Non-stationary Heat Exchange Research Articles

Experimental setup for comprehensive study of non-stationary heat transfer in complex liquid media.

This paper presents a setup for studying non-stationary heat exchange in liquid media on a scale of small times and sizes at high heat flux densities created using a high-speed precision power controller. A platinum wire with a diameter of 20μm and a length from 0.5 to 1cm is used as a probe. The heating time ranged from 1 to 300ms, and the heat flux densities from 1 to 20 MW/m2 were achieved in the experiments. Strictly specified conditions for heat release in the probe are confirmed by maintaining a constant power in a series of pulses with an accuracy of 0.05%. Acquiring a primary electrical signal of thermograms is synchronized with high-speed video recording, allowing an accurate relationship between the applied thermal load and the mechanics of processes in the studied liquid. The setup capabilities were shown by comparing the boiling patterns of a simple substance, such as ethanol, and a solution with a lower critical solution temperature, a 30 vol.% solution of polypropylene glycol-425 in water, which have similar thermograms. As a result, a qualitative difference between the heating and boiling patterns observed using high-speed video and the presented setup fitting its purpose has been shown.

Modeling of Non-stationary Heat Transfer of Floor Structures of Rooms with Artificial Ice

Statement of the problem. The purpose of the scientific article is to develop an adjusted methodology and program for calculating the non-stationary heat exchange of floor structures with artificial ice, taking into account the asymmetric radiation cooling of heat exchange surfaces to determine the time they reach the critical temperature. Results. An important characteristic of the adjusted technique is the use of the average coefficient of irradiation of the surfaces of the structure, as well as taking into account the displacement of the thermal center during its asymmetric heating. The technique takes into account the radiant component from the ice surface in the determining generalized heat transfer coefficient. Conclusions. The considered features make it possible to increase the accuracy of calculating the cooling time of the surface of the structure to critical values, which can be used to adjust the operating modes of microclimatic systems of rooms with artificial ice.

Modeling of thermal processes in vertical heat exchangers of ground-source heat pump

The basic directions for perfection heat supply systems are the tendency of transition to the low-temperature heating systems based at application heat pump installations. We consider heat supply systems with ground-source heat pump. The paper considers modeling the efficiency of ground-source heat pumps with the account of Ukrainian climate conditions. The paper presents the energy performance criteria which show the way to rational implementation of ground-source heat pumps for heating. A computational model based at non-stationary heat exchange processes in elements of ground-source heat pumps is developed. Generalization and analysis of theoretical and experimental dates allow establishing nature of dependence thermophysical properties on temperature insoil around ground heat exchangers during long term operation of the system. Numerical simulation of thermal processes in vertical ground pipes for ground-source heat pump working with low-temperature heating systems has been worked out. The results of numerical modeling demonstrated perfection and technical adaption of ground-source heat pump for low-temperature heating systems with rational using ground energy potential. The novelty of our method is complex usage theoretical and experimental results which enable to prevent freezing in the ground during long term exploitation This provides insights at effects of different resource mixes and may serve as a new approach to analysis of future heat pump systems using ground source energy development.The proposed method contributes to the field of creation and implementation the sustainable heating technologies using renewable energy sources. The ground heat exchange pipes quantities that are necessary for effective operation heat pump installation, capacity and coefficient of performance with the account of climate conditions are calculated.

Analytical and Numerical Solution of the Problem on Nonstationary Heat Exchange of Counterflows

Analytical and Numerical Solution of the Problem on Nonstationary Heat Exchange of Counterflows

Нестаціонарний теплообмін — визначення коефіцієнта тепловіддачі стаціонарним методом та методом регулярного теплового режиму

Нестаціонарний теплообмін — визначення коефіцієнта тепловіддачі стаціонарним методом та методом регулярного теплового режиму

Evaporation of a water layer under local non-isothermal heating

• Water layer evaporation under local heating was investigated. • Gas convection effect was several times greater than diffusion and buoyancy of vapor. • Local heating allowed achieving high values of the heat transfer coefficient. • Obtained results were important for the development of new methods of heat exchange enhancement. The evaporative behavior of a water layer and the heat transfer under local heating were investigated. Non-stationary and non-isothermal heat exchange conditions were created by laser heating. The thermal field on the free surface of the layer was registered using thermal imaging technology. Changes in the liquid mass due to evaporation were determined using the gravimetric method. The key factors affecting water evaporation were estimated and the characteristic heat fluxes associated with evaporation, radiation and convection are compared. The effect of free convection in the gas phase on the evaporation rate of the water film is several times higher than the effect of diffusion and buoyancy of vapor. The local heating allowed achieving high values of the heat transfer coefficient and heat fluxes at considerably smaller liquid temperatures and heating area, which is advantageous from an energy point of view. The obtained results were important for the development of new methods of heat exchange enhancement, as well as for the prediction of heat fluxes and hydrodynamic flows and evaluation of non-isothermal, non-stationary and inhomogeneous heat exchange.

Вільна конвекція — дослідження регулярного теплового режиму в різних шарах по висоті рідини

The rate of cooling (heating) of experimental liquids is investigated separately for five thermocouples located at different heights of the experimental probe. Research is carried out on an experimental stand in the system "environment I — body II", where "environment I" is water, and "body II" is the investigated liquid medium in a thin metal cylindrical shell under conditions of free convection. The rate of cooling (heating) is studied for test liquids: water, sunflower oil, distilled glycerin. The analysis of cooling (heating) processes during free convection of non-stationary heat exchange is carried out. This paper presents research carried out on an experimental stand, which consists of an external metal vessel, containing the environment with a temperature of t1, an internal cylindrical metal vessel containing an experimental liquid medium with a temperature of t2. Accordingly, probes with five thermocouples are placed in hot and cold liquids to take temperature values at a fixed time Non-stationary heat conduction processes occur during heating or cooling of bodies, substances, gaseous media and are accompanied by a change in enthalpy. Non-stationary and stationary processes of body heating due to the internal heat source, the electric heater, are considered in laboratory practice to determine the thermophysical properties of materials, bodies of substances and where cooling occurs due to the surrounding air. The analysis of the excess temperature, carried out separately for each thermocouple, placed along the height of the cylindrical volume, shows that the linearity of the dependence Ln ϑ = f(τ) is preserved, while the influence of free convection on the value of m is manifested. The value of m depends on the height of the liquid layer in which the temperature is measured.

The behavior of heat transfer and entropy in a thin layer of liquid under laser heating

Experimental studies of heat flux, Nusselt number, heat transfer coefficient and entropy generation under local laser heating in the range of water layer heights from 1.4 to 5 mm have been carried out. The laser heats a graphite spot on the glass wall, which warms up the liquid. To date, most studies in the fields of the velocity rotor and entropy generation have been carried out theoretically, and in the absence of surface Marangoni forces. There is extremely little experimental data on the measurement of these fields, especially at non-stationary three-dimensional heat exchange. The temperature field on the free surface of the liquid layer is strongly inhomogeneous and asymmetrical at a layer height of 1.4 mm. At a layer height of 4–5 mm, the symmetry of the temperature field increases. Using the optical method of Particle Image Velocimetry, instantaneous fields of velocity and vorticity have been studied. As the layer height increases, the characteristic size of vortex structures on the liquid surface rapidly decreases with a sharp decrease in the correlation function of the vortex flow. Local laser heating of the layer creates a spatially uneven heat exchange, which is formed during self-organization of various convective vortex structures. For the first time it is shown that the behavior of entropy generation during local heating of the layer differs from the case with uniform heating of the wall. The transition of a symmetric velocity field from two vortices to an asymmetric field with many vortices leads to a decrease in entropy generation with an increase in the Rayleigh and Marangoni numbers. The heat transfer coefficient in the liquid decreases with increasing layer height from 1.4 mm to 5 mm.

Cooling and heating of the fluid in the cylindrical volume

Experimental studies of the non-stationary heat exchange in the system «environment I – body II» have been carried out. It is established that in the body II, which consists of the fluid and thin-walled metal envelope, the characteristic features of the regular thermal mode occur, i.e., cooling (heating) rate of the body II- m = const; heat transfer coefficient between the water (environment I) and body II is practically stable α1 = const; uneven temperatures distribution coefficient in the body II ψ = const. This new notion of the heat transfer regularities in the body II is planned to apply for further development of the experimental-calculation method for the forecasting of the heat exchange intensity in the compound fluid media with limited information regarding thermophysical and rheological properties.

INFLUENCE OF UNSTEADY CONDITIONS ON HEAT EXCHANGE DURING A SHARPY TRANSITION TO FILM BOILING

Boiling is one of the main physical processes, which that take place in heat exchange equipment designed for various purposes. The problem of removing large thermal loads from the heated surface is important for nuclear energy, chemical industry, metallurgy, electronics and other areas where intense heat is released. Boiling processes in process equipment perform important protective functions and can control its effectiveness.
 According to the boiling curve, with increasing temperature power, the flow passes through five regions, starting from the single-phase region of free convection and ending with the region of developed film boiling.
 The purpose of this article is an analytical study of heat transfer at spontaneous transition to the film boiling (explosive type of boiling), taking into account the unsteady nature of this process.
 In order to achieve the aim of this research, two analytical approaches were used, namely, the symmetry method and the Laplace method. As a result of mathematical transformations, expressions for the nonstationary temperature distribution and the Nusselt number are obtained. The obtained expressions make it possible to analyze the dynamics of non-stationary heat exchange processes. The results of analytical and numerical modeling were also compared. It was found that the results of the self-similar solution have a better comparison with numerical data compared to the results according to the Laplace method.

Heat Exchange and Charging of a Metallic Particles Surrounded by Condensed Dispersed Phase of its Oxide

The time of setting of a stationary charge of particles of copper and tantalum, heated up to the melting temperature due to the electric exchange with a condensed dispersion phase of its own oxide, is determined. It is also shown that, for every time moment, the falling metallic particle can be considered as the one having aquasistationary electric charge, due to the non-stationary heat exchange with the surrounding. Experimental data on the charge of copper particles and its cooling characteristics are presented.

Нестаціонарний теплообмін у вертикальному циліндричному об’ємі, заповненому рідиною

The paper analyzes the conditions of convective heat transfer in the "limited volume" and in the "large volume". It is established that the process of heat exchange in the elements of the basic experimental setup in the system of experimental calculation method corresponds to heat exchange in a "large volume" under conditions of free convection. An experimental stand for the study of nonstationary heat exchange in the system "environment I ― body II", the main elements of which are two working cavities — the outer filled with water, volume V1, and the inner filled with the studied liquid medium volume V2, and V1 is larger V2 3 times. The results of experimental determination of heat transfer between the inner surface of a thin metal cylinder and the investigated liquid medium in a limited space for the system “environment I — body II” under the conditions of laminar regime of both the environment and the investigated liquid medium are presented. The heat transfer coefficient was investigated using the calculation-experimental method during heating and cooling of refined sunflower oil, distilled glycerin, sugar solution with a concentration of 50 %, 60 % under conditions of free convection. The experimental results are compared with the results of studies by well-known authors for "large volume" conditions. The conditions of the heat exchange process during the experiment at different directions of heat exchange in a "limited volume" (inside a thin-walled metal cylinder with the liquid under study) are described. It is substantiated that at the surface of a cylindrical metal wall in the process of heat exchange a thermal boundary layer is formed, within which the coolant temperature changes and heat transfer within the boundary layer occurs due to thermal conductivity. The criterion equations for determining the heat transfer coefficient during heating and cooling of the investigated liquid medium under free convection conditions are obtained.

Heat Exchange of the Cylindrical Liquid Body of a Limited Height with the Environment

The experimental investigations of the intensity of the heat exchange between the internal surface of the thin-wall metal cylinder and the studied liquid medium were carried out in conditions of its cooling (heating), i.e. under nonstationary heat exchange conditions. The existence of the regular thermal mode in the liquid medium surrounded by the thin-wall metal cylinder has been established. Local in time heat loss coefficients were derived using appropriate dimensionless equations for the stationary mode conditions of heat-exchange in a large volume. Heat loss coefficients were determined using regular thermal mode methods and computational-&-experimental heat loss coefficients. The changes in the relative values of the heat loss coefficients were analyzed using the method of regular thermal mode and computational-&-experimental heat loss coefficients. The deviations in the values of given coefficients in time are mainly within ± 10 %. Relative values of the heat loss coefficients deviate within ± 40 % using appropriate dimensionless equations for the conditions of the stationary mode of heat exchange in a large volume. This conclusion is natural because the cooling (heating) process is nonstationary.

An Apparatus for Studying Nonstationary Heat Exchange in Liquid Media

An Apparatus for Studying Nonstationary Heat Exchange in Liquid Media

Heat transfer from the heated by an electric current vertical rod in the regime of non-stationary natural convection

The heat transfer from a vertical silicon rod heated by passing an electric current, placed in a cylindrical container filled with gas, with isothermal cold side walls, in the regime of non-stationary conjugate natural convection heat exchange, is studied numerically by the finite element method in a three-dimensional formulation of the problem. On the surface of the rod, a control point is selected, at which a constant temperature is maintained by changing the voltage of the electric current. The equations of thermogravitational convection are solved in the Boussinesq approximation in the terms of temperature, vortex, and vector field potential. The spatial form of convective flows is investigated. Temperature fields in a gas and in a solid are investigated. The calculations were performed with the Prandtl number equal to 0.67 and the Rayleigh number range from 104 to 5•105.

Calculation of non-stationary heat exchange in multi-layer media by means of the theory of Markov chains

Calculation of non-stationary heat exchange in multi-layer dissimilar media is the problem, which is often found in many industries including high temperature metallurgic technologies of production and treatment of metals, alloys and their products. The similar problems arise during the design of buildings and structures. The models used for calculation are mainly based on approximate solutions of classical heat conduction equations. The drawback of such models is a complex computational procedure, associated with the need to solve a large number of equations and to define a large number of identification parameters. At present, the calculation of one mode of heat transfer between the sections of a composite structure requires 6–8 hours of operation of a computer of average power. In this regard, the development of discrete models that require less computer time is of current importance. The proposed model of the process is based on the theory of Markov chains. A multi-layer medium is presented as a chain of small but finite cells. Each of them contains a certain amount of heat that can be transferred to the neighboring cells. The part of the transferred heat is directly proportional to the heat conduction coefficient and in inverse proportion to the material heat capacity, material density and the cell length. The matrix model to describe heat transfer between cell in a multi-layer media based on the theory of Markov chains is developed. Construction of the matrix of transition probabilities is described, evolution of the state vectors i.e. distribution of heat and temperature is carried out, and non-uniformity of the heater temperature is taken into account. Comparison of calculated and experimental data has showed the adequate description of the real process using the model. Analysis of identification parameters has given a satisfactory result.

ON THE TECHNIQUE OF DEFINITION OF THE HEAT MASS EXCHANGE NONSTATIONARY COEFFICIENTS IN WORKINGS

The simple formulas for calculation of non-stationary factors heat and mass exchange in workings are offered. The dimensionless potentials, necessary for calculations, of carry of energy and mass are received by a method of mathematical modeling.

CRITERION CURVES FOR HEAT CALCULATION IN MINES WITH HOT WATER CURRENTS

The problem of building a nonstationary temperature field round the working in a massif with deep hot water currents available has been solved by applying the similarity theory. Criterion curves for temperature determination at the walls of the working when various means of drainage are used are presented, as well as the calculation formula for the nonstationary heat exchange coefficient.

On liquid phase hydrates formation in pipelines in the course of gas non-stationary flow

Nowadays, when the emphasis is on alternative means of energy, natural gas is still used as an efficient and convenient fuel both in the home (for heating buildings and water, cooking, drying and lighting) and in industry together with electricity. In industrial terms, gas is one of the main sources of electricity generation in both developed and developing countries. Pipelines are the most popular means of transporting natural gas domestically and internationally. The main reasons for the constipation of gas pipelines are the formation of hydrates, freezing of water plugs, pollution, etc. It is an urgent task to take timely measures against the formation of hydrates in the pipeline. To stop gas hydrate formation in gas transporting pipelines, from existing methods the mathematical modelling with hydrodynamic method is more acceptable. In this paper the problem of prediction of possible points of hydrates origin in the main pipelines taking into consideration gas non-stationary flow and heat exchange with medium is studied. For solving the problem the system of partial differential equations governing gas non-stationary flow in main gas pipeline is investigated. The problem solution for gas adiabatic flow is presented.

ОЦІНЮВАННЯ ВОГНЕСТІЙКОСТІ ВОГНЕЗАХИЩЕНИХ СТАЛЕВИХ КОНСТРУКЦІЙ

Purpose. Evaluation of fire resistance of fire-resistant steel structures using the developed calculation and experimental method. Methods. Finite difference method, landfill fire test method, mathematical and computer modeling of non-stationary heat exchange processes, determination of thermophysical characteristics of fire-retardant coatings based on solving direct and inverse thermal conductivity problems. Results. Geometric, physical, computer models have been developed, with the help of which the fire resistance of fire-resistant steel structures has been evaluated by the calculation-experimental method. The adequacy of the developed method for assessing the fire resistance of fire-resistant steel structures in assessing the fire resistance of fire-resistant I-beam steel column has been checked. The analysis of tests on fire resistance of fire-resistant steel columns exposed to fire at the standard temperature of the fire without the load applied to them has been carried out. A computer model of the “steel column – reactive flame retardant coating” system has been built for numerical simulation of non-stationary heating of such a system. The fire resistance of fire-resistant steel columns of I-beam section without load applied to them has been evaluated using the calculation-experimental method. Verification of results of experimental research with results of numerical modeling has been carried out. Scientific novelty. The convergence of the results of experimental data on the duration of fire exposure at the standard temperature of the fire to reach the critical temperature of steel with the results of numerical simulations has been determined. Based on the comparison of the experimental results and numerical modeling, the adequacy of the developed model to the real processes that occur when heating fire-retardant steel columns without applying a load under fire conditions at a standard fire temperature has been confirmed. The efficiency of the proposed calculation and experimental method for assessing the fire resistance of fire-resistant steel structures has been confirmed. Practical significance. It consists in the implementation of the results on objects of different purposes in assessing the fire resistance of fire-resistant steel structures by evaluating the effectiveness of fire-retardant coatings of steel building structures.