detection and Location of the Degraded Bearings Based on Monitoring the Longitudinal Expansion Performance of the Main Girder of the Dashengguan Yangtze Bridge

AbstractTemperature field data from the steel truss arch girder and longitudinal displacement data from six groups of rubber bearings were collected by the structural health monitoring system for the Dashengguan Yangtze Bridge. By using long-term monitoring data, two correlations are investigated: the linear correlation between the longitudinal displacement and the temperature field (including uniform temperature and temperature gradient); and the linear correlation of the longitudinal displacements in different locations. A multivariate linear regression equation is used to model the first correlation, and a Lagrange polynomial interpolation is used to model the second correlation. The final mathematical models, representing the healthy state of the bearings, can be applied to simulate the longitudinal displacements of the main girder. Furthermore, the change regularity of longitudinal displacements for the degraded rubber bearings is revealed taking advantage of a hysteretic model and presumed envelope ...

Onset of convection in a vertical porous cylinder with a permeable and conducting side boundary

The onset of natural convection in a vertical porous cylinder saturated by a fluid is studied. The lateral confinement of the porous cylinder is due to an external porous medium having a permeability much smaller than that of the cylinder. Thus, the vertical side boundary of the cylinder is permeable and constrained by given pressure and temperature distributions. The lower and upper plane boundaries of the cylinder are impermeable walls. The lower wall is subject to a uniform heat flux, while the upper wall has a uniform temperature. The basic motionless state displays a uniform and vertical temperature gradient oriented downward. The linear stability analysis is carried out by using an analytical dispersion relation. The allowed modes of perturbation are determined as solutions of the Helmholtz–Dirichlet problem. First, the natural convection problem is formulated for a circular cylinder. Then, the investigation is generalised to an arbitrary cross-sectional shape of the cylinder. The sample case of an elliptical cylinder is studied in detail, by adopting an analytical solution based on Mathieu functions.

Effect of Porous Graphite for High Quality SiC Crystal Growth by PVT Method

The effect of the porous graphite plate above the source material on properties of silicon carbide (SiC) crystals grown by Physical Vapor Transport method has been investigated. The porous graphite plate was inserted on source powder to produce a more C-rich for the polytype stability of 4H-SiC crystal and a uniform radial temperature gradient. The dendrite structure obtained from SiC source powder in the crucible with porous graphite plate was more densely formed than that in the conventional crucible. The crystal quality of 4H-SiC single crystals grown in porous graphite inserted crucible was revealed to be better than that of crystal grown SiC crystals in the conventional crucible.

Spatial variability in the coefficient of thermal expansion induces pre-service stresses in computer models of virgin Gilsocarbon bricks

In this paper, the authors test the hypothesis that tiny spatial variations in material properties may lead to significant pre-service stresses in virgin graphite bricks. To do this, they have customised ParaFEM, an open source parallel finite element package, adding support for stochastic thermo-mechanical analysis using the Monte Carlo Simulation method. For an Advanced Gas-cooled Reactor brick, three heating cases have been examined: a uniform temperature change; a uniform temperature gradient applied through the thickness of the brick and a simulated temperature profile from an operating reactor. Results are compared for mean and stochastic properties. These show that, for the proof-of-concept analyses carried out, the pre-service von Mises stress is around twenty times higher when spatial variability of material properties is introduced. The paper demonstrates that thermal gradients coupled with material incompatibilities may be important in the generation of stress in nuclear graphite reactor bricks. Tiny spatial variations in coefficient of thermal expansion (CTE) and Young's modulus can lead to the presence of thermal stresses in bricks that are free to expand.

Magnetoelectrets prepared by using temperature gradient method

A novel Temperature Gradient method for preparation of magnetoelectret is proposed. Non uniform magnetic field and temperature gradient are expected to be the main cause for the formation of magnetoelectrets (MEs). Being bad conductors of heat, during their formation, there is a possibility for the existence of a temperature gradient along the dielectric electrode interface. In this condition, the motion of, molecules and charge carriers are dependent on Temperature Gradient in a preferred direction. To increase this temperature gradient on both sides of the sample novel method for the preparation of MEs is developed for the first time. For this method the special sample holders are designed in our laboratory. MEs are prepared in such a way that one surface is cooled and the other is heated, during the process. With the help of XRD analysis using Type-E orientation pattern and surface charge studies on magnetoelectrets, the two main causes Non uniform magnetic field and temperature gradient for the formation of magnetoelectrets (MEs), are authenticated experimentally.

고품질 4H-SiC 단결정 성장을 위한 다공성 흑연 판의 역할

The present research is focused on the effect of porous graphite what is influenced on the 4H-SiC crystal growth by PVT method. We expect that it produces more C-rich and a change of temperature gradient for polytype stability of 4H-SiC crystal as adding the porous graphite in the growth cell. The SiC seeds and high purity SiC source materials were placed on opposite side in a sealed graphite crucible which was surrounded by graphite insulator. The growth temperature was around and the growth pressure was 10~30 Torr of an argon pressure with 5~15 % nitrogen. 2 inch off-axis 4H-SiC with C-face (000-1) was used as a seed material. The porous graphite plate was inserted on SiC powder source to produce a more C-rich for polytype stability of 4H-SiC crystal and uniform radial temperature gradient. While in case of the conventional crucible, various polytypes such as 6H-, 15R-SiC were observed on SiC wafers, only 4H-SiC polytype was observed on SiC wafers prepared in porous graphite inserted crucible. The defect level such as MP and EP density of SiC crystal grown in the conventional crucible was observed to be higher than that of porous graphite inserted crucible. The better crystal quality of SiC grown using porous graphite plate was also confirmed by rocking curve measurement and Raman spectra analysis.

Thermo-Electro-Mechanical Buckling of Shear Deformable Hybrid Circular Functionally Graded Material Plates

Buckling analysis of perfect circular functionally graded plates with surface-bounded piezoelectric layers based on the first-order shear deformation theory is presented in this article. The material properties of the functionally graded (FG) layer are assumed to vary continuously through the plate thickness by distribution of power law of the volume fraction of the constituents. The plate is assumed to be under constant electrical field and two types of thermal loadings, namely, the uniform temperature rise and nonlinear temperature gradient through the thickness. Also, the stability of a plate under radial mechanical compressive force is examined. The equilibrium and stability equations are derived based on the first-order shear deformation plate theory using a variational approach. The boundary condition of the plate as an immovable type of the clamped edge is considered. Resulting equations are employed to obtain the closed-form solution for the critical buckling temperature for each loading case. The effects of electric field, piezo-to-host thickness ratio, and power law index of functionally graded plates subjected to thermo-mechanical-electrical loads are investigated. The results are compared with the classical plate theory and verified with the available data in the open literature.

Numerical investigation of the thermocapillary actuation behavior of a droplet in a microchannel

The thermocapillary actuation behavior of a silicone droplet in a microchannel is numerically investigated in the present paper. The finite element method with the two-phase level set technique, which is ideally suitable for tracking the interfaces between two immiscible fluids, is employed to solve the Navier–Stokes equations coupled with the energy equation. The lower wall of the microchannel is subjected to a uniform temperature gradient, while the upper one is either adiabatic or isothermal. The thermocapillary flow inside the droplet is significantly affected by the thermal condition of the upper wall. When the upper wall is set to be adiabatic, a pair of asymmetric thermocapillary convection vortices initially occurs inside the droplet but these turn into a sole thermocapillary vortex once enough time has passed. For the isothermal case, a pair of asymmetric thermocapillary convection vortices always appears inside the droplet. The droplet initially accelerates for both the adiabatic and isothermal cases. The droplet velocity then decreases dramatically for the adiabatic case while it decreases slowly for the isothermal one. The dynamic contact angle of the droplet in a microchannel is strongly affected by the passage of the air flow over the droplet which is induced by the thermocapillary convection and the presence of the upper wall. The actuation velocity is enhanced by a higher temperature gradient, a reduction of microchannel height and a smaller contact angle for both adiabatic and isothermal cases.

Error Suppression via Tension for Flexible Square Antenna Panels and Panel Arrays

The concept of passive surface error control via tension is explored for flexible square panels and for a rectangular array of such panels suspended with four catenaries. The panels are 1 mm thick 1 m square graphite-epoxy composite plates with a four-ply symmetric layup. Individual panel response to a uniform through-thickness temperature gradient field both alone and combined with in-plane tension (stretch) are examined. When integrated into a grid (a simple phased array configuration), the effects of a transient slew are considered. Characteristic responses are identified and studied symbolically as well as numerically. The results demonstrate the feasibility of combined global and component-level error suppression for the considered structure, exercised singularly by global prestress that also maintains the integrity of the tension structure. Limited attention is paid to mission-specific issues such as stowage and deployment. Assuming an X-band radar context, surface errors are related to a 1 mm limit. Out-of-flatness is evaluated with three metrics relevant to different aspects of signal processing: the maximum depth of the deformed surface, its maximum lateral deviation from the best-fit plane, and the phased-array radiometric rms surface error.

Onset of Electrothermoconvection in a Dielectric Fluid Saturated Porous Medium in a Modulated Electric Field

The onset of electroconvection in a horizontal dielectric fluid saturated with a densely packed porous layer subjected to uniform temperature gradient and an alternating electric field is investigated. The dielectric constant is assumed to be a linear function of temperature. The boundaries are considered to be free-free and isothermal. Modified Darcy equation is used to describe the flow in porous medium and solution to the resulting eigenvalue problem is obtained using a regular perturbation method with small amplitude approximation. The shift in the electric Rayleigh number is calculated as a function of Darcy number, Rayleigh number, frequency of electric modulation and Prandtl number and their effects on the stability of the system are discussed.

Freezing colloidal suspensions: periodic ice lenses and compaction

AbstractRecent directional solidification experiments with aqueous suspensions of alumina particles (Anderson & Worster, Langmuir, vol. 28 (48), 2012, pp. 16512–16523) motivate a model for freezing colloidal suspensions that builds upon a theoretical framework developed by Rempel et al. (J. Fluid Mech., vol. 498, 2004, pp. 227–244) in the context of freezing soils. Ice segregates from the suspension at slow freezing rates into discrete horizontal layers of particle-free ice, known as ice lenses. A portion of the particles is trapped between ice lenses, while the remainder are pushed ahead, forming a layer of fully compacted particles separated from the bulk suspension by a sharp compaction front. By dynamically modelling the compaction front, the growth kinetics of the ice lenses are fully coupled to the viscous flow through the evolving compacted layer. We examine the periodic states that develop at fixed freezing rates in a constant, uniform temperature gradient, and compare the results against experimental observations. Congruent with the experiments, three periodic regimes are identified. At low freezing rates, a regular periodic sequence of ice lenses is obtained; predictions for the compacted layer thickness and ice-lens characteristics as a function of freezing rate are consistent with experiments. At intermediate freezing rates, multiple modes of periodic ice lenses occur with a significantly diminished compacted layer. When the cohesion between the compacted particles is sufficiently strong, a sequence of mode-doubling bifurcations lead to chaos, which may explain the disordered ice lenses observed experimentally. Finally, beyond a critical freezing rate, the regime for sustained ice-lens growth breaks down. This breakdown is consistent with the emergence of a distinct regime of ice segregation found experimentally, which exhibits a periodic, banded structure that is qualitatively distinct from ice lenses.

Thermophoretic Motion of a Cylindrical Particle with Chemical Reactions

The thermophoresis of a circular cylindrical particle bearing a chemical reaction in a gas prescribed with a uniform temperature gradient in the direction perpendicular to its axis is analyzed. The Knudsen number is assumed to be moderately small so that the fluid motion is in the slip-flow regime with effects of temperature jump, thermal creep, frictional slip, and thermal stress slip at the particle-gas interface. The appropriate governing equations of heat conduction/generation and fluid motion are solved analytically and the thermophoretic velocity of the particle is obtained in closed forms. The thermophoretic velocity is a linear function of the thermal stress slip coefficient whose effect increases with an increase in the Knudsen number. When the composition-dependent factor of the chemical reaction within the particle does not depend on position, the thermophoretic velocity is diminished as the reaction is endothermic and augmented as the reaction is exothermic. When this factor is a function of p...

Modelling thermocapillary migration of a microfluidic droplet on a solid surface

A multiphase lattice Boltzmann model is developed to simulate immiscible thermocapillary flows with the presence of fluid–surface interactions. In this model, interfacial tension force and Marangoni stress are included by introducing a body force term based on the concept of continuum surface force, and phase segregation is achieved using the recolouring algorithm proposed by Latva-Kokko and Rothman. At a solid surface, fluid–surface interactions are modelled by a partial wetting boundary condition that uses a geometric formulation to specify the contact angle, and a colour-conserving boundary closure scheme to improve the numerical accuracy and suppress spurious velocities at the contact line. An additional convection–diffusion equation is solved by the passive scalar approach to obtain the temperature field, which is coupled to the hydrodynamic equations through an equation of state. This model is first validated by simulations of static contact angle and dynamic capillary intrusion process when a constant interfacial tension is considered. It is then used to simulate the thermocapillary migration of a microfluidic droplet on a horizontal solid surface subject to a uniform temperature gradient. We for the first time demonstrate numerically that the droplet motion undergoes two different states depending on the surface wettability: the droplet migrates towards the cooler regions on hydrophilic surfaces but reverses on hydrophobic surfaces. Decreasing the viscosity ratio can enhance the intensity of thermocapillary vortices, leading to an increase in migration velocity. The contact angle hysteresis, i.e., the difference between the advancing and receding contact angles, is always positive regardless of the contact angle and viscosity ratio. The contact angle hysteresis and the migration velocity both first decrease and then increase with the contact angle, and their minimum values occur at the contact angle of 90 degrees.

English

2 ABSTRACT: The problem of mass distribution in an unsteady two dimensional flow of a viscous, incompressible, thermally and electrically conducting binary fluid mixture between two infinite vertical parallel plates under the influence of chemical reaction, uniform magnetic field and temperature gradient is studied. The magnetic field lines are assumed to be fixed relative to the moving plates. The governing partial differential equations are first transformed into ODEs by the method of separation of variables and then solved numerically by using MATLAB's built in solver bvp4c. The solution for the concentration of the rarer and lighter component of the binary fluid mixture for different values of Reynolds number, chemical reaction parameter, Schmidt number, thermal diffusion number and Peclet number are presented graphically to analyse their effects on mass distribution of the components of the binary fluid mixture.

Longitudinal spin current induced by a temperature gradient in a ferromagnetic insulator

Based on the solution of the stochastic Landau-Lifshitz-Gilbert equation discretized for a ferromagnetic chain subject to a uniform temperature gradient, we present a detailed numerical study of the spin dynamics with a focus particularly on finite-size effects. We calculate and analyze the net longitudinal spin current for various temperature gradients, chain lengths, and external static magnetic fields. In addition, we model an interface formed by a nonuniformly magnetized finite-size ferromagnetic insulator and a normal metal and inspect the effects of enhanced Gilbert damping on the formation of the space-dependent spin current within the chain. A particular aim of this study is the inspection of the spin Seebeck effect beyond the linear response regime. We find that within our model the microscopic mechanism of the spin Seebeck current is the magnon accumulation effect quantified in terms of the exchange spin torque. According to our results, this effect drives the spin Seebeck current even in the absence of a deviation between the magnon and phonon temperature profiles. Our theoretical findings are in line with the recently observed experimental results by M. Agrawal et al., Phys. Rev. Lett. 111, 107204 (2013).

Applicability and failure of the flux-gradient laws in double-diffusive convection

AbstractDouble-diffusive flux-gradient laws are commonly used to describe the development of large-scale structures driven by salt fingers – thermohaline staircases, collective instability waves and intrusions. The flux-gradient model assumes that the vertical transport is uniquely determined by the local background temperature and salinity gradients. While flux-gradient laws adequately capture mixing characteristics on scales that greatly exceed those of primary double-diffusive instabilities, their accuracy rapidly deteriorates when the scale separation between primary and secondary instabilities is reduced. This study examines conditions for the breakdown of the flux-gradient laws using a combination of analytical arguments and direct numerical simulations. The applicability (failure) of the flux-gradient laws at large (small) scales is illustrated through the example of layering instability, which results in the spontaneous formation of thermohaline staircases from uniform temperature and salinity gradients. Our inquiry is focused on the properties of the ‘point-of-failure’ scale ($\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}H_{pof}$) at which the vertical transport becomes significantly affected by the non-uniformity of the background stratification. It is hypothesized that$H_{pof} $can control some key characteristics of secondary double-diffusive phenomena, such as the thickness of high-gradient interfaces in thermohaline staircases. A more general parametrization of the vertical transport – the flux-gradient-aberrancy law – is proposed, which includes the selective damping of relatively short wavelengths that are inadequately represented by the flux-gradient models. The new formulation is free from the unphysical behaviour of the flux-gradient laws at small scales (e.g. the ultraviolet catastrophe) and can be readily implemented in theoretical and large-scale numerical models of double-diffusive convection.

Large thermal biasing of individual gated nanostructures

We demonstrate very large and uniform temperature gradients up to about 1 K every 100 nm, in an architecture which is compatible with the field-effect control of the nanostructure under test. The temperature gradients demonstrated greatly exceed those typically obtainable with standard resistive heaters fabricated on top of the oxide layer. The nanoheating platform is demonstrated in the specific case of a short-nanowire device.

Thermal behaviors of concrete and steel bridges after slab replacements with GFRP honeycomb sandwich panels

During the last several decades, fiber-reinforced polymer (FRP) panels have been applied as one of the alternatives for slab retrofits and replacements in bridge engineering. Even though with many benefits, e.g., high strength, light weight, long-term durability, and good corrosion resistance, the FRP structures have not been widely adopted. It is partly due to the lack of national design guidelines and the uncertainties on long-term performances. Many studies have been conducted to investigate the static and dynamic performances of FRP bridges, while the studies on their thermal responses are not adequate. Therefore, the present study demonstrates an investigation on the thermal behavior of a FRP bridge based on a slab replacement project conducted by the Kansas State Department of Transportation, i.e., a steel deck replaced by glass FRP (GFRP) panels. The temperatures of the bridge and ambient air are recorded from December 2002 to July 2004. Firstly, a finite element model is developed using a sub-structuring method. This method is particularly efficient for modeling structures with complex configurations and subjected to the traffic and temperature loadings. Then, the model is verified by comparing the predicted live load distribution factors with that of a live load test; and it is further adopted to study the thermal responses using the measured temperatures from January 24 to July 13 in 2004. Finally, a parametric study is performed to study two general slab replacement cases, i.e., bridges with concrete beams and steel girders, respectively. Different loading conditions are considered, including the uniform temperature variations, temperature gradients, self-weights, and HS-20 live loads. Results from this study indicate that careful attentions should be paid to the thermal responses of bridges. The temperature gradients and uniform variations evidently affect the performance of bridges after replacing the concrete slabs with GFRP panels, such as the induced vertical deflections, horizontal movements, and stresses of steel girders or concrete beams.

Instabilities of natural convection in a periodically heated layer

AbstractNatural convection in an infinite horizontal layer subject to periodic heating along the lower wall has been investigated using a combination of numerical and asymptotic techniques. The heating maintains the same mean temperatures at both walls while producing sinusoidal temperature variations along one horizontal direction, with its spatial distribution characterized by the wavenumber $\alpha $ and the amplitude expressed in terms of a Rayleigh number $R{a}_{p} $. The primary response of the system takes the form of stationary convection consisting of rolls with the axis orthogonal to the heating wave vector and structure determined by the particular values of $R{a}_{p} $ and $\alpha $. It is shown that for sufficiently large $\alpha $ convection is limited to a thin layer adjacent to the lower wall with a uniform conduction zone emerging above it; the temperature in this zone becomes independent of the heating pattern and varies in the vertical direction only. Linear stability of the above system has been considered and conditions leading to the emergence of secondary convection have been identified. Secondary convection gives rise to either longitudinal rolls, transverse rolls or oblique rolls at the onset, depending on $\alpha $. The longitudinal rolls are parallel to the primary rolls and the transverse rolls are orthogonal to the primary rolls, and both result in striped patterns. The oblique rolls lead to the formation of convection cells with aspect ratio dictated by their inclination angle and formation of rhombic patterns. Two mechanisms of instability have been identified. In the case of $\alpha = O(1)$, parametric resonance dominates and leads to a pattern of instability that is locked in with the pattern of heating according to the relation ${\delta }_{cr} = \alpha / 2$, where ${\delta }_{cr} $ denotes the component of the critical disturbance wave vector parallel to the heating wave vector. The second mechanism, the Rayleigh–Bénard (RB) mechanism, dominates for large $\alpha $, where the instability is driven by the uniform mean vertical temperature gradient created by the primary convection, with the critical disturbance wave vector ${\delta }_{cr} \rightarrow 1. 56$ for $\alpha \rightarrow \infty $ and the fluid response becoming similar to that found in the case of a uniformly heated wall. Competition between these mechanisms gives rise to non-commensurable states in the case of longitudinal rolls and the appearance of soliton lattices, to the formation of distorted transverse rolls, and to the appearance of the wave vector component in the direction perpendicular to the forcing direction. A rapid stabilization is observed when the heating wavenumber is reduced below $\alpha \approx 2. 2$ and no instability is found when $\alpha \lt 1. 6$ in the range of $R{a}_{p} $ considered. It is shown that $\alpha $ plays the role of an effective pattern control parameter and its judicious selection provides a means for the creation of a wide range of flow responses.

Compact reactor for Fischer–Tropsch synthesis based on hierarchically structured Co catalysts: Towards better stability

A compact reactor for Fischer–Tropsch (F–T) synthesis based on hierarchically structured Co catalysts has been developed. The SiO2 coated carbon nanofiber (CNF)/carbon felt (CF) has several advantages, such as high surface area, low pressure drop, high thermal conductivity and good mass transport properties. CNFs were grown on the microfibers of CF, which greatly increased the surface area of the composite. An in situ sol–gel method has been developed to coat one thin layer of SiO2 to form CNF/CF@SiO2 core shelled tubes. The efficient 3-D thermal conductive network in the composite provided a relatively uniform temperature gradient during the F–T synthesis without adding any diluents. The prepared Co/(CNF/CF@SiO2) catalysts have higher activity and C5+ selectivity. The catalyst stability is also greatly enhanced by manipulating the Co-support interface by means of the coated SiO2 layer on CNFs. Reduction–oxidation (Redox) cycles indicate that the SiO2 layer decreases the oxidation potential of the Co nanoparticles, with enhanced catalytic stability as a result.