In nature, there are three types of heat transfer: 1) thermal conductivity; 2) convection; 3) radiation.
Thermal conductivity
Thermal conductivity is the transfer of heat from one body to another when they come into contact or from a warmer part of the body to a cold one.
Various substances have different thermal conductivity. All metals have great thermal conductivity. Gases have low thermal conductivity; a vacuum has no thermal conductivity (in a vacuum there are no particles that would provide thermal conductivity).
Substances that conduct heat poorly are called heat insulators.
Artificially created heat insulators include stone wool, polystyrene foam, foam rubber, metal ceramics (used in the production of spaceships).
Convection
The spread of heat by moving streams of gas or liquid is called convection.
During convection, heat is transferred by the substance itself. Convection is observed only in liquids and gases.
Thermal radiation
Distribution of heat from a warm body using infrared rays called thermal radiation.
Thermal radiation is the only type of heat transfer that can occur in a vacuum. The higher the temperature, the stronger the thermal radiation. Thermal radiation is produced, for example, by people, animals, the Earth, the Sun, a stove, a fire. Infrared radiation can be imaged or measured with a thermograph (heat camera).
| Infrared thermal cameras sense invisible infrared or thermal radiation and provide accurate, non-contact temperature measurements. Infrared thermography allows thermal radiation to be fully visualized. The figure shows the infrared radiation of a human palm. | |
............................................................................. During thermographic inspection of buildings and structures, it is possible to detect structural areas with increased thermal permeability, check the quality of connections of various structures, and find areas with increased air exchange. |
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Examples of 15-20 thermal phenomena indicating which one (radiation; convection; heat transfer)
Heating and cooling, evaporation and boiling, melting and solidification, condensation are all examples of thermal phenomena.
The main source of heat on Earth is the Sun. But also, people use a lot artificial sources heat: fire, stove, water heating, gas and electric heaters, etc.
It was not immediately possible to answer the question of what heat is. Only in the 18th century did it become clear that all bodies are made of molecules, that molecules move and interact with each other. Then scientists realized that heat is related to the speed of movement of molecules. When bodies are heated, the speed of molecules increases, and when they cool, they decrease.
You know that if you put a cold spoon into hot tea, after a while it will heat up. In this case, the tea will give up some of its heat not only to the spoon, but also to the surrounding air. It is clear from the example that heat can be transferred from a more heated body to a less heated body. There are three methods of heat transfer - thermal conductivity, convection, radiation.
Heating a spoon in hot tea is an example of conduction. All metals have good thermal conductivity.
Convection transfers heat in liquids and gases. When we heat water in a saucepan or kettle, the lower layers of water warm up first, they become lighter and rush upward, giving way to cold water. Convection occurs in a room when the heating is on. Hot air the battery rises and the cold one falls. But neither thermal conductivity nor convection can explain how, for example, the Sun, far from us, heats the Earth. In this case, heat is transferred through airless space by radiation (heat rays).
A thermometer is used to measure temperature. You usually use room or medical thermometers.
When we talk about Celsius temperature, we mean a temperature scale in which 0°C corresponds to the freezing point of water, and 100°C is its boiling point.
In some countries (USA, UK) the Fahrenheit scale is used. In it, 212°F corresponds to 100°C. Converting temperature from one scale to another is not very simple, but if necessary, each of you can do it yourself. To convert a Celsius temperature to a Fahrenheit temperature, multiply the Celsius temperature by 9, divide by 5, and add 32. To do the reverse conversion, subtract 32 from the Fahrenheit temperature, multiply the remainder by 5, and divide by 9.
In physics and astrophysics, another scale is often used - the Kelvin scale. In it, the most low temperature in nature (absolute zero). It corresponds to -273°C. The unit of measurement in this scale is Kelvin (K). To convert Celsius temperature to Kelvin temperature, you need to add 273 to degrees Celsius. For example, Celsius is 100°, and Kelvin is 373 K. To convert back, you need to subtract 273. For example, 0 K is -273°C.
It is useful to know that the temperature on the surface of the Sun is 6000 K, and inside it is 15,000,000 K. The temperature in outer space far from stars is close to absolute zero.
We think that you don’t need to be convinced of how important thermal phenomena are. Knowledge about them helps people design home heaters, heat engines (internal combustion engines, steam turbines, jet engines, etc.), predict the weather, melt metal, create thermal insulation and heat-resistant materials that are used everywhere - from building houses to space ships.
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Here you can download a lesson summary for grade 8 “Thermal conductivity, convection, radiation” for the subject: Physics. This document will help you prepare good and high-quality material for the lesson.
Subject: Physics and Astronomy
Class: 8 rus
Lesson type: Combined
Purpose of the lesson:
Technical means training: ___________________________________________________
_______________________________________________________________________
Lesson structure
1.Organization of the lesson (2 min.)
Greeting students
2. Homework survey (15 min) Topic: Internal energy. Ways to change internal energy.
3. Explanation of new material. (15 min)
These types of heat transfer have their own characteristics, but the transfer of heat in each of them always goes in one direction: from a more heated body to a less heated one. In this case, the internal energy of a hotter body decreases, and that of a colder body increases.
The phenomenon of energy transfer from a more heated part of the body to a less heated one or from a more heated body to a less heated one through direct contact or intermediate bodies is called thermal conductivity.
In a solid body, particles are constantly in oscillatory movement, but do not change their equilibrium state. As the temperature of a body increases when it is heated, the molecules begin to vibrate more intensely, as their kinetic energy increases. Part of this increased energy is gradually transferred from one particle to another, i.e. from one part of the body to neighboring parts of the body, etc. But not all solids transfer energy equally. Among them there are so-called insulators, in which the mechanism of thermal conduction occurs quite slowly. These include asbestos, cardboard, paper, felt, granite, wood, glass and a number of other solids. Medb and silver have greater thermal conductivity. They are good heat conductors.
Liquids have low thermal conductivity. When a liquid is heated, internal energy is transferred from a more heated region to a less heated one during collisions of molecules and partly due to diffusion: faster molecules penetrate into a less heated region.
In gases, especially rarefied ones, the molecules are located at fairly large distances from each other, so their thermal conductivity is even less than that of liquids.
Vacuum is a perfect insulator, because there are no particles in it to transfer internal energy.
Depending on the internal state, the thermal conductivity of different substances (solid, liquid and gaseous) is different.
It is known that the thermal conductivity of water is low, and when the upper layer of water is heated, the lower layer remains cold. Air is an even worse conductor of heat than water.
Convection is a heat transfer process in which energy is transferred by jets of liquid or gas. Convection in Latin means “mixing.” Convection does not exist in solids and does not occur in a vacuum.
Covection, widely used in everyday life and technology, is natural or free.
A heat sink is a device that is a flat cylindrical container made of metal, one side of which is black and the other shiny. There is air inside it, which, when heated, can expand and escape out through the hole.
Absorption is the process of converting radiation energy into internal energy of a body.
Black surface is the best emitter and best absorber, followed by rough, white and polished surfaces.
4. Consolidation: (10 min) self-test questions, assignments and exercises
specific tasks: 1) Comparison of the thermal conductivity of metal and glass, water and air, 2) Observation of convection in a living room.
6. Assessment of student knowledge. (1 min)
Basic literature: Physics and astronomy grade 8
Additional reading: N. D. Bytko “Physics” parts 1 and 2
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Here you can download Thermal Conductivity. Convection. Radiation, 8th grade for the subject: Physics. This document will help you prepare good and high-quality material for the lesson.
Physics lesson notes in 8th grade
Koshikova Victoria Alexandrovna,
physics teacher
MBOU secondary school No. 47 of the city of Belgorod, Belgorod region
Lesson topic: “Thermal conductivity. Convection. Radiation".
Thermal conductivity. Convection. Radiation
The purpose of the lesson: to organize activities for the perception, comprehension and primary memorization of new knowledge and methods of activity.
Lesson progress
1. Organizational stage
2. Checking homework
Testing (2 options)
1. Temperature is physical quantity, characterizing...
a) ...the ability of bodies to do work.
b) ...different states of the body.
c) ...degree of body heating.
2. What air temperature was recorded by the thermometer shown in the figure? What is the error in measuring temperature?
a) 30.5 °C; 0.5 °C. b) 32 °C; 0.5 °C.
c) 32 °C; 1 °C. d) 30 °C; 1 °C.
3. One glass contains warm water(No. 1), in another - hot (No. 2), in the third - cold (No. 3). In which of them is the water temperature the highest, in which do the water molecules move at the lowest speed?
a) No. 2; No. 3. b) No. 3; No. 2. c) No. 1; No. 3. d) No. 2; No. 1
4. Which of the following phenomena are thermal?
a) Falling onto half a spoon. b) Heating the soup on the stove.
c) Snow melting in the sun. d) Swimming in the pool.
5. What molecules of the body are involved in thermal motion? At what temperature?
a) Located on the surface of the body; at room temperature.
b) All molecules; at any temperature,
c) Located inside the body; at any temperature.
d) All molecules; at high temperature.
6. In the room in identical vessels under the piston there are equal masses carbon dioxide. In which vessel does the gas have the greatest energy at the positions of the pistons shown in the figure?
7. In which of the following cases does the internal energy of a body change?
a) A stone, falling off a cliff, falls faster and faster.
b) The dumbbells are lifted from the floor and placed on a shelf.
c) The electric iron was plugged in and the laundry began to be ironed.
d) The salt was poured from the bag into the salt shaker.
8. The change in internal energy of which body occurs as a result of heat transfer in the above situations?
a) Heating of the drill bit when making a hole with a drill.
b) A decrease in gas temperature as it expands.
c) Cooling a stick of butter in the refrigerator,
d) Heating of the wheels of a moving train.
Test on the topic:
1. Temperature unit...
a) ...joule. b) ...pascal. c) ...watt. d) ...degree Celsius.
2. Body temperature depends on...
a) ...him internal structure. b) ...the density of its substance.
c) ...the speed of movement of its molecules. d) ...the number of molecules in it.
3. How do the molecules of hot tea differ from the molecules of the same tea when it has cooled?
a) Size. b) Speed of movement.
c) The number of atoms in them. d) Color.
4. What movement is called thermal?
a) The movement of a body during which it heats up.
b) Constant chaotic movement of the particles that make up the body.
c) The movement of molecules in the body at high temperature.
5. Internal energy is the energy of body particles. It consists of...
a) ...the kinetic energy of all molecules.
b) ...potential energy of interaction between molecules.
c) ...kinetic and potential energies of all molecules.
6. What energy does the balloon launched by meteorologists have?
a) Kinetic. b) Potential.
c) Internal. d) All these types of energy.
7. In what ways can you change the internal energy of the body?
a) By setting it in motion. b) By performing work on the body or on it.
c) Raising it to a certain height. d) By heat transfer.
8. In what example does the internal energy of a body change as a result of performing mechanical work?
a) A teaspoon is dropped into a glass of hot water.
b) When the truck braked sharply, a burning smell came from the brakes.
c) Water is boiling in the electric kettle.
d) A person warms his frozen hands by pressing them to a warm radiator.
"Thermal movement. Temperature. Internal energy"
"Thermal movement. Temperature. Internal energy"
3. Updating the subjective experience of students
Internal energy
Ways to increase internal energy
Heat transfer
Types of Heat Transfer
4. Learning new knowledge and ways of doing things
1. Thermal conductivity is the phenomenon of transfer of internal energy from one part of the body to another or from one body to another upon their direct contact.
Fig.7,8 (Peryshkin textbook)
Liquids and gases have low thermal conductivity, because the distance between molecules is greater than that of solids.
The following materials have poor thermal conductivity: wool, hair, paper, bird feathers, cork, vacuum.
2. Convection - transfer of energy by jets of gas or liquid.
In order for convection to occur in gases and liquids, they must be heated from below.
3. Radiation – transfer of energy by various rays, i.e. in the form of electromagnetic waves.
5. Initial check of understanding of what has been learned
6. Consolidation of what has been learned
Work on the collection of Lukashik problems No. 945-955
7. Results, homework
items 4-6, exercises 1-3
8. Reflection
List of used literature
1. Peryshkin A.V. Physics. 8th grade. - M.: Bustard, 2009.
2. Gromov S.V., Rodina N.A. Physics. 9th grade - M.: Prosveshchenie, 2002.
3. Chebotareva V.A. Physics tests. 8th grade – Publishing house “Exam”, 2009.
4. Lukashik V.I., Ivanova E.V. Collection of problems in physics grades 7-9 - M.: Prosveshchenie, 2008.
docbase.org
Topic: Thermal conductivity, convection, radiation.
Lesson type: Combined
Purpose of the lesson:
Educational: introduce the concept of heat transfer, types of heat transfer, explain that heat transfer with any type of heat transfer always goes in one direction; that depending on the internal structure, the thermal conductivity of various substances (solid, liquid and gaseous) is different, that a black surface is the best emitter and the best absorber of energy.
Developmental: develop cognitive interest in the subject.
Educational: to develop a sense of responsibility, the ability to competently and clearly express one’s thoughts, be able to behave and work in a team
Intersubject communication: chemistry, mathematics
Visual aids: 21-30 drawings, thermal conductivity table
Lesson structure
1.Organization of the lesson (2 min.)
Greeting students
Checking student attendance and class readiness for class.
2. Homework survey (10 min) Topic: Internal energy. Ways to change internal energy.
3.Physical dictation (mutual testing) (5 min)
4. Explanation of new material. (15 min)
The method of changing internal energy in which particles of a more heated body, having greater kinetic energy, in contact with a less heated body, transfer energy directly to the particles of a less heated body is called heat transfer. There are three methods of heat transfer: thermal conductivity, convection and radiation.
These types of heat transfer have their own characteristics, but the transfer of heat in each of them always goes in one direction: from a more heated body to a less heated one. In this case, the internal energy of a hotter body decreases, and that of a colder body increases.
The phenomenon of energy transfer from a more heated part of the body to a less heated one or from a more heated body to a less heated one through direct contact or intermediate bodies is called thermal conductivity.
In a solid body, particles are constantly in oscillatory motion, but do not change their equilibrium state. As the temperature of a body increases when it is heated, the molecules begin to vibrate more intensely, as their kinetic energy increases. Part of this increased energy is gradually transferred from one particle to another, i.e. from one part of the body to neighboring parts of the body, etc. But not all solids transfer energy equally. Among them there are so-called insulators, in which the mechanism of thermal conduction occurs quite slowly. These include asbestos, cardboard, paper, felt, granite, wood, glass and a number of other solids. Copper and silver have greater thermal conductivity. They are good heat conductors.
Liquids have low thermal conductivity. When a liquid is heated, internal energy is transferred from a more heated region to a less heated one during collisions of molecules and partly due to diffusion: faster molecules penetrate into a less heated region.
In gases, especially rarefied ones, the molecules are located at fairly large distances from each other, so their thermal conductivity is even less than that of liquids.
A vacuum is a perfect insulator because it lacks particles to transfer internal energy.
Depending on the internal state, the thermal conductivity of different substances (solid, liquid and gaseous) is different.
Thermal conductivity depends on the nature of energy transfer in a substance and is not related to the movement of the substance itself in the body.
It is known that the thermal conductivity of water is low, and when the upper layer of water is heated, the lower layer remains cold. Air is an even worse conductor of heat than water.
Convection is a heat transfer process in which energy is transferred by jets of liquid or gas. Convection in Latin means “mixing”. Convection does not exist in solids and does not occur in a vacuum.
Convection, widely used in everyday life and technology, is natural or free.
When liquids or gases are mixed with a pump or stirrer to uniformly mix them, convection is called forced convection.
A heat sink is a device that is a flat cylindrical container made of metal, one side of which is black and the other shiny. There is air inside it, which, when heated, can expand and escape out through the hole.
In the case when heat is transferred from a heated body to a heat sink using heat rays invisible to the eye, the type of heat transfer is called radiation or radiant heat transfer
Absorption is the process of converting radiation energy into internal energy of a body.
Radiation (or radiant heat transfer) is the process of transferring energy from one body to another using electromagnetic waves.
The higher the body temperature, the higher the radiation intensity. The transfer of energy by radiation does not require a medium: heat rays can also propagate through a vacuum.
Black surface is the best emitter and best absorber, followed by rough, white and polished surfaces.
Good energy absorbers are good energy emitters, and bad energy absorbers are bad energy emitters.
5. Consolidation: (10 min) self-test questions, assignments and exercises
7. Assessment of students' knowledge (1 min). Reflection.
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The formation of steam at the liquid-vapor interface occurs due to heat supplied from the heating surface through the vapor layer through thermal conductivity and radiation.
The interaction of flammable vapors with oxygen in the air occurs in the combustion zone, into which flammable vapors and air must continuously flow. This is possible if the liquid receives a certain amount of heat necessary for evaporation. Heat during the combustion process comes only from the combustion zone (flame), where it is continuously released. Heat from the combustion zone to the surface of the liquid is transferred through radiation. Heat transfer by thermal conductivity is impossible, since the speed of movement of vapors from the surface/liquid to the combustion zone is greater than the speed of heat transfer through them from the combustion zone to the liquid. Heat transfer by convection is also impossible; tan is a vapor flow
The distribution of heat inside a body is possible in two ways: thermal conduction and convection. In the first method, heat spreads due to collisions of molecules, and the molecules of the hotter part of the body, which have on average greater kinetic energy, transfer part of it to neighboring molecules. Thus, heat can spread in a body even in the absence of obvious movement of its parts, for example in a solid body. In liquids and gases, along with thermal conductivity, heat distribution usually also occurs by convection, i.e., by direct transfer of heat by more heated masses of the liquid, which during movement occupy the places of less heated masses. In gases, it is also possible for heat to spread from one part of the gas to another through radiation.
Heat from the combustion zone to the surface of oil waste is transferred mainly through radiation. There is no thermal conductivity towards the evaporating layer, since the speed of movement of vapors from the surface of the liquid to the combustion zone is greater than the speed of their transfer of heat from the combustion zone to the liquid.
Heat transfer by convection from the surface solid to a liquid (gas) or vice versa occurs in cases where particles of a gas or liquid change their location relative to a given surface and at the same time act as heat carriers. The movement of such particles is caused either by the movement of the entire mass of liquid (gas) under the influence of an external influence (forced convection), or is a consequence of the difference in the densities of the substance at different points in space, caused by the uneven distribution of temperatures in the mass of the substance (natural or free convection). Convection is always accompanied by heat transfer through conduction and radiation.
If energy transfer occurs simultaneously in a medium through radiation and thermal conductivity, then the quantity characterizing the intensity of this transfer at a given point will be the vector Chx = Chl Ch, where
When considering a number of applied problems, it is interesting to study the process of heat transfer in periodic media containing vacuum layers or cavities, where heat transfer is carried out only by radiation. In other cases, these cavities are filled with gas with negligible thermal conductivity and absorption coefficients. In this case, one can often neglect the presence of gas and consider these cavities as vacuum. Structures and materials containing interlayers and nolo-
Loose materials with low volumetric weight, such as powders and fibers filled with gas at atmospheric pressure, are used to insulate air liquefiers, liquid oxygen and nitrogen tanks, gas separation columns and other equipment whose temperature does not fall below the boiling point liquid nitrogen. In such insulating materials, the ratio of the volume of the gas space to the volume of the solid material can be from 10 to 100. In FIG. Figure 5.53 shows the thermal conductivity coefficients of some common loose materials. The thermal conductivity of the best examples of these materials approaches that of air, indicating that the air occupying the space between the particles carries most of the heat. This explains the principle of gas-filled insulation, the solid material of which prevents heat transfer by radiation and convection. In an ideal case, heat transfer due to thermal conductivity of the solid material is negligible, and heat is transferred only by the gas. In actual insulation, some heat passes directly through the powder particles or fibers, and the resulting thermal conductivity is usually somewhat greater than that of the gas. The exception is very fine powders, the distances between the particles of which are so small that the average free path of gas molecules is greater than these distances; the thermal conductivity of the gas in this case decreases, as with decreasing pressure. Thus, the thermal conductivity of powder insulation, even if the powder is filled with gas at atmospheric pressure, can be less than the thermal conductivity of the gas filling the space between the particles.
In a good vacuum, heat transfer by the residual gas is negligible. Therefore, when designing vessels, they strive to reduce the heat flow through the supporting elements and heat transfer through radiation. The heat flow through the insulating supports is determined design features and mechanical strength of supports general solution this question is impossible. If the dimensions of the vessel are not limited, then by increasing the length of the supports and using a material with low thermal conductivity, it is possible to ensure a very small heat transfer along the supports. Even in limited spaces, an experienced designer can usually find a way to increase the thermal resistance of the supports. In contrast, radiative heat transfer weakly depends on the thickness of the insulating space; with a small thickness of the vacuum space, its insulating properties are even slightly improved due to the approximation
Heat transfer through any wall from a hotter coolant to another, colder coolant is a relatively complex phenomenon. If we take, for example, a tube bundle of an evaporator, which is heated by flue gases, then there are three elementary methods of heat transfer, which are considered as the main ones. The heat of the flue gases is transferred to the beam tubes through conduction, convection and radiation. Heat is transferred through the walls of the tubes only through thermal conductivity, and from the inner surface of the tube to
Thermal conductivity is concerned with the transfer of heat through the movement and collision of atoms and molecules that make up a substance. It is similar to the process of diffusion, in which material is transferred using a similar mechanism. Convection is the transfer of heat through the movement of large aggregates of molecules, i.e., in essence, it is similar to the mixing process. It is obvious that heat transfer by convection can only occur in liquids and gases, while thermal conduction is the main type of heat transfer in solids. In liquids and gases, along with convection, thermal conductivity is also observed, but the first is a much faster process and usually completely masks the second process. Both conduction and convection require a material medium and cannot occur in a complete vacuum. This highlights the main difference between these two processes and the process of radiation, which occurs best in a vacuum. The exact process by which the transmission of energy by radiation through empty space is effected has not yet been established, but for our purpose it will be convenient to regard it as occurring by means of wave motion in a purely hypothetical medium (the ether). It is believed that the internal energy of a substance is transferred to the wave motion of the ether; this motion propagates in all directions, and when a wave collides with a substance, the energy can be transmitted, reflected or absorbed. When absorbed, it can increase the body's internal energy in three ways: 1) by causing a chemical reaction,
In such high-temperature processes as glass melting, brick firing, aluminum smelting, etc., where the temperature of the exhaust flue gases is inevitably high, the amount of useful fuel heat in the overall combustion heat balance is a small part (in the previous example - 36% without taking into account losses due to radiation from the furnace walls). Therefore, in in this case Fuel savings can be achieved by using heat recovery devices, such as recuperators to preheat the air supplied to fuel combustion or waste heat boilers to generate additional steam, as well as by improving thermal insulation to reduce losses by radiation, heat conduction and convection outer surface furnace walls into the surrounding space.
Heat exchange in the core, intermediate medium and at the boundaries between them is carried out through thermal conductivity of the element of the solid skeleton of the material, heat transfer from one solid particle to the neighboring one in places of their direct contact, molecular thermal conductivity in the medium filling the gaps between particles, heat transfer at the boundaries of solid particles with external environment radiation from particle to particle through the intermediate medium, convection of gas and moisture contained between the particles.
Layers condensed in vacuum are extremely sensitive to the conditions of their formation, in particular to the temperature of the substrate, the intensity of condensation, the temperature of the condensed gas, the power of the heat flow supplied to the condensation surface by radiation and through the thermal conductivity of the residual gas.
In connection with the above, it is clear that the thermal conductivity coefficient of condensate in equation (5.52) is a thermal characteristic not of a monolithic body, but of a highly dispersed material. This material - condensate - consists of a skeleton - a skeleton, which is a collection huge amount solid particles - crystals separated from each other by spaces filled with residual gas. In such a complex material, heat transfer is no longer limited to the thermal conductivity of a solid body, but is carried out through heat transfer along individual particles - an element of the solid skeleton of the heat transfer material, thanks to thermal conductivity from one solid particle to the neighboring one in places of their direct contact, thermal conductivity of the residual gas in the pores and voids between particles of radiation from particle to particle.
General provisions. In technology, we often have to deal with such cases of heat exchange, when the temperature of the environment with which this surface exchanges heat is specified, and not the temperature of the wall surface. Compared with the issues of thermal conductivity and thermal radiation by solids, the problem of heat transfer from the surrounding liquid or gaseous medium to the wall surface through convection is much more complex, and therefore, to a large extent, is still far from resolved to this day. When we are dealing with the transfer of heat from a solid to a liquid or gas, the heat exchange due to thermal conductivity recedes in magnitude compared to heat exchange due to convection. The latter, as mentioned above, is that in a moving layer of liquid or gas adjacent to the wall, due to the flow existing in this
Heat transfer from one body to another can occur through conduction, convection and thermal radiation.
Many solid and liquid polymers are almost completely impenetrable to infrared radiation, so incident energy is absorbed by the body and converted into heat at its surface. However, some heat is still immediately lost to the environment through convection and radiation. The absorbed heat spreads into the body through the process of conductive heat transfer. The temperature distribution in a body heated by radiant energy depends not only on the heat flow, but also on the thermal conductivity of the substance and convective heat losses from the surface.
Heat transfer can be accomplished through one of the following three methods or a combination of them. These methods are barely 1) thermal conduction, 2) convection and 3) radiation
One of the most common and oldest (proposed in 1880) is the thermal conductometric method. The operation of thermal conductometric gas analyzers is based on the dependence of the electrical resistance of a conductor with a large temperature coefficient of resistance on the thermal conductivity of the mixture surrounding the conductor. Heat is transferred through a gaseous medium through conduction, convection and radiation. The thermal conductivity of a gas is related to its composition. They strive to reduce or stabilize the proportion of heat transfer by convection and radiation.
Thus, the circulating water in a particular cooler is cooled through heat transfer atmospheric air, and part of the heat is transferred as a result of surface evaporation of water - the conversion of part of the water into steam and the transfer of this steam by diffusion into the air, the other part - due to the difference between the temperatures of water and air, i.e. heat transfer by contact (thermal conduction and convection). A very small amount of heat is also removed from the water by radiation, which is usually not taken into account in the heat balance. At the same time, there is a heat influx to the cooled water from solar radiation, which is so small that it is neglected in the heat balance of cooling towers and spray pools.
Heat is transferred from more heated bodies to less heated bodies through thermal conductivity, convection and thermal radiation. -
Comparison of heat transfer processes due to radiation and thermal conductivity. Thermal conductivity is caused by the movement of microparticles of the body; heat exchange by radiation is carried out through electromagnetic waves or photons. There is no thermal conductivity in the void. Heat exchange by radiation between bodies occurs both in the presence and absence of a material medium. If the medium does not absorb radiation, then its temperature does not affect the heat transfer process in any way. For example, you can set a wooden object on fire by focusing the sun's rays using a lens made of ice.
The combustion of fuel is accompanied by the release and transfer of heat, as well as losses, or more precisely, the dissipation of heat into the surrounding environment. The transfer of heat occurs by convection, that is, it is directly moved by a rising gas flow, as well as by a flow of solid particles. In addition, heat transfer occurs within gas and particle flows through conduction and radiation. Thermal conductivity in gas and particle environments, as well as molecular diffusion, takes place regardless of their movement. Pot1 and mass and heat due to diffusion and thermal conductivity arise together in the presence of gradients - temperature and concentrations (more precisely, chemical potential x) - and are determined by mutual linear functions and y7 (see Chapters V and VI). But in practice, heat transfer due to the concentration gradient, as well as mass transfer due to the temperature gradient (thermal diffusion) can be neglected.
For an isothermal flow T - onst and from the relation p = pRT follows the formula (Za) at - 1. In the case of an adiabatic flow, it is assumed that heat is transferred only through convection (there is no thermal conductivity or radiation) and we have dQ = O in the formula ( 21). For a single
Several kilowatts. Using an auxiliary circuit, a spark is created that generates a number of ions, and then a strong ring current is generated in the ionized gas by magnetic induction. The resulting plasma is heated to several tens of thousands of degrees Kelvin, which is significantly higher than the temperature at which quartz glass softens. Obviously, it is necessary to find a way to protect the source from self-destruction, which is achieved using an argon current, which acts as a cooler. Argon is supplied tangentially from the outer tube at high speed (Fig. 9-6), which creates a vortex flow (shown in the figure) and the temperature decreases. Hot plasma tends to stabilize at some distance from the walls in the shape of a toroid, which also prevents overheating. The sample is sprayed into a nebulizer (not shown in the figure) and carried away by a slow current of argon to the center (to the hole in the pie). Here it is heated by thermal conductivity and radiation up to 7000 K and is completely atomized and excited. The loss of detectable atoms due to ionization (a source of difficulty in plasma AAS) does not play a major role in ICP spectroscopy due to the presence of more easily ionized argon atoms.
The gas mixture flows through the channels between the catalyst granules. In this case, heat and mass transfer occurs between the particles and the flow. In the core of the flow, mass and heat transfer are carried out mainly due to convection, since the flow is usually turbulent. Near the surface there is a laminar boundary layer, the gas velocity in which drops to zero at the surface of the granule. The transport of reagents and reaction products through it in the direction normal to the surface is carried out by molecular diffusion, and heat by thermal conductivity. Heat transfer can also occur through thermal conduction from particle to particle through the contact surface and by radiation between particles.
There are three types of heat transfer: conduction, convection and thermal radiation. Thermal conductivity is the phenomenon of heat transfer through direct contact between particles of different temperatures. This type includes heat transfer in solids, for example, through the wall of an apparatus. Convection is the phenomenon of heat transfer by transporting particles of liquid or gas and mixing them with each other. Heat exchange can also be carried out through radiation - the transfer of energy like light in the form of electromagnetic waves.
An important role for the process of combustion (gasification) of fuel is played by the direction of mutual movement of the solid and gas phases. There are two known schemes for organizing the movement of gas and fuel flows, co-current and counter-current. In the direct-flow scheme of gas and fuel flows, the thermal preparation of the reagents occurs less intensively, without the participation of hot gases and mainly through the transfer of heat from the combustion zone by heat transfer and radiation. In the counter-fire scheme, more reliable ignition of the fuel is achieved, since heat is transferred to heat it by convection from hot gases and thermal conductivity from hot surfaces.
It should be noted that in relation to dispersed materials, the term thermal conductivity can be used only conditionally, if by this concept we mean not only conductive heat transfer (i.e., thermal conductivity itself), but also heat transfer through convection and radiation. Thus, the thermal conductivity coefficient determined for dispersed media is a certain value equivalent to the thermal conductivity coefficient in the Fourier equation, if in general this equation is applicable under given conditions (i.e., if the process of heat transfer through the listed mechanisms can be described quite accurately by this equation) . Therefore, it is more correct to call this quantity the equivalent thermal conductivity coefficient (see section II, etc.). With this in mind, we will, however, retain the generally accepted term thermal conductivity for the sake of brevity.
These researchers compared their data with an expression for the effective thermal conductivity of particle aggregates. They say, as does Mayer, that the effective thermal conductivity through any surface is equal to the average thermal conductivity of air and fuel with respect to the portion of the surface covered by each, and that an equivalent thermal conductivity is obtained from black body radiation through voids. Using this equation, with some simplification he allowed, Mayer was able to express the effective thermal conductivity of the fuel layer in terms of the true thermal conductivity of the fuel, the volume of voids, the temperature in the fuel layer and the diameter of the largest particles. The tenloic conductivity of the gas filling the voids is included in the analysis data of its various parts and cannot be detected directly. As an indicator of the order of magnitude obtained from this expression, the effective thermal conductivity of a coke layer at a temperature of 815° with a void volume of 50% and with an upper limit of grain size of 2.54 C/I is given, which was determined to be 0.00414. The true thermal conductivity of the fuel is such a small fraction (about 5%) of the effective that the effective thermal conductivity of the entire layer is largely independent of the fuel used.
General provisions. In technology, we often have to deal with cases of heat transfer when the ambient temperature is specified, rather than the wall surface temperature. Compared with thermal conductivity and thermal radiation, the transfer of heat through convection from the surrounding liquid or gaseous medium to the wall surface is a much more complex and far from studied process. When heat transfers from a solid to a liquid or gas, heat exchange due to thermal conductivity recedes in magnitude compared to heat exchange due to convection. The latter is that in a moving layer of liquid or gas adjacent to the wall, due to the flow existing in this layer, everything comes into contact with the wall. time new. and new particles, which thus either take heat with them or give it away to the wall with which they come into contact. This convective transport
TO a known temperature and placed in place of the burner. In this way it was possible to obtain the value of the spectral brightness of the flame and from here, according to Kirchhoff's law, also the spectral brightness of the black body at the same temperature as the temperature of the flame. This temperature was compared with the temperature of the flame, measured as follows: a thin platinum-rhodium wire located outside the flame was heated by passing a current and the energy of its radiation was measured by a thermal column at different temperatures. The latter were measured using an optical pyrometer. Based on this, a radiation energy curve was constructed (in watts per centimeter length of wire) as a function of temperature. Then the wire was introduced into the flame, and its temperature was measured for various values imparted to it. electrical energy. From here, another curve was constructed, expressing the energy supply (in watts per centimeter of wire length) as a function of temperature. For a certain temperature value these curves intersect. For radiation from the wire, the flame is almost transparent. This follows from the relatively low emissivity of the wire in the region of the infrared absorption bands of the flame, and, in addition to i jro, it was confirmed by direct experiment. Therefore, at this temperature, the amount of energy emitted by the millet is equal to the amount of transmitted electrical energy. This can only take place if energy is not lost and is not transferred to the wire by thermal conduction or convection, i.e. if the temperatures of the wire and the gas flame are the same. Therefore, the intersection point determines the temperature of the gas flame.
As the drop evaporates, it cools. In view of the analogy between the phenomena of thermal conductivity and diffusion (neglecting heat transfer through convection and radiation, considering the thermal conductivity coefficient R of a gaseous medium to be independent of temperature and vapor concentration, i.e., considering l = onst), we can write the equations for the stationary temperature distribution around a spherical drop, similar (4.3)
Muraur did not give a complete quantitative theory, but rather related the results large number experiments with a qualitative picture of the combustion process. The surface decomposition of the fuel, producing a combustible gas mixture, is treated as the rate-determining stage of combustion, and parameters such as pressure, initial temperature, flame temperature, heat of explosion, and radiation are interpreted as if they influenced this initial decomposition. The transfer of energy from the flame to the surface of the fuel occurs through the process of thermal conduction, the rate of which is proportional to pressure, and the process of radiation, which is independent of pressure. This gives the following law for the burning rate
10/22/16 03:50:35 PM
Types of Heat Transfer
Physics 8th grade.
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THERMAL CONDUCTIVITY
transfer of energy from more heated areas of the body to less heated ones due to thermal movement and the interaction of microparticles (atoms, molecules, ions, etc.), which leads to equalization of body temperature.
Different materials have different thermal conductivities
Copper Steel
THERMAL CONDUCTION IN HOUSEHOLD
Good thermal conductivity
Poor thermal conductivity
CONVECTION
This is the transfer of energy by jets of liquid or gas. During convection, matter is transferred.
CONVECTION CAN BE:
NATURAL
ARTIFICIAL
(FORCED)
Convection in everyday life
Home heating
Home cooling
In both thermal conductivity and convection, one of the conditions for energy transfer is the presence of matter. But how is the heat of the Sun transferred to us on Earth? outer space– vacuum, i.e. there is no substance there, or it is in very sparse condition?
Therefore, there is some other way to transfer energy
RADIATION
Radiation is the process of emitting and propagating energy in the form of waves and particles.
All bodies around us emit heat to one degree or another.
Sunlight
A night vision device allows you to capture the weakest thermal radiation and convert it into an image
Light (mirror) surfaces – reflect thermal radiation
This way you can reduce heat loss or direct heat to the right place
Dark surfaces absorb thermal radiation
A solar collector is a device for collecting thermal energy from the Sun (solar plant) transferred by visible light and near-infrared radiation. Unlike solar panels, which produce electricity directly, solar collector produces heating of the coolant material.
Under natural conditions, the transfer of internal energy to heat exchange always occurs in a strictly defined direction: from a body with a higher temperature to a body with a lower temperature. When the temperatures of the bodies become the same, a state of thermal equilibrium occurs: the bodies exchange energy in equal quantities.
The set of phenomena associated with the transition of thermal energy from one part of space to another, which is caused by the difference in the temperatures of these parts, is generally called heat exchange. There are several types of heat transfer in nature. There are three ways to transfer heat from one body to another: thermal conductivity, convection and radiation.
Thermal conductivity.
Place the end of a metal rod in the flame of the alcohol lamp. We attach several matches to the rod at equal distances from each other using wax. When one end of the rod is heated, the wax balls melt and the matches fall off one after another. This indicates that internal energy is transferred from one end of the rod to the other.
Figure 1 Demonstration of the thermal conduction process
Let's find out the reason for this phenomenon.
When the end of the rod is heated, the intensity of movement of the particles that make up the metal increases, and their kinetic energy increases. Due to the randomness of thermal motion, they collide with slower particles of the adjacent cold layer of metal and transfer part of their energy to them. As a result of this, internal energy is transferred from one end of the rod to the other.
The transfer of internal energy from one part of a body to another as a result of the thermal movement of its particles is called thermal conductivity.
Convection
The transfer of internal energy by thermal conduction occurs mainly in solids. In liquid and gaseous bodies, the transfer of internal energy is carried out in other ways. Thus, when water is heated, the density of its lower, hotter layers decreases, while the upper layers remain cold and their density does not change. Under the influence of gravity, denser cold layers of water fall down, and heated ones rise up: mechanical mixing of cold and heated layers of liquid occurs. All the water warms up. Similar processes occur in gases.
The transfer of internal energy due to mechanical mixing of heated and cold layers of liquid or gas is called convection.
The phenomenon of convection plays a big role in nature and technology. Convection currents cause constant mixing of air in the atmosphere, due to which the composition of the air in all places on Earth is almost the same. Convection currents provide a continuous supply of fresh portions of oxygen to the flame during combustion processes. Due to convection, the air temperature in living quarters is equalized during heating, as well as air cooling of devices during the operation of various electronic equipment.
Figure 2 Heating and equalization of air temperature in residential premises during heating due to convection
Radiation
The transfer of internal energy can also occur through electromagnetic radiation. This is easy to discover through experience. Let's plug in the electric heating stove. It warms our hand well when we bring it not only from above, but also from the side of the stove. The thermal conductivity of the air is very low, and convection currents rise upward. In this case, the energy from the helix heated by electric current is mainly transferred by radiation.
The transfer of internal energy by radiation is carried out not by particles of matter, but by particles of the electromagnetic field - photons. They do not exist “ready-made” inside atoms, like electrons or protons. Photons arise when electrons move from one electron layer to another, located closer to the nucleus, and at the same time carry with them a certain portion of energy. Reaching another body, photons are absorbed by its atoms and completely transfer their energy to them.
The transfer of internal energy from one body to another due to its transfer by particles of the electromagnetic field - photons, is called electromagnetic radiation. Any body whose temperature is higher than the ambient temperature radiates its internal energy into the surrounding space. The amount of energy emitted by a body per unit time increases sharply with increasing temperature.
Figure 3 Experiment illustrating the transfer of internal energy of a hot kettle through radiation
Figure 4 Radiation from the Sun
Transport phenomena in thermodynamically nonequilibrium systems. Thermal conductivity
In thermodynamically nonequilibrium systems, special irreversible processes arise, called transfer phenomena, as a result of which spatial transfer of energy, mass, and momentum occurs. Transport phenomena include thermal conductivity (caused by energy transfer), diffusion (caused by mass transfer), and internal friction (caused by momentum transfer). For these phenomena, the transfer of energy, mass and momentum always occurs in the direction opposite to their gradient, i.e. the system approaches a state of thermodynamic equilibrium.
If in one region of the gas the average kinetic energy of molecules is greater than in another, then over time, due to constant collisions of molecules, a process of equalization of the average kinetic energies of molecules occurs, i.e., in other words, equalization of temperatures.
The process of energy transfer in the form of heat obeys Fourier’s law of thermal conductivity: the amount of heat q that is transferred per unit time through a unit area is directly proportional 
- temperature gradient equal to the rate of temperature change per unit length x in the direction of the normal to this area:
, (1)
where λ is the thermal conductivity coefficient or thermal conductivity. The minus sign shows that during thermal conduction, energy is transferred in the direction of decreasing temperature. Thermal conductivity λ is equal to the amount of heat transferred through a unit area per unit time with a temperature gradient equal to unity.
It is obvious that the heat Q passed by thermal conductivity through the area S during time t is proportional to the area S, time t and temperature gradient 
:
It can be shown that
(2)
where with V - specific heat capacity of gas at constant volume(the amount of heat required to heat 1 kg of gas by 1 K at constant volume), ρ - gas density,<υ>- arithmetic average speed of thermal movement of molecules,<l> - average free path.
Those. it is clear on what reasons the amount of energy transferred by thermal conduction, for example, from a room through a wall to the street, depends. Obviously, the more energy is transferred from the room to the street, the more larger area wall S, the greater the temperature difference Δt in the room and outside, the longer the time t for heat exchange between the room and the street, and the smaller the wall thickness (thickness of the layer of substance) d: 
~
.
In addition, the amount of energy transferred by thermal conduction depends on the material from which the wall is made. Different substances under the same conditions transfer different amounts of energy by thermal conduction. The amount of energy that is transferred by thermal conduction through each unit area of a layer of a substance per unit time when the temperature difference between its surfaces is 1 ° C and when its thickness is 1 m (unit length) can serve as a measure of the ability of a substance to transfer energy by thermal conductivity. This value is called the thermal conductivity coefficient. The higher the thermal conductivity coefficient λ, the more energy is transferred by the layer of substance. Metals have the greatest thermal conductivity, liquids have somewhat less. Dry air and wool have the lowest thermal conductivity. This explains the heat-insulating properties of clothing in humans, feathers in birds and wool in animals.
THERMAL CONDUCTIVITY: Pour into aluminum and glass pans of equal capacity hot water. Which pan will heat up faster to the temperature of the water poured into it? Aluminum conducts heat faster than glass, so an aluminum pan will heat up faster to the temperature of the water poured into it.
CONVECTION In industrial refrigerators, air is cooled using pipes through which cooled liquid flows. Where should these pipes be located: at the top or bottom of the room? To cool the room, the pipes through which the cooled liquid flows must be located at the top. Hot air, in contact with cold pipes, will cool and fall down under the influence of the Archimedes force.
Type of heat transfer Features of heat transfer Figure Thermal conductivity Requires a certain time The substance does not move Atomic-molecular energy transfer Convection The substance is transferred by jets Observed in liquid and gas Natural, forced Warm up, cold down Radiation Radiated by all heated bodies Carry out in a complete vacuum Emitted, reflected, absorbed
Heat transfer is a spontaneous irreversible process of energy transfer from more heated bodies or parts of the body to less heated ones. Heat transfer is a way of changing the internal energy of a body or system of bodies. Heat transfer determines and accompanies processes in nature, technology and everyday life. There are three types of heat transfer: conduction, convection and radiation.
