Non-stationary Temperature Conditions Research Articles

RESEARCH OF HEAT RESISTANCE OF CARBON-CARBON COMPOSITE MATERIALS WITH PROTECTIVE COATINGS OBTAINED USING FUNCTIONALLY ACTIVE CHARGES

This study focuses on the development of protective coatings for carbon-carbon composite materials (CCСM) used in high-temperature processes in aerospace engineering. CСCM have limitations due to their susceptibility to oxidation, erosion, and burnout in gas streams. Our research is aimed at creating protective coatings using functionally active charges (FAC) obtained under non-stationary temperature conditions and improving the performance of composites. The aim of our study is to identify the optimal compositions of powders for chromium-alloyed protective coatings using functionally active charges to increase the heat resistance of the working surface. We analyzed various methods of obtaining protective coatings, including chemical-thermal and liquid-phase saturation methods, to find out their peculiarities in interaction with the CCСM matrix and changes in their mechanical properties. In addition to the traditional methods, we have investigated the method of saturating the surface with a solid phase in an active gas environment using FAС, which are obtained under non-stationary temperature conditions. This method provides high quality coatings, reduces processing time and makes it possible to work at high temperatures depending on the composition of the spray coatings. Much attention was paid to the problem associated with chemical interaction and formation of carbide phases, which are important for ensuring the stability of coatings under high temperature conditions. Experimental studies include a factorial experiment to determine the compositions of powder mixtures that provide high heat resistance. Various independent variables, such as the content of chromium, silicon, titanium, and aluminum, were considered, taking into account their influence on the physical and mechanical properties of coatings. Particular attention was paid to the optimization of the parameters of thermal autoinitiation of the functionally active charge under process conditions. Regression equations are presented to evaluate the dependence of the wear resistance of coatings on the autoinitiation parameters and the content of alloying elements. The analysis of the study results includes the construction of three-dimensional graphical dependencies for optimizing the composition of powdered CBA in the Cr-Al-Ti and Cr-Al-Si systems. In practice, chromium-aluminum-siliconized coatings obtained under isothermal conditions, when tested for heat resistance, have a more porous surface through which oxygen penetrates to the surface of the HCCM. Compared to the coatings obtained using FAS, the heat resistance is 1.5-1.7 times higher, which can be explained by the higher concentration of chromium, aluminum, silicon, and titanium, which contribute to the formation of protective oxide membraness. The low-porosity surface of the coatings obtained using functionally active layers prevents oxygen from entering the material, contributing to the formation of oxide protective membranes such as SiO2, TiO2, Cr2O3, Al2O3.

Kinetics of Chemisorption on the Surface of Nanodispersed SnO2–PdOx and Selective Determination of CO and H2 in Air

In this work, the kinetics and mechanisms of the interaction of carbon monoxide and hydrogen with the surface of a nanosized SnO2-PdOx metal oxide material in air is studied. Non-stationary temperature regimes make it possible to better identify the individual characteristics of target gases and increase the selectivity of the analysis. Recently, chemometric methods (PCA, PLS, ANN, etc.) are often used to interpret multidimensional data obtained in non-stationary temperature regimes, but the analytical solution of kinetic equations can be no less effective. In this regard, we studied the kinetics of the interaction of carbon monoxide and hydrogen with atmospheric oxygen on the surface of SnO2-PdOx using semiconductor metal oxide sensors under conditions as close as possible to classical gas analysis. An analysis of the influence of catalytic surface temperature on the mechanisms of chemisorption processes allowed us to correctly interpret and mathematically describe the electrophysical characteristics of the sensor in the selective determination of carbon monoxide and hydrogen under nonstationary temperature conditions. The reaction mechanism is applied as well to the analysis of the operation scheme of the CO sensor TGS 2442 of Figaro Inc.

Specifics of integrated thermal imaging monitoring modern civil buildings and thermophysical properties of enclosing structures made of building materials of mass production

Objective. The purpose of the study is to improve the technical procedure and increase the accuracy of determining the key thermophysical characteristics of building materials and products in a stationary thermal regime, as well as expanding the possibilities of using integrated thermal imaging monitoring in assessing the heat-conducting qualities of heterogeneous single-layer building structures, which are, including in non-stationary temperature conditions. Method. The methodological basis of the study is the fundamental provisions of the theories of heat transfer, thermal control and infrared diagnostics, methods of qualitative and quantitative analysis of thermograms.Result. The scheme of complex non-destructive diagnostics of the thermal and technical condition of heat-protective shells of capital and completed construction facilities, engineering and technical systems and subsystems of life support for various functional, technological, operational purposes based on the results of thermography and qualitative and quantitative assessment of thermal images is presented. Examples of the use of thermal imaging equipment and other auxiliary control and measuring equipment in the field examination of translucent structures and non-translucent ventilated facade systems of a real civil building are given, as well as the results of thermal imaging identification of the main thermal properties of a fragment of an enclosing building structure in the form of a wall of silicate brick. Conclusion. The presented method of active thermal non-destructive determination of the main thermophysical properties of structural building materials and products, as well as an experimental setup for its implementation, make it possible to study the entire range of thermal characteristics under various conditions and modes using a thermal imager and related instrumentation.

Thermal resistance of building envelopes with heat-conducting elements in summer period

The analysis of the current methods and techniques of solving the problem of heat resistance of building envelopes with heat-conducting elements shows the solution of a onedimensional problem of heat resistance. One of the possible methods for determining the temperature fluctuation amplitude on the inner surface of the building envelopes with heatconducting elements is the modeling of non-stationary temperature conditions in the computer program. However, this solution causes great difficulties, as it transfers the specified calculation from engineering to scientific and cannot be recommended for practical application. The second method of solving this problem is the application of the convergence coefficient, which can be obtained empirically. The selection of the convergence coefficient allows for the influence of the heat-conducting elements on the weighted average surface temperature depending on the envelope configuration.The structural analysis of the building envelopes and their impact on the averaged amplitude of oscillations on the inner surface are conducted. The arrangement of heat-conducting elements at the outer edge is characterized by a negligible influence of the vibration amplitude on the averaged amplitude over the structural surface. The arrangement of heat-conducting elements greatly affects the heat-conducting elements. According to the comparative analysis, the convergence coefficient is preferable in harmonics of the average temperature fluctuations on the inner surface.

Semiconductor metal oxide sensor for hydrogen sulphide operating under non-stationary temperature conditions

The aim of the work was to create a selective gas sensor for hydrogen sulphide. As a result of adding ammonia to the zinc acetate solution, centrifuging the obtained zinc hydroxide and subsequent calcination, a polydisperse zinc oxide powder with a grain size of 5–50 nm was obtained. The material was characterized using X-ray phase analysis and transmission electron microscopy. Subsequently, silver nitrate and terpeniol were added to the zinc oxide nanopowder to form a paste. The gas-sensitive material was obtained by applying the resulting paste on a special dielectric substrate and subsequent calcination, as a result of which the terpeniol burned out, and the silver nitrate turned into an oxide (the mass fraction of the silver was 3%). A non-stationary temperature mode for the operation of the sensor was selected, in which, after rapidheating of the sensor to 450 °C (2 seconds), slow (13 seconds) cooling to 100 °C occurred. Each subsequent heating-cooling cycle with a total period of 15 seconds began immediately after the end of the previous cycle. The use of an unsteady temperature mode in combination with the selection of the composition of the gas-sensitive layer made it possible to obtain a response of 200 for a hydrogen sulphide concentration of 1 ppm. Along with an increase in sensitivity, a significant increase in selectivity was also observed. The cross-sensitivity for the determination of hydrogen sulphide and other reducing gases (CO, NH3, H2) was more than three orders of magnitude. Thus, this sensor can be used to detect hydrogen sulphide even in the presence of interfering components. The use of highly selective sensors in the tasks of qualitative andquantitative analysis can significantly simplify the calibration in comparison with “electronic nose” devices. Devices based on highly selective sensors do not require the use of mathematical methods for processing multidimensional data arrays.

Assessment of constant and time-varying temperature schemes on the convective drying characteristics of hemp leaves

In the present study, selected mathematical drying models are evaluated for describing the convective drying of hemp (Cannabis sativa L.) leaves. Drying kinetics have been experimentally investigated in a laboratory scale convective dryer for a constant airflow velocity of 1 m s-1 and air-drying temperatures of 40, 50 and 60 °C. Non-stationary temperature conditions were also investigated for the same temperature range, under different advancing time-varying temperature profiles. Weibull formula was the most suitable mathematical expression for describing the drying kinetics for both stationary and non-stationary drying conditions. Drying at increased heating rates led to a lower final moisture content compared with the constant drying temperature levels, exposing the raw material for a shorter duration to the higher air-drying temperature.

Advances in Material Wide Range Temperature Determination by Dual-Color Emissivity Free Methodology in Long-Mid-near Infrared Ranges at Non-stationary Conditions

Dual color emissivity free methodology by thermography allows to obtain 2D (two-dimensional) temperature maps by using local grey body hypotheses and narrowband filters. By using a suitable pair of filters is possible to obtain the ratio between two thermal camera input signals that depend only on the temperature and not on the emissive properties of the investigated surface. The aim of this concise review paper is to summarize and discuss the developments and applications from long- to mid-near infrared ranges and in a wide range of temperature values of the dual-color thermographic technique that has been analysed through the use of an analytical model based on the integration of Planck’s law and attenuated with the transmission curves of sensors, optics, filters, and attenuators during the last years. Moreover, the applicability to the non-stationary temperature conditions and finalized to the materials mainly used in the aerospace plasma wind tunnel (PWT) re-entry are shown.

OPERATIONAL PROPERTIES OF CONSTRUCTION MATERIALS WITH PROTECTIVE COATINGS OBTAINED UNDER NON-STATIONARY TEMPERATURE CONDITIONS

The structure and phase composition of chromium-alloy coatings consisting of the following phases are investigated in the work: when boron doping the coating on brass C68700 consists of phases: Cu3Al, CuB, CuZn2 and zones of solid solution Cr, Al, B in copper; during titanium doping Cu3Al and CuAl phases in the transition zone: phases CuTi and CuTi2 When tested on A572 steels, in conditions of sliding friction, the best wear resistance among the considered protective coatings are doped with titanium and boron. Their wear resistance is 1,8 – 2,1 times higher than that of coatings obtained under isothermal conditions. Under non-stationary temperature conditions, the microhardness reaches the following values: when doped with boron H100 = 16500 – 17500 MPa, when doped with silicon H100 = 13500 – 14500 MPa, when doped with titanium H100 = 15000 – 16000 MPa. The obtained results correlate with the indicators of wear resistance. A comparative analysis of the corrosion resistance of protective coatings obtained under non-stationary temperature conditions and isothermal conditions shows that they have a weight loss of 1.7–2.1 times less. It was found that when tested in a 30% aqueous solution of sulfuric acid, all protective coatings have good stability. Thus, when doped with boron, the weight loss is 15,2 g/m2, при when doped with silicon - 10,8 g/m2, and when doped with titanium - – 9,9 g/m2.

MODELING PRODUCTION OF CORROSION RESISTANT DIFFUSION LAYERS UNDER NON-STATIONARY TEMPERATURE CONDITIONS

The paper discusses methods for obtaining alloyed chromium-alloyed coatings and the search for optimal powder mixtures that allow the formation of protective coatings on CCCM under non-stationary temperature conditions. Refractory compounds, primarily carbides, borides, nitrides and silicides, as well as alloys based on them, can be promising materials for applying protective coatings. In addition to protection against oxidation, coatings made of refractory compounds have high hardness and wear resistance. The implementation of the application of protective coatings on high-carbon materials is difficult for a number of reasons. This concerns the low wettability of the carbon surface by refractory metals, low adhesion of the coating to the substrate, which largely depends on the thermophysical properties of the coating, and also the choice of coating components depending on the operating conditions of the finished product. Under non-stationary temperature conditions, it is possible to obtain coatings of various chemical compositions by varying the amount and content of alloying additives. The gas-transport method of applying coatings using UTC makes it possible to obtain CCCM with enhanced physical and mechanical properties, without requiring high energy costs and time.

MODELING OF DIFFUSION COATINGS STRUCTURE WITH VANADIUM AND TUNGSTEN IN COMPOSITION-FILLING ENVIRONMENTS

The process of modeling the production of vanadium and tungsten coatings is an effective method of increasing the reliability and durability of machine parts, tools by creating protective layers on the surface of the workpiece, which have a unique set of physico-chemical properties.One of the most effective methods of developing protective layers having high mechanical, physical and chemical characteristics is the diffusion saturation of the surface of the layers using composite powder materials.The formation of protective coatings in composite saturation media occurs under non-stationary temperature conditions. The method of diffusion saturation of the solid phase surface in the active gas medium is based on the analysis of the requirements for the diffusion layer and the method of metal saturation is selected. The rates of chemical processes are determined by kinetic regularities that depend on both temperature and diffusion factors. This method provides high surface quality, is the simplest and most used in laboratory practice, reproduces well in production conditions, and does not require special sophisticated equipment to implement it.

The Structure of Iron Powder Aggregation Obtained by Selective Laser Melting

A powder of different fractional composition was obtained from the iron ingot using the melt atomization method, from which samples for further studies were grown on the EOSint M270 unit using the selective laser melting (SLM) method. When considering the thin sections of the samples, we observed equiaxed recrystallized grains up to 20 μm in size, randomly distributed particles in the form of fragments up to 5 μm in size and equally oriented non-axial crystallites, forming groups in the form of arcuate layers with a front width of 800 μm and depth from 10 to 30 microns. Between them, narrow zones of increased chemical content were found, which are interpreted as probable traces of the stoppages of thermal fronts. We recorded traces of mass localized microplastic flow in the form of extended (up to 350x40 µm) and curved microregions with unequal-sized grains bordering microregions filled with equiaxial recrystallized grains due to the action of non-uniform thermal fields. Thus, in metal objects produced by the SLM, the complex morphology of the structure indicates a combination of different-scale mechanisms of structural changes. It has been established that in non-stationary temperature conditions of structure formation, the role of kinetic processes increases.

Comparison of the Stress-State Level for the Inhomogeneous Rock Mass with a Spherical Cavity in the Stationary and Non-Stationary Temperature Conditions

The paper contains comparative analysis of solutions for problems with the rock-mass stress-strain state in the vicinity of a spherical cavity for the case of stationary and non-stationary temperature conditions. Such a cavity was created by blasting and further was used as storage for products that tend to self-heating. So, both problems considered the process inhomogeneity of the rock mass soil and the inhomogeneity caused by an action of various high-temperature fields. The results obtained prove the significant impact of the inhomogeneity on the stress state of the rock mass for any types of temperature conditions. The development of methods for calculation of underground structures that would fully consider the conditions of their creation and operation as well as actual properties of materials is one of crucial areas in structural mechanics.

Superheater tube element temperature regime

Physico-mathematical model for equation of non-stationary temperature conditions of superheater are defined in this work. This model can be used for calculation of temperature regime for both clean surface tube and tube with pollution on its outer and inner surface. Validation of numerical results, which were taken during calculation, are made. Numerical results were compared with results of thermalphysic calculation and results obtained by other authors. Error of numerical results is not more than 2 %.

Compensating distortion effects in repeated measurements under non-stationary conditions

In this paper a compensation technique is presented for applications using repeated measurements under moderate non-stationary measurement conditions. The assumption is that the measurement conditions are stationary during a single repeated measurement but non-stationary over the entire measurement time. The objective with repeated measurements is often to retrieve an estimate of the (noise reduced) signal and its uncertainties. In order to obtain accurate estimates of these two quantities, stationary measurement conditions must be guaranteed under the measurement time. A condition that in many situations is difficult to achieve, e.g., during long measurement times or in a rapidly changing environment. The proposed method compensates for linear dynamic changes during the measurement time, where the dynamical changes, the underlying signal waveform and the noise covariance are considered unknown. The theoretical effect of moderate non-stationary conditions on repeated measurements is analyzed and experimentally validated. Estimation results using the proposed technique are presented for repeated ultrasonic measurements under non-stationary temperature conditions. The results show accurate signal estimation and noise characterization with uncorrelated normally distributed residuals, in contrast to standard synchronization techniques.

Nonstationary temperature conditions of windings of transformers-betatrons with a limited repeat number of heating-cooling

The exact analytical solution of the nonstationary problem of thermal conductivity for a rod-type heat-generating element is obtained.

Enhanced Microstructured Reactor Performance under Forced Temperature Oscillations

The forced variation of reaction parameters is a known method to improve the performance of catalytic reactors leading to process intensification. The most often experimentally varied parameters so far were the reactant concentrations or pressure. Due to the high thermal inertia of conventional reactors it was almost impossible to achieve fast periodic reproducible temperature changes. However, it has been proven theoretically that fast periodic temperature variations may increase the reaction rate compared to the stationary temperature conditions.The possibility to thermally cycle microstructured stainless steel reactors in a periodic way with temperature differences of up to 60 K and a frequency as high as 0.06 Hz has been demonstrated. This gives the opportunity to study the influence of fast temperature changes on heterogeneously catalyzed gas phase reactions. The catalytic CO oxidation over Pt supported on Al2O3 was chosen as a test reaction. The concentrations of CO, O2 and CO2 were monitored online at the reactor outlet using FTIR and mass spectrometry. The experimental measurements under non-stationary temperature conditions have shown an increase in the measured CO2 concentration compared with the one under temperature stationary conditions. For a temperature oscillation frequency of 0.048 Hz with an amplitude of 38 K, the mean CO2 concentration is 1.72 times higher than the mean value obtained under quasi-stationary conditions. A possible explanation for this phenomenon is an increase of the reaction rate due to the presence of a transitional reactive species surface coverage resulting from the temperature oscillations.

Forced periodic temperature cycling of chemical reactions in microstructure devices

In this publication, several stainless steel microstructure reactors specially designed to obtain rapid and periodic temperature changes are presented. Different microstructure reactor designs have been manufactured and tested for their thermal behaviour and equally by running a test reaction under stationary and non-stationary temperature conditions. The devices were continuously electrically heated and periodically cooled by a deionized water flow. The objective of the experimental measurements was to demonstrate that non-stationary temperature conditions may lead to an increase in the reaction rate compared to the stationary conditions. The heterogeneously catalysed oxidation of CO was chosen as the test reaction. The catalyst used was a dispersion of platinum on a porous alumina support generated by sol–gel technology. The experiments realized under non-stationary temperature conditions with a temperature oscillation amplitude of 41 K and a period duration of 21 s show an increase in the mean CO 2 concentration of a factor 1.72 compared to the mean concentration under quasi-stationary temperature conditions. The simulations of a simple monomolecular reaction under non-stationary temperature conditions indicate that the presence of a transitional surface coverage generated by the temperature oscillations may be a possible explanation for the observed phenomenon.

Novel design of a microstructured reactor allowing fast temperature oscillations

Two types of stainless steel microstructured reactors for catalytic gas-phase reactions have been developed and characterized with respect to their thermal behaviour under non-stationary temperature conditions. One of the reactors used (FTC-I) allowed periodic temperature changes up to 100 K with a frequency of 0.05 Hz. However, a broad temperature gradient of 80 K developed inside the reactor. A second reactor (FTC-II) enabled periodic temperature variations of maximum 60 K with a frequency of 0.06 Hz while avoiding temperature non-homogeneity. The CO oxidation taken as a test reaction was carried out over a Pt/Al2O3 catalyst in the FTC-II reactor. In this way it was possible to study the effect of non-stationary temperature conditions on the reactor performance. A significant increase in CO conversion was observed with periodic temperature cycling as compared to values obtained under steady-state conditions. © 2007 Elsevier B.V. All rights reserved.

ANFIBE: A Comprehensive Model for Swelling and Tritium Release from Neutron-Irradiated Beryllium—I: Theory and Model Capabilities

A mechanistic model for the description of helium swelling and tritium release in neutron-irradiated beryllium is presented. Initially aimed at predicting the mechanical stability and the tritium retention capacity of beryllium in a fusion reactor blanket, the ANFIBE code was finally extended to provide an exhaustive description of the properties of this material under fast neutron irradiation. In-solid diffusion and precipitation of helium and tritium, radiation re-solution, and bubble growth and coalescence in different structural domains of the material are taken into account and formulated in a compact rate equation system, enabling the evolution of swelling and release to be calculated under stationary and nonstationary irradiation and temperature conditions. A particular feature of the model is the treatment of the growth of gas bubbles and pores in the interactive compressive stress field created by the gas precipitated in cavities of different sizes and at different pressures, enabling a realistic and accurate calculation of the stress-sensitive intergranular-swelling components and of the related pore-venting effects. The salient physical hypotheses of the model are discussed, as well as the formalism adopted for the description of helium and tritium diffusion precipitation and swelling.

Study of temperature fields and deformations in internal combustion engine components under non-stationary temperature conditions

Study of temperature fields and deformations in internal combustion engine components under non-stationary temperature conditions