Thermostressed State Research Articles

Automation studies of thermo-stressed state of the core when exposed to local heat fluxes, heat transfer

Many load-bearing elements of power plants are influenced by local heat flows and convective heat exchangers. Because of this, a deformed thermostress state is formed in the bearing elements in the form of a rod of limited length. Therefore, it is important to develop special computational algorithms and corresponding programs for automated research of rods of limited length. At the same time, the developed system must take into account the physical-mechanical and geometric characteristics of the studied core and the simultaneous presence of local convective heat exchange.This work uses the fundamental laws of energy saving. The developed calculation algorithm is implemented in the form of an IBM PC Python program. The results are presented in tables.

Analysis of the Thermo-Stressed State of an Uncooled Deformable Laser Mirror

Analysis of the Thermo-Stressed State of an Uncooled Deformable Laser Mirror

Optimization of the Temperature and Thermo-Stressed State of a Concrete Dam Constructed from Particularly Lean Roller-Compacted Concrete

In this paper, we consider the optimization of temperature conditions and the thermo-stressed state of a concrete gravity dam made of extra lean roller-compacted concrete constructed in climate conditions corresponding to the Pskem HPP in the Republic of Uzbekistan. We show the need to take into account the effect of solar radiation on the heating of the concrete mass during the layer-by-layer construction of a gravity dam. A methodology was used to estimate solar radiation, considering cloud cover and the use of field and satellite observations. The seasonality of the concrete work and the terrain surrounding the construction site were also examined. We assessed the degree of influence of the factors acting on the formation of the temperature regime and the thermo-stressed state using the factor experiment technique. Regression equations, which allowed us to estimate the values of temperature and thermal stress arising in the structure during the construction period, were obtained. The created numerical model has been used for the estimation and optimization of the thermo-stressed state of the Pskem HPP dam option made of low-cement roller-compacted concrete. On the basis of calculations of the stress–strain state in elastic and elastic-plastic formulations, the possible cracking of concrete was evaluated. The competitiveness of the considered concrete option of the dam with the ground one is shown.

УПРАВЛЕНИЕ ГРАДИЕНТОМ ТЕМПЕРАТУРЫ ПО СЕЧЕНИЮ МАССИВНЫХ КОНСТРУКЦИЙ СПОСОБОМ ПОСЛОЙНОЙ УКЛАДКИ БЕТОННОЙ СМЕСИ

The paper considers a method for depositing concrete mixtures into massive structures, which provides a favorable thermo-stressed concrete state. The thermo-stressed state was assessed according to the temperature gradient arising during thermo-curing along the cross-section of a prismatic structure with a surface modulus of 3.1 m–1. The paper analyzes domestic and foreign literature on the maximum allowable values of the temperature gradient and recommends accounting for the influence of the structural reinforcement ratio on allowable temperature gradients along the reinforcement cross-section. It also discusses other means of achieving a favorable thermo-stressed state of concrete studied in previous works. The paper presents the unsatisfactory results of a computer simulation of temperature distribution without concrete mixture layering. These results show the necessity of using effective set retarding admixes to ensure concrete layering without the formation of construction joints. Layering the concrete distributes the heat release process over time and reduces its peak values due to a smaller amount of cement in each layer compared to continuous layering of the entire volume.

Thermomechanical behavior of a solid electroconductive ball under the action of amplitude modulated radioimpulse

A physical and mathematical model for determining the thermostressed state of an electroconductive solid ball under the action of an amplitude modulated radioimpulse is proposed. The centrally symmetric problem of thermomechanics for the considered ball is formulated. The azimuthal component of the magnetic field strength vector, temperature, and the radial component of the displacement vector were chosen as the determining functions. To construct solutions of the formulated components of the initial-boundary value problems of electrodynamics, heat conductivity, and thermoelasticity, a polynomial approximation of the determining functions over the radial variable is used. As a result, the initial-boundary value problems on the determining functions are reduced to the corresponding Cauchy problems on the integral characteristics of these functions over the radial variable. General solutions of Cauchy problems under homogeneous nonstationary electromagnetic action are obtained. Based on these solutions, the change in time of Joule heat, ponderomotor force, temperature and stresses in the ball under the action of amplitude-modulated radioimpulse depending on its amplitude-frequency characteristics and duration is numerically analyzed.

STUDY OF THERMOSTRESSED STATE AND THERMOSTABILITY OF SLAG CARRIAGE BOWLS

Impact of high temperatures on slag bowls causes considerable elastoplastic deformations resulted in appearance of cracks in them. To prevent destruction of the bowls casing, it is necessary to undertake measures for stabilization of thermostressed state of slag carriage bowls and increasing their thermostability. Results of experimental and computer studies of temperature fields and thermostressed state of slag carriage bowls of various design presented. The study of the thermostressed state of slag carriage bowls was carried out by a tensometric method based on standard equipment. The measuring of temperature and thermal stresses was carried out in the most critical bowls sections. To study the thermostressed state of bowls, a methodology of computer simulation and engineer analysis based on Solid Works/Cosmos Works software was proposed. Results of comparative evaluation of the thermostressed state and thermostability of the forming bowl casing, used in the domestic industry and abroad presented. Basic measures to stabilize the thermostressed state of slag carriage bowls and ways of their thermostability increasing were proposed. Reliability of the results obtained was confirmed by results of experimental studies.

Influence of incompatibility of thermomechanical parameters of bearing layers of a bridge structure on its thermo-stressed state

On the basis of the theory of thermoelasticity, the problem of the thermally stressed state of a two-layer fragment of a bridge structure, consisting of a metal base and an asphalt-concrete upper layer, under conditions of a change in the ambient temperature at different values of the coefficients of thermal linear expansion of the layers is considered. Using the finite element method, the fields of thermal stresses, deformations and displacements are constructed for various values of the thermomechanical characteristics of the layers. The analysis of the influence of the values of thermomechanical parameters on the stress-strain state of the system is carried out. It is shown that with an increase in the incompatibility of these characteristics, the intensities of stresses and strains increase. The zones of concentration of these functions are found. It is recommended to use materials with close values of their thermomechanical parameters when designing bridges to avoid their premature destruction.

Mathematical modelling and development of a computational algorithm for the study of thermo-stressed state of a heat-resistant alloy

The problem of increasing the thermal stability of structural elements made of heat-resistant metals and alloys operating in a complex force and thermal field is one of the key priorities of modern high technology research. The most important case is the study of the thermal stability of structural elements in real conditions of heat fluxes with varying intensity, with a complex configuration of heat-insulated local surfaces and internal point heat sources. Many basic load-bearing structural elements operating in a large thermal field (elements of gas turbine and jet engines, etc.), are made of heat-resistant alloys. The physical feature of such alloys is that the coefficient of thermal expansion and the modulus of elasticity of the material strictly depends on the temperature distribution field, that is, the coefficients are a function of temperature. The purpose of this study is to simulate a thermo-stressed state in rod elements of a structure based on the law of conservation of energy, in the presence of a heat flux applied on the lateral surface, which varies along the coordinate in a linear manner. To solve the outlined problem, a potential energy minimisation method is used in combination of a quadratic finite element with three nodes. As a result, from the condition of the minimum of the functional defining the potential energy, a resolving system of linear algebraic equations is obtained. All possible natural boundary conditions are taken into account. In this system, all integrals used are calculated analytically. Moreover, the law of conservation of energy is fulfilled for each of the equations of the resulting system. As a result, the values of displacement, deformation and stresses were calculated, as well as the values of elastic temperature and thermoelastic components of deformations and stresses for a specific example.

Stress-Strain State of Rockfill Dam Concrete Face Due to Temperature Impact During Reservoir Impoundment

During the reservoir initial impoundment, a rockfill dam concrete face maybe subject to intensive thermal effect. Thermal stresses may considerably change the concrete face stress-strain state (SSS) and induce crack formation in it. With the aid of numerical modeling the author studied the concrete face thermal stresses and deformations for three alternatives of temperature distribution along the face thickness. It was shown that the character of temperature distribution plays an important role in formation of the face thermo-stressed state. At uniform cooling (or heating) due to rockfill low stiffness the face can partially compensate temperature deformations and thermal stresses. Thermal stresses makeup only 15÷30% of the potential ones in constrained conditions. More dangerous is the case of non-uniform distribution of stresses: it causes the face bending deformations. Stresses from bending deformations actually are not compensated. Combination of linear and bending deformations causes high thermal stresses in the face, which may become the reason of crack formation in the face. To provide the face favorable thermo-stressed state without impact from bending deformation sit is necessary to execute the impoundment of the reservoir during a long period. At designing rockfill dams with concrete faces it is necessary to take into account the fact that the less is the face thickness the more is the value of thermal stresses caused by uniform cooling (heating). It is reasonable to revise the recommendations for assigning the face thickness.

Thermomechanical behavior of an electrically conductive cylindrical implant under the action of external unstable electromagnetic fields

A physical and mathematical model for determining the thermostressed state of an electrically conductive cylindrical implant under the action of external unstable electromagnetic fields, in particular the impulse with amplitude modulation, is proposed. This model allows predicting the maximum allowable values (according to physiological norms) of temperature and stress intensities in the considered implant depending on the parameters of external unstable electromagnetic fields and the time of their action.

Thermomechanical behavior of an electrically conductive cylindrical implant under the action of external unstable electromagnetic fields

A physical and mathematical model for determining the thermostressed state of an electrically conductive cylindrical implant under the action of external unstable electromagnetic fields, in particular the impulse with amplitude modulation, is proposed. This model allows predicting the maximum allowable values (according to physiological norms) of temperature and stress intensities in the considered implant depending on the parameters of external unstable electromagnetic fields and the time of their action.

CCP maneuverability increasing algorithm realized on DCS with the metal allowable thermo-stressed state provision

Power generating companies’ main task today is to obtain the highest possible profit. To accomplish this task, it is necessary to have maneuverable equipment that able to reach the maximum power value in minimal time and maintain metal acceptable working conditions. Metal high thermo-stressed state can lead to premature ageing and emergency situations that require investment in overhaul or replacement of units. The authors describe the method of increasing maneuverability of combined-cycle power plant. The main considered input parameter is thermo-stressed state of ‘critical’ elements of waste heat recovery boiler and steam turbine. Based on calculated thermo-stressed values, the algorithm informs the operator of the most optimal steam turbine loading speed. The algorithm does not require any outside software and is performed in the distributed control system installed at the power plant. Also, the authors present the power plant start-up trends with thermo-stressed state calculated values to show the stress reserve, which should be reduced, thus making the start-up faster and increasing the economic performance of the power unit.

Influence of the Elastic Modulus of the Bed on the Thermal Stress State of a Concrete Block

The effect of the elastic modulus of the bed on the thermostressed state of a concrete block is analyzed. Numerical results on the thermostressed state of the block caused by the exothermic heating due to the hydration of cement are presented. The numerical analysis was performed using Midas Civil 2011 software. The time dependence of the thermal stress is obtained for different ratios of elastic modulus of the completely hardened concrete to the elastic modulus of the bed: n = Ec/Eb = 0.5, 1.0, 2.0, 3.0, 4.0, 5.0. The ratio has a strong effect on the thermal stresses in the concrete block near the bed and should be taken into account in designing and constructing massive concrete structures.

Creepage of the Inner Shell of the High-Pressure Cylinder of the Steam Turbine К-325-23.5

The body parts are the most critical elements of steam turbines that operate being exposed to the action of the complicated thermal power loads. These parts are not subject to their replacement during the entire period of their operation. The defects are accumulated in them due to the low-cycle fatigue and creepage. For the reliable determination of the thermostressed state of turbine casings we need to solve thermomechanical problems with variable boundary conditions and temperature-dependent material properties. Special FEM (finite element method) – based computation technique and software package were developed to carry out the 3D simulation of the kinetics of thermostressed state peculiar for power equipment elements. The calculation studies were carried out for the thermostressed state of the inner shell of the high-pressure cylinder used by the steam turbine К-325-23.5. To take into account its structural properties the available software was improved through the introduction of the new types of finite elements. The temperature field distribution was derived from the solution of the heat conductivity problems using the boundary heat exchange conditions of a third kind on appropriate surfaces. The creepage was defined by the theory of aging using isochoric steel curves for 200 thousand hours. The obtained data showed that a significant redistribution of equivalent stresses was observed in the high-temperature zone of the casing due to the metal creepage. Maximum equivalent tensile and compressive creepage strains have actually the same level and do not exceed 0.31 % for the given time. It means that the inner shell has a considerable in-service life reserve.

MATHEMATICAL MODELING OF STRENGTH-DEFORMED CONDITION OF PIPE CONCRETE CONSTRUCTIONS AT SUSTAINABLE TEMPERATURE

The mathematical modeling of the thermo-stressed state of pipe-concrete structures under conditions of stationary thermal conductivity is carried out. During the study, the most common structural type of TBC was selected and mathematical models of the stress-strain state of TBA under heating conditions were taken into account, taking into account their geometric dimensions and thermophysical characteristics of metal and concrete. The main feature of the TBC study is that the heat exchange in the structure, as well as the corresponding force response, are investigated independently of each other, whereas the problem of fire resistance should be posed as a classical problem of elasticity, taking into account the mutual influence of temperature and mechanical stresses.
 In two-layer structures (concrete - metal), the largest radial stresses that occur in concrete work in tension. Tensile stresses occur in the thickness of the outer tube. The stresses occurring on the inner surface of the steel sleeve coincide with the stresses in the concrete. When approaching the outer surface, they decrease and on the surface r = R2 equal to zero. In the case where the coefficients of linear extension αt (i = 1, 2) are equal to each other, the maximum tensile stresses are reduced within 0≤r≤R1 .; if νi (i = 1, 2) within 0≤ r≤R2, the stresses will also decrease. Axial stresses work on compression. They reach maximum value in the outer shell. For equal values αt (1) = αt (2), the magnitude of the stresses does not change, and at ν1 = ν2 the stresses in the metal will decrease. Ring stresses in the region 0≤ r≤R1 are tensile stresses and in the region R1≤ r≤R2 are compression stresses, and the compression stresses are greater than the tensile stresses in concrete. For αt (1) = αt (2), the stresses in the concrete decrease and for ν1 = ν2.

Термопружна поведінка біматеріалу з міжфазною щілиною, заповненою нестисливою рідиною

Thermo-stressed state of the bimaterial with the interfacial crack filled with an incompressible fluid is investigated. The bimaterial with zero Dundurs parameter is considered. The thermo-elastic problem is reduced to nonlinear systems of singular integro-differential equations for a temperature jump between the cracks faces and an opening crack. The pressure of the fluid is determined from the equation of the incompressible fluid. Using the method of successive approximations a numerical solution of the problem is constructed. The opening crack and temperature jump are determined by the finite sum for Chebyshev polynomials of the second kind. The coefficients for Chebyshev polynomials are determined from the system of linear algebraic equations. Having made numerical calculations, the dependences of the pressure of the fluid and the opening crack from the mechanical loads and density and direction of the heat flow have been constructed. The influence of fluid pressure on the stress intensity factor mode I are analyzed.

Про деякі способи досягнення відсутності термонапружень у неоднорідному за товщиною безмежному шарі при стаціонарному тепловому навантаженні

A method for determining the characteristics of functional gradient materials (FGM) for providing zero thermal stresses in an infinite layer with given constant thermal loads is proposed. We assume that the classical convective conditions of heat transfer are given on the surfaces of the layer, the temperature field is stationary, the characteristics of the FGM are described by the model of a simple mixture, the characteristics of the thermo-stressed state and the material depend only on the transverse variable. Precise analytical expressions were obtained for the distribution of the concentration of one of the materials on the thickness of the layer in the absence of mass forces and heat sources, which provides zero longitudinal stresses.

ACCOUNTING OF THE OWN THERMO-STRESSED STATE OF THE SOLID CONCRETE WHILE PROVIDING THE REQUIRED CONSUMER PROPERTIES OF THE CRIMEAN BRIDGE CONSTRUCTIONS

Introduction.The paper deals with the issues related to the determination and registration of the own thermo-stressed state formed in concrete while hardening and used to provide the required consumer properties in the Crimean bridge construction. Therefore, the problem is relevant because of the deadlines for the construction of such facility in the conditions of rough terrain and dry hot climate, and also taking into account the development of the unique non-class facilities in Russia.Materials and methods.The research is carried out in real-time conditions with the usage of modern measuring systems in practice, allowing to control the process of temperature and strength of hardening concrete in time. The modern computational and analytical complex, which is repeatedly tested under real conditions and by thermophysical calculations of hardening concrete of various class objects and massiveness, is used.Results.The results of the intrinsic thermo-stressed state of hardening concrete are presented in determining the permissible temperature gradient, which make it possible to accelerate the process of erecting the object with observance of the necessary consumer properties.Discussion and conclusion.The main conclusions and suggestions are made taking into account the intrinsic thermally stressed state of the concrete when it was hardened during the construction of the Crimean bridge. The paper would be interesting and useful for engineers and technical workers engaged in real production conditions, and for specialists dealing with the issues of the thermophysical processes occurring in concrete hardening and with the problems of ensuring the high consumer concrete properties in structures.

Features of peripheral heating of monolithic reinforced concrete structures

The winter period in Russia lasts quite a long time, several months. During this time, at construction sites the works, such as laying a concrete mix into formwork structures, continues. During heat treatment of concrete, an uneven distribution of temperature across structural section is observed. The formed quality of concrete structures is related to heat treatment parameters. A favourable thermostressed state in concrete is achieved using methods, which assure that, when hardening, the temperature in the central part of concrete is higher than at the periphery. These are thermos and pre-heating methods. The modern method of using a heating wire ensures even temperature distribution across structural section, but the steel core remains in the concrete block. The peripheral heating method using a heating formwork makes heat treatment with soft impact on concrete possible. A repeated use of a heating formwork allows cost reduction in comparison with the method of heating wire. The mathematical program developed at the Department of Building Construction and Structural Theory makes it possible to determine the parameters of heat treatment. The economic effect of the proposed method is shown.

INFLUENCE OF A TYPE OF CONTROLLING FUNCTIONS OF NON-STATIONARY PROBLEM OF THERMAL CONDUCTING ON THE RESOURCE STRENGTH OF THE PISTON

The analysis of calculation and experimental data of the temperature and thermo-stressed state of the piston in the transient processes of the engine is performed. The control functions are determined in the zone of the first and second porous rings. The results of the temperature state of the piston have been obtained taking into account certain control functions. The obtained results coincide with the experimental data with an error in the admissible mediums. A one-step change in control functions is considered in comparison with the obtained type of control functions. The influence of the type of control functions of the non-stationary heat conduction problem on the temperature and thermo-stressed state of the edge of the combustion chamber of the piston is shown. The simplified variants of control functions are offered on the basis of one-step change of the type of control functions. The estimation of the resource strength of the piston is taken into account taking into account the peculiarities of the change of simplified control functions. A simplified version of control functions is defined, which corresponds to the concept of guaranteed maintenance of the resource of the edge of the combustion chamber of the piston.