Nonlinear Thermal Properties Research Articles

On-line heat flux estimation of a nonlinear heat conduction system with complex geometry using a sequential inverse method and artificial neural network

This paper presents a method for on-line estimating a time-varying surface heat flux of a nonlinear heat conduction system with complex geometry from transient temperature measurements. The study consisted of two problems: the forward problem and the inverse problem. For the forward problem, a fast and accurate method based on artificial neural network (ANN) was developed to nonlinearly map any known heat flux to the corresponding temperatures. The training data was obtained by off-line finite element simulations. For the inverse problem, an adaptive sequential Tikhonov regularization (ASTR) method was proposed to estimate the boundary heat flux, which shows superiority in on-line applications due to its independence of future measurement. At each time step of the on-line process, the trained ANN was called by the ASTR procedure to calculate the temperature and sensitivity coefficient. The coupled method, referred to as ASTR-ANN, was tested numerically in a three-dimensional solid system with nonlinear thermal properties and complex geometry. Comparisons with several existing nonlinear inverse methods were also conducted, and the results demonstrated the validity of the proposed ASTR-ANN method.

Nonlinear electrical conductivity and thermal properties of AgNPs/BN/EPDM composites for cable accessory

To reduce the electric field distortion and increase the thermal conductivity of the dielectric materials in cable accessories, this paper proposes a method to prepare composites with good nonlinear electrical conductivity and high thermal conductivity. Silver nanoparticles grown on hexagonal boron nitride (AgNPs/BN) were prepared as a powder and introduced into ethylene propylene diene monomer (EPDM) to obtain AgNPs/BN/EPDM composites. The nonlinear electrical conductivity, thermal conductivity and breakdown field strength of the composites were investigated at different temperatures. The nonlinear electrical conductivity and thermal conductivity of the composites increased with increasing AgNPs content grown on BN. Moreover, the breakdown field strength decreased with increasing AgNPs, and exhibited a weaker temperature dependence than neat EPDM. Simulations showed that the composites could improve the electric field distribution in a cable accessory.

FIRE RESISTANCE PROVIDING OF HOLLOW‐CORE CONCRETE SLABS USING SHEET BUILDING MATERIALS

Introduction. This paper deals with the analysis of world experience in fire resistance providing of hollow‐core concrete slabs. To protect concrete structures are used many structural applications: thermal coatings and materials. The research first analyzes main methods and hypothesis using to make temperature analysis of solution fire resistance of concrete structures. Problem of making temperature analysis of hollow‐core concrete slabs are nonlinear thermal material properties and radiation heat transfer in the hollow-cores. The aim of this paper is to establish the temperature distribution in hollow‐core concrete slab considering radiation heat transfer in the hollow-cores in case of fire in compartment that is spreading by standard temperature-time curve. In addition, the aim is to substantiate the possibility of using gypsum panels to provide fire resistance of hollowcore concrete slabs. Material statement. The paper reports the results of modeling the process of heat transfer in hollow‐core concrete slab, between compartment space and slab surface and in hollow‐cores. To calculate temperature fields in hollow‐core concrete slab considering nonlinear thermal material properties and radiation heat transfer in the hollow-cores was used finite element model. At addition, the results of finite elements simulations show temperature fields in hollow‐core concrete slab and gypsum panels that installed under concrete slab. Scientific novelty. The paper reports results of theoretic substantiated of possibility of using gypsum panels to protect of hollow‐core concrete slabs considering nonlinear thermal material properties, radiation heat transfer between surfaces and radiation heat transfer in the hollow-cores. The results indicate that using gypsum panels to protect of hollow‐core concrete slabs reduces speed heating of concrete elements to critical temperatures that increase fire resistance of hollow‐core concrete slabs to 20.4 %.

Calculation of linear induction motor features by detailed equivalent circuit method taking into account non-linear electromagnetic and thermal properties

The main purpose of the work is to suggest the technique of solving multi-physics problems (calculation of magnetic, temperature and velocity fields) for the purpose of units with forced cooling system and complex shapes. The general problem is the presence of narrow region in which it is necessary to calculate the velocity field of a cooling liquid. The problem is solved by a simplified method based on a detailed equivalent circuit, which significantly reduces required computational resources in comparison with numerical methods (finite element and finite volume methods). The paper presents the transition from partial differential equations to algebraic equations, created on the basis of detailed equivalent circuits. The results of the model are analyzed on a real linear induction motor applied in the transport system. Thermal modes are being considered depending on cooling intensity and magnitude of power supply parameters of the motor. It is proposed to reduce the computation time by simplified calculation of the velocity field using an analytical coefficient obtained from the analysis of the equivalent circuits. The vectorization of cycles intended for building the stiffness matrix, storage of the matrix in sparse form, and rational sequences of matrix calculations in the course of the solution made it possible to reduce the computational time almost twice as well. The comparison of the accuracy of the results obtained is presented, depending on the discretization of the computational domain and chosen type of interpolation. Verification results of electromagnetic and thermal calculations obtained from experimental data are also presented.

Experimental and numerical study of a PCM solar air heat exchanger and its ventilation preheating effectiveness

This article presents a PCM solar air heat exchanger integrated into ventilated window developed to maximize the use of the solar energy to pre-heat the ventilated air. The system is designed to improve the indoor air quality and thermal comfort by continuous pre-heated air supply at a reduced energy use through the capturing and storing of solar energy. This study examines the thermodynamic behavior of the system both experimentally and numerically. This entails a full-scale experiment in climate boxes to study the thermal storage and heat release ability of the facility. Accordingly, a numerical model combining heat transfer and buoyancy derived laminar flow and nonlinear thermal properties of the PCM is built and validated with the experimental data. The model is then used for configuration optimization of the PCM solar air heat exchanger to maximize the solar energy storage and the ventilation pre-heating effectiveness. The results show that for a 6-h solar charging period, the optimum PCM plate depth is 90 mm and the optimum air gap thickness is 6 mm. The same configuration can be used for both summer night cooling and winter solar energy storage applications. The total stored/released latent heat after one charging period is 93.31 MJ/m3.

Fast explicit dynamics finite element algorithm for transient heat transfer

This paper presents a novel methodology for fast simulation and analysis of transient heat transfer problems. The proposed methodology is suitable for real-time applications owing to (i) establishing the solution method from the viewpoint of computationally efficient explicit dynamics, (ii) proposing an element-level thermal load computation to eliminate the need for assembling global thermal stiffness matrix, leading to (iii) an explicit formulation of nodal temperature computation to eliminate the need for iterations anywhere in the algorithm, (iv) pre-computing the constant matrices and simulation parameters to facilitate online calculation, and (v) utilising computationally efficient finite elements to efficiently obtain thermal responses in the spatial domain, all of which lead to a significant reduction in computation time for fast run-time simulation. The proposed fast explicit dynamics finite element algorithm (FED-FEM) employs nonlinear thermal material properties, such as temperature-dependent thermal conductivity and specific heat capacity, and nonlinear thermal boundary conditions, such as heat convection and radiation, to account for the nonlinear characteristics of transient heat transfer problems. Simulations and comparison analyses demonstrate that not only can the proposed methodology handle isotropic, orthotropic, anisotropic and temperature-dependent thermal properties but also satisfy the standard patch tests and achieve good agreement with those of the commercial finite element analysis packages for numerical accuracy, for three-dimensional (3-D) heat conduction, convection, radiation, and thermal gradient concentration problems. Furthermore, the proposed FED-FEM algorithm is computationally efficient and only consumes a small computation time, capable of achieving real-time computational performance, leading to a novel methodology suitable for real-time simulation and analysis of transient heat transfer problems.

Improving structural, UV–visible, SHG efficiency, third order nonlinear optical, mechanical, thermal and dielectric properties of ZTC crystal using π-bonded ligand of oxalic acid

Growth of pure and oxalic acid (OA) added zinc thiourea chloride (ZTC) crystals was achieved by slow evaporation method. The structural parameters of these have been confirmed by single crystal XRD. Incorporation/presence of OA in ZTC crystal was established by EDS. Optical transparency of grown crystals has been evaluated in 200–900 nm region and linear optical constants are examined in visible region using the UV–visible transmittance data. SHG efficiency of OA:ZTC crystal is observed to be 4.51 times that of ZTC crystal tested by Kurtz-Perry method. Z-scan measurement was done at 632.8 nm to confirm the nature of third order nonlinearity of OA:ZTC crystal and third order nonlinear susceptibility (χ(3)), refraction (n2) and absorption coefficient (β) were evaluated. Mechanical behavior of OA:ZTC crystal is investigated using Vicker's microhardness studies. Dielectric properties of crystals were comparatively investigated at different temperature. The thermal stability and melting point of OA doped ZTC crystal material has been determined employing thermogravimetric and differential scanning calorimetric analysis.

Enhancement of nonlinear optical and thermal properties of polyurethanes by modifying the chromophores with fused heterocyclic and pyrimidine rings

This article describes the synthesis of substituted triazolo[3,4‐b]‐thiadiazole heterocycles based nonlinear optical (NLO) responsive chromophores. To enhance the NLO properties of the chromophores, the pyrimidine ring was introduced with an appropriate π‐bonding. With these chromophores, different series of polyurethanes were prepared by condensation with tolylene‐2,4‐diisocyanate and 3,3′‐dimethoxy‐4,4′‐biphenylene diisocyanate. The structure of the resulting chromophores and polyurethanes was confirmed using Fourier transform infrared, proton nuclear magnetic resonance, and Carbon Hydrogen Nitrogen (CHN) analyzer. The inherent viscosities (ηinh) of the polyurethanes measured by Ubbelohde viscometer were in the range of 0.2324–0.2760 dl/g. Thermal behavior of the polyurethanes was investigated using differential scanning calorimetry and thermogravimetric analysis. The molecular orientation in polyurethane films was induced by a corona poling, and this was ascertained using ultraviolet‐visible spectrophotometer and atomic force microscope. The thicknesses and refractive indices of thin films were determined by an Ellipsometer. Polyurethanes exhibited excellent solubility in most of the common organic solvents. The second harmonic generation (SHG) coefficients (d33) of the poled polyurethanes ranged from 63.20 and 95.32 pm/V at 532 nm. None of the polyurethanes showed SHG decay below 100°C, and retained 95% of the signal even up to 500 h. The resulting enhanced NLO efficiency and longer temporal stability make these polyurethanes as promising candidates for photonic devices. POLYM. ENG. SCI., 59:500–509, 2019. © 2018 Society of Plastics Engineers

Nonlinear thermal and electronic optical properties of graphene in N-methylpyrrolidone at 800 nm with femtosecond laser pulses

Graphene is a useful saturable absorber in a variety of lasers working in mode-locking or Q-switch regimes. The optical performance of chemically synthesized graphene is still not completely characterized. In this study, the saturable absorption and the nonlinear refractive index of graphene flakes in N-methylpyrrolidone, in both liquid and solid phases, have been studied at 800 nm with the z-scan technique using femtosecond laser pulses. The results obtained using a Ti:sapphire laser oscillator in the mode-locking regime (6 fs, 78 MHz) or in the continuous wave shows that the optical properties of graphene have a thermal origin, while at the lower repetition rate and higher energy and intensity of a Ti:sapphire amplifier (95 fs, 1 kHz), it shows the electronic Kerr effect. Solid samples with very high optical densities, equivalent to 60 layers of graphene grown by chemical vapor deposition (CVD), can be fabricated. They show a higher saturation intensity (Is≈100 GW cm–2) than CVD-grown (74 MW cm–2) or epitaxially grown (4 GW cm–2) graphene and intensity-dependent changes in transmission from 25% to 43%. This change in transmission in multilayer solid samples points to a good performance as a saturable absorber in laser cavities.

Numerical analysis on the behaviour of reinforced concrete frame structures in fire

Numerical approach using finite element method has been used to evaluate the behaviour of reinforced concrete frame structure subjected to fire. The structure is previously designed in accordance with Eurocode standards for the design of structures for earthquake resistance, for the ductility class M. Thermal and structural response are obtained using a commercially available software ANSYS. Temperature-dependent nonlinear thermal and mechanical properties are adopted according to Eurocode standards, with the application of constitutive model for the triaxial behaviour of concrete with a smeared crack approach. Discrete modelling of concrete and reinforcement has enabled monitoring of the behaviour at a global, as well as at a local level, providing information on the level of damage occurring during fire. Critical regions in frame structures are identified and assessed, based on temperatures, displacements, variations of internal forces magnitudes and achieved plastic deformations of main reinforcement bars. Parametric analyses are conducted for different fire scenarios and different types of concrete aggregate to determine their effect on global deformations of frame structures. According to analyses results, the threedimensional finite element model can be used to evaluate the insulation and mechanical resistance criteria of reinforced concrete frame structures subjected to nominal fire curves.

Predictions of transverse thermal conductivities for plain weave ceramic matrix composites under in-plane loading

A coupled temperature-displacement steady-state analysis of plain weave ceramic matrix composites along through-thickness direction was reported in this paper. Based on the formulation of Binary Model, a meso-scale 2 × 2 unit cell model of plain weave composites was established. The periodic displacement boundary conditions and through-thickness temperature gradient were imposed. The non-linear strain-dependent thermal properties of constituent materials have been discretised by multi-linear curves, which have been implemented by the user defined subroutine, USDFLD of the finite element package, Abaqus. The degradation of the composite through-thickness thermal conductivity with uni-axial straining has been predicted. The predictions considering tow waviness have a good agreement with experimental results. The analysis indicates that the degradation of through-thickness thermal conductivities is due to the combination of shear failure and wake debonding mechanisms. The matrix/medium played an important role in the through-thickness heat flow of plain weave ceramic matrix composites.

Nanoparticles and nonlinear thermal radiation properties in the rheology of polymeric material

The present analysis is related to the dynamics of polymeric liquids (Oldroyd-B model) with the presence of nanoparticles. The rheological system is considered under the application of nonlinear thermal radiations. Energy and concentration equations are presented when thermophoresis and Brownian motion effects are present. Bidirectional form of stretching is considered to interpret the three-dimensional flow dynamics of polymeric liquid. Making use of the similarity transformations, problem is reduced into ordinary differential system which is approximated by using HAM. Influence of physical parameters including Deborah number, thermophoresis and Brownian motion on velocity, temperature and mass fraction expressions are plotted and analyzed. Numerical values for local Sherwood and Nusselt numbers are presented and discussed.

Numerical exploration of the combined effects of non-linear thermal radiation and variable thermo-physical properties on the flow of Casson nanofluid over a wedge

PurposeThe effect of non-linear thermal radiation and variable thermo-physical properties are investigated in the Falkner-Skan flow of a Casson nanofluid in the presence of magnetic field. The paper aims to discuss this issue.Design/methodology/approachSelected bunch of similarity transformations are used to reduce the governing partial differential equations into a set of non-linear ordinary differential equations. The resultant equations are numerically solved using Runge-Kutta-Fehlberg fourth-fifth-order method along with shooting technique.FindingsThe velocity, temperature and concentration profiles are evaluated for several emerging physical parameters and are analyzed through graphs and tables in detail.Research limitations/implicationsThis study only begins to reveal the research potential and pitfalls of research and publishing on boundary-layer flow, heat and mass transfer of Casson nanofluid past and the moving and static wedge-shaped bodies.Originality/valueIt is found that the presence of non-linear thermal radiation and variable properties has more influence in heat transfer. Furthermore, temperature profile increases as the radiation parameter increases.

Asymptotic analysis of heat transfer in composite materials with nonlinear thermal properties

We study heat transfer through a composite with periodic microstructure. The thermal conductivity of the constituents is assumed to be temperature-dependent, and it is modeled as a polynomial in terms of the temperature. The thermal resistance between the constituents is taken to be nonlinear. In order to determine the effective thermal properties of the material, we apply the asymptotic homogenization method. We discuss different approaches to determine these effective properties for the different volume fractions of the inclusions. For high volume fractions of the inclusion, we apply the lubrication theory. In the case of low volume fractions of the inclusions, we apply the three-phase model. Comparing some special cases of our results to existing ones in the literature shows a good accuracy.

Synthesis, Crystal Structures, and Nonlinear Optic and Thermal Properties of Two Diiodocarbazole Derivatives

Two 3,6-diiodocarbazole derivatives were prepared from the iodination of the corresponding phenylcarbazole. 3,6-Diiodo-9-phenylcarbazole crystallises in the chiral space group P21 and shows good second-harmonic generation effects. Thermogravimetric analysis of the two compounds shows high thermal stabilities, in which the decomposition temperature for 1 and 2 are 273 and 308 °C, respectively.

Crystallographic Investigations into Properties of Acentric Hybrid Perovskite Single Crystals NH(CH3)3SnX3 (X = Cl, Br)

The hybrid perovskites with special optoelectronic properties have attracted more attention to the scientific and industrial applications. However, because of the toxicity and instability of lead complexes, there is interest in finding a nontoxic substitute for the lead in the halides perovskites and solving the ambiguous crystal structures and phase transition of NH(CH3)3SnX3 (X = Cl, Br). Here, we report the bulk crystal growths and different crystal morphologies of orthorhombic hybrid perovskites NH(CH3)3SnX3 (X = Cl, Br) in an ambient atmosphere by bottom-seeded solution growth (BSSG) method. More importantly, detailed structural determination and refinements, phase transition, band gap, band structure calculations, nonlinear optical (NLO) properties, XPS, thermal properties, and stability of NH(CH3)3SnX3 (X = Cl, Br) single crystals are demonstrated. NH(CH3)3SnCl3 single crystal undergoes reversible structural transformation from orthorhombic space group Cmc21 (no. 36) to monoclinic space group Cc (n...

Effect of interaction in the Ga–As–O system on the morphology of a GaAs surface during molecular-beam epitaxy

A thermodynamic analysis of processes of interphase interaction in the Ga–As–O system has been performed and their theoretical laws have been determined, taking into account nonlinear thermal physical properties of the compounds, the oxide film compositions, and modes of molecular-beam epitaxy of GaAs. The processes of interaction of the native oxide of GaAs with the substrate material and also with Ga and As4 from a vapor gaseous phase have been studied experimentally. The experimental results correlate with the results of the thermodynamic analysis. The laws of influence of the removal of the proper oxide on the evolution of the GaAs surface morphology under conditions of the molecular-beam epitaxy have been proposed.

Analysis of the quench propagation along Nb3Sn Rutherford cables with the THELMA code. Part II: Model predictions and comparison with experimental results

To improve the technology of the new generation of accelerator magnets, prototypes are being manufactured and tested in several laboratories. In parallel, many numerical analyses are being carried out to predict the magnets behaviour and interpret the experimental results. This paper focuses on the quench propagation velocity, which is a crucial parameter as regards the energy dissipation along the magnet conductor. The THELMA code, originally developed for cable-in-conduit conductors for fusion magnets, has been used to study such quench propagation. To this purpose, new code modules have been added to describe the Rutherford cable geometry, the material non-linear thermal properties and to describe the thermal conduction problem in transient regime. THELMA can describe the Rutherford cable at the strand level, modelling both the electrical and thermal contact resistances between strands and enabling the analysis of the effects of local hot spots and quench heaters. This paper describes the model application to a sample of Short Model Coil tested at CERN: a comparison is made between the experimental results and the model prediction, showing a good agreement. A comparison is also made with the prediction of the most common analytical models, which give large inaccuracies when dealing with low n-index cables like Nb3Sn cables.

Coordination polymers assembled from semirigid fluorene-based ligand: A couple of enantiomers

A couple of Mg(II)-based coordination polymer enantiomers [MgL(DMF)(H2O)3]n (R-MgL and S-MgL), and a Zn(II)-based coordination polymer [ZnL(DMF)]n (ZnL) have been synthesized by the solvothermal reactions between the achiral ligand 4,4′-(9,9-dimethyl-9H-fluorene-2,7-diyl)dibenzoic acid (H2L) and the corresponding metal salts. The MgL was obtained as the racemic conglomerate from the one pot reaction. The single crystal X-ray structural analyses reveal that MgL crystallize in the chiral space group P21 and possesses the right- or left-handed homochiral 1D Mg–O–C helical chain. The ZnL crystallize in the non-centrosymmetrical space group Aba2 and possesses the 2D network comprised of 1D Zn–O–C meso-helical chains and ligands. The MgL and ZnL complexes exhibit strong coordination-perturbed ligand-centered blue emissions when excited at 320nm. Their second-order nonlinear optical effects and thermal properties have also been studied.

Coupled Thermal and Electromagnetic Analysis of the NAFASSY Magnet

The paper presents an analysis of the current distribution and electromagnetic losses in the NAFASSY magnet carried out with the THELMA code, thanks to a brand-new thermal module coupled with the pre-existing electromagnetic module. The non-linear thermal and electrical properties of both superconducting and copper strands, depending on the local temperature, current density and magnetic field, are taken into account. The model analyses a single turn of the magnet, located in the highest field zone, focusing on the current distribution in the cable, the coupling AC and DC losses during ramped waveforms. The results are then extrapolated to estimate the behaviour of the overall magnet. A description of the models is given, together with a parametric analysis of different boundary conditions and cable discretizations. The analysis shows that, in nominal working conditions, no thermal instability should take place. However, local current redistribution among the strands may occur, mainly driven by the interstrand contact pattern, the local magnetic field and the strand current density.