Theory Of Thermal Conductivity Research Articles

Numerical Simulation of Coupled Water and Salt Transfer in Soil and a Case Study of the Expansion of Subgrade composed by Saline Soil

Salt expansion is one of the major problems of subgrade involving saline soils. Salt expansion is induced by the crystallization of salt when the soil water content or temperature decreases. A case of salt expansion in subgrade composed by saline soil is introduced. It is located in G314 Road, which is one of the national highways in China, from Urumqi to Khunjerab. There is 3.40 ∼5.91% salt within 1.0 depth in the ground surface of the G314 (K587 + 035), which belongs to strong sulphate salty soil. The maximum salt expansion is up to 20cm in G314 Road, which has a serious impact on the road. Salt expansion is majorly controlled by the salt crystallization and the transmission of water and salt. Based on the solute transport theory in porous media, the water flow theory and thermal conduction theory, the govern differential equations of coupled salt crystallization, salt transmission and water flow are established in this paper. The coupled water and salt transfer problem of saline soil is solved by a software named as COMSOL with the use of a secondary development module. To verify the method for simulating the coupled water and salt transmission process, the water and salt transfer process in a 1D soil column is simulated and the results are compared with the theoretical solution. The results show that coupled model of water and salt established in this paper can accurately predict salt migration and crystallization of saline soil.

Self-Similar Theory of Thermal Conduction and Application to the Solar Wind.

We propose a self-similar kinetic theory of thermal conductivity in a magnetized plasma, and discuss its application to the solar wind. We study a collisional kinetic equation in a spatially expanding magnetic flux tube, assuming that the magnetic field strength, the plasma density, and the plasma temperature decline as power laws of distance along the tube. We demonstrate that the electron kinetic equation has a family of scale-invariant solutions for a particular relation among the magnetic-, density-, and temperature-scaling exponents. These solutions describe the heat flux as a function of the temperature Knudsen number γ, which we require to be constant along the flux tube. We observe that self-similarity may be realized in the solar wind; for the Helios data 0.3-1 AU we find that the scaling exponents for density, temperature, and heat flux are close to those dictated by scale invariance. We find steady-state solutions of the self-similar kinetic equation numerically, and show that these solutions accurately reproduce the electron strahl population seen in the solar wind, as well as the measured heat flux.

Determination of soil parameters to calculate soil resistivity

This paper is dedicated to the development of techniques allowing for design solutions to be refined on grounding devices. The resistivity is used in the design of grounders in dependence on the soil humidity, temperature, and density. To determine soil humidity at the depth of soil occurrence, two models are used: depending on the season at a depth of up to 1 m and depending on the level of groundwaters at depths of more than 1 m. The models are developed based on the fuzzy logic theory and data from water stations and geological exploration. The technique and corresponding dependencies are used to find the temperature of soil layers during a year and calculate soil resistivity. Temperature was simulated based on the Fourier theory of thermal conductivity and data from weather stations. It is shown that these methods and dependencies allow one to calculate more precisely the resistances of grounders at the design stage. The obtained in vitro dependencies of soil resistivity on the soil humidity and density are considered. These dependencies allow one to refine the dynamics of change in the grounder resistance from the time of installation to complete soil subsidence.

A REFINED MATHEMATICAL MODEL OF MULTIPHYSICS PROCESSES FOR MAGNETIC PULSE TREATMENT OF MATERIALS

Introduction. The complexity of the theoretical description of the magnetic pulse treatment of the material is in the mutual coupled processes of electromagnetic and thermal fields with plastic deformation of the material and processes in an electrical circuit. The paper deals with the combined transient mathematical model of the system of equations of the electromagnetic field, theory of elasticity, thermal conductivity and electrical circuit. Purpose. Research and testing of the developed mathematical model and assess the impact of various parameters on the process of deformation of the work piece. Methodology. Investigation of nonlinear mathematical model is carried out by the finite element method using a special software package. Results. The resulting solution of the transient mathematical model allows studying the influence of parameters of the circuit, the speed and the characteristics of the material to plastic deformation and heating of the work piece, which allows to select the optimum process parameters. Originality. This is an integrated approach to the development of a mathematical model, which includes the electromagnetic field equations, the theory of elasticity, thermal conductivity and electrical circuit equations with a storage capacitor. Conclusions. A comprehensive mathematical model and its solution are obtained. It is established a small effect of heating temperature on the amount of strain. Currents caused by movement of the work piece must be taken into account in the calculations. Inertial forces significantly affect the nature of the deformation. During the deformation it is necessary to consider the nonlinearity of elasticity modulus. Thermal deformation of the work piece is much less mechanical strain and opposite in sign to them, but the surface temperature stresses due to the high temperature gradient equal to 20 % of the yield strength of the work piece.

Gas outburst with sediments because of tetrahydrofuran hydrate dissociation

SummaryMethane hydrate (MH) is a new energy resource in the 21st century. But the dissociation of MH from sediments during the MH exploration or oil/gas exploration under a hydrate layer accompanied by the softening of soils and formation of excess pore gas pressure may lead to ground failures and environmental disasters. In this study, experiments on modeling the weakening and failure of the sediment by heat‐induced dissociation of tetrahydrofuran (THF) hydrate were presented. The failure mode of gas outburst was observed. Gas outbursts is a process where gas and soils in hydrate‐dissociation zone burst out after the continuous skeleton of over‐layer is fractured during the expansion of the dissociation zone and the formation of gas zone and excess pore gas pressure. An analytical method is presented by decoupling heat transfer and soil deformation. The geometrical and mechanical similarities for gas outburst are obtained. An empirical criterion for the occurrence of outburst is proposed using the theory of thermal conduction, rigid plastic mechanics, and the experimental data. Copyright © 2015 John Wiley & Sons, Ltd.

The theory of electric and thermal conductivity in bubble gas-liquid media

The theory of electric and thermal conductivity in bubble gas-liquid media

Investigation of thermoelectric SiC ceramics for energy harvesting applications on supersonic vehicles leading–edges

Utilizing thermoelectric technology to aerodynamic heat harvesting on the leading-edge is worth noticing in the thermal protection systems. In this paper, a nose-tip model in a supersonic flow field is developed to predict the thermoelectric performance of SiC ceramics structures. The generation performance is numerically investigated in terms of the computational fluid dynamics and the thermal conduction theory. The output power and energy efficiency of the nose-tip model are obtained with Mach number varying from 2·5–4·5. The generated power reaches 1·708 W/m2at a temperature difference of 757 K at M = 4·5. With respect to the Thomson effect, the output power decreases rapidly. However, larger output power and energy efficiency would be obtained with the increase of Mach number, with or without considering the Thomson heat. Moreover, under the higher Mach numbers, larger range of output current value is available.

SHS in a flow of inert gas: Effect of flow direction

62 As is widely known, combustion of SHS systems as a rule is accompanied by partial or complete melting of reagents and/or products [1, 2]. It has been suggested that the above melts move under the action of ther mocapillary forces [3] and that the capillary forces in typical “gasless” systems are markedly stronger [4] compared to other forces affecting the melt motion: a change in the surface tension coefficient within the combustion zone was found to attain a value of 0.1 N/m [4]. In order to rationalize some specific fea tures of solid flame combustion, the convection–con duction mechanism has been proposed [5]. In terms of this model, burning velocity U is a sum of two compo nents: a value defined by classical thermal conductiv ity theory of SHS [1] and a “convective” term calcu lated under the assumption that there exists a piston of the melt that is pushed forward by a pressure difference ahead and behind the combustion wave (Fig. 1).

Effect of Mach number on thermoelectric performance of SiC ceramics nose-tip for supersonic vehicles

This paper focus on the effects of Mach number on thermoelectric energy conversion for the limitation of aero-heating and the feasibility of energy harvesting on supersonic vehicles. A model of nose-tip structure constructed with SiC ceramics is developed to numerically study the thermoelectric performance in a supersonic flow field by employing the computational fluid dynamics and the thermal conduction theory. Results are given in the cases of different Mach numbers. Moreover, the thermoelectric performance in each case is predicted with and without Thomson heat, respectively. Due to the increase of Mach number, both the temperature difference and the conductive heat flux between the hot side and the cold side of nose tip are increased. This results in the growth of the thermoelectric power generated and the energy conversion efficiency. With respect to the Thomson effect, over 50% of total power generated converts to Thomson heat, which greatly reduces the thermoelectric power and efficiency. However, whether the Thomson effect is considered or not, with the Mach number increasing from 2.5 to 4.5, the thermoelectric performance can be effectively improved.

A discussion of the relationship between heat input parameter and angular distortion by considering moving heat source effect

In recent years, it has become much more necessary to control or reduce weld distortion, which has negative influences on structural integrity, without loss of manufacturing efficiency. Higher accurate prediction of weld distortion brings in one of the contributions for improvement of manufacturing efficiency. Commonly, angular distortion, which is controlled by temperature distribution along the direction of plate thickness, is controlled by heat input parameter (Qnet/h2). However, it is also known that there are several concerns against applicability of conventional heat input parameter for prediction of angular distortion. One of the concerns is the effect of welding speed. This is because the conventional heat input parameter is derived from welding thermal conduction theory without considering moving of the weld heat source. Actually, a pre-study by experiment about the effect of welding conditions, including welding speed, on angular distortion has shown that angular distortion is not always controlled by heat input parameter in the case of different welding speed. Then, in order to clarify the effect of welding speed on that, more detailed study is further required. In this paper, a numerical study on the effect of welding speed on the relationship between heat input parameter and angular distortion is performed based on the past experimental results. In addition, the reason why the relationship between heat input parameter and angular distortion is affected by welding speed is discussed from a viewpoint of distribution of the maximum temperature rise and residual plastic strain. It is concluded that the effect of welding speed on the relationship between heat input parameter and angular distortion is understood by the difference in distribution of mechanical melting temperature, which identifies the distribution of residual plastic strain for angular distortion, in plate thickness section.

Thermal conductivity of the single-crystal monoisotopic 29Si in the temperature range 2.4–410 K

The temperature dependence of the thermal conductivity κ(T) of single-crystal silicon highly enriched in 29Si (99.919%) isotope has been measured in the temperature range 2.4–410 K. At low temperatures (T < 6 K) in the boundary phonon scattering regime, the thermal conductivity of the 29Si crystal is higher than that of the 28Si (99.983%) crystal. At high temperatures where the thermal conductivity is determined by anharmonic processes of phonon scattering, the thermal conductivity of 29Si is lower than that of 28Si. The conclusions of the theory of phonon thermal conductivity on the mass dependence of κ(T) agree with the experimental results.

Numerical analysis of Nd:YAG pulsed laser polishing CVD self-standing diamond film

Chemical vapor deposited (CVD) diamond film has broad application foreground in high-tech fields. But polycrystalline CVD self-standing diamond thick film has rough surface and non-uniform thickness that adversely affect its extensive applications. Laser polishing is a useful method to smooth self-standing diamond film. At present, attentions have been focused on experimental research on laser polishing, but the revealing of theoretical model and the forecast of polishing process are vacant. The paper presents a finite element model to simulate and analyze the mechanism of laser polishing diamond based on laser thermal conduction theory. The experimental investigation is also carried out on Nd:YAG pulsed laser smoothing diamond thick film. The simulation results have good accordance with the results of experimental results. The temperature and thermal stress fields are investigated at different incidence angles and parameters of Nd:YAG pulsed laser. The pyramidal-like roughness of diamond thick film leads to the non-homogeneous temperature fields. The temperature at the peak of diamond film is much higher than that in the valley, which leads to the smoothing of diamond thick film. The effect of laser parameters on the surface roughness and thickness of graphite transition layer is also carried out. The results show that high power density laser makes the diamond surface rapid heating, evaporation and sublimation after its graphitization. It is also found that the good polish quality of diamond thick film can be obtained by a combination of large incident angle, moderate laser pulsed energy, large repetition rate and moderate laser pulse width. The results obtained here provide the theoretical basis for laser polishing diamond film with high efficiency and high quality.

Research on the Inversion of Equivalent Surface Heat Transfer Coefficient of Mass Concrete by Calculating its Heat Flux

Most of the cracks on concrete dam are external ones, while external heat preservation is an important measure to prevent cracking. In order to obtain the actual thermal parameters, according to thermal conduction theory and the temperature distribution conditions of optical fiber on concrete surface, the surface temperature distribution of concrete pouring deck was real-time monitored by setting optical fiber in different depths; then the surface heat flux of mass concrete was calculated, thereby the equivalent surface heat transfer coefficient, which varied as time goes, was inversed. It is indicated that the inversion process is relatively simple and reliable, and the heat transfer coefficient obtained can well reflect the real performance of the insulation materials. Meanwhile, it is also indicated that the heat transfer coefficient of equivalent surface varies as time goes, which can contribute to back analysis calculation and actual engineering practice.

Anisotropic Mechanical and Thermal Properties of Nd&lt;sub&gt;2&lt;/sub&gt;SrAl&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt;

For a long time since the anisotropy basically confined to a single crystal, used as a polycrystalline ceramic materials generally considered to be isotropic. In this paper, the anisotropic mechanical, thermal expansion coefficient and thermal conductivity of a double perovskite slab-rocksalt layer Nd2SrAl2O7 was studied by first principles as an example. The method is using density functional theory (DFT) and crystal parameters, which has been used to calculate the modulus of elasticity of anisotropic in a three-dimensional space. While combined with traditional thermal conductivity theory, we have obtained the tensor of thermal diffusion, thermal conductivity in 3D space for the first time in the world within no. The results are in good agreement with the experiment. The advantage of method is avoiding the difficulty of experimental measurement, reducing the time and obtaining relatively accurate results.

Synthesis and thermal conductivity of Y6UO12

Y6UO12 was synthesized by solid-state reactions of Y2O3 and U3O8. The high-density pellet of Y6UO12 was prepared by the spark plasma sintering followed by heat treatment in air for oxygen supplementation. The thermal conductivity (κ) was evaluated using the laser flash method from room temperature to 1173 K. The κ of Y6UO12 decreased with increasing temperature in the whole temperature range, indicating that the phonon contribution was predominant. The room temperature κ value of Y6UO12 was 4.90 Wm−1K−1. The magnitude relationship of κ among Y6UO12, Y6WO12, and Yb6WO12, i.e. κ of Yb6WO12 < κ of Y6UO12 < κ of Y6WO12, was discussed based on the general lattice thermal conductivity theory.

Theory of thermal transport in multilayer hexagonal boron nitride and nanotubes

We present a theory for the lattice thermal conductivity ${\ensuremath{\kappa}}_{L}$ of single-walled boron nitride nanotubes (BNNTs) and multilayer hexagonal boron nitride (MLBN), which is based on an exact numerical solution of the phonon Boltzmann equation. Coupling between layers in MLBN and nanotube curvature in BNNTs each break a phonon scattering selection rule found in single-layer hexagonal boron nitride (SLBN), which reduces ${\ensuremath{\kappa}}_{L}$ in these systems. We show that out-of-plane flexural phonons in MLBN and out-of-tube phonons in BNNTs provide large contributions to ${\ensuremath{\kappa}}_{L}$, qualitatively similar to multilayer graphene (MLG) and single-walled carbon nanotubes (SWCNTs). However, we find that the ${\ensuremath{\kappa}}_{L}$'s in BNNTs and MLBN are considerably smaller compared to similar SWCNTs and MLG structures because of stronger anharmonic phonon scattering in the former. A large and strongly temperature-dependent isotope effect is found reflecting the interplay between anharmonic and isotope scattering phonons. Finally, we also demonstrate convergence of BNNTs into SLBN for large-diameter nanotubes and MLBN to bulk hexagonal boron nitride within a few layers.

NONLINEARITY EFFECT OF TRANSPORT PROPERTIES ON COMBUSTION OF VOLATILE ORGANIC PARTICLES

This paper presents the structure of laminar, one-dimensional, and steady-state flame propagation in a uniform cloud of volatile organic particles. In this model, it is assumed that particles vaporize to yield a gaseous fuel. In order to study the effects of thermal conductivity and thermal diffusion on combustion of organic particles, an asymptotic analysis, based on the large Zeldovich number or high rate of reaction, is used. In the preheat zone, the rate of chemical reaction is small, and transfer phenomena play a significant role in temperature and mass distributions. Therefore, according to the kinetic theory of gases, thermal conductivity (λ) and diffusion coefficient (D) are considered to be proportional to T1/2 and T3/2, respectively. Finally, flame characteristics for different equivalence ratios of solid mixture for different Lewis numbers are reported. Results show that both flame temperature and burning velocity increase with rise in the Lewis number.

Nanostructure model of thermal conductivity for high thermoelectric performance

The effective medium theory of thermal conductivity of two-phase composites studied by Nan et al. has been extended to investigate concentrated nanocomposites. Due to the presence of inter-particle phonon scattering processes in concentrated nanocomposites, the effective lattice thermal conductivity keff varies more rapidly with the volume fraction of second-phase inclusions in the composite. Applying the new keff expressions to monolithic material systems, the results are found to capture the experimental trend of monolithic nanostructured materials. In particular, it is noted that the dimensionless figure of merit, ZT, is nearly doubled by only reducing the lattice thermal conductivity. Two-phase nanocomposites have also been evaluated, demonstrating that these latter systems are very suited for high thermoelectric performance. Present study leads to several strategies for obtaining ZT ∼ 2 or higher in nanocomposites.

A Fractal Approach to Assess the Risks of Nitroamine Explosives

To the best of our knowledge, this work represents the first thermal conductivity theory for fractal energetic particle groups to combine fractal and hot-spot theories. We considered the influence of the fractal dimensions of particles on their thermal conductivity and even on the sensitivity of the explosive. Based on this theory, two types of nitroamine explosives (hexahydro-1,3,5-trinitro-1,3,5-triazine [RDX] and hexanitrohexaazaisowurtzitane [HNIW]) with different sizes, size distributions, and microscale morphologies were prepared using wet milling, solvent/nonsolvent, and ridding methods. The dependence of the explosive sensitivity on the fractal characteristics of the particles was investigated. Specifically, the size distributions and scanning electron microscopy (SEM) images of the samples were used to obtain the fractal dimension (D) and surface fractal dimension (Ds), respectively, by using a least-squares method and fractal image processing software (FIPS). The mechanical sensitivity and thermal stability of the samples were characterized using mechanical sensitivity tests and differential scanning calorimetry (DSC) and were further compared with the previous results upon the investigation about HMX (octahydro-1.3.5.7-tetranitro-1,3,5,7-tetrazocine). The results indicate that the sensitivity of nitroamine explosives largely depends on the fractal dimensions of the particles. Specifically, the sample with a higher D value is more insensitive to impact action, whereas the sample with a higher Ds value is more sensitive to friction action. In addition, the sample with both higher D and Ds values has less heat release and a slower rate of thermal decomposition. All of the above observations can be attributed to the alternation of the formation of hot spots that was controlled by heat mass and thermal conductivity due to the increase of D and Ds values caused by changes in parameters such as fine particle content, specific surface area, porosity content, surface protruding points, and surface roughness. Therefore, the data in these studies were used to develop a thermal conductivity theory for fractal energetic particle groups that could be applied to the prediction of the sensitivity of energetic materials.

Online Calculating Models of Temperature and Thermal Stresses for Turbine Rotors

This paper presents the faster calculating models of temperature and thermal stress in a rotor based on two-dimensional axis-symmetry thermal conduction and thermo-elastic theory. The models can deal with nonlinear factors caused by temperature-dependant properties using modified Newton-Raphson iterative method. An example has been given to compare the results calculated by the models with those calculated by finite element method. It is shown that the analytical models gained have high accuracy. The iterative models can be used for online monitoring and control of turbine.