HEAT TRANSFER METHODS.
When performing thermal drying, two processes are distinguished:
1) evaporation of the moisture to be removed;
2) removal of the generated steam from the surface of the material.
For the evaporation of 1 kg of moisture, it is necessary to bring a quite certain amount of heat to the area of vaporization. Therefore, heat transfer is the basis of the working processes that take place in drying plants. In practice, to a greater or lesser extent, all three main forms of heat transfer are realized: 1) thermal conductivity; 2) convection; 3) radiation.
In addition, in many dryers, a special type of heat transfer is of great importance, namely, heat transfer by short-term contact, which occurs, for example, in roller dryers, on the heating grates of vacuum dryers and in drum dryers when cold material interacts with heated elements of internal devices.
The approach to heat transfer problems in drying technology differs from the approach in other branches of engineering. In mechanical engineering, the shape and dimensions of heat-transferring and heat-receiving elements are in most cases well known (pipes, plates, etc.). In drying plants, the geometric shape of most agricultural products subjected to drying is extremely diverse, so it is difficult to describe it with a sufficient degree of accuracy by analytical dependencies.
Another difficulty is that the moisture evaporation zone in the material is constantly moving and depends on the process conditions. For this reason, in drying plants, more than in any other technical field, experimental studies form the basis for the calculation and design of devices.
The basic laws of heat transfer set forth below will be presented to the extent necessary for a complete understanding of the processes occurring in agricultural dryers.
Thermal conductivity as a method of heat transfer
Heat transfer by conduction occurs within solids, stationary liquids and gases due to the transfer of energy in the form of heat from one elementary particle to another. Heat is transferred from an area of high temperature to an area of lower temperature. In steady state, the heat flux density between two parallel surfaces of the body depends on the temperature difference, wall thickness and thermophysical constant - thermal conductivity K (Fig. 3.13):
Rice. 3.13. Thermal conductivity of a flat wall
q is the heat flux density, kcal/(m2 h);
λ – thermal conductivity, kcal/(m h ºС);
U1, U2 – temperature on the first and second surfaces, ºС;
s – wall thickness, m
In the case of a homogeneous body bounded flat surfaces, the temperature between them in the steady state thermal regime falls according to a linear law. For
bodies of a complex structure, the process in a layer of infinitely small thickness ds is described by an equation of the form
where dυ is the temperature difference in a layer of infinitesimal thickness, °С. The minus sign in the equation indicates that the heat flow is directed towards lower temperature.
In order to draw conclusions about the process in the entire body based on the consideration of the process in a layer of infinitely small thickness, it is necessary to carry out integration under certain boundary conditions.
Convection (heat transfer method)
Heat transfer by convection essentially involves two processes (Figure 3.17):
1) heat transfer by thermal conduction from the surface of a solid body through a laminar boundary layer to the vicinity of the turbulent flow core;
2) heat transfer by turbulent transfer from the laminar boundary layer to the core of the turbulent flow.
Drying is characterized by the reverse direction of the heat flow: from the drying agent to the surface of the solid. The heat transfer equation relates the temperature difference between the flow and the surface of the body with the heat flux density:
where is the heat transfer coefficient, kcal/(m2 h °C);
UL;U0 - temperature on the wall and in the core of the flow, °C.
Rice. 3.17. Temperature profile during heat transfer from a turbulent flow to the surface of a solid body through a laminar boundary layer: UL - temperature in the core of the flow; U0 - temperature on the surface of the body
To understand the processes of convective heat transfer, it is necessary to distinguish between elementary processes (flow around single bodies) and complex processes (heat transfer in a layer of bulk materials, counter- and forward flow, etc.).
Laminar boundary layer, turbulent flow core, heat transfer by thermal conduction and turbulent mixing, as well as mass transfer in the boundary layer in the forward and backward directions, are interconnected and have a variety of effects on each other. These processes can be described using the balance equations of energy and mass exchange. For the description, it is expedient to introduce dimensionless criteria that connect many physical and technological parameters. With the help of such criteria, real physical dependencies can be described more simply and more clearly, while refusing to directly use the physical parameters that characterize the process.
radiation heat transfer by radiation
Heat transfer by radiation (for example, with infrared heating) occurs when energy is transferred. electromagnetic waves from one body to another. In this case, neither a solid, nor a liquid, nor a gaseous carrier is involved in the transfer of energy by radiation. In accordance with the Stefan-Boltzmann law, the energy radiated by a body into the surrounding space is proportional to its temperature (in degrees Kelvin) to the fourth power:
q is the radiation energy flux density, kaal/(m2 x);
C is the emissivity of the body;
T - temperature, K.
If we bring two bodies with different temperatures closer to each other (Fig. 3.21), then the difference between the absorbed and radiated energy of each of these bodies is estimated by the equation
Q = A1 С12[( T 1 / 100)4 – (T2 / 100)4] = A2 C21[( T 1 / 100)4 – (T2 / 100)4],
where Q- heat flux of radiation energy, kcal/h; A1, A2 - radiating surface of bodies 1 and 2; C12, C21 - radiation coefficients, kcal/[m2-h (K/100)4]. The coefficients C12 or C21, based on the representation of the emissivity of individual bodies, are obtained from the following equations:
1 / C12 \u003d 1 / C1 + A1 / A2 (1 / C2 - 1 / Cs);
1 / C21 \u003d 1 / C2 + A2 / A1 (1 / C1 - 1 / Cs);
Rice. 3.22. Radiation anergy flux density between bodies heated to different temperatures (at C=4.0)
Figure 3.23. Temperature distribution in a ceramic plate when heated by a stream of infrared rays (according to the work)
where Cs is the black body emissivity; Cs= 4.96 kcal/[m2-h (K/100)4].
The tables often give the value of the relative characteristic (Table 3.10)
On fig. Figure 3.22 shows the dependence of the radiation energy flux density on temperature υ1 and υ2 under the assumption that C12 = C21 = 4 kcal/[m2-h (K/100)4]. It can be seen from the graphs that at large temperature differences, the radiation energy depends only on the temperature of the hotter body.
Of particular interest is the process of heat supply with the help of radiation in drying installations, which is due to the possibility of penetration of radiation energy into various media. The depth of penetration of heat fluxes during radiation depends on the type of material and the type of radiation. For capillary-porous bodies of organic origin, this depth is 0.1–2 mm.
Owing to the fact that required heat is released partially inside the body, and not only on its surface, under certain conditions on the surface, the heat flux density can be increased many times over.
Table 3.10 The emissivity of a substance according to Schmidt
|
SUBSTANCE |
Temperature, °C |
Emissivity ε = C/ Cs |
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Gold, silver, copper polished |
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polished, slightly oxidized |
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sanded |
||
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blackened (oxidized) |
||
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cleanly ground |
||
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highly oxidized |
||
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Clay burnt |
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Ice is smooth, water |
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Ice, rough surface |
||
According to A. V. Lykov, the energy flux density, for example, can be increased from 750 kcal/(m2-h) for convection to 22,500 kcal/(m2-h) for radiation. On fig. 3.23 shows in graphical form the process of heating the body with the help of radiation energy. From the graph, it is clearly seen that thermal energy is initially released only inside the body, since otherwise the maximum temperature would have to be on the surface of the body.
Contact heat exchange
Contact heat transfer is observed when two bodies having different temperatures at the initial moment of time come into contact with each other, as a result of which the temperature of these bodies tends to some common average temperature . In practice, heat transfer of this kind can be found on heated or heated surfaces during pouring, vibration, sliding of the dried material.
At the first moment of time after the contact of two bodies, which initially had different temperatures, an average temperature is established on the surface of their contact, denoted by U0. The value is called the thermal activity of the body. Wherein:
The average value of the reduced heat transfer coefficient, referred. to the time interval t and the temperature difference U0-U∞ (where - U∞ is the initial temperature of the cold body), is calculated by the formula.
With short-term contact, the average value of the reduced heat transfer coefficient can be quite high.
Heat transfer during heating in an alternating electromagnetic field.
If two metal plates at a certain distance from each other are placed in an alternating electromagnetic field, then an alternating current will appear between them, depending on the field strength and capacitance
Figure 3.25. Change in permittivity v and dielectric loss tangent tgδ as a function of frequency f variable electromagnetic field and moisture content of pine wood (according to the work)
If a material is placed between the capacitor plates, then the capacitive current will increase in proportion to the permittivity ε of the material. The water contained in agricultural products, compared to their dry mass, has a high dielectric constant (at a temperature of 0 ° C ε = 80), so the constant e can be used to measure the moisture content of the material.
The purely capacitive current does not heat up the wet material. The phase-shifted currents inside the material also have an active component. The value expressed by the ratio of the active and capacitive components is called the tangent of the dielectric loss angle:
IR is the active component of the current strength, A; IC - capacitive component of the current strength, A; U - operating voltage, V; R - active resistance, Ohm; w- circular frequency, 1/s; C - capacity, F; ε - dielectric constant; f- frequency Hz.
The release of heat in the material is due only to the active component of the current:
If we express the voltage in terms of the field strength E (voltage per centimeter of the distance separating the plates), then we can obtain an expression characterizing the power of volumetric heat release:
Q - heat release, kcal/h; V is the volume of the condenser, cm3; E - tension electric field, V/cm.
The losses determined by tgδ and the dielectric constant e are largely dependent on the moisture content of the material and the frequency of change electromagnetic field(Fig. 3.25) . Already at a relatively low moisture content, both of the above parameters increase significantly. This creates the necessary conditions for the so-called dielectric drying. At the same time, heat generation becomes especially large where moisture is contained the most. As a result, in such places, moisture evaporates faster. Besides, in this case the material is dehydrated first from the inside, which has great importance to prevent its destruction from shrinkage stresses (when wood is dried), observed during conventional drying methods, when the material dries first on the outside, and then on the inside.
At atmospheric pressure, the temperature inside the wet material rises to about 100°C and remains constant at that level. If moisture evaporates in such a large amount that the material is in the hygroscopic region, then the temperature will rise further. As a result, the core of the material may be charred while its outer layers are still wet.
Dielectric, or high-frequency drying, is not widely used not only because of the large capital investments and the cost of highly qualified maintenance, but also because of the high energy intensity of the process. The thermal energy necessary for the evaporation of moisture is obtained as a result of the conversion of electrical energy, while the energy conversion is associated with noticeable losses.
The theory of heat transfer studies the patterns of distribution and transfer of thermal energy. The exchange of energy in the form of heat occurs in the presence of a temperature difference between individual bodies or parts of the same body and continues until the temperature of both bodies is equal. Since temperature is a measure of internal energy, therefore, during heat transfer, an increase in the internal energy of one (cold) body occurs due to its decrease in another body (hot).
The process of heat transfer is natural and irreversible, that is, it always proceeds in one direction: from a hot body to a cold one.
There are three types of heat transfer: conduction, convection and radiation.
Thermal conductivity- the process of heat propagation in solids and liquids at rest. In dielectrics (in materials that do not conduct electricity), thermal energy is transferred by oscillations crystal lattice, and in metals - mainly due to the movement of free electrons in the lattice. Thermal conductivity in its pure form is observed only in solids.
Convection- transfer of heat during the movement of individual masses and volumes of liquid and gaseous bodies.
Usually, convection and heat conduction occur simultaneously. Such a process is called convective heat transfer. The transfer of heat from one body to another during convection and heat conduction is carried out only when they come into contact.
Radiation-heat exchange between bodies at a distance in the form lu-clean energy. The carriers of radiant energy are electromagnetic waves (photons). When radiating, the thermal energy of a heated body turns into radiant energy, spreads in the surrounding space, falls on another body and again turns into thermal energy.
The solution of heat transfer problems always has a specific character, unambiguously determined by the conditions of the processes.
These conditions include:
- geometric features of the surfaces of bodies and the space surrounding them (shapes, sizes);
- features of the process in time;
- boundary features of the heat transfer process, i.e. the value and distribution physical quantities at the interfaces of bodies involved in heat transfer;
- physical and Chemical properties and parameters of the medium in which the heat transfer takes place.
However, these uniqueness conditions do not always allow one to obtain an analytical solution of problems in the theory of heat transfer. Therefore, for the study of heat transfer processes, physical experiments and generalization of their results are of exceptional importance.
Thermal conductivity
Features of the phenomena of heat conduction are associated with the distribution of temperature in bodies. In the general case, the temperature of bodies can change at all points in space over time. The set of instantaneous temperature values at all points of the space under study is called temperature field.
The temperature field is homogeneous, if the temperature is the same at all points in space, and heterogeneous if it is different. Surfaces with points of the same temperature are called isothermal, and the cross section of these surfaces - isotherms(Fig. 3.1). Heat does not spread along isothermal surfaces. The most rapid temperature change occurs in the direction along the normal to isothermal surfaces.
Rice. 3.1. temperature field
The limit of the ratio of the temperature difference of two isotherms to the distance between them along the normal, when n tends to zero is called gradient
temperature volume and denoted by grad t.
Gradient - a measure of the greatest intensity of temperature change; it is a vector quantity. The direction in which the temperature rises is considered positive. Quantitatively, the intensity of heat transfer is characterized by heat flux density, that is, the amount of heat passing through a unit surface per unit time. According to the Fourier law - the basic law of thermal conductivity - the heat flux density, W / m 2, is determined by the formula
where Q- amount of heat, J; F- Area, m2 ; τ - time, h
Fourier's law states that the heat flux density is proportional to the temperature gradient
where λ - coefficient of thermal conductivity characterizing the intensity of heat propagation, i.e. the amount of heat passing as a result of thermal conductivity per unit time through a unit heat exchange surface when the temperature drops by 1 degree per unit length of the normal to the isothermal surface, W / m K .
The minus sign on the right side indicates the opposite direction of the heat flow and temperature changes in the body. The thermal conductivity coefficient depends on chemical composition bodies, their structure, density, humidity, pressure, temperature and is on the order of 0.01 to 400 W / (m K).
Bodies that have λ <0,2 Вт/(м·К), называются heat insulators. Good conductors of heat are bodies that have λ >20 W/(m K).
Smallest values gases have a thermal conductivity coefficient (from 0.01 to 1 W / (m K)), the largest - metals (silver - 410, copper -
360, aluminum - 200-300, steel - 45-55 W / (m K)).
The Fourier heat equation is a mathematical description of the process of temperature change with time at any place of the body, caused by the resulting heat transfer.
The heat conduction equations are usually analytically solved for specific process conditions using known uniqueness conditions.
In practice, one has to meet with various problems of heat conduction, which are conventionally divided into three groups:
1) stationary thermal conductivity, when the temperature distribution in the body remains unchanged in time and, accordingly, the heat flux density is constant. Heat exchange processes in heating devices and apparatuses, enclosing structures of building structures at long-term constant temperatures of outdoor and internal environment can be considered independent of time;
2) non-stationary thermal conductivity, when the temperature field changes with time. Non-stationary thermal conductivity is observed, for example, during heating and cooling of bodies, when before the onset of thermal exposure the entire body mass had the same temperature;
3) temperature waves in bodies subjected to periodic thermal effects. For example, annual fluctuations in temperature in the surface layer of the earth, daily fluctuations in the temperature of the outside air and, under their influence, the temperature of the surfaces of the enclosing structures.
Below is a particular solution of the Fourier equation for two problems of stationary heat conduction.
1. One-dimensional distribution of heat in a flat wall (Fig. 3.2). The heat flux in a flat wall is equal to
news F 1 and F 2, °С.
For sandwich wall with layer thicknesses δi and thermal conductivity coefficients λ i the heat flow equation is generalized as follows:
where α - coefficient of convective heat transfer, characterizing the intensity of heat transfer by convection, W / (m 2 K); t- liquid temperature away from the wall, °С; t st- wall surface temperature, °С; F- heat-receiving surface of the body, m 2.
One of the main tasks of the theory of convective heat transfer is to determine the value of the heat transfer coefficient for specific process conditions.
By the amount α many factors influence, the main of which are the nature of convection, the mode of motion, the physical properties of the liquid, the geometric features of the surface of the bodies involved in heat exchange.
Convection is called free, if it arises due to the pressure difference (density) due to the inhomogeneity of the temperature field of the liquid. The phenomenon of free convection can be observed over the surface of heated bodies, when air particles located near these surfaces, heating up, rise upwards, and cold air masses rush in their place (Fig. 3.4).
Free convection occurs naturally in any volume where there are bodies with different temperatures, and proceeds the more intensely, the higher the temperature difference.
Rice. 3.4. Free convection: a- vertical heated wall; b- horizontal plate; in– horizontal stove, heated from below
forced convection called heat transfer during the movement of a liquid under the action of external forces, for example, created by a pump, fan, compressor. In this case, the intensity of heat transfer is the higher, the greater the speed of the flow of the liquid washing the surfaces of the bodies.
The reason for the increase in the intensity of heat transfer with an increase in the flow velocity is to change the mode of fluid movement, the transition from laminar to turbulent motion (see Fig. 3.1).
In a laminar flow, thermal energy is transferred by heat conduction and transverse mass diffusion. The intensity of such energy transfer depends on the properties of the medium, and the less, the greater the thickness of the flow. In a turbulent flow, energy is transferred from the liquid to the wall by mixing masses, and only in the boundary layer - by thermal conductivity. Therefore, the intensity of heat transfer in a turbulent flow is higher than in a laminar one.
Laminar and turbulent fluid flows can be observed both under forced and under free movement. However, in the latter case, these modes are created exclusively by the conditions of thermal action, while in forced motion, artificial methods of influencing the fluid flow are used.
The intensity of convective heat transfer also depends on the physical properties of the liquid, characterized by the value of the coefficients of thermal conductivity and thermal diffusivity, heat capacity, coefficients of volumetric expansion and kinematic viscosity.
The geometric conditions of convective heat transfer are determined by the shape of the body, its dimensions, and the nature of the surface flowed around by the fluid.
According to the geometric conditions, heat transfer is distinguished during the internal flow of fluid in pipes, channels ( internal task) and external washing of surfaces by a stream (external task). With an external flow, the flow can be longitudinal with respect to largest size surface or transverse (for example, when flowing around a bundle of pipes located perpendicular to the direction of flow).
In all cases, geometric conditions have a significant impact on the distribution of velocities and temperatures in the flow, on the mode of motion, changing the intensity of heat transfer. To take into account these factors, it is necessary to specify the characteristic dimensions and shape of the body.
The values of heat transfer coefficients in various tasks convective heat transfer is determined by solving criterion equations, with the help of which the data of experimental studies are generalized, for example, for free convection, an equation of the form
where Nu l -Nusselt criterion; α -coefficient of convective heat-
Grashof; g- acceleration of gravity, m/s 2 ; β - volumetric coefficient
Reynolds; FROM, n, m- experimental coefficients, - fluid velocity, m/s.
Electrothermal processes are associated with the conversion of electrical energy into thermal energy with the transfer of thermal energy inside the body (solid, liquid, gaseous) or from one volume to another according to the laws of heat transfer.
Heat transfer (heat exchange) is the transfer of heat from one part of space to another, from one body to another, or inside the body from one part of it to another. An indispensable condition for heat transfer is the presence of a temperature difference between individual bodies or sections of bodies.
There are stationary and non-stationary heat transfer (Fig. 2.1).
There are three types of heat transfer, three different methods of heat transfer (Fig. 2.2).
Thermal conductivity is due to thermal motion and energy interaction of microparticles (molecules, atoms, electrons), particles with higher energy (more heated and, therefore, more mobile) give part of their energy to less heated (less mobile). The rate of heat transfer in this case depends on physical properties substance, in particular on its density. For dense bodies (metal), the heat transfer rate is higher, for porous ones (polystyrene) - less.
The heat flux through a flat wall at steady state (determined by the Fourier law) is proportional to the temperature difference of the wall surface and inversely proportional to the thermal resistance of the wall.
When heat is transferred by radiation, energy is transferred in the form electromagnetic waves. This type of heat transfer can take place only in a medium transparent to these rays.
Each opaque heated body in a transparent medium radiates radiant energy in all directions, propagating at the speed of light. When meeting with other completely or partially opaque bodies, this radiant energy is again converted (in whole or in part) into heat, heating these bodies. Consequently, radiant heat transfer is accompanied by a double transformation of energy - thermal energy into radiant energy and then again radiant energy into thermal energy.
If the temperatures of the bodies between which radiative heat exchange is carried out are different, then as a result of heat exchange between them, heat will be transferred from a more heated body to a less heated one, one of them will heat up, and the other will decrease its temperature.
When a heated body radiates into an unlimited space (with one-sided heat transfer), the radiant heat flux is proportional to the constant emissivity of a completely black body, the degree of blackness of the body, numerically equal to its absorbing capacity, and the absolute temperature of the heated body.
Rice. 2.2. Classification of heat transfer according to the method of heat transfer
The analytical solution of problems associated with convective heat transfer presents significant difficulties, since this process is described complex system differential equations. Therefore, the problems of convective heat transfer are solved using experimentally obtained constants and quantities. The heat flux of convective heat transfer is determined on the basis of the Newton-Richmann law. According to this law, the heat flow is directly proportional to the washing surface, the mode of movement of the coolant (heat transfer coefficient) and the temperature difference between the wall and the gas or liquid.
Today we will try to find the answer to the question “Heat transfer is?..”. In the article, we will consider what the process is, what types of it exist in nature, and also find out what is the relationship between heat transfer and thermodynamics.
Definition
Heat transfer is a physical process, the essence of which is the transfer. An exchange occurs between two bodies or their system. In this case, a prerequisite is the transfer of heat from more heated bodies to less heated ones.
Process features
Heat transfer is the same type of phenomenon that can occur both with direct contact and with separating partitions. In the first case, everything is clear; in the second, bodies, materials, and media can be used as barriers. Heat transfer will occur in cases where a system consisting of two or more bodies is not in a state thermal equilibrium. That is, one of the objects has a higher or lower temperature compared to the other. This is where the transfer of heat energy takes place. It is logical to assume that it will end when the system comes to a state of thermodynamic or thermal equilibrium. The process occurs spontaneously, which can tell us
Kinds
Heat transfer is a process that can be divided into three ways. They will have a basic nature, since within them real subcategories can be distinguished, having their own characteristic features along with general patterns. To date, it is customary to distinguish three. These are thermal conductivity, convection and radiation. Let's start with the first one, I guess.
Ways
This is the name of the property of this or that material body perform energy transfer. At the same time, it is transferred from the hotter part to the colder one. This phenomenon is based on the principle of chaotic motion of molecules. This is the so-called Brownian motion. The higher the temperature of the body, the more actively the molecules move in it, since they have more kinetic energy. Electrons, molecules, atoms participate in the process of heat conduction. It is carried out in bodies, different parts of which have different temperatures.
If a substance is capable of conducting heat, we can talk about the presence of a quantitative characteristic. In this case, its role is played by the coefficient of thermal conductivity. This characteristic shows how much heat will pass through unit indicators of length and area per unit of time. In this case, the temperature of the body will change exactly by 1 K.
It was previously believed that the exchange of heat in various bodies (including the heat transfer of enclosing structures) is due to the fact that the so-called caloric flows from one part of the body to another. However, no one found signs of its actual existence, and when the molecular-kinetic theory developed to a certain level, everyone forgot to think about caloric, since the hypothesis turned out to be untenable.
Convection. Water heat transfer
This method of heat energy exchange is understood as transfer by means of internal flows. Let's imagine a kettle of water. As you know, hotter air currents rise to the top. And cold, heavier ones sink down. So why should water be any different? It's exactly the same with her. And in the process of such a cycle, all layers of water, no matter how many there are, will heat up until a state of thermal equilibrium occurs. Under certain conditions, of course.
Radiation
This method is based on the principle of electromagnetic radiation. It comes from internal energy. We will not go into the theory much, we will simply note that the reason here lies in the arrangement of charged particles, atoms and molecules.
Simple heat conduction problems
Now let's talk about how the calculation of heat transfer looks in practice. Let's solve a simple problem related to the amount of heat. Let's say we have a mass of water equal to half a kilogram. The initial temperature of the water is 0 degrees Celsius, the final temperature is 100. Let's find the amount of heat we spent to heat this mass of matter.
To do this, we need the formula Q \u003d cm (t 2 -t 1), where Q is the amount of heat, c is the specific m is the mass of the substance, t 1 is the initial, t 2 is the final temperature. For water, the value of c is tabular. Specific heat will be equal to 4200 J / kg * C. Now we substitute these values into the formula. We get that the amount of heat will be equal to 210,000 J, or 210 kJ.
First law of thermodynamics
Thermodynamics and heat transfer are interconnected by some laws. They are based on the knowledge that changes in internal energy within a system can be achieved in two ways. The first is mechanical work. The second is the communication of a certain amount of heat. By the way, the first law of thermodynamics is based on this principle. Here is his formulation: if a certain amount of heat was imparted to the system, it will be spent on doing work on external bodies or on increasing its internal energy. Mathematical notation: dQ = dU + dA.
Pros or cons?
Absolutely all quantities that are included in the mathematical notation of the first law of thermodynamics can be written both with a plus sign and with a minus sign. Moreover, their choice will be dictated by the conditions of the process. Assume that the system receives some amount of heat. In this case, the bodies in it heat up. Therefore, the gas expands, which means that work is done. As a result, the values will be positive. If the amount of heat is taken away, the gas cools, and work is done on it. The values will be reversed.
Alternative formulation of the first law of thermodynamics
Let's assume that we have some periodically operating engine. In it, the working body (or system) performs a circular process. It is commonly called a cycle. As a result, the system will return to its original state. It would be logical to assume that in this case the change in internal energy will be zero. It turns out that the amount of heat will be equal to the work done. These provisions allow us to formulate the first law of thermodynamics in a different way.
From it we can understand that a perpetual motion machine of the first kind cannot exist in nature. That is, a device that does work in a larger amount compared to the energy received from outside. In this case, actions must be performed periodically.
First law of thermodynamics for isoprocesses
Consider first the isochoric process. It keeps the volume constant. This means that the change in volume will be zero. Therefore, the work will also be equal to zero. Let us discard this term from the first law of thermodynamics, after which we obtain the formula dQ = dU. This means that in an isochoric process, all the heat supplied to the system goes to increase the internal energy of the gas or mixture.
Now let's talk about the isobaric process. Constant value there is pressure in it. In this case, the internal energy will change in parallel with the work. Here is the original formula: dQ = dU + pdV. We can easily calculate the work done. It will be equal to the expression uR(T 2 -T 1). By the way, this is physical meaning universal gas constant. In the presence of one mole of gas and a temperature difference of one Kelvin, the universal gas constant will be equal to the work done in an isobaric process.
Heat exchange- this is the process of changing internal energy without doing work on the body or the body itself.
Heat transfer always occurs in a certain direction: from bodies with a higher temperature to bodies with a lower.
When the temperatures of the bodies equalize, heat transfer stops.
Heat exchange can be carried out in three ways:
- thermal conductivity
- convection
- radiation
Thermal conductivity
Thermal conductivity- the phenomenon of the transfer of internal energy from one part of the body to another or from one body to another with their direct contact.
Metals have the highest thermal conductivity- they have hundreds of times more than water. The exceptions are mercury and lead., but even here the thermal conductivity is tens of times greater than that of water.
When lowering a metal needle into a glass with hot water very soon the end of the spoke became hot too. Consequently, internal energy, like any kind of energy, can be transferred from one body to another. Internal energy can also be transferred from one part of the body to another. So, for example, if one end of a nail is heated in a flame, then its other end, which is in the hand, will gradually heat up and burn the hand.
The heating of a pan on an electric stove occurs through heat conduction.
Let us study this phenomenon by doing a series of experiments with solids, liquids and gases.
Let's bring the end of a wooden stick into the fire. It will ignite. The other end of the stick, which is outside, will be cold. Means, wood has poor thermal conductivity.
We bring the end of a thin glass rod to the flame of an alcohol lamp. After a while, it will heat up, while the other end will remain cold. Therefore, and glass has poor thermal conductivity.
If we heat the end of a metal rod in a flame, then very soon the entire rod will become very hot. We can no longer hold it in our hands.
Means, metals conduct heat well, that is, they have a high thermal conductivity. Silver and copper have the highest thermal conductivity..
Thermal conductivity at various substances different.
Wool, hair, bird feathers, paper, cork and other porous bodies have poor thermal conductivity. This is due to the fact that air is contained between the fibers of these substances. Vacuum (space freed from air) has the lowest thermal conductivity. This is explained by the fact that thermal conductivity is the transfer of energy from one part of the body to another, which occurs during the interaction of molecules or other particles. In a space where there are no particles, heat conduction cannot take place.
If there is a need to protect the body from cooling or heating, then substances with low thermal conductivity are used. So, for pots, pans, plastic handles. Houses are built from logs or bricks, which have poor thermal conductivity, which means they are protected from cooling.
Convection
Convection is a heat transfer process carried out by the transfer of energy by flows of liquid or gas.
An example of the phenomenon of convection: a small paper pinwheel, placed over a candle flame or an electric light bulb, begins to rotate under the influence of rising heated air. This phenomenon can be explained in this way. Air, in contact with a warm lamp, heats up, expands and becomes less dense than the cold air surrounding it. The Archimedes force acting on warm air from the cold side upwards is greater than the force of gravity acting on warm air. As a result, the heated air "floats", rises, and cold air takes its place.
In convection, energy is transferred by the jets of gas or liquid themselves.
There are two types of convection:
- natural (or free)
- forced
In order for convection to occur in liquids and gases, it is necessary to heat them from below.
Convection cannot occur in solids.
Radiation
Radiation- electromagnetic radiation emitted due to internal energy by a substance at a certain temperature.
The thermal radiation power of an object that satisfies the criteria of a blackbody is described by the Stefan-Boltzmann law.
The ratio of the emissive and absorptive abilities of bodies is described Kirchhoff's radiation law.
The transfer of energy by radiation is different from other types of heat transfer: it can be carried out in full vacuum.
All bodies radiate energy: both strongly heated and weakly, for example, the human body, a stove, an electric light bulb, etc. But the higher the body temperature, the more energy it transmits by radiation. In this case, the energy is partially absorbed by these bodies, and partially reflected. When energy is absorbed, bodies heat up in different ways, depending on the state of the surface.
Bodies with a dark surface absorb and radiate energy better than bodies with a light surface. At the same time, bodies with a dark surface are cooled faster way radiation than bodies with a light surface. For example, in a light teapot hot water retains heat longer than in the dark.
