Non-stationary Heat Exchange Research Articles

Efficiency of fire extinguishing with general purpose fire extinguishing powder in case of non-stationary interaction of its particles with burning material

Evaluation of the effectiveness of fire extinguishing by jet systems of powder fire extinguishing in conditions of non-stationary heat exchange processes and heterogeneous inhibition of active flame centers by powder particles was the aim of the work. The theoretical dependence of the amount of heat, absorbed by the particles of fire extinguishing powder, and the reaction rate of heterogeneous active centers of flame, inhibiting them, in non-stationary conditions of heat transfer, as well as inhibition reaction for fire extinguishing ink jet systems were obtained. The extinguishing of a flame with a fire extinguishing powder under non-stationary conditions is more effective, the smaller is the effective size of the powder particles, the longer is their stay in the combustion zone, and the shorter are the characteristic times of heat transfer and inhibition reaction. Comparison of the estimates of the characteristic duration of heat transfer and inhibition reaction for widely used fire extinguishing powders has shown a large inertia of the thermal mechanism of fire extinguishing, which greatly reduces its effectiveness at high speeds of powder particles in the combustion zone.

Influence of thermal conductivity of the partition wall on non-stationary conjugate natural convective heat exchange and temperature fields in the walls of a rectangular fuel tank

The non-stationary conjugate heat transfer in a rectangular model of a fully filled thin-walled tank, divided by an internal vertical partition wall into two compartments of 1/3 and 2/3 of the tank volume is studied numerically in the conjugate formulation. The outer surfaces of the end walls and the upper wall of the tank are heat-insulated. The lower wall suddenly warms up under the influence of a uniform heat flux. The calculations are performed at the ratio of the thermal conductivity of solid walls to the thermal conductivity of fuel λs/λf = 1041.3 and λs/λf = 1 (low-heat partition wall) and thermal parameters of fuel T1 (Prandtl number Pr = 25.66). The influence of internal vertical partitions wall on the spatial shape of convective flows and the regularities of non-stationary conjugate heat exchange is studied. The temperature fields in the liquid and in the tank walls are calculated.

Effect of thermal conductivity of thin walls limiting the inclined liquid layer on non-stationary conjugate natural convective heat exchange and temperature fields in thin walls

Non-stationary conjugate convective heat transfer in a rectangular cavity at an angle of 45 degrees is studied numerically in the conjugate formulation. A liquid layer with a Prandtl number of 25.66 is located in a cavity with thin metal or low-heat walls. The outer surface of the lower wall is suddenly heated, and the outer surface of the upper wall is maintained at the original temperature. The system of equations of non-stationary thermogravitation convection is solved in the Boussinesq approximation in a dimensionless form. The time evolution of convective flows and temperature fields in the liquid and in the walls is studied. It is shown that the thermal conductivity of walls significantly affects the spatial shape of convective flows and the regularities of conjugated convective heat exchange.

The laser heat-treatment process control by the non-stationary convective heat exchange modeling

Various technological processes allow to obtain high quality indicators, such as laser cutting, welding, hardening, etc. The article presents the results of modeling for laser cutting of thick-walled structures. The creation of computational tools for express analysis in a mathematical setting and a quantitative assessment of temperature fields in spatially variable and non-stationary conditions of laser exposure are of undoubted practical interest to minimize experimental costs.

Modeling of the process of non-stationary heat exchange in the lining of a rotating unit

Rotating heat units, or rotary kilns, are widely used in many industries where they are used for heat treatment of bulk materials. The efficiency of use largely depends on the reliable operation of the main structural elements. This is especially true of the lining of the furnace, the reliability of which significantly affects the performance, safety and long-term and reliable operation of the furnace unit. Therefore, the work devoted to the improvement of the theoretical model of heat exchange in a rotary kiln is extremely relevant as it contributes to the intensification of its work and makes it possible to implement more intensive modes of operation of the heat unit. The calculation of the temperature regime in the lining of the rotating furnace and filled with the processed material is performed. The differential equation for no stationary thermal conductivity is used. Numerical calculation is performed iteratively using the finite difference method according to an explicit scheme. AutoLisp programs in the AutoCAD environment are used to visualize and display graphical information. The movement of the material is modelled by changing the value of the heat transfer coefficient to the inner surface of the lining in the place where it is located. This takes into account the movement of the material along the inner surface of the lining. The change in position depends on the speed of rotation and the amount of material. The shift of the material layer by the angle Δφ occurs after determining by iteration the total time period for which the indicated displacement occurs. At the beginning of the rotation of the furnace, the temperature of the lining area free of material is evenly heated to the maximum value. After contact of the lining with the material begins the redistribution of temperatures in the surface layers. The inner surface of the lining of the furnace is heated by a gas stream, and when it gets under the layer of material - is cooled to 1246 °C, compared with the initial 1465 °C. At the time of exit, the temperature of the lining rises quite rapidly, after which the temperature rise slows down. After one revolution, the lining reaches the initial temperature level. This phenomenon explains the essence of the process of heat recovery by lining. In general, the amplitude of temperature fluctuations on the outer surface of the lining is 218.7 °C. At the same time intensive change of temperatures promotes emergence of the largest gradients and accordingly thermal stresses especially in a zone where there is a processed material. The depth of penetration of the variable temperature into the lining is defined as the ratio at which the amplitude of temperature fluctuations is 0.01 from the amplitude of oscillations on the surface. It is possible to determine that in this case the temperature change occurs at a depth of up to 70 mm, with a total lining thickness of 230 mm. The layers of the lining, which are deeper, have significantly smaller temperature fluctuations. Determining the intensity of heat flow allows to characterize the influence of technological parameters of the unit on the temperature and thermal regime of the lining and the rotary kiln as a whole. Analysis of the data of temperature fields and heat flux shows that the output to the quasi-stationary heat regime occurs after 4-5 revolutions of the furnace. The result of this research is the development of a mathematical model, algorithms and software for calculating and studying the process of non-stationary heat transfer in the lining of rotating units. Numerical calculations are performed and the evolution and distribution of temperatures and heat fluxes in the lining are determined. The depth of penetration and the amplitude of alternating temperature fluctuations that lead to the destruction of the surface layer of the lining refractorines due to thermal stresses are determined. The results of the calculation are also given in the electronic appendix to the article in the form of video files.

Influence of the Homochronism Number of the Liquid in a Spherical Layer on the Heat Exchange in it and its Magnetic Hydrodynamics

Results of numerical simulation of the nonstationary convective heat exchange in the electrically conducting liquid flowing in the space between two concentric isothermal spheres are presented. The influence of the homochronism number of the liquid on the structure of its flow, the temperature and magneto-induction fields in it, and the distribution of local Nusselt numbers in the spherical liquid layer was investigated.

Modeling of Thermal Protection System Based on Thermionic Technology

A high-speed aircraft, capable of maneuvering when moving in dense layers of the atmosphere, is subjected to high dynamic and thermal loads, which can lead to destruction of its shape and changes in its aerodynamic characteristics. Consequently, the main design objective is the thermal protection of structural elements of aircraft. Electron transpiration cooling is a prospective method for the solution of this problem. A mathematical model of the process of nonstationary heat exchange in the electron transpiration cooling system during convective heating has been formulated and studied. The effect of the evaporation (emission) of electrons from the emitter surface upon lowering the temperature of the composite shell of thermionic emission systems is shown. The influence of different coolants on the heat transfer in the thermionic converter was studied.

Numerical validation of a method determining thermal diffusivity based on a measurement of a temperature profile

The knowledge of dynamic thermal properties of building elements is necessary to investigate temperature and heat flux changes in natural daily and annual cycles. The basic dynamic parameter is thermal diffusivity. A method for determining its value for real objects has been proposed. This method is based on measuring the temperature in the element’s volume and on assuming that the obtained results meet the Fourier equation. Validation by a numerical experiment was made. The wall of the building with known thermal parameters was assumed and the temperature distribution was calculated over time in the process of non-stationary heat exchange. From the results, the diffusivity value was calculated and compared with the data entered into the model. Validations were performed for several accuracy of the temperature value and for two forms of function which approximated the temperature values obtained from calculation. A preliminary analysis of errors has been carried out. Keywords: measurements of thermal diffusivity, temperature distribution in a building element, approximation, heat transfer

Strength and Mechanism of Adhesion to the Substrate Layer while Applying Plasma Coatings in Oxidizing Environments

It has been examined that when an oxide layer is treated under atmospheric conditions on a treated substrate surface, in the contact zone, an oxide layer is always present. This mixed oxide layer influences the contact temperature and the adhesion strength of the particles that are being coated to the substrate surface.Henceforth the process of heating of the contact layer of surface oxides was investigated. The experimentation was demonstrated by coupling of the particles of the coating layer with the surface layer when wetting is provided by melting of the oxide layer. The conditions of the wettable contact under nonstationary heat exchange were determined depending on the thermophysical properties of the contact pair, the temperatures of the coating particles and the substrate. The wetting time was evaluated depending on the thickness of the oxide layer. The identity of the adhesion strength of the coating and the cohesive strength of the oxide layer was substantiated. It was evaluated that for the inner surface of the coating elastic modulus and breaking stress is about 20% higher than for the external. The detected difference is related to the presence of the temperature gradient across the thickness for coating.

Problem on Conjugate Nonstationary Heat Exchange in Supersonic Flow over a Blunt-Nosed Cone at an Angle of Attack

The flow of a gas over a conic body, having a nose blunted over a sphere, with a supersonic velocity at different angles of attack was considered. The possibility of control of the temperature regime of such a body in the case of three-dimensional gas flow over it was investigated. The dependence of the temperature of the surface of the indicated body on the material of which it is made, the angle of attack of the body, and other constitutive parameters of the problem on conjugate nonstationary heat exchange between the body and the gas fl ow over it was determined for standard conditions of an aerodynamic experiment. Results of numerical solution of this problem for different angles of attack of the body were processed in the criteria form. The dependences obtained allow one to estimate the degree of decrease in the maximum temperature at the surface of a blunt-nosed conic body depending on the thermophysical parameters of the material of which this surface is made, including in the case of nonstationary gas fl ow over it. It is shown that a uniform heating of the peripheral part of such a body and a marked decrease in the temperature of its windward part at large angles of attack can be provided with the use of highly heat-conducting materials for these parts of the body.

Study on non-stationary heat exchange in combined packages of water-resistant protective clothing of firefighters

The results of research of non-stationary heat exchange in combined packages intended for creation of special water- and heat-resistant protective clothing of firefighters from dangerous and harmful factors during emergency rescue and other urgent works, with participation of non-toxic substances, acid solutions, alkalis, oil and petroleum products, liquid toxic substances, as well as during operation in water with temperature from 0 to 70 °С are presented. The stability of clothing material packs has been investigated as a transient heat exchange process in a multilayer plate with ideal thermal contact at the joints of the layers. The unlimited plate is heated on both sides under different heat exchange conditions according to Newton’s Law, with constant action of the heat source on one of the surfaces of the hot liquid contacting through the waterproof thin surface. Second surface of the plate interacts with external medium, temperature of which varies according to linear law. At solving the equation of non-stationary thermal conductivity with nonlinear transport coefficients, linearization methods are used based on the approximation of nonlinear coefficients, such that nonlinear equations become approximately linear. The entire heat transfer process is divided into a plurality of small-time intervals within which the transfer coefficients are constant. The zonal method of investigation of non-stationary thermal conductivity in clothing packages establishes equations for calculation of temperature, densities of thermal flows, distribution of temperature across thickness of clothing packages. It has been shown that under accepted calculation simplifications, parameter values are well consistent with the experiment. The composition of the clothing package is proposed, which meets the technical requirements of TУ BY 101114857.082-2015 “Personal Protective Kits”.

Model of Thermal Mechanism of Subclass A1 Fire Extinguishing with General Purpose Fire Extinguishing Powder in Non-Stationary Heat Exchange Conditions

The aim of the paper was to develop a model of thermal extinguishing mechanism using dry chemical powder taking into account the inertia of heat transfer to powder particles during unsteady heat exchange to identify the optimal conditions for extinguishing of A1 class fires with powders.The method of experimental and mathematical modelling of fire extinguishing process using dry chemical powder with short-term effect on the fire was used to achieve the goal. The experimental dependences of the extinguishing time and unit consumption of the extinguishing powder on the intensity of the powder supply to the combustion zone in extinguishing of subclass A1 fire in same area and in a limited volume were obtained. The mathematical model of a thermal extinguishing mechanism using dry chemical powder has been developed, taking into account the inertia of heat transfer to powder particles during unsteady heat exchange.Analysis of the regularities of extinguishing the subclass A1 fire using powder with a short feeding it into the fire indicates the presence of optimum values of unity consumption of powder on the fire from the intensity of feeding it into the fire. The presence of this optimum is due to the inertia of extinguishing the subclass A1 fire using powder due to the finiteness of the heat transfer time to the particles of the extinguishing powder and the limited time of interaction of particles with the combustible material.The theoretical analysis of the fire extinguishing process over the area taking into account the inertia of heat transfer to the powder particles at non-stationary heat exchange are carried out. The results of the analysis are in qualitative agreement with the results of the experimental study of the regularities of extinguishing of model fire foci of subclass A1 with General-purpose fire extinguishing powder.

Аналитическое исследование теплообмена в термогидроциклоне для промысловой депарафинизации нефти

The article considers modernization of the design of thermohydrocyclone (THC), where cooling, solidification and heating mechanisms are carried out in one continuous process. This reduces the time of the complete cycle of oil removal from paraffin, increases the productivity and efficiency of the plant. The technological scheme of continuous operation THC is presented. The general factors and simplifying allowances for mathematical modeling of phase transition processes (melting and solidification) are considered. Differential equations for heat transfer in the course of paraffin melting and solidification in a conical layer have been solved analytically. Formulas for determining thickness of the conical layer of the new phase, time of formation of a new solid phase, and average density of the heat flow in the conical layer on a THC wall have been derived. The effect of centrifugal inertia forces is taken into account. The formulas for calculating the basic parameters are summarized in the form of power criterion equations linking Fourier criteria (Fo), Kosovich criteria (Ko) and Pomerantsev criteria (Po) under the quasiconductive non-stationary heat exchange in THC.

Modeling the Thermal Response of a Cast-Iron–Concrete System under Active Thermal Non-Destructive Control

In this paper, we perform the mathematical modeling of the thermophysical experiment of diagnosing a space under a cast-iron tubing of a vertical tubing support using optical lock-in thermography. We present the formulation and numerical solution of a two-dimensional boundary value problem of non-stationary heat exchange, including heat transfer due to thermal conductivity, natural convection, and radiation. The defect in the under-tubing space is detected based on an analysis of the distribution of the phase characteristics of the temperature fluctuations at the tubing boundary (which is available for observation) calculated by the digital lock-in correlation. The effect of the heating frequency, heating time, and noise on the distribution of phase characteristics is studied to optimize the process of active thermal non-destructive control. Based on the use of the reference temperature distribution at the tubing boundary with a defect-free tubing space, an algorithm for mathematically processing noisy data is proposed. The algorithm includes the Kalman filter, the Rauch-Tung-Striebel smoothing procedure, and a smoothing spline method with the criterial selection of the smoothing parameter. The results of the computational experiments illustrate the effectiveness of the presented approach.

Mathematical Simulation of the Heat and Mass Exchange in the Process of Convective-Radiant Heating of a Blunt-Nosed Body

A numerical analysis of the nonstationary heat and mass exchange in the heat-resistant composite material (a carbon-filled plastic), forming a cover of the conic part of a blunt-nosed body exposed repeatedly to a pulsed laser radiation, has been performed on the basis of the heat model of destruction of such a material. Different regimes of thermochemical destruction of the carbon-filled plastic due to the action of a moderate-intensity laser radiation on it under the conditions where the porous material of the spherical bluntness of the body is not melted were determined. It was established that the gaseous products of pyrolysis of the carbon material as well as its condensed-phase particles and vapor play a dominant role in the shielding of the surface of the conic part of the body from the laser radiation.

МАТЕМАТИЧЕСКОЕ МОДЕЛИРОВАНИЕ ПРОЦЕССОВ ТЕПЛОПЕРЕНОСА ПРИ РАБОТЕ ТЕПЛОНАСОСНЫХ СИСТЕМ ИСПОЛЬЗОВАНИЯ ГЕОТЕРМАЛЬНОЙ ЭНЕРГИИ

Актуальность. Использование теплонасосных установок для отопления вместо традиционных систем, которые получают энергию в процессе сжигания различных видов топлива, имеет ряд экологических и экономических преимуществ. Тепловые насосы могут получать энергию из воздуха, грунта и воды. Их сферы применения разнообразны: горячее водоснабжение и кондиционирование помещений, нагрев и охлаждение воды для различных нужд, сушки/осушения воздуха, производства пара, испарения, дистилляции. При применении природных водоёмов (озёра, пруды, водохранилища) в качестве низкопотенциального источника энергии теплонасосных установок на поверхности трубки испарителя образуется лёд. Важно рассматривать закономерности и характеристики процессов теплообмена между водой и трубкой испарителя при образовании льда на её поверхности. Цель: математическое моделирование нестационарного конвективного теплообмена между водой и трубками испарителя теплонасосных установок в условиях формирования льда на их поверхности. Объект: теплообменник испарителя теплового насоса, погружённый в воду. Методы: численное решение задач конвективного теплообмена в условиях фазового перехода воды методом конечных элементов в среде COMSOL. Результаты. Установлены закономерности нестационарного конвективного теплопереноса вблизи трубок испарителя водяного теплового насоса с температурой, при которой образуется лёд на их поверхности. Показана необходимость учета влияния термогравитационной конвекции в воде на тепловой поток и процесс образования льда на поверхности трубки испарителя теплонасосной установки. Получены зависимости числа Нуссельта от характеристики конвективного теплообмена в воде (чисел Рэлея, Фурье и Стефана). Выявлено, что перепад температур в воде вблизи трубки увеличивался при уменьшении глубины её расположения относительно поверхности водного источника при показателях температур воды выше 277 К. При температурах воды ниже 277 К тепловой поток достигал максимального значения у поверхности трубки, которая находилась глубже.

The characteristics of a 4 K Gifford-McMahon cryocooler with a second stage-regenerator packed with cenospheres

Designing efficient Gifford- McMahon (GM) cryocoolers that would allow cooling to temperatures below 4.2 K is considered an important step in the development of helium temperature technologies. Modern two-stage GM machines ensure cold production up to 4.2 K, but with relatively low efficiency, due mainly to the low heat capacity of the material used for the second-stage regenerator packing of GM machines. Two urgent problems are solved, in order to create 4 K GM cryocoolers: the non-stationary heat exchange processes in GM regenerators are described; the efficiency indicators of a GM second-stage regenerator based on cenospheres filled with helium are identified. A comparative analysis of the various types of regenerator packing, which make it possible to obtain temperatures below 4.2 K, is performed, and the advantages of helium-filled cenosphere packing are substantiated. The operation of a two-stage GM cryorefrigerator with a cooling capacity of 0.2 W at a temperature level of 4.2 K is analyzed using the wave approach to regenerator modeling. It is shown that the GM machine can be improved by using a regenerator filled with cenospheres in its second stage.

Conjugate Problem on Nonstationary Heat Exchange in Supersonic Flow over a Blunt-Nosed Cone

A conjugate problem on the nonstationary heat exchange in the supersonic gas flow past a conic body with a nose blunted over a sphere at a zero angle of attack was solved for the case where the stagnation flow temperature is as high as 2500 K. The possibility of control of the temperature regime of such a body in the case where a laminar flow is realized in the boundary layer on its surface was investigated. The results of calculations were generalized in the form of criterion dependences allowing one to determined the maximum temperature on the surface of the body with a good accuracy, including for a nonstationary flow over it, which makes it possible to estimate the ranges of thermophysical and geometric parameters of the body in which the temperature of its surface can be decreased to a required level.

Численный анализ влияния нестационарности процессов теплопереноса в зонах размещения подземных теплопроводов на их тепловые режимы и тепловые потери

Актуальность исследования обусловлена необходимостью разработки новых подходов к анализу тепловых режимов и тепловых потерь подземных трубопроводов и подтверждается основными положениями Энергетической стратегии России на период до 2030 г. Подземные трубопроводы широко используются при транспортировке жидкостей в различных областях, например, таких как водоснабжение и теплоснабжение, нефтепроводы и газопроводы, технологические трубопроводы промышленных предприятий. При проектировании систем такого рода необходимо учитывать нестационарность теплообмена между грунтом и подземным трубопроводом, что во многих случаях оказывает существенное влияние на экономичность транспортировки энергоносителей. Цель: численный анализ нестационарных тепловых режимов и тепловых потерь подземных теплоопроводов и исследование температурных полей и закономерностей нестационарного теплопереноса в зонах размещения подземных бесканальных трубопроводов. Объекты: типичные для систем транспортировки энергоносителей подземные двухтрубные бесканальные трубопроводы. Трубопроводы изолированы пенополиуретаном и защитным покровным гидроизоляционным слоем из полиэтилена. Температуры на внутренней поверхности труб равны среднегодовым температурам энергоносителей в подающих и обратных трубопроводах водяных тепловых сетей при их работе по температурному графику 95/70 °С. Сезонное изменение температуры окружающей среды вычислялось по закону простого гармонического колебания. Исследования проводились для климатических условий г. Томск. Средний коэффициент теплоотдачи на поверхности раздела «грунт - окружающая среда» составлял 5 Вт/(м2  К). Методы: численное решение задач теплопереноса методом конечных элементов с использованием аппроксимации Галеркина, неравномерной конечно-элементной сетки, количество элементов которой выбирается из условий сходимости решения, сгущение сетки проводится методом Делоне. Результаты. Установлены масштабы тепловых потерь и закономерности нестационарного теплопереноса в зонах размещения подземных бесканальных трубопроводов. Показана необходимость учета нестационарности теплопереноса в зоне размещения подземных теплопроводов при прогностическом моделировании их тепловых режимов и тепловых потерь. Нестационарность теплопереноса в зоне размещения подземных тепловых сетей может оказать заметное влияние, например, в тех случаях, когда в зоне теплового влияния подземных трубопроводов расположены смежные коммуникации или инженерные сооружения.

Application of substances with phase transitions for thermal energy accumulation

This publication is devoted to creation of energy-efficient systems for cold supply and heat supply using heat energy accumulators. Thermal energy accumulation increases the efficiency of heat power systems including cooling and air conditioning systems, reduces peak power consumption and capacities of thermal installations at variable loads. It is shown that substances with phase transition (SPT) are widely used for thermal energy accumulation. They are mainly of the solid body-liquid type providing volume and mass density of heat storing and cold energy that is 5–14 times higher in comparison with accumulating liquids. Requirements to SPT with regard to thermal energy accumulators are formulated. We have given an overview of the SPT recommended for application to which organic compounds belong (paraffins, fatty acids), hydrates of salts, eutectics (may include organic and inorganic compounds in their structure). The advantages and disadvantages of each group of substances are shown. The information on the properties of certain SPT in relation to air conditioning systems is presented. It is shown that SPT having industrial applications are hidden under trademarks. It is noted that the creation of heat energy accumulation systems made in Russia requires carrying out a fundamental and applied research complex. We have presented application examples of thermal energy accumulation using SPT in air conditioning systems. The designs of thermal energy accumulators are described, their advantages and disadvantages are noted. We have carried out the analysis of calculation methods for systems with thermal energy accumulators available in literature including solutions of Stefan problem about non-stationary heat exchange at phase transitions in relation to the thermal energy accumulation. The conclusion is drawn on the numerical method advantages for solution of this problem. The research directions are formulated which implementation will allow for developing Russian systems of heat and cold supply with heat energy storage devices.