Thermophysical Coefficients Research Articles

Thermophysical Coefficients of Mustard Seeds Grain-Air Mixture under High-Temperature Conditions

The paper highlights that modeling the drying process of mustard seed samples using thermogravimetric moisture measurement enables the identification of the most effective design options for drying chambers. In this process, mustard seeds form a grain-air mixture with specific thermophysical properties. (Research purpose) To measure the key thermophysical coefficients of the mustard grain-air mixture at different moisture content levels and high temperatures. (Materials and methods) Mustard seed samples with moisture content ranging from 3.24 to 15.07 percent were used. The cylindrical layer method enables uniform heat distribution by positioning the heater at the center of the material layer. Experimental setups were designed to measure the thermal conductivity and thermal diffusivity coefficients of the mustard seeds, while the volumetric and specific heat capacity coefficients were calculated. (Results and discussion) Graphs illustrating the dependence of thermal conductivity, thermal diffusivity, volumetric and specific heat capacity, as well as the bulk density of the mustard seed grain-air mixture on moisture content were constructed. Approximating functions for these dependencies were identified within the studied range. (Conclusions') In the moisture content ranged from 3.24 to 15.07 percent, the thermal conductivity coefficient of the mustard seed grain-air mixture increases from 0.156 to 0.176 kW/(m·K); the thermal diffusivity coefficient rises from 6.29·10-8 to 7.70·10-8 m2/s; while the volumetric heat capacity of the mixture decreases from 2490.8 to 2286.9 kJ/(m3·K). It was found that the bulk density initially increases within the same moisture content range, reaching a maximum at around 7.5 percent, and then decreases. Conversely, the specific heat capacity decreases to a minimum point at approximately 9 percent moisture content, and then begins to rise.

Analysis of the Main Factors Controlling the Formation of Subaerial Taliks, Using a One-Dimensional Mathematical Model. A Case Study for the Shestakovka River Basin (Central Yakutia)

Received March 9, 2023; revised September 7, 2023; accepted October 2, 2023This study presents a mathematical model of heat transfer in a subaerial talik. The model is based on the concepts presented in classical works on permafrost, as well as on the results of geological and geophysical research carried out in the Shestakovka River basin (Central Yakutia). This model is based on the solution of the classical Stefan problem on the moving of the phase transition boundaries for a multilayer and multiphase medium. The solution was calculated on a unstructured mesh. When the phase boundaries move, thawed or frozen layers of soil are formed or wedged out. The layers include: snow cover, seasonally thawed soil, seasonally frozen and frozen sand deposits, as well as soil-vegetative layer. Published empirical relationships were used to calculate thermophysical coefficients, which are presented in this article. Simple variants of the model were considered to clarify the contribution of various factors to the process of formation and evolution of taliks. It has been established that the presence of snow cover and soil-vegetative layer have the most significant effect on the formation of taliks. Calculations show that taliks are formed in the first years of the modeled period, in the presence of snow and the absence of soil-vegetative layer. The soil-vegetative layer, depending on its composition and moisture content (ice content), can prevent the formation and development of taliks. The authors do not consider cases where shrubs contribute to snow accumulation. The humidity and iciness of the layer of sand sediments located in Central Yakutia have practically no effect on this process.

Heat and mass transfer in the processes of drying thin natural leathers pasted on smooth surfaces

The results of the study of drying thin natural leather pasted on a smooth surface are presented. The results of solving the differential equation of non-stationary heat conduction with constant thermophysical coefficients are used to determine the average temperature during the period of decreasing drying rate. The calculated values of temperatures for moisture-proof and moisture-free skin surfaces are given. Methods for simplifying solutions of nonlinear equations with the aim of linearizing these equations for approximating solutions with variable transport coefficients are considered. The use of the method of piecewise stepwise approximation of the transfer coefficients with constant values of these coefficients over short time intervals showed that for low-intensity drying processes, the solutions of the linear heat equation are implemented quite accurately, confirming the regularities obtained empirically. The calculation of the duration of drying according to the method of B. S. Sazhin is given. The results of the processing of experimental curves for drying leathers pasted are presented. The reliability of the obtained equations is verified and the experiment is compared with the calculated values using the formulas. The obtained approximate analytical solutions with reliable values of the transfer coefficients, confirmed by experimentally established regularities, are of practical importance. Together with experimental methods, analytical methods make it possible to establish optimal drying regimes and more accurately generalize experimental data.

The Effect of Laser Shock Peening on the Thermophysical Parameters of Metals

Using Ti64 titanium alloy as an example, the article discusses the change in the thermophysical parameters of metals under the influence of a hardening method such as laser shock peening the essence of which is the formation of residual compressive stresses in the material under the influence of high-intensity laser radiation. The experiments are carried out with plates made of Ti64 titanium alloy, which is one of the most common construction materials in modern industry. One of the plates is a control specimen, and the surface of the second one is subjected to laser processing. The thermophysical parameters of specimens are determined using the infrared thermography method, the advantage of which is the ability to simultaneously measure two coefficients, thermal diffusivity and thermal conductivity of an explored material. The specimen is heated by the laser for some time, which can be perceived as a point heat source on the surface of the plate. Simultaneously with the laser action, the surface temperature of the specimen is recorded by an infrared camera. The thermophysical coefficients are determined as optimization parameters when matching experimental data with the analytical solution of the heat equation for a geometry similar to the experimental setup.

Modeling of Thermal Processes in a Technological System during Electromechanical Processing

The article presents mathematical models of thermal processes of combined methods of electromechanical processing, which are based on the heat conduction equations, which take into account thermophysical properties (thermal diffusivity, heat capacity, thermal conductivity and heat transfer), initial and boundary conditions, technological and other features of the studied processing methods. In view of the fact that electromechanical processing (EMT) is characterized by a process with a volumetric heat release zone, with sufficiently small dimensions and high intensity, it is assumed that the determination of temperature fields can be carried out using the superposition principle. In this case, the main factors affecting the amount of heat propagating into the contacting bodies are the intensity of heat removal, the thermophysical properties of the contacting bodies, and the speed of their relative movement. Dependences are proposed that allow one to describe the temperature fields in the volumes of parts subjected to thermal deformation during electromechanical processing, which have significant geometric parameters that ensure sufficient heat removal from the treated surfaces into the part. It has been established that in the case of processing parts of small dimensions, with insignificant heat removal, it becomes necessary to take into account the accumulation of heat, which will lead to a general increase in temperature in the technological system during processing, and a decrease in the efficiency of the hardening process. The derived mathematical dependencies in the form of heat conduction equations with the established restrictions make it possible to determine the parameters of thermal fields during electromechanical processing in the system “working tool” - “local microvolume of the surface layer”, while these presented mathematical relationships are presented in a linear formulation with thermophysical coefficients. It has been theoretically established and experimentally confirmed that in the process of processing the main factors affecting the amount of heat propagating into the contacting bodies are the intensity of heat removal, the thermophysical properties of the contacting bodies, and the speed of their relative movement. The performed experimental studies have confirmed the adequacy of the mathematical dependencies used to calculate the temperatures in the local microvolumes of the treated surface layer.

Математическое моделирование тепловых процессов в системе «Бетон-грунт»

The calculation of concrete curing modes and the development of mathematical models are greatly influenced by the temperature field in the concrete of channel linings. The problem is reduced to solving a nonlinear heat equation that takes into account the exothermic heat release of concrete and the phase transformation of moisture, and time-varying boundary conditions that allow taking into account the impact on concrete of the external environment during laying and maintenance. Variable thermophysical coefficients make it possible to take into account the inhomogeneity of the medium (in the case of laying concrete on the ground) and the change in the aggregate state of the substance when the phase transformation temperature is reached. Since it is impossible to obtain an analytical solution in a general form, a numerical solution method is used, based on a combination of a finite-difference solution with a method for calculating heat release and concrete strength from the corresponding fields of isothermal curves obtained experimentally. When constructing a difference scheme, an integro-interpolation method (balance method) is used, based on the law of conservation of heat. For an extended body of sufficiently large dimensions, the process of heat transfer in it is assumed to be linear, and the coordinate system with the center is taken to be on the axis of the body. The presented mathematical model of thermal processes in the “concrete-soil” system makes it possible to predict the modes of holding monolithic concrete to achieve the necessary technological requirements, as well as to apply the most economical modes.

Thermophysical Properties of Chernozem Soils Growing Ornamental Plants in Summer

Currently, there is no data on the transfer of heat and moisture of arboretum soils that grow such ornamental plants as Meyer lilac (Syringa meyeri), rowan “Alaya” (Sorbus aucuparia), and dwarf thuja “Danica” (Thuja occidentals). We experimentally determined that the thermophysical qualities of chernozems in lilac plantations changed with the water regime in the growing season. The sample maximum values of volumetric heat capacity and thermal conductivity were observed in the dense illuvial horizon. The sample minimums were registered in the AB transition horizon. Thermal diffusivity was the highest at the lowest humidity. In rowan plantations, the moisture content in transitional and illuvial horizons AB and B changed insignificantly. Such moisture distribution corresponds to the slope aspect and the features of rowan. The thermophysical coefficients also slightly fluctuated in rowan plantations, especially in the underlying horizons AB and B. The same horizons displayed the sample maximum of thermal diffusivity. It corresponded to the moisture content of capillarity disruption (MCCD from now onward). MCCD is a value lying between the minimal water potential of soil and the Briggs-Shantz wilting coefficient. It denotes the water content, at which moisture mobility and conductivity sharply decrease. In thuja plantations, the indexes of volumetric heat capacity and thermal conductivity in the upper soil horizon were minimal. In the illuvial horizon, these indexes were 42% and 12% higher, respectively. However, the uniform distribution of moisture in the soils that grow thuja has equalized thermal diffusivity in the AB and B horizons. Overall, the soil profile of rowan plantations contained less moisture than that of lilac and thuja plantations. Therefore, the volumetric heat capacity and thermal conductivity were lower, since they corresponded to the soil moisture content.

Enhancement of thermophysical coefficients in nanofluids: A simulation study

Molten salts constitute one kind of PCMs (Phase Change Materials) widely used in concentrating solar power facilities for heat storage and heat transfer. This paper aims to simulate nanofluid PCMs with molecular dynamics method. Concretely, the thermophysical properties of a nanofluid of KNO3 doped with SiO2 nanoparticle are investigated by equilibrium and nonequilibrium molecular dynamics simulations. For the first time, these properties of a nanofluid in the family of PCMs are calculated. The density, thermal expansion coefficient, specific heat capacity, thermal conductivity, and viscosity are characterized as functions of the SiO2 nanoparticle concentration. The effect of the SiO2 nanoparticle size on the nanofluid’s properties is also investigated. The simulation results present an enhancement of the thermophysical properties, especially for the specific heat capacity, in good agreement with the existing experimental results on a representative nanofluid PCM, and open prospects for the understanding of microscopic mechanism leading to such enhancements.

Modeling of thermal conductivity of reed products

The present work researches processes of heat transfer by samples of mats manufactured from reed. Due to the unique properties of the reed, such as medium density, low thermal conductivity, relatively high weather resistance, high chemical resistance, possibility to produce parts on-site, cost-effectiveness and others, reed products are widely used in building construction despite the high rate of development of new technologies. The mechanisms of heat insulation in the result of energy transfer through the material are established, which makes it possible to influence this process. It is proven, that the mechanisms are conditioned upon the increased porosity of the material. Thus, decreasing volume weight results in decreasing thermal conductivity, and vice versa. The simulation of the heat transfer process with the flameproof coating is carried out, the dependencies of the thermophysical coefficients on the temperature are determined. Based on the obtained dependencies, the coefficient of thermal conductivity for the products made of dry pine wood is calculated and makes 0.056 W / (m · K). Features of slowing down the process of heat transfer to material made of wood wool and glue binding agent with the formation of pores were studied. This is explained by the fact that there is no movement of air in large pores, accompanied by heat transfer. Thus, there are grounds to argue for the possibility of directed regulation of the processes of the formation of thermal insulation products using the reeds characterized by voids in the stems.

Mass transfer in irrigated plane-parallel channels for cocurrent laminar flow of liquid and gas

Isothermal absorption of components from dilute gaseous mixtures by falling film of liquids in irrigated plane-parallel channels for co-current laminar flows of liquid and gas were modeled taking into account the processes occurring at the gas entrance. A solution was obtained in the form of analytical expression and reduced to the solution of particular limited problems for the cases when the resistance is concentrated within the gas or the liquid. For visual representation of fluxes the plane of hydrodynamic parameters was suggested. The calculations can be carried out inside of a limited area of the plane. Outside of the area, some simplifications can be adopted: (a) mass transfer resistance is concentrated in a gas or in a liquid and (b) the concentration changes in transversal directions can be neglected or there is a boundary diffusion layer in the gas or the liquid. It was shown that in calculating of the process of gas cooling by the transversal temperature gradients in the liquid can be neglected. The restrictions on temperature difference at the entrance were found, according to them the corresponding thermophysical coefficients can be assumed to be constants and the heat exchange can be calculated using the relationships derived for isothermal absorption. The method can be applied to a wide class of problems which can be reduced to solving of convection diffusion equations with more difficult interphase boundary conditions. Compared to existing numerical and analytical methods, our approach advantage is in its simplicity, visual representation, and shorter computation time.

Analysis of empirical correlations of thermophysical properties of water suspensions of aluminum oxide nanoparticles

Here, we consider the empirical relationships presented earlier in the literature that describe the thermophysical properties of H2O + Al2O3 nanofluids, such as viscosity and heat conductivity coefficients. The main parameters affecting these properties of nanofluid are considered to be the volume fraction of particles ϕ, fluid temperature T, and particle size dp. The suitability of approximation formulas for calculating the viscosity and heat conductivity coefficients is determined by comparing the data calculated by these formulas with the experimental results. The behavior of the analytical curves of thermophysical coefficients is analyzed in the following ranges of influencing parameters: 0 < ϕ ≤ 0.1, 280 K ≤ T ≤ 360 K, 1 nm ≤ dp ≤ 100 nm. Estimates of the degree of dependence of calculation results on the values of these parameters are given. Conclusions on the qualitative and quantitative reliability of the correlation formulas proposed in the literature, as well as on the limits of their applicability in the ranges of variation of the influencing parameters are drawn.

Расчет температур в алюминиевой пластине по диаметрам расплавленных точек

The concept of equilibrium effective power of welding heat sources is substantiated. This is the difference between the total effective power and loss of ablation for heat transfer to the environment and to the equipment. The use of equilibrium power allows determining the parameters of the heat source and thermophysical coefficients in a mathematical model by the size of the weld pool. A computer program has been developed for calculating the size of the weld pool and thermal cycles during the action of a normal circular heat source on the plate surface. An example is given of determining the parameters of a normal-circular source using the diameters of the molten points on a plate made of an aluminum alloy by a compressed arc with opposite current pulses. Thermal cycles are calculated for a point on the axis of the heat source at different times of its action. Estimates of the time of crystallization of the molten metal after switching off the arc were made. The calculated radial temperature distribution was obtained. With this model, the temperatures in the center of the weld pool are close to two aluminum melting points. The developed method of calculating temperatures can be used to determine the optimal modes of spot arc welding, precluding the formation of crystallization cracks

Temperature modes of the contactor contacts during the test for the limiting breaking capacity

The processes of electric arc quenching of an electromagnetic contactor during testing for ultimate breaking capacity are considered. The conditions for facilitating the successful arc quenching when turning off the limiting currents are shown: by reducing the phase shift between current and voltage, by reducing the amplitude of the restriking and recovery voltages. The processes of anode heating during arcing (heat saturation mode), after the change of polarity and transition of the current through zero, processes on the cathode in the temperature equalization mode are considered. The mathematical models of cathode thermal processes adressed the heat fluxes of the ionic component and evaporation. The mathematical models of anode thermal processes in the temperature equalization mode took into account the heat fluxes of the ionic component, thermionic emission, and evaporation. The calculations were carried out for the averaged values of thermophysical coefficients for copper, since the arc base moves from the contacts to the contact holders, which are made of copper or its alloys. The calculation results showed that the used mathematical models of thermal processes are appropriate both for the cathode and for the anode. This was confirmed by the results of previously performed and published experimental and theoretical studies of thermal processes at the switching contacts of electrical devices.

The phase-change heat conduction analysis during solidification processes by a hybrid generalized FDM

In this paper, the single-domain enthalpy model is adopted for heat transfer analysis of phase change during solidification processes. The resulting second-order parabolic partial differential equations (PDEs) with varying thermophysical coefficients is numerically solved by a hybrid generalized finite difference method (GFDM) under mixed boundary conditions. The spatial derivatives in the PDEs are approximated by the Taylor series expansions combining with the moving-least squares technique. The temporal derivative is evaluated with a six-point symmetric difference by the classical Crank-Nicholson technique. The Newton-Raphson iteration method is used to solve the resulting nonlinear algebraic equations. Finally, the transient temperature field and the moving phase-change interface are obtained by analysing the nodal temperature distribution. Several examples are presented for verify the stability and effectiveness of this meshless method.

Determination of thermal and physical characteristics of dead pine wood thermal insulation products

The studies allowed manufacturing dead pine wood thermal insulation materials for the arrangement of premises. Raw materials for their production are wood fibers formed as flat boards. The mechanisms of the thermal insulation process during energy transfer through the material, which makes it possible to influence this process are determined. It is proved that the processes of thermal insulation consist in reducing the porosity of material. So, with a decrease in material density, thermal conductivity decreases, and vice versa. Modeling of the heat transfer process in the swelling of the fireproof coating is carried out, temperature dependences of thermophysical coefficients are determined. Based on the obtained dependences, thermal conductivity for dead pine wood products, reaching 0.132 W/(m·K) is estimated. In case of adhesive bonding of wood products, it decreases to 0.121 W/(m∙K) and when creating wood wool thermal insulation boards it decreases to 0.079 W/(m∙K), respectively. Features of inhibition of the process of heat transfer to the adhesive bonded wood wool material are associated with the formation of pores. This is because in small pores there is no air movement, accompanied by heat transfer. Thermal conductivity of homogeneous material depends on density. So, with a decrease in the material density to 183 kg/m3, thermal conductivity decreases 1.67 times, and vice versa, when using the board, thermal conductivity decreases only 1.1 times. This allows confirming the compliance of the discovered real mechanism of thermal insulation with the revealed conditions for the formation of properties of the inorganic and organic-mineral bonded wood wool material, as well as practical attractiveness of low-quality wood. The latter, in particular, relate to determining the amount of the binder component. Thus, there is reason to argue about the possibility of directed regulation of the processes of formation of wood thermal insulation materials using wood wool and inorganic and organic-mineral binder, which can form a fire-retardant film on the material surface

The effect of bioresorbable additives and micro-bioobjects on gel formation, stabilization and thermophysical properties

The same properties of agarose gels containing neutral bioresorbable additives and living microorganisms which are important for use in additive technologies of bioreactors creation were considered. Data on the kinetics of gel formation from the solution during cooling were obtained by spectroscopic measurement by measuring the shift of the maximum spectrum of light passing through the gel, depending on the temperature. The dynamics of aging was investigated for gels of different concentrations of agarose, bioresorbable additives and living cells. The time dependences of the decrease in the optical transparency of such gels during the aging process, characterizing the changes in their structure, were obtained. Special attention was paid to the effect of liquid evaporation from gels in the process of gel formation and during long-term storage on relaxation processes leading to their spontaneous increase in density. Experiments were performed to determine the dynamics of the temperature fields simultaneously with heat flux measurements during the formation of studied gels with different concentrations. On the basis of the obtained experimental data and previously developed method, the thermophysical coefficients of agarose gels containing an admixture of starch and living yeast cells were calculated.

Evaporation and boiling crisis of droplets alcohol solution

Evaporation and boiling crisis of droplets ethanol aqueous solution were studied experimentally. The evaporation intensity depends on the nucleate boiling, solution diffusion, a change in physical characteristics with time and droplet interfacial surface area. At nucleate boiling in a droplet, most evaporation relates to a growth in the droplet surface area and only 20 % relates to the diffusion effect and a variation in the thermophysical coefficients. At boiling crisis, experimental dependence for vapor layer height on overheating was observed. At Leidenfrost temperature, the height of the vapor layer was many times higher than the surface microroughness value of the wall. There are oscillates of liquid-vapor interface, and this increases the transitional temperature range associated with a boiling crisis of droplets.

Non-stationary heat transfer in gels applied to biotehnology

Unsteady heat transfer in agarose gels of various concentrations was studied in order to make a breakthrough in the technology of 3-D additive bioprinting. Data on the kinetics of the phase transformation was obtained using spectroscopy as a function of temperature during the formation of agarose hydrogel. The dynamics of aging was investigated for gels of different densities. The time dependence of the structural changes was obtained. Particular attention was paid to the changes in the structure of the gel due to the processes of evaporation of the liquid during the gel formation and during long-term storage. Experiments were performed to determine the dynamics of the temperature fields simultaneously with heat flux measurements during the formation of agarose gels from different initial concentrations. A technique based on experimental data for the computations of the thermophysical coefficients of agarose gels was developed.

Experimental Identification of the Equivalent Conductive Resistance of a Thermal Elementary Model of an Induction Machine

This paper proposes a basic thermal model to estimate the temperature in different points of an induction motor, totally enclosed with external ventilation, for different loads at steady state. This basic model consists simply of a conductive thermal resistance for each point considered in the machine. Thereafter, the intermediates thermal resistances of conduction of the model are deduced. This approach is very easy to implement, requiring no geometrical data, or thermo-physical coefficients, or complex methods of implementation of a thermal model. Indeed, by knowledge of total losses in the machine, the temperature of the carcass, and the temperature of any point inside of the latter allows to deduct the equivalent thermal resistance of conduction of the different points and so the corresponding temperature.

A method for solving a conjugate problem of heat transfer. Part 2

An analytical solution of the spatial conjugate problem of heat transfer with constant thermophysical coefficients has been obtained. Conditions for the single-valued solvability of the nonlinear boundary-value problem have been found, and the rate of convergence of iteration process has been estimated. The results of test checkings of the analytical solution from Part 1 of the present article are given, and the assessment of the accuracy of formulas when compared with the well-known numerical method and analytical solution is presented.