Problem Of Thermal Conductivity Research Articles

THERMOPHYSICAL CHARACTERISTICS OF THE HVAF COATING FROM QUASICRYSTALLINE ALLOY OF THE Al-Cu-Fe SYSTEM

The article deals with the results of determining the thermal conductivity coefficient from quasicrystalline coatings of the Al-Cu-Fe system in the temperature range up to 900 °C. The Al-Cu-Fe quasicrystalline alloy acquires a great interest for practical use as a material for protective coatings. The Al-Cu-Fe alloy is used to produce thermal barrier coatings in internal combustion engines, non-stick coatings on chemical synthesis equipment and in the food industry and to prevent the icing in aviation. The Al-Cu-Fe quasicrystals have low density, high hardness, high elasticity modulus, high values of corrosion resistance and wear resistance, low coefficient of friction, lowered adhesion, low thermal conductivity in combination with the coefficient of thermal expansion, which is close by its value to some metals. The water atomized Al63Cu25Fe12 powder with a dispersion of +40/-63 μm that has a content of the quasicrystalline phase of about 60 wt. % was used for spraying. The coating was sprayed to the butt of a cylindrical substrate from steel 45 (diameter – 25 mm, height – 10 mm), which before spraying was subjected to jet-abrasive treatment by corundum powder with a determining particle size of 1 mm at air pressure of spraying of 0.5 MPa. The Al-Cu-Fe coating with a thickness of more than 0.8 mm was made by high-speed air-fuel (HVAF) spraying using a burner GVO-RV12 with the following spraying mode: the pressure in the combustion chamber of the burner is 1.0 MPa; the oxidant excess coefficient a ≈ 1.2, the spraying distance is 270 mm. The samples were installed on the side surface of the drum (diameter 120 mm), which rotates at a speed of 2.0 rev/s (the speed of movement of the sputtering spot is 0.8 m/s). Spraying was done in three steps of 10 seconds each with a 30 second cooling time between them. Determination of the temperature dependence of the thermal conductivity of the coating was carried out by solving the inverse problem of thermal conductivity by one-dimensional temperature fields in samples obtained by single-sided jet heating with an industrial hot air torch (at surface temperatures up to 450 °C) and an oxygen-propane welding torch (at temperatures above 450 °C). It is shown that the values of the thermal conductivity coefficient of Al-Cu-Fe quasicrystalline coatings in the range of 20 °С…900 °С vary within λ = 1.9 – 2.31 W/(m×K).

Choice of Regularization Methods in Experiment Processing: Solving Inverse Problems of Thermal Conductivity

This work is aimed at numerical studies of inverse problems of experiment processing (identification of unknown parameters of mathematical models from experimental data) based on the balanced identification technology. Such problems are inverse in their nature and often turn out to be ill-posed. To solve them, various regularization methods are used, which differ in regularizing additions and methods for choosing the values of the regularization parameters. Balanced identification technology uses the cross-validation root-mean-square error to select the values of the regularization parameters. Its minimization leads to an optimally balanced solution, and the obtained value is used as a quantitative criterion for the correspondence of the model and the regularization method to the data. The approach is illustrated by the problem of identifying the heat-conduction coefficient on temperature. A mixed one-dimensional nonlinear heat conduction problem was chosen as a model. The one-dimensional problem was chosen based on the convenience of the graphical presentation of the results. The experimental data are synthetic data obtained on the basis of a known exact solution with added random errors. In total, nine problems (some original) were considered, differing in data sets and criteria for choosing solutions. This is the first time such a comprehensive study with error analysis has been carried out. Various estimates of the modeling errors are given and show a good agreement with the characteristics of the synthetic data errors. The effectiveness of the technology is confirmed by comparing numerical solutions with exact ones.

About Calculation and Forecast of Temperature in the Layer Cell of Self-Heating of Raw Materials in a Silo

The article clarifies an approximate variant for calculating the temperature in an inhomogeneous layer cell without a clear boundary under the assumption of significant distance in its center between the upper and lower ends of the silo when heat exchange conditions have little effect on the development of temperature in the layer cell due to poor thermal conductivity. The normal Gaussian law concerning distribution of thermal sources in the cell on an axis of a silo is accepted. The integral cosine of the Fourier transform is used to construct the analytical solution of the nonstationary thermal conductivity problem. A compact formula for calculating the increase in excess temperature in the center of the self-heating cell over time is derived and used to identify the parameters of the cell. The change in temperature at other points of the raw material is expressed through incomplete gamma function that is reduced to the probability integral. Calculations show that for the selected distribution of thermal sources, the temperature increase slows down rapidly with separation from the center of the cell. The possibility of determining the pattern distribution of the localized field of excess ambient temperature over time is proved. Examples of density identification of thermal sources are given. After identification, the calculation formulas become consistent with the experiment and suitable for the theoretical prediction of temperature rise in the raw material. The approbation of the proposed mathematical expressions to identify the parameters of the self-heating process of raw materials showed high accuracy relative to experimental data with a deviation of 0.01–0.015%. It is possible not only to determine the parameters of the self-heating cell but also to predict the time of reaching a flammable temperature in it.

Microsphere Structure Composite Phase Change Material with Anti‐Leakage, Self‐Sensing, and Photothermal Conversion Properties for Thermal Energy Harvesting and Multi‐Functional Sensor

AbstractThe low thermal conductivity and liquid melt leakage of phase change materials are long‐standing bottlenecks for efficient and safe thermal energy harvesting. Although high thermal conductivity materials combined with phase change materials can address the thermal conductivity problem, ensuring no leakage and no reduction in latent heat in the meantime remains challenging. Here, a strategy to synthesize microsphere‐structured phase change composites by encapsulating phase change materials in graphene via emulsion polymerization (no additional emulsifier) and chemical reduction is proposed. Multiple graphene sheets are connected to construct an efficient thermally conductive (increase 58.5 times in thermal conductivity) and electrically conductive network. The composite microspheres exhibit no leakage (<0.5%) and superior phase transition behavior after 1500 heating‐cooling cycles, and sense external environments such as temperature changes and water drops falling, allowing them to be engineered into devices for temperature monitoring. In addition, it converts electrical energy into thermal energy to achieve rapid temperature increases. The incorporation of polydopamine improves the photothermal efficiency of the phase change microspheres and senses light irradiation, offering a promising route to extend the single source of thermal energy. This method provides new insight into the photothermal integration and intelligent sensing of phase‐change materials.

Novel magnetocaloric composites with outstanding thermal conductivity and mechanical properties boosted by continuous Cu network

Magnetic refrigeration based on the magnetocaloric effect is expected to trigger technological revolution in the refrigeration industry due to the merits of high energy efficiency and complete elimination of greenhouse gas emissions. However, the implementation of this emerging technology is hindered by some challenges in the magnetocaloric materials, e.g., low thermal conductivity (λ) and poor mechanical properties. This paper reports a novel magnetocaloric composite that is characterized by continuous Cu networks within the (Mn,Fe)2(P,Si) magnetocaloric matrix. This unique microstructure is designed with the aid of finite-element simulations and experimentally realized by hot pressing the (Mn,Fe)2(P,Si)/Cu core/shell powders. High-resolution transmission electron microscope revealed good interfacial coherence between the matrix and the binder, which is highly desirable to reduce the interfacial thermal resistance. The magnetocaloric composites with such a novel microstructure exhibit a high λ of 20.4 Wm−1K−1 and a large maximum compressive strength of 570 MPa, which are the best comprehensive properties for room-temperature magnetocaloric materials ever reported. Consequently, this work offers a promising way to tackle the bottleneck problems of low thermal conductivity and the brittleness of the magnetocaloric materials, which may accelerate the practical application of magnetic refrigeration.

Experimental study on preparation and thermal storage properties of expanded graphite/paraffin wax as a shape-stabilized phase change material

As an organic phase change material (PCM), paraffin wax (PW) has the problem of low thermal conductivity. It has been applied in the heat storage engineering. To improve the thermal performance of paraffin wax adopted in heat storage, it was mixed with expanded graphite (EG) via melt-blending method. The EG/PW was compressed into a shape-stabilized phase change material (SSPCM). The DSC curve was measured and the relationship between pressure and density of the SSPCM was tested. The SSPCM was vacuum encapsulated in aluminum bags and placed in a packed bed heat storage system to measure the thermal storage properties. The results showed that the thermal conductivity of the SSPCM was 7.06 W/(m °C), and the maximum density of the SSPCM was 1.07 g/cm 3. The average thermal efficiency of the heat storage system was 95.3%. The number of heat storage units was positively correlated with the heat release power. This study explored the feasibility of a simplified heat storage system.

Synthesis and comparative analysis of biochar based form-stable phase change materials for thermal management of buildings

Scientific community has paid attention to organic phase change materials (PCM) as a solution for the passive thermal management of buildings because they are chemically stable, phase transition range falls within thermal comfort range, non-toxic, non-corrosive, and readily available. However, low thermal conductivity and leakage problem during phase transitions are the primary limitations associated with organic PCM that require attention in order to use in many applications. In the present investigation, suitable materials (porous biochars) for providing shape stability to organic PCM have been systematically explored. Simple impregnation method has been used for the preparation of form-stable PCM green composites. The biochar is derived from three abundant biomass, namely, sugarcane bagasse (SCB), water hyacinth (WH), and yellow oleander (YO). Binary and ternary blends of three biochars derived from the mentioned biomasses have also been studied for their candidature as supporting matrix. The morphological and structural studies confirmed that the biochar is suitable for the encapsulation of melted PCM. The composites are compared based on the heat of fusion and thermal conductivity. The highest heat of fusion is found to be 99 J/g for the PCM with a binary mixture of SCB and WH. It has been observed that thermal conductivity is highly dependent on the properties of the biochar matrix. Furthermore, thermal conductivity is found to increase by 23 % in the case of ternary mixture of biomass with PCM and decreased by 15.8 % for the case of SCB with PCM mixture than that of the pure PCM, respectively.

Двойственная вариационная модель стационарного процесса теплопроводности, учитывающая пространственную нелокальность

Microcontinuum theories boast a great potential for simulating structurally sensitive materials. There exists a sufficiently large number of works delineating the basics of non-local mechanics using the theory of elasticity as an example. Estimating the investigative capacity of non-local mechanics is at present particularly relevant to simulating nanodevices, nanoelectromechanical systems (NEMS), and media featuring complex internal micro- and nanostructures. Typically, analysing these simulations involves overcoming certain difficulties caused by the necessity to solve integro-differential equations numerically. Variational methods may be successfully applied to analysing mathematical models of continuous media as an additional tool. The paper describes plotting an alternative functional for the problem of steady-state thermal conductivity in a homogeneous body, taking into account non-locality effects and featuring a temperature-independent thermal conductivity coefficient. We show that the stationary conditions for this functional do not differ from those in the absence of non-locality. The alternative functional combined with the fundamental functional presented previously constitute a dual variational model. We quantitatively analyse the problem of an infinite planar plate featuring constantly active internal heat sources. The dual variational formulation of the problem allows us not only to obtain an approximate solution to the problem under consideration, but also to estimate its error, as well as to reduce this error by selecting other approximating functions if necessary

Enhancement of solar thermal storage properties of phase change composites supported by modified copper foam

Metallic foams, especially copper foams (CF), have been investigated to solve the problems of leaking and low thermal conductivity of phase change materials (PCMs), which helps to promote the application in solar thermal energy storage and thermal management. In this paper, the surface and pore structure of commercial CF was modified by in-situ formed copper nanowires and the introduction of flake graphite (FG). The copper nanowires and embedded FG not only provide greater capillary absorption force to prevent the leakage of liquid paraffin but also enhance the thermally conductive and photothermal conversion efficiency of the phase change composites. The thermal conductivity of the modified CF supported composite PCM reaches 4.1 W m−1 k−1, which is 1.6 times of the un-modified CF supported composite PCM and 20.5 times of paraffin. These results indicate the great application prospects in the fields of solar thermal energy storage and thermal management.

DEVELOPMENT OF MEASURES TO INCREASE RELIABILITY TURBOCHARGER BEARING UNIT AUTOTRACTOR DIESEL ENGINE

To increase the reliability of small turbochargers, in particular the bearing unit, it is proposed to use in the automatic mode of local cooling of the bearing with compressed air. The design of the turbocharger with the central case which houses the bearing and to which engine oil from the engine lubrication system is brought under excess pressure is considered. This design is the most common among turbochargers of tractor engines. Forced engine modes can be critical for the bearing, accompanied by fluctuations in the exhaust gas temperature, for example, due to an uncontrolled increase in cyclic supply, a sharp increase in load. Such modes lead to an increase in temperature deformations of the turbine wheel, rotor, reduce the reliability of the turbocharger. Heat dissipation from the rotor through the bearing assembly into the lubrication system is insufficient, additional short-term local cooling is required. The study simulated heat transfer processes in the bearing assembly of a small turbocharger using the developed mathematical model based on the finite element method. To clarify the model, namely the boundary conditions of the thermal conductivity problem, a series of non-motorized experiments with a locally cooled bearing were performed. In the course of non-motorized experiments, the algorithm of the automatic control system operation was worked out, some of its structural elements were selected and tested in practice. Conducted non-motorized experiments and the results of mathematical modeling confirmed the effectiveness of using the system of automatic local cooling of the bearing assembly. These measures increase the reliability of small turbochargers.

Calculation model of thermoelectric system for cooling discrete semiconductor devices

Objective. The aim of the study is to develop a computational model of a thermoelectric system (TPS) for cooling discrete semiconductor devices (DSD), implemented in the Elcut application software package, and to conduct a corresponding numerical experiment on it.Method. A new type of DPP cooling system has been developed, which uses several TEM sections that provide heat removal from both the lower and upper surfaces of the fuel elements, which increases the heat exchange surface between the cold source and the REE element and increases the heat removal efficiency. Using the finite element method, a computational model of this system is constructed based on the solution of the problem of thermal conductivity for a multi-element structure of a complex configuration, each component of which is characterized by thermal and electrical parameters.Result. A numerical experiment was carried out using the calculation model. As a result of it, the results of a numerical experiment were obtained in the form of a three-dimensional picture of the temperature field of the DPP - TPP system when it reaches the stationary mode, as well as graphs of temperature changes over time in the center of discrete semiconductor devices at various values of the cooling capacity of the thermoelectric module (TEM).Conclusion. As a result of the numerical experiment, it was found that the proposed design of the cooling TPP fully allows solving the problem of ensuring the required temperature regime of the DPP in the range of its power up to 75 W. At the same time, the use of sections of thermoelectric modules from thermoelements identical in their geometric, electrical and thermal characteristics makes it possible to increase the manufacturability of the device, as well as to ensure its operation in optimal current modes.

Finite fractal dimension of pullback attractors for semilinear heat equation with delay in some domain with moving boundary

In this article we study a semilinear mathematical model with delay term for thermal conduction problems defined on a one-dimensional moving boundary domain. The goals of this work are to prove existence, regularity, and finite fractal dimension of the pullback attractors on tempered universes that depends on a non-increasing function η:R→(0,+∞).

Solution to the problem of thermal conductivity in a two-dimensional formulation on a graphics processing unit using parallel computing

Subject: technology and algorithms of parallel programming.
 Objective: to compare the execution speed of a sequential algorithm on a central processor with a parallel algorithm on a graphics processor when solving a two-dimensional thermal conductivity problem.
 Methods: the Crank Nicholson method modified by the author for solving the problem of two-dimensional thermal conductivity on a graphics processor.
 Research results: 1. The solution of the two-dimensional thermal conductivity problem according to the Crank Nicholson scheme is not absolutely parallel and the maximum possible acceleration is not achieved 2. With a matrix dimension of 16 by 16 and single precision, the execution time on the GPU turned out to be 1.32 1.72 times faster than on the CPU. With a 32 by 32 matrix dimension, the execution time on the GPU turned out to be 3.66 6.07 times faster than on the CPU. 3. When calculating with double precision, the greatest acceleration is observed at 71.89 with 10 iterations of calculation, if there are more than 104 iterations, then the acceleration in calculations on a GPU with double precision approaches calculations with single precision.

Performance Characterization of Form-Stable Carbon-based Network Microcapsules for Thermal Energy Storage

In order to solve the problems of easy leakage and low thermal conductivity of paraffin in molten state. In this study, modified phase-change microcapsules with paraffin as the core material, melamine-formaldehyde resin as the wall material, flake graphite (FG), multilayer graphene (GNP) and expanded graphite (EG) as heat conduction additives were prepared by in-situ polymerization. The results show that the thermal conductivity of the composite phase change material gradually increases with the increase of the mass fraction of carbon additives, and the heat transfer enhancement effect of EG/MPCMs composite phase change material is the most obvious, with a thermal conductivity of 1.9 W/(m⋅K), which is 9.5 times that of pure paraffin. The addition of FG/MPCMs, GNP/MPCMs and EG/MPCMs carbon-based materials have little effect on the latent heat of phase change microcapsules, and the latent heat retention rates are 83.8%, 78.8% and 86.4% respectively. Microcapsules with phase change have good leakage prevention performance, and there are two obvious temperature buffering stages in the process of melting and condensation, so which have good thermal regulation ability. Compared with the previous studies, the prepared composite phase change microcapsules have high thermal storage capacity while ensuring high thermal conductivity, and have potential applications in industry.

Computational Studies of the Thermal Stress State of Multilayer Glazing with Electric Heating

Ensuring the efficient and reliable safe operation of ground special equipment of various types, aircrafts is an important and urgent problem. Preservation of the strength, protective properties and transparency of the glazing of machinery cabins in a wide range of temperatures under various strength influences is one of the key components of this problem. Multilayer packages made of different types of glass are used for special equipment glazing. These packages are connected to each other with adhesive polymer materials. Electrically heated glazing, which allows to avoid the special equipment icing, as well as to protect the viewing area from fogging, is used for reliable and failure-free operation of special equipment at low temperatures. Based on this, an important problem that affects the efficiency of the use of special equipment is to ensure the reliable operation of electric glass heating. With the help of a software package developed on the basis of the finite element method for the analysis of the structures thermal stress in 3D formulation, which allows to consider a wide class of practical problems of varying complexity, the problems of non-stationary and stationary thermal conductivity and thermal elasticity for a trapezoidal frontal electrically heated multilayer glazing are solved. A study of the thermal stress state of glazing with an electric heating system, which allows to avoid freezing of glass operating at low temperatures, was carried out. The reasons for which the delamination of the multi-layer glazing may occur (impermissible temperature modes, mechanical strength effects, violation of operating conditions) are determined. Multilayer glazing with an electric heating system is used for aircrafts, military equipment, land transport, which can be operated at different temperatures. In view of this, the study of their thermal stress state and determination of possible causes of delamination allows to ensure the operation reliability and increase the efficiency of the use of special equipment in different climatic conditions. It is planned to carry out further studies of the glass block thermal stress state taking into ac-count the thermostat operation and determining the temperature sensors location points, as well as changes in the physical properties of materials and the power of the heating element with temperature changes.

THE PROCESS OF STATIONARY THERMAL CONDUCTIVITY IN THE ROD IN THE PRESENCE OF EXTERNAL HEAT EXCHANGE

The article provides a solution to the problem of thermal conductivity for a rod with a small cross-section, taking into account external heat transfer. In this case, the boundary conditions are set by the boundary conditions. The solution is given in a Cartesian coordinate system using hyperbolic functions.

Form-stable phase change materials enhanced photothermic conversion and thermal conductivity by Ag-expanded graphite

Phase change materials (PCMs), which exhibit significant capacity of latent heat absorption and release during the phase change process, have been promisingly used for solar energy conversion and storage. However, the application of organic PCMs for solar energy storage remains a great challenge owing to their low thermal conductivity and liquid leakage problem. In this work, a form-stable PCM with polyethylene glycol (PEG) and epoxy resin (EP) was prepared by melt blending (the mass ratio is 1:1). The experimental results indicated that the leakage problem of PEG was solved by the cross-linking of the molecular chains of PEG and EP. Meanwhile, in order to improve the solar energy conversion capability and thermal conductivity, an extremely low loading (4 wt%) of expanded graphite (EG) modified with Ag nanoparticle was added into the PEG/EP matrix. The results suggested that Ag nanoparticles were uniformly dispersed by electroless silver plating in the surface of EG, and the electroless modified EG was evenly distributed in the PEG/EP matrix. Moreover, the composite fillers exhibited prominent solar energy conversion capability (90.7 %) and higher thermal conductivity (121 %). Therefore, this novel composite has favorable potential in practical applications of solar energy storage.

Сalculation of fire resistance of fire protected reinforced concrete structures

A finite-element model was developed for thermal engineering calculation of a fire-resistant multi-cavity reinforced concrete floor in the ANSYS software complex. With the help of the developed model, a thermal engineering calculation of a fire-resistant reinforced concrete multi-hollow floor slab was carried out, the essence of which was to solve the problem of non-stationary thermal conductivity and was reduced to determining the temperature of the concrete of the reinforced concrete floor at any point of the cross section at a given time (including at the place of installation of the fittings).A comparison of the results of numerical modeling with the results of an experimental study of fire resistance was carried out. An approach is proposed that allows taking into account all types of heat exchange by specifying cavities as a solid body with an equivalent coefficient of thermal conductivity. The model makes it possible to study stationary and non-stationary heating of both unprotected and fire-protected reinforced concrete structures. At the same time, with the help of the developed model, it is possible to take into account various factors affecting fire-resistant reinforced concrete structures: fire temperature regimes, thermophysical characteristics of reinforced concrete structures, coatings for fire protection of reinforced concrete structures. The adequacy of the developed model was tested, as a result of which it was established that the calculated values of temperatures satisfactorily correlate with experimental data. The largest area of deviation in temperature measurement is observed at the 100 th minute of calculation and is about 3 ºС, which is 9 %. The workability of the developed model for evaluating the fire resistance of fire-resistant reinforced concrete structures and its adequacy to real processes that occur during heating of fire-resistant reinforced concrete structures with the application of a load under the conditions of fire exposure under the standard fire temperature regime have been proven.

EXPERIMENTAL DATA AND THE NONLINEAR INVERSE PROBLEM OF HEAT TRANSFER

In the paper the development a method for finding the nonlinear heat-conducting characteristics of the soil is being presented. Two-layer container complexes have been created, the side faces of which are thermally insulated so that the 1D heat equation can be used. In order not to solve the boundary value problem with a contact discontinuity and lose the accuracy of the method’s solution, a temperature sensor was placed at the junction of two media, and a mixed boundary value problem is solved in each area (container). To provide the initial data with an inverse coefficient problem, two temperature sensors are used: one sensor was placed at the open boundary of the container and recorded the soil temperature at this boundary, and the second sensor was placed at a short distance from the boundary, which recorded the air temperature. The measurements were carried out on the time interval  First, the initial-boundary value problem of thermal conductivity with nonlinear coefficients of thermal conductivity, heat capacity, heat transfer, and material density are studied numerically. The nonlinear initial-boundary value problem is solved by the finite difference method. Based on the measured data of the complex, special functionals are constructed and the thermal conductivity coefficient , density , specific heat capacity , heat transfer coefficient are found, which depend on the temperature of the material. Based on the experimentally measured data, the corresponding functional is minimized on each time interval using the gradient descent method. All thermophysical characteristics for a container with clay were found with a relative error of 5%.  

Application of the calculating scheme for rock freezing depth during geotechnical monitoring on the Anzob pass (Tajikistan)

Rock instability in high-altitude areas is an important risk factor for humans and infrastructure, especially in the context of climate change. The presence of water in the rock leads to a clear increase in the strength of the rock, the effect is enhanced at lower temperatures and a higher degree of saturation. Since the decrease in rock strength due to the phase transition from frozen to thawed state has a great impact on the strength and stability of the rock soil, the paper proposes a model and a numerical method for estimating the depth of rock soil freezing based on data on air temperature and the thickness of the snow cover. The paper also evaluates the effect of air temperature and snow cover on the rock soil stability and depth of freezing according to the proposed calculation algorithm which is applied for the winter periods 2010/2011-2019/2020 at the Anzob pass in Tajikistan. The calculating scheme is constructed according to the three-layer medium thermal conductivity problem (snow, frozen and thawed rock) with a phase transition on the boundary frozen and thawed rock soil. The equation of heat balance contains the heat of the phase transition, the energy of inflow from the thawed rock and the outflow into the frozen rock and, in the presence of snow cover, the outflow into the atmosphere air through it.