The modeling and simulation of the thermal analysis on the hydrogenerator stator winding insulation

This paper presents the modeling and simulation of the thermal analysis on the hydrogenerator stator winding. The insulation aging is predetermined first by the insulation temperatures that, in turn, are influenced by the environmental conditions and second by the speed increase of the high temperature chemical reaction in materials. By increasing the temperature in the electro-insulated material, many molecules enter in chemical reaction accelerating the insulation aging. The heat transfer is a natural process, caused by inner energy, between bodies with high temperature and bodies with lower temperature. This process can also take place between parts of the same body that have different temperatures. The heat is transmitted by conduction, convection, and radiation. The heat transfer and especially the thermal conduction are a domain in which the finite element method is successfully applied. The thermal conduction problems will be solved by the finite elements method. The analysis of the thermal transfer process was made using the modeling and simulation program with finite elements ANSYS, and the results of the simulations were compared with measurement values. The analyzed stator winding is supplied with high voltage of 11 kV that is used for a high power hydrogenerator. To realize the thermal analysis of the winding stator, the coil will be supplied with 11 kV. The results of the analysis on a prototype model present the thermal transfer in coil–insulation–air system when the coil is hot.     

Prediction of the radiation term in the thermal conductivity of crosslinked closed cell polyolefin foams

The thermal conductivity and the cellular structure as well as the matrix polymer morphology of a collection of chemically crosslinked low‐density closed cell polyolefin foams, manufactured by a high‐pressure nitrogen gas solution process, have been studied. With the aid of a useful theoretical model, the relative contribution of each heat‐transfer mechanism (conduction through the gas and solid phases and thermal radiation) has been evaluated. The thermal radiation can be calculated by using a theoretical model, which takes into account the dependence of this heat‐transfer mechanism with cell size, foam thickness, chemical composition, and matrix polymer morphology. A simple equation, which can be used to predict the thermal conductivity of a given material, is presented. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 993–1004, 2000     

Electrospun nanofibrous membranes embedded with aerogel for advanced thermal and transport properties

The primary purpose of cold weather clothing is to shield the wearer from the extremities of the external environment. The thermal properties of nanofibers and their potential applications have tremendous scope and application in this area. The objective of this study was to investigate the mechanisms of heat transfer through fibrous insulation where the fiber diameter was less than 1 μm. Electrospinning process was used to produce flexible polyurethane and polyvinylidene fluoride nanofibers embedded with silica aerogel. The thermal and transport behavior of the samples was evaluated, and results were statistically analyzed. Presence of aerogel particles were confirmed through microscopic examination. Thermal behavior was investigated by using thermogravimetric analysis and differential scanning calorimetry. The results showed that the polyvinylidene fluoride nanofibrous membranes embedded with aerogel obtained a good thermal stability with lower weight loss than polyurethane nanofibrous membranes. The glass transition and melting point was not affected by the aerogel content in the layers, validating that polymers are not miscible. The increase in duration of electrospinning led to higher web thickness, which resulted in considerable decrease in air permeability. Considerable improvement of thermal insulation was observed by increasing the number and the weight per unit area of both nanofibrous membranes. The results confirmed increase in thermal insulation by embedding silica aerogel in nanofibrous membranes. With reference to the results, it could be concluded that nanofibers embedded with aerogel are good for thermal insulation in cold weather conditions. Thermal insulation battings incorporating nanofibers could possibly decrease the weight and bulk of current thermal protective clothing.     

Description and modeling of polyurethane hydrolysis used as thermal insulation in oil offshore conditions

Polymers are widely used for passive thermal insulation coatings on steel pipe in offshore oil and gas production. In this industry, structures used in deep sea have to be reliable, as they are in service for more than 20 years in a very severe environment: sea water, hydrostatic pressure and temperature gradient. One of the main questions is how to test and predict the lifetime of such structures in the laboratory? This study presents one approach that has been developed to characterize and predict the degradation of polymers used as thermal insulation materials.This paper is dedicated to polyurethane (polyether based) degradation in sea water at high temperature. Ageing has been performed in natural sea water under hydrostatic pressure at temperatures ranging from 70 to 120 °C on 2 mm thick samples. Water diffusion in the material and hydrolysis have been characterized using mass evolution and tensile tests. Based on these results, a model for the urethane hydrolysis reaction is proposed.     

Heat transfer in microsphere insulation

The results of an investigation of heat transfer in a new type of insulation (microsphere insulation) are presented. The effects of the microsphere diameter, the concentration of metallized microspheres and the residual gas pressure on the thermal conductivity of the insulation were investigated. Measurements were made of the thermal conductivity at 77 to 300 K of microspheres with differing diameters (e.g. 95, 130 and 270 μm) and of samples with silver metallized microsphere concentrations of 7 and 32%. Measurements of average thermal conductivity (77–296 K) were made at residual gas pressuresk(p) in the range from 10^?3 Pa to 10⁵ Pa for pure nitrogen. The component of heat transfer by gas,k _gc (p), was estimated.     

Numerical modelling of sample–furnace thermal lag in dynamic mechanical analyser

Due to dynamic nature of processes taking place during the experiment (chemical reaction and physical processes, heat flow, gas flow, etc.) the results obtained by thermal methods may considerably depend on the conditions used during the experiment. Therefore, whenever the results of thermal analysis are reported, the experimental conditions used should be stated. In this paper we have studied the heat transfer from the furnace to the sample and through the sample during dynamic mechanical analysis measurements. Numerical modelling of the heat transfer was done using an own computer program based on the heat conduction equation, solved numerically applying the finite difference methods. The calculated values of the thermal lag between the furnace and the sample were compared with the values experimentally determined on samples of a composite polymeric energetic material (double-base rocket propellant). Also, the temperature distribution within the sample as a function of the heating rate was analysed using the same numerical model. It was found out that using this model and temperature dependent heat transfer coefficient, experimentally obtained values of the thermal lag between the furnace and the sample can be satisfactory described. It was also shown that even at slow heating rates, such is, e.g. 2 °C min⁻¹, the thermal lag between the furnace and the sample can reach several degrees, while the thermal gradient within 3-mm thick rectangular sample can reach 0.4 °C.     

Influence of thermal process on microstructural and physical properties of ambient pressure dried hydrophobic silica aerogel monoliths

The experimental results of thermal process on the microstructural and physical properties of ambient pressure dried hydrophobic silica aerogel monoliths are reported and discussed. With sodium silicate as precursor, ethanol/hexamethyldisiloxane/hydrochloric acid as surface modification agent, the crack-free and high hydrophobic silica aerogel monoliths was obtained possessing the properties as low density (0.096 g/cm³), high surface area (651 m²/g), high hydrophobicity (~147°) and low thermal conductivity (0.0217 Wm/K). Silica aerogels maintained hydrophobic behavior up to 430 °C. After a thermal process changing from room temperature to 300 °C, the hydrophobicity remained unchanged (~128°), of which the porosity was 95.69% and specific density about 0.094 g/cm³. After high temperature treatment (300–500 °C), the density of final product decreased from 0.094 to 0.089 g/cm³ and porosity increased to 96.33%. With surface area of 466 m²/g, porosity of 91.21% and density about 0.113 g/cm³, silica aerogels were at a good state at 800 °C. Thermal conductivities at desired temperatures were analyzed by the transient plane heat source method. Thermal conductivity coefficients of silica aerogel monoliths changed from 0.0217 to 0.0981 Wm/K as temperature increased to 800 °C, revealed an excellent heat insulation effect during thermal process.     

Evaluation method for heat transfer coefficient of a contact interface across a microscale thin liquid polymer film

The research presented here evaluates the heat transfer coefficient of the contact interface of a thin liquid polymer film between a pair of columnar aluminum bodies with an initial temperature difference of approximately 160 K. We measured the unsteady temperature changes inside the columns. The heat transfer test was performed with three types of liquid polymers: squalane, oleic acid, and silicone oil. The heat transfer coefficient of the polymer films as a fitting parameter was obtained by ensuring the numerically computed time evolution of the columns’ temperature corresponded with the experimentally measured data. The interfacial heat transfer coefficients of the thin polymer films (mean thickness: 60 μm) for all three polymers used were 1.75 kW/m²·K, 2.75 kW/m²·K, and 4.10 kW/m²·K. The present estimating method for determining interfacial heat transfer coefficients was suitable for a material-polymer film-material contact model. The time evolution of the temperature at the contact surfaces was also effectively evaluated using the numerical simulation.     

Thermal analysis of loop heat pipe used for high-power LED

The goal of this study is to improve the thermal characteristics of high-power LED (light emitting diode) package by using a loop heat pipe. The heat-release characteristics of high-power LED package are analyzed and a novel loop heat pipe (LHP) cooling device for high-power LED is developed. The thermal capabilities, including start-up performance, temperature uniformity and thermal resistance of loop heat pipe under different heat loads and incline angles have been investigated experimentally. The obtained results indicates that the thermal resistance of the heat pipe heat sink is in the range of 0.19–3.1 K/W, the temperature uniformity in the evaporator is controlled within 1.5 °C, and the junction temperature of high-power LED could be controlled steadily under 100 °C for the heat load of 100 W.     

Synthesis and thermal behavior of 4,5-dihydroxyl-2-(dinitromethylene)-imidazolidine (DDNI)

A novel energetic material, 4,5-dihydroxyl-2-(dinitromethylene)-imidazolidine (DDNI), was synthesized by the reaction of FOX-7 and glyoxal in water at 70 °C. Thermal behavior of DDNI was studied with DSC and TG-DTG methods, and presents only an intense exothermic decomposition process. The apparent activation energy and pre-exponential factor of the decomposition reaction were 286.0 kJ mol⁻¹ and 10^31.16 s⁻¹, respectively. The critical temperature of thermal explosion of DDNI is 183.78 °C. Specific heat capacity of DDNI was studied with micro-DSC method and theoretical calculation method, and the molar heat capacity is 217.76 J mol⁻¹ K⁻¹ at 298.15 K. The adiabatic time-to-explosion was also calculated to be a certain value between 14.54 and 16.34 s. DDNI presents lower thermal stability, for its two ortho-hydroxyl groups, and its thermal decomposition process becomes quite intense.     

Surface modification of boron nitride via poly (dopamine) coating and preparation of acrylonitrile‐butadiene‐styrene copolymer/boron nitride composites with enhanced thermal conductivity

A biology‐inspired approach was utilized to functionalize hexagonal boron nitride (h‐BN), to enhance the interfacial interactions in acrylonitrile‐butadiene‐styrene copolymer/boron nitride (ABS/BN) composites. The poly (dopamine), poly (DOPA) layer, was formed on the surface of BN platelets via spontaneously oxidative self‐polymerization of DOPA in aqueous solution. The modified BN (named as mBN) coated with poly (DOPA) was mixed with ABS resin by melting. The strong interfacial interactions via π‐π stacking plus Van der Waals, both derived from by poly (DOPA), significantly promoted not only the homogeneous dispersion of h‐BN in the matrix, but also the effective interfacial stress transfer, leading to improve the impact strength of ABS/mBN even at slight mBN loadings. A high thermal conductivity of 0.501 W/(m·K) was obtained at 20 wt% mBN content, reaching 2.63 times of the value for pure ABS (0.176 W/(m·K)). Meanwhile, the ABS/mBN composites also exhibited an excellent electrical insulation property, which can be expected to be applied in the fields of thermal management and electrical enclosure.     

Exceptional thermal stability and thermodynamic properties of lithium based metal–organic framework

A three-dimensional lithium-based metal–organic framework Li₂(2,6-NDC) (2,6-NDC = 2,6-naphthalene dicarboxylate) has been synthesized solvothermally and characterized by X-ray powder diffraction, elemental analysis, FT-IR spectroscopy, thermogravimetry and mass spectrometer analysis (TG–MS). The framework has exceptional stability and is stable to 863 K. The thermal decomposition characteristic of this compound was investigated through the TG–MS from 293 to 1250 K. The molar heat capacity of the compound was measured by temperature modulated differential scanning calorimetry (TMDSC) over the temperature range from 195 to 670 K for the first time. The thermodynamic parameters such as entropy and enthalpy versus 298.15 K based on the above molar heat capacity were calculated.     

Mechanically robust conductive carbon clusters confined ethylene methyl acrylate–based flexible composites for superior shielding effectiveness

Arresting of conducting carbon black into polymeric matrix to develop flexible and light weight composite has been a widely practiced platform. Extensive development of telecommunication creates a major vexations regarding radiation pollution. Thereafter, we have been motivated to develop low‐cost, flexible composites of specialty carbon black VXC (Vulcan XC 72)–filled ethylene methyl acrylate (EMA) by mechanical mixing technique. Developed composite has significant conductivity (6.67 × 10⁻⁴ S cm⁻¹) with promising mechanical and thermal properties. Dispersion of high‐structured carbon blacks in EMA was investigated by small angle X‐ray scattering to reflect the dependency of conducting mesh formation on dispersion. Interconnected filler network development has been visualized by field emission scanning electron microscope and high‐resolution transmission electron microscope. Electromagnetic interference shielding value in the X band has calculated to be 30.8 dB. Such features can make this EVXC (EMA‐Vulcan XC 72) composite a useful alternate for flexible electromagnetic interference shielding material.     

Analysis of the first thermal response test in Algeria

Ground source heat pumps (GSHPs) mean attractive heating and cooling systems. Optimum design of a borehole heat exchanger (BHE), as the outer part of a GSHP heating system, requires knowledge of the thermal properties of the soil. Those data, the effective thermal conductivity of the soil λ_eff and the average temperature of the soil T ₀ enable us to determine the necessary number and depth of boreholes. The determination of thermal conductivity of the soil in laboratory experiments does not usually coincidence with the data under in situ conditions. Therefore, an in situ method of experimental determination of these parameters, thermal response testing (TRT) is used primarily for in situ determination of design data for BHEs. In this study, which was the first TRT in Algeria (Tlemcen site), the purpose was to determine the effective ground thermal conductivity. Measured data were evaluated by the line source model. Used method and performed evaluation are presented for a borehole drilled in clay, silt, and sand. The resulting effective ground thermal conductivity was 1.364 W/m K and the borehole thermal resistance was 0.18 K/(W/m).     

A new PvT device for high performance thermoplastics: Heat transfer analysis and crystallization kinetics identification

The quality of thermoplastic parts strongly depends on their thermal history during processing. Heat transfer modelling requires accurate knowledge of thermophysical properties and crystallization kinetics in conditions representative of the forming process. In this work, we present a new PvT apparatus and associated method to identify the crystallization kinetics under pressure. The PvT-xT mould was designed for high performance thermoplastics: high temperature (up to 400 °C), high cooling rate (up to 200 K/min) and very high pressure (up to 200 MPa). Specific volume measurements were performed at a low cooling rate to avoid a thermal gradient. The crystallization kinetics under pressure can be identified for a wide range of cooling rates by an inverse method taking into account the thermal and crystallinity gradients. Since identification is based on volume variations, the proposed methodology is non-intrusive. Furthermore, the enthalpy released by the crystallization was measured during the experiment by a heat flux sensor located in the moulding cavity.     

Development of anisotropic ferromagnetic composites for low-frequency induction heating technology in medical applications

Modern medical applications such as varicose treatment, hyperthermia, or even endovenous thermal ablation require to bring heat flux locally through the human body. The challenge behind such techniques resides in converting electrical power into heat flux and transfer it directly to the targeted area without contaminating and damaging the surrounding tissues. Low-frequency induction heating (LFIH) of catheters made out of biocompatible magnetic composites is an elegant solution. By inserting the catheter through the varicose to be treated and by exciting it through LFIH, it seems possible to reach a temperature high enough to properly heal the damaged area while preserving the surrounding healthy ones. Although recent published results seem promising, an optimized procedure is still required to achieve further improvements. Many directions lying on the active material formulation have been largely explored in the past (variations of particle content, nature, size, and shape). In this work, we propose an alternative solution, which involves the processing of ferromagnetic composites under a constant homogeneous magnetic field, leading to the strong anisotropic behavior due to particles alignment. Remarkably, experimental results demonstrate that by exciting such anisotropic composites along the alignment direction enhances the LFIH effect by more than 30%. Moreover, improvements can also be noticed in the perpendicular direction, meaning that the structured distribution is enough to increase the ferromagnetic properties. Furthermore, the resulting composite is highly flexible, making it easier to be integrated in several medical devices (e.g. endovenous thermal catheter, electromagnetic tracking system, and so on).     

导热塑料的研究与应用

随着科学的进步导热塑料应用领域不断扩大,尤其近些年来蓬勃发展的信息产业,为导热塑料提供了新的发展空间.本文对比了高分子材料、金属材料及金属氧化物导热性能,介绍了聚合物的导热机理,并对不同填充含量可适用的导热模型进行了介绍.讨论了提高塑料导热性能的途径和近年来提高导热性能新的研究方法,对非绝缘导热塑料、绝缘导热塑料的应用研究和最新进展作了综述,提出了导热塑料目前存在的问题,展望了导热塑料的应用前景.     

Heat capacities,third-law entropies and thermodynamic functions of the negative thermal expansion material Zn2GeO4 from T=(0 to 400) K

The molar heat capacity of Zn₂GeO₄, a material which exhibits negative thermal expansion below ambient temperatures, has been measured in the temperature range 0.5⩽(T/K)⩽400. At T=298.15 K, the standard molar heat capacity is (131.86 ± 0.26) J · K⁻¹ · mol⁻¹. Thermodynamic functions have been generated from smoothed fits of the experimental results. The standard molar entropy at T=298.15 K is (145.12 ± 0.29) J · K⁻¹ · mol⁻¹. The existence of low-energy modes is supported by the excess heat capacity in Zn₂GeO₄ compared to the sums of the constituent binary oxides.     

Combination of RSM and NSGA-II algorithm for optimization and prediction of thermal conductivity and viscosity of bioglycol/water mixture containing SiO2 nanoparticles

The fluids containing nanoparticles have enhanced thermo-physical characteristics in comparison with conventional fluids without nanoparticles. Thermal conductivity and viscosity are thermo-physical properties that strongly determine heat transfer and momentum. In this study, the response surface method was firstly used to derive an equation for the thermal conductivity and another one for the viscosity of bioglycol/water mixture (20:80) containing silicon dioxide nanoparticles as a function of temperature as well as the volume fraction of silicon dioxide. Then, NSGA-II algorithm was used for the optimization and maximizing thermal conductivity and minimizing the nanofluid viscosity. Different fronts were implemented and 20th iteration number was selected as Pareto front. The highest thermal conductivity (0.576 W/m.K) and the lowest viscosity (0.61 mPa.s) were obtained at temperature on volume concentration of (80 °C and 2%) and (80 °C without nanoparticle) respectively. It was concluded that the optimum thermal conductivity and viscosity of nanofluid could be obtained at maximum temperature (80 °C) or a temperature close to this temperature. An increase in the volume fraction of silicon dioxide led to the enhancement of thermal conductivity but the solution viscosity was also increased. Therefore, the optimum point should be selected based on the system requirement.     

Determination of thermal parameters of one-dimensional nanostructures through a thermal transient method

We present an improved methodology for a thermal transient method enabling simultaneous measurement of thermal conductivity and specific heat of nanoscale structures with one-dimensional heat flow. The temporal response of a sample to finite duration heat pulse inputs for both short (1 ns) and long (5 μs) pulses is analyzed and exploited to deduce the thermal properties. Excellent agreement has been obtained between the recovered physical parameters and computational simulations through choosing an optimized pulse width.