Any material body has such a characteristic as heat, which can increase and decrease. Heat is not a material substance: as part of the internal energy of a substance, it arises as a result of the movement and interaction of molecules. Since the heat of different substances may differ, there is a process of transferring heat from a hotter substance to a substance with less heat. This process is called heat transfer. We will consider the main and mechanisms of their action in this article.
Definition of heat transfer
Heat transfer, or the process of temperature transfer, can occur both within matter and from one substance to another. At the same time, the intensity of heat transfer largely depends on the physical properties of matter, the temperature of substances (if several substances participate in heat transfer) and the laws of physics. Heat transfer is a process that always proceeds unilaterally. The main principle of heat transfer is that the hottest body always gives off heat to an object with a lower temperature. For example, when ironing clothes, a hot iron gives off heat to the trousers, and not vice versa. Heat transfer is a time-dependent phenomenon that characterizes the irreversible distribution of heat in space.
Heat transfer mechanisms
The mechanisms of thermal interaction of substances can take different forms. There are three types of heat transfer in nature:
- Thermal conductivity is a mechanism of intermolecular heat transfer from one part of the body to another or to another object. The property is based on the inhomogeneity of temperature in the substances under consideration.
- Convection - heat exchange between fluid media (liquid, air).
- Radiation exposure is the transfer of heat from bodies (sources) heated and heated due to their energy in the form of electromagnetic waves with a constant spectrum.
Let us consider the listed types of heat transfer in more detail.
Thermal conductivity
Most often, thermal conductivity is observed in solids. If, under the influence of any factors, areas with different temperatures appear in the same substance, then thermal energy from a hotter area will pass to a cold one. In some cases, this phenomenon can even be observed visually. For example, if we take a metal rod, say, a needle, and heat it on fire, then after some time we will see how thermal energy is transferred through the needle, forming a glow in a certain area. At the same time, in a place where the temperature is higher, the glow is brighter and, conversely, where t is lower, it is darker. Thermal conductivity can also be observed between two bodies (a mug of hot tea and a hand)
The intensity of heat transfer depends on many factors, the ratio of which was revealed by the French mathematician Fourier. These factors primarily include the temperature gradient (the ratio of the temperature difference at the ends of the rod to the distance from one end to the other), the cross-sectional area of the body, and the thermal conductivity coefficient (for all substances it is different, but the highest is observed in metals). The most significant coefficient of thermal conductivity is observed in copper and aluminum. It is not surprising that these two metals are more often used in the manufacture of electrical wires. Following the Fourier law, the heat flux can be increased or decreased by changing one of these parameters.
Convection types of heat transfer
Convection, which is characteristic mainly of gases and liquids, has two components: intermolecular heat conduction and movement (propagation) of the medium. The mechanism of action of convection occurs as follows: with an increase in the temperature of a fluid substance, its molecules begin to move more actively, and in the absence of spatial restrictions, the volume of the substance increases. The consequence of this process will be a decrease in the density of the substance and its upward movement. A striking example of convection is the movement of air heated by a radiator from a battery to the ceiling.
There are free and forced convective types of heat transfer. Heat transfer and mass movement in the free type occurs due to the heterogeneity of the substance, that is, the hot liquid rises above the cold one in a natural way without the influence of external forces (for example, heating a room through central heating). With forced convection, the movement of the mass occurs under the action of external forces, for example, stirring tea with a spoon.
Radiant heat transfer
Radiant or radiative heat transfer can occur without contact with another object or substance, therefore, even in radiation heat transfer is inherent in all bodies to a greater or lesser extent and manifests itself in the form of electromagnetic waves with a continuous spectrum. A prime example of this is the sun. The mechanism of action is as follows: the body continuously radiates a certain amount of heat into the space surrounding it. When this energy hits another object or substance, part of it is absorbed, the second part passes through, and the third part is reflected into the environment. Any object can both radiate heat and absorb, while dark substances are able to absorb more heat than light ones.
Combined heat transfer mechanisms
In nature, types of heat transfer processes are rarely found separately. Much more often they can be seen together. In thermodynamics, these combinations even have names, for example, thermal conductivity + convection is convective heat transfer, and thermal conductivity + thermal radiation is called radiative-conductive heat transfer. In addition, there are such combined types of heat transfer as:
- Heat transfer is the movement of thermal energy between a gas or liquid and a solid.
- Heat transfer is the transfer of t from one matter to another through a mechanical obstacle.
- Convective-radiant heat transfer is formed by combining convection and thermal radiation.
Types of heat transfer in nature (examples)
Heat transfer in nature plays a huge role and is not limited to the heating of the globe by the sun's rays. Extensive convection currents, such as the movement of air masses, largely determine the weather throughout our planet.
The thermal conductivity of the Earth's core leads to the appearance of geysers and the eruption of volcanic rocks. This is only a small part on a global scale. Together, they form the types of convective heat transfer and radiative-conductive types of heat transfer necessary to sustain life on our planet.
The use of heat transfer in anthropological activities
Heat is an important component of almost all industrial processes. It is difficult to say which type of heat exchange is used by man most of all in the national economy. Probably all three at the same time. Through heat transfer processes, metals are smelted, producing a huge number of goods, from everyday items to space ships.
Thermal units capable of converting thermal energy into useful power are extremely important for civilization. Among them are gasoline, diesel, compressor, turbine units. For their work, they use various types of heat transfer.
Fundamentals of the theory of heat transfer.
Heat transfer- a science that studies heat transfer between bodies and the distribution of temperature in bodies.
The main forms of heat transfer:
1. Thermal conductivity.
2. Convective heat transfer.
3. Radiant heat transfer.
Thermal conductivity is the process of heat transfer by direct contact of bodies or individual parts of the body that have different temperatures. In this case, the process of heat transfer occurs due to the transfer of the energy of micromotion of some particles to others.
In its pure form, thermal conductivity is observed in solids, as well as in stationary gases and liquids in the case when there is no convection in them.
Heat flow , .
Fourier's Law: The heat flow is proportional to the temperature and area gradient, i.e. .
Heat flux density , .
Thermal conductivity coefficient - the amount of heat that passes per unit time through a unit surface through a unit wall thickness at a temperature drop of one degree,.
Convective heat transfer- the process of heat transfer, which is carried out in space (in volume), due to the movement of macro particles.
In this process, there is a joint action of convection (movement) and heat transfer due to heat conduction.
Newton's equation: , where is the thickness of the boundary layer in which heat transfer occurs due to heat conduction; - coefficient of convective heat transfer, .
Radiant heat transfer- heat transfer is carried out in space due to the energy of electromagnetic waves.
Stefan-Boltzmann's law: , where is the intensity of the radiation of an absolutely black body.
Newton-Richmann equation: , where is the radiant heat transfer coefficient.
Thermal conductivity.
temperature field- a set of temperature values at individual points of the body depending on time and spatial coordinates.
Mathematical notation of a non-stationary three-dimensional temperature field: . Mathematical notation of a stationary three-dimensional field: . This field is called stationary because .
Isothermal surface is the locus of points having the same temperature.
Isotherm is the intersection of an isothermal surface with a perpendicular plane.
An isothermal surface either closes inside the body or breaks off at its boundary.
temperature gradient is a vector directed along the normal to the isothermal surface in the direction of increasing temperature and numerically equal to the limit of the ratio of temperature change to the distance between isotherms along the normal ( 0 S/m)
Fourier Law:
Heat flow: , .
Heat flux density: , , .
Tasks of the theory of heat conduction:
1. Find a non-stationary three-dimensional temperature field, .
2. Find the heat flux and heat flux density, , .
Question #32
Differential equation of heat conduction.
Conventions:
1. Thermophysical properties of the system: , , .
2. Microparticles of the body are motionless.
3. Internal sources of heat are evenly distributed in the body.
Where is the coefficient of thermal diffusivity characterizing the rate of temperature change at any point of the body, ;
is the heat capacity of the body; is the density of the body; is the bulk heat release density, wm/m 3; - temperature; is the Laplace operator.
(for polar coordinates , , ), .
Uniqueness conditions– mathematical description of the particular features of the process under consideration.
Solving the equation , we obtain a general solution, which, together with the uniqueness conditions, will give us particular solutions.
Conditional uniqueness:
1. Geometric conditions:
a. Body Shape:
i. Flat body.
ii. Cylindrical body.
iii. spherical body.
b. Limited body.
c. Unlimited body.
2. Physical conditions:
a. The nature of the change in physical parameters:
i. The nature of the change.
ii. The nature of the change.
iii. The nature of the change.
iv. The nature of the change.
3. Initial conditions (temporary):
4. Boundary conditions:
a. Boundary conditions of the first kind - the law of temperature change at the boundary of the body:
b. Boundary conditions of the second body - the law of change in the temperature flow in the wall of the body:
c. Boundary conditions of the third kind:
i. The law of change in ambient temperature.
ii. The law according to which the body heat exchanges with the environment.
d. Boundary conditions of the fourth kind, .
Question #33
1. Flat wall.
Given: , , .
Find: , , .
Solution:
Common decision: .
Border conditions: .
Thermal resistance of a flat wall - .
The ratio is called the thermal conductivity of the wall.
Question #34
Consider the transfer of heat by thermal conduction through flat three-layer wall(Fig. 2b) under the following conditions: wall layer thickness , , ;
coefficients of thermal conductivity of materials, respectively, , , ; the contact between the walls is ideal and the temperature at the boundary of adjacent layers is the same. Heat transfer occurs under stationary conditions - the heat flux density over all layers of the wall has the same value ( q=idem). In these conditions:
Let us single out from this series of equalities the temperature difference (temperature drop across the layers of the wall)
Adding the left and right parts of the temperature difference equations, we obtain on the left the temperature change in the wall , on the right - the product of the heat flux density q and total thermal resistance
Thus, for the heat flux density during heat transfer by thermal conductivity through a flat three-layer wall, we obtain the following expression:
In general, for a wall consisting of n - layers, this expression will be written like this:
Where R is the total thermal resistance of the multilayer wall.
Question #35
The amount of heat given off by a liquid to a solid wall or perceived by the liquid from the wall is determined by the Newton–Richmann equation
and the heat flux density is as follows
where α is the coefficient characterizing the conditions of heat exchange between the liquid and the surface of the solid, called heat transfer coefficient, W/(m 2 °C); - temperature difference, 0 С.
In accordance with formula (61), in its physical meaning, the heat transfer coefficient is the heat flux density ( q) on the surface of the body, referred to the temperature difference between the surface of the body and the environment. The heat transfer coefficient is numerically equal to the heat flux density at a temperature difference equal to unity.
The heat transfer coefficient depends on many factors. In the most general case, it is a function of the shape and size of the body, the mode of fluid movement, the physical properties of the fluid, the position in space and the state of the heat exchange surface, and other quantities. The process of heat transfer, depending on the nature of the movement of the liquid, proceeds differently.
Question #36
Radiant heat transfer.
Solids radiate and absorb energy in the entire range of wavelengths by the surface layer. The intensity of radiation depends only on temperature. Liquids behave in a similar way. Gases radiate and absorb energy in a limited range of wavelengths throughout their volume. The emission intensity of gases depends on the temperature, layer thickness and partial pressure of the components.
radiant energy is the energy emitted by the body in the entire wavelength range, .
Radiation intensity is the amount of energy radiated from a unit surface, .
Radiant energy can be found by the formula: .
Law of energy conservation: .
Where is the reflection coefficient, is the absorption coefficient, is the transparency coefficient.
If , that is, then the body is called absolutely white.
If , that is , then the body is called absolutely black.
The density of the integral radiation, referred to the considered wavelength range, is called spectral intensity of radiation(W/m3):
Angular intensity: .
Spectral angular intensity: .
Planck's law establishes the dependence of the radiation intensity of a black body E 0λ from the wavelength λ and temperature T
Stefan-Boltzmann law: .
Degree of blackness: .
Kirchhoff's law is formulated like this: the ratio of the density of the hemispherical integral radiation to the absorption capacity is the same for all bodies having the same temperature and is equal to the density of the integral hemispherical radiation of a completely black body at the same temperature: , where is the absorption coefficient.
The amount of heat that will remain in one of the two bodies:.
Wien's displacement law says - the wavelength, which corresponds to the maximum value of the radiation intensity (E 0λ =max), is inversely proportional to the absolute temperature Fig.11
Question #37
Heat transfer by radiation between solids.
Based on the laws of radiation, the calculation equation for radiant heat transfer between a body 1 of arbitrary shape and the surface of another, larger body 2 covering it is obtained (Fig. 14)
Where Q 1.2 is the heat flux transmitted by radiation from body 1 to body 2, W;
ε 1.2 is the reduced emissivity of bodies 1 and 2, determined from the expression
F1 And F2 are the surface areas of bodies 1 and 2, m2; T 1 and T 2- absolute temperature of the surfaces of bodies 1 and 2, K.
Such a case is also called heat transfer by radiation between the body and its shell; the inner body is always body 1.
A special case of the considered heat transfer is heat transfer between two parallel unlimited walls (Fig. 15). When F 1 = F 2 = F, the calculation equation for heat transfer by radiation is used, and the reduced degree of emissivity is determined from the expression
Equation (2.57) can be used to calculate the radiant heat exchange between two bodies of any shape and their arbitrary location, only in each particular case to determine the reduced emissivity and surface (for ε 1.2 And F 1.2) have their own calculation expressions.
Question #38
Heat transfer through flat single-layer and multi-layer
flat wall
Heat equation: .
Boundary conditions of the first kind: .
Boundary conditions of the third kind: , .
In this series of equalities, the first equation determines the amount of heat transferred by convection (and radiation) from the hot coolant to the wall; the second equation is the same amount of heat transferred by thermal conductivity through the wall; the third equation is the transfer of the same amount of heat transferred by convection (and radiation) from the wall to the cold coolant.
Let us single out from this series of equalities the temperature difference
Adding the left and right parts of the equations characterizing the temperature difference and taking into account that we get the expression for the final temperature difference
where is the thermal resistance of a flat wall ( m 2 0 С\Bm)
From here, follows the expression for the heat flux density and heat flux (heat transfer equation of a flat wall)
Where q is the heat flux density ( W/m2);
Q is the heat flux ( W);
k=1/R- heat transfer coefficient of a flat wall (W / m 2 ºС)
where is the thermal resistance of heat transfer of a flat wall (m 2 ºС / W);
; - thermal resistances of heat transfer from the side of the hot coolant, thermal conductivity of the flat wall and thermal resistances of heat transfer from the side of the cold coolant, respectively.
The temperature of the inner and outer surface of the wall is determined from the following considerations:
hence we have
In the case of a multilayer wall
Question #39
Heat transfer- transfer of heat from one carrier to another through a solid surface separating them.
Stationary process- a process in which the temperatures of the media do not change, that is, .
Non-stationary process- a process in which the temperatures of the media change, that is, .
For curved walls, the heat transfer coefficient is usually determined by the same equation as for a flat wall. In this case, for curved walls, the calculated heat transfer surface is determined from the expression
The water equivalent of the heat transfer surface.
For cylindrical walls: .
Linear heat transfer coefficient: .
Heat transfer coefficient for inner wall: .
Heat transfer coefficient for outer wall: .
Question #40-41
Classification of heat exchangers.
1. By type of action:
a. Surface type devices - devices in which heat transfer occurs in the presence of a solid surface.
i. Regenerative devices are surface-type devices in which a solid surface is alternately washed by hot and cold coolants. These devices are used in cases where the heat carriers have high temperatures, or when the heat carriers are not clean.
ii. Recuperative devices are surface-type devices in which a solid surface is washed continuously with hot and warm heat carriers through separating surfaces.
1. Shell and tube heat exchangers.
2. Devices of the "pipe in pipe" type:
a. Single-flow devices of the "pipe in pipe" type.
b. Multi-flow devices of the "pipe in pipe" type.
b. Mixing type devices - devices in which there is a direct mixing of hot and cold heat carriers.
Scheme of the apparatus of the "pipe in pipe" type:
Apparatuses of this type have a simple design and high flow rates, however, to obtain large capacities of the apparatus, it is necessary to install a large number of structural elements and the apparatus itself will take up a lot of space.
Scheme of the apparatus of the shell-and-tube type:
In such devices, it is possible to create direct-flow, counter-flow, cross-flow, U-shaped symmetrical and other flows.
Thermal balance of the heat exchanger: , where is the efficiency coefficient of the thermal apparatus, .
1. (hydraulic resistance is low), then , , , at .
2. Capacitor.
3. Evaporator.
The power of the thermal apparatus (Grosgof's equation): , where is the average temperature difference.
For forward flow: , .
For countercurrent: , .
Where and are the water equivalents of the heat exchange surface.
For any scheme, it can be determined in accordance with two methods:
1. Classical technique: , where - coefficient depending on the type and properties of the warm apparatus, is determined from the graphs of functions and .
2. Belokon's method. Countercurrent index:
For forward flow.
For countercurrent.
For a U-shaped symmetrical circuit.
For any scheme, the average temperature difference: .
Question #42
There are two types of calculations for thermal devices:
1. Calculation of the first kind (constructive). Known: , , , , , , , . Task: Selection or design of a heat exchanger ( , ). , and - condensation temperature.
1. Vapor-compression refrigeration machines, in which the working fluid is steam, and the working process takes place in the compressor.
2. Air chillers, in which the working fluid is air.
3. Absorption refrigerators, in which vapors are absorbed by aqueous solutions.
4. Steam jet refrigeration machines with injectors as an actuator.
Working process of vapor compression refrigeration unit:
1-2 - adiabatic compression; 4-5 - throttling process.
Diagram of a vapor compression refrigeration unit:
Such installations operate in the following temperature range: .
8. Steam jet refrigeration machines with injectors as an actuator.
HEAT EXCHANGE
HEAT EXCHANGE(thermal energy transfer), the process of transferring heat from one object to another. Transfer occurs during the time when two or more bodies at different temperatures are in thermal contact. There are three types of heat transfer: HEAT CONVENTION, CONVECTION and RADIATION. With heat conduction, the transfer of heat occurs from molecule to molecule within the body, as, for example, with an iron rod inserted into a fire. In convection, heat is transferred by the circulation of a liquid or gas, as in boiling. When radiated, heat is transferred in the form of electromagnetic waves, like sunlight. Heat exchange processes are an integral part of many production processes, when heat energy from one source is transferred to another without their combination. The simplest example of heat transfer is the use of heat transfer, when a pipe system with a developed outer surface and a hot liquid flowing inside is immersed in a container through which another cold liquid flows, and as a result of heat transfer, heat is transferred from hot to cold liquid.
Three types of heat transfer can be seen when a pan is heated: (A) conduction through the metal walls of the pan (1), convective movement of the fluid (2) and radiation from the heat source transferred to the pan (3). In theory, a well-insulated conductor of heat, one end of which is placed in ice and the other in boiling water, changes temperature along its length (B) linearly, like a straight line on a graph. The temperature change characteristic of a poorly insulated conductor is shown by a curved dotted line. Thermos(C) contains a vacuum (4) between the walls to prevent heat conduction and convection, and silver plated walls to avoid heat loss through radiation.
Scientific and technical encyclopedic dictionary.
Synonyms:See what "HEAT EXCHANGE" is in other dictionaries:
Heat transfer ... Spelling Dictionary
Spontaneous irreversible process of heat transfer due to temperature gradient. In the general case, the transfer of heat can also be caused by the inhomogeneity of the fields of other physical. values, eg. concentration gradient (see DUFOUR EFFECT). Distinguish… … Physical Encyclopedia
HEAT TRANSFER, along with work in thermodynamics, is one of the types of energy exchange of a thermodynamic system (a physical body) with surrounding bodies, occurring through the processes of heat conduction, convection or radiation and not accompanied by ... ... Modern Encyclopedia
A spontaneous irreversible process of heat transfer from more heated bodies (or parts of bodies) to less heated ones (in the general case, heat transfer can also be caused by the inhomogeneity of fields of other physical quantities, for example, the difference in concentrations m. ... ... Big Encyclopedic Dictionary
Heat transfer, heat transfer, heat transfer Dictionary of Russian synonyms. heat exchange n., number of synonyms: 4 exchange (55) ... Synonym dictionary
HEAT EXCHANGE- a spontaneous irreversible process of distribution of thermal energy from more heated bodies or parts of the body to less heated ones without doing work. There are the following types of T.: (see), thermal conductivity (see) and heat transfer using radiation ... ... Great Polytechnic Encyclopedia
HEAT EXCHANGE, husband. (specialist.). The process of irreversible transfer of heat from hotter bodies to cooler ones. Heat exchange regulation. | adj. heat exchanger, oh, oh. Explanatory dictionary of Ozhegov. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 ... Explanatory dictionary of Ozhegov
heat exchange- Spontaneous irreversible heat transfer process due to a temperature gradient [Terminological dictionary for construction in 12 languages (VNIIIS Gosstroy of the USSR)] Topics thermodynamics EN heat exchangeintercambio térmico DE… … Technical Translator's Handbook
Heat exchange- - spontaneous process of heat transfer from more heated parts of concrete to less heated ones. [Terminological dictionary for concrete and reinforced concrete. Federal State Unitary Enterprise "Research Center" Construction "NIIZHB them. A. A. Gvozdeva, Moscow, 2007, 110 pages] Heading ... ... Encyclopedia of terms, definitions and explanations of building materials
Heat exchange- spontaneous irreversible transfer of heat in space with a non-uniform temperature field, characterized by a temperature gradient. Heat transfer occurs from more heated bodies to less heated ones and is characterized by a vector ... ... Encyclopedic Dictionary of Metallurgy
Books
- Heat transfer in single-phase media and during phase transformations. Textbook, V. V. Yagov, The content of the textbook corresponds to the program of the discipline "Heat and Mass Transfer", which is read to students studying in the field of thermophysics as part of the training direction "140700. ... Category: Thermodynamics and statistical physics Publisher: MPEI,
- Heat transfer and thermal testing of materials and structures of aerospace equipment under radiation heating , Victor Eliseev , The monograph is devoted to the problems of heat transfer and thermal testing of materials and structures of aerospace equipment using high-intensity radiation sources. The results are presented... Category: Educational literature Publisher:
Methods of heat transfer - heat is always transferred from bodies that are hotter to less hot. The methods of transferring heat from a solid body (wall) to a liquid or gas flowing around it are called heat transfer. The methods of transferring heat from one medium to another, separated by a partition (wall), are called heat transfer. There are three methods of heat transfer: conduction, convection and radiation (radiation).
Thermal conductivity is the process of heat propagation in a body (one) by transferring kinetic energy from more heated molecules to less heated ones that are in contact with each other. In its pure form, thermal conductivity takes place in solids in very thin, motionless layers of liquid and gas.
Heat transfer methods propagate through the walls of the boiler. The thermal conductivity of various substances is different. Metals are good conductors of heat. The thermal conductivity of air is very low. Weakly conduct heat porous bodies, asbestos, felt and soot.
Convection is the transfer of heat due to the movement of molar volumes of the medium. Usually, the convective method of heat transfer occurs together with thermal conductivity and is carried out as a result of free or forced movement of molar volumes of liquid or gases (natural or forced convection). Natural convection spreads heat from stoves, heating appliances, when heating water in steam boilers, cooling boiler linings and other heating devices. The free movement of liquids or gases is due to the different densities of heated and cold particles of the medium. For example, the air near the surface of the furnace heats up, becomes lighter, rises, and heavier, colder air enters in its place. As a result, air circulation occurs in the room, which transfers heat.
Heat transfer methods include convection. Forced convection occurs when heat is transferred from the inner wall of the boiler to the water moving under the action of the pump.
Radiation (radiation) is the transfer of heat from one body to another by means of electromagnetic waves through a medium transparent to thermal radiation. This process of heat transfer is accompanied by the transformation of thermal energy into radiant energy and, conversely, radiant energy into thermal energy. Radiation transfers heat from the flame of burning fuel to the surface of cast-iron sections or steel pipes of the boiler. Radiation is the most efficient way to transfer heat, especially if the radiating body has a high temperature, and the rays from it are directed perpendicular to the heated surface.
The concept of heat transfer. The three types of heat transfer discussed above in their pure form are very rare. In most cases, one species is accompanied by another. An example of this is the transfer of heat from the gaseous products of combustion to the wall of a hot water boiler (Fig. 7). On the left, its surface is in contact with hot gaseous combustion products and has a temperature t 1 on the right it is washed by water and has a temperature t 2 The temperature in the wall decreases in the direction of the x axis.
Rice. 7. Transfer of heat from gaseous products of combustion to the wall of the boiler.
In this case, the heat from the gas to the wall is transferred simultaneously by convection, heat conduction and radiation (radiant heat transfer). The simultaneous transfer of heat by convection, conduction and radiation is called complex heat transfer.
The result of the simultaneous action of individual elementary phenomena is attributed to one of them, which is considered the main one. So, radiation (radiation), also called direct recoil, plays a dominant role in the transfer of heat in the combustion chamber from the flue gases to the outer heating surface of the boiler, although along with it, both convection and thermal conductivity participate in the transfer of heat.
Methods for transferring heat from the outer heating surface to the inner one through a layer of soot, a metal wall and a layer of scale are carried out only by heat conduction. Finally, from the inner heating surface of the boiler to the water, heat is transferred only by convection. In the gas ducts of the boiler, the process of heat exchange between the wall of the section and the gases washing it is also the result of the combined action of convection, thermal conductivity and radiation. However, convection is taken as the main phenomenon.
The quantitative characteristic of the transfer of heat from one coolant to another through the wall separating them is the heat transfer coefficient K. For a flat wall, the coefficient K is the amount of heat transferred per unit time: from one liquid to another over an area of 1 m 2 with a temperature difference between them of one degree. - is determined by the formula:
K \u003d (1 / α 1 + δ 3 / λ 3 + δ st / λ st + δ n / λ n + 1 / α 2) -1
where α 1 is the coefficient of heat transfer from gases to the wall of the heating surface, W / (m 2 × deg); δ 3 - thickness of ash or soot deposits (the so-called external pollution), m; δ st - wall thickness of sections or pipes, m; δ n - scale thickness (the so-called internal pollution), m; λ 3 , λ st, λ in - the corresponding coefficients of thermal conductivity of ash or soot, walls and scale, W / (m × deg); α 2 -. heat transfer coefficient from the wall to water / W / (m 2 × deg).
In accordance with the above example of complex heat transfer (see Fig. 7), the total heat transfer coefficient, and from gases to the boiler wall, respectively, is equal to:
α 1 \u003d α k + α l
where α to and α l - coefficients, heat transfer by convection and radiation.
The reciprocal of the heat transfer coefficient is called the thermal resistance to heat transfer. For this case:
R \u003d 1 / K \u003d 1 / α 1 + δ 3 / λ 3 + δ st / λ st + δ n / λ n + 1 / α 2
Different substances have different thermal conductivity coefficients.
Thermal conductivity coefficient K - the amount of heat transferred through a unit area of the heating surface per unit time with a temperature difference of 1 degree and a wall thickness of 1 m. When using off-system units (kcal per hour), the dimension of the thermal conductivity coefficient is kcal × m / (m 2 × h × deg), in the SI system - W / (m × deg).
The coefficients of thermal conductivity of various materials, most often found in heating and boiler equipment, are given below, W / (m × deg).
The amount of heat Q transferred through the wall is determined by the formula:
where K - heat transfer coefficient, W / (mg × deg); ∆t is the average temperature difference between the heating and heated media or the average logarithmic temperature difference, deg; H is the heating surface area, m 2 .
The mean logarithmic temperature difference ∆t is determined by the formula:
∆t = ∆t - ∆t m /2.31 g (∆t 0 /∆t m)
where ∆t g and ∆t m are the largest and smallest temperature differences between the heating and heated medium.
Rice. 8. The nature of the change in the temperatures of working fluids at
a - forward flow; b - countercurrent.
Character of change of temperatures of working liquids is shown in fig. 8. If the heating and heated fluids flow in the same direction in the heat exchanger, then such a flow pattern is called forward flow (see Fig. 8, a), and in the opposite direction - counterflow (see Fig. 8, b).
For a unit area of the heat transfer surface, the specific flux, denoted by q, will be equal to:
From the above formulas it can be seen that the amount of heat transferred is the greater, the larger the heating surface area H and the greater the average temperature difference or temperature difference and heat transfer coefficient K. The presence of scale, ash or soot on the boiler wall significantly reduces the heat transfer coefficient (see example below ).
The determining factor in the transfer of heat by radiation is the temperature of the radiating body and the degree of its blackness. Therefore, in order to intensify the transfer of heat by radiation, it is necessary to increase the temperature of the radiating body by increasing the surface roughness.
Heat transfer by convection depends on: the speed of movement of gases, the temperature difference between the heating and heated medium, the nature of the flow of gases around the heating surface - longitudinal or transverse, the type of surface - smooth or ribbed. The main ways to intensify heat transfer by convection are: increasing the speed of gases, swirling them in gas ducts, increasing the heating surface area due to its finning, increasing the temperature difference between the heating and heated media, and countercurrent (countercurrent) washing.
Example. Consider the effect of scale and soot on heat transfer in a boiler using the data in this section. We accept the wall thickness of the cast-iron boiler section δ 1 \u003d 8 mm, and the layer of scale deposited on it with a thickness of δ 2 \u003d 2 mm and the soot layer δ 3 \u003d 1 Gmm. The coefficients of thermal conductivity of the wall λ 1 , scale λ 2 and soot λ 3, respectively, are taken equal to 54; 0.1 and 0.05 kcal / (m × h × deg) (√62.7; 0.116 and 0.058 W / (m 2 × K). Values of heat transfer coefficients: from, gases to the wall α 1 \u003d 20 kcal / ( m 2 × deg); from the wall to the water α 2 = 1000 kcal / (m 2 × h × deg). The temperature of the gases is taken equal to t gas = 800 ° C, water t = 95 C.
We make calculations for clean and contaminated walls of a cast-iron boiler.
A. The boiler wall is clean.
Find the heat transfer coefficient:
K \u003d (l / α 1 + δ / λ + l / α 2) -1 \u003d (1/20 + 0.008 / 54 + 1/1000) -1 \u003d 1 / 0.0512 \u003d 19.5 kcal / (m 2 × h × deg) = 22.6 W / (m 2 × deg) and heat flux through the wall.
q \u003d K∆t \u003d 19.5 (800-95) \u003d 13700 kcal / (m 2 × h) \u003d 15850 W / (m 2).
Let us determine the temperature of the outer surface of the wall of the cast-iron section using the formula
q \u003d α 1 (t gas - t st) -1 q \u003d α 1 t gas - α 1 t st; α i t st = α 1 t gas
t st \u003d t gas - q / α 1 \u003d 800 - 13700/20 \u003d 115 ° С.
It can be seen from the calculation that with a clean wall of the boiler, its temperature differs little from the temperature of the water inside the boiler.
B. Boiler wall dirty.
Repeating the whole calculation, we find:
K \u003d (l / α 1 + δ 1 / λ 1 + δ 2 / λ 2 + δ 3 / λ 3 + 1 / α 2) -1 \u003d (1/20 + 0.008 / 54 + 0.002 / 0.1 (+ 0.001 / 0.05+ 1 \u003d 1000) -1 \u003d (0.0912) -1 \u003d 11 kcal / (m 2 × h × 1 × hail) \u003d 12.7 W / (m 2 × hail)
q \u003d 11 (800 - 95) \u003d 7750 kcal / (m 2 × h) \u003d 8960 W / (m 2), t st \u003d 800 - 7750/20 \u003d 412C.
It can be seen from the calculation that the deposition of soot is undesirable because, having a low thermal conductivity, it makes it difficult to transfer heat from the flue gases to the boiler walls. This leads to an excessive consumption of fuel, a decrease in the production of steam or hot water by boilers.
Scale, having a low thermal conductivity, significantly reduces the transfer of heat from the boiler wall to the water, as a result of which the walls become very overheated and in some cases; burst, causing boiler failures.
Comparing the results of the calculation, we see that the heat transfer through the contaminated wall has almost halved, the wall temperature of the cast-iron section during scale has increased to dangerous limits, according to the strength of the metal, which can lead to section rupture. This example clearly shows the need for regular cleaning of the boiler from both scale and soot or ash.
Instruction
Thermal conductivity is the transfer of heat from more heated parts of a substance to less heated parts, leading to equalization of the temperature of the substance. Molecules of a substance with more energy transfer it to molecules with less energy. Thermal conductivity refers to the Fourier law, which is the relationship between the temperature gradient in the medium and the heat flux density. The gradient is a vector showing the direction in which the scalar field changes. Deviations from this law can occur at very strong shock waves (large gradient values), at very low temperatures and in rarefied gases, when the molecules of a substance more often collide with the vessel walls than with each other. In the case of rarefied gases, the heat transfer process is considered not as heat transfer, but as heat transfer between bodies in a gaseous medium.
This is the transfer of heat in liquids, gases or solids, according to kinetic theory. The essence of the kinetic theory is that all bodies (material) consist of atoms and molecules that are in continuous motion. Based on this theory, convection is the transfer of heat between substances at the molecular level, provided that the bodies are under the influence of gravity and unevenly heated. A heated substance, under the action of gravity, moves relative to a less heated substance in the opposite direction to gravity. The warmer substances rise and the colder substances sink. The weakening of the effect of convection is observed in cases of high thermal conductivity and a viscous medium, and also the degree of its ionization and the magnetic field strongly affect convection in ionized gases.
Thermal radiation. A substance, due to internal energy, creates electromagnetic radiation with a continuous spectrum, which can be transmitted between substances. The position of the maximum of its spectrum depends on how hot the substance is. The higher the temperature, the more energy the substance releases and, therefore, the more heat can be transferred.
Heat transfer can occur through a thin partition or wall between bodies, from a warmer substance to a less warm one. A more heated substance transfers part of the heat to the wall, after which the process of heat transfer occurs in the wall and heat is transferred from the wall to a less heated substance. The intensity of the amount of heat transferred directly depends on the heat transfer coefficient, which is defined as the amount of heat transferred through the unit surface area of the partition per unit time at a temperature difference between substances of 1 Kelvin.
