Thermal conductivity- this is a type of heat transfer in which there is a direct transfer of energy from particles (molecules, atoms) of a more heated part of the body to particles of its less heated part.
Consider a series of experiments with heating solid body, liquid and gas.
Radiant heat transfer.
Radiant heat transfer- this is heat transfer, in which energy is transferred by various beams.
It can be Sun rays, as well as the rays emitted by heated bodies around us.
So, for example, sitting near a fire, we feel how heat is transferred from the fire to our body. However, the cause of such heat transfer cannot be either thermal conductivity (which is very small for the air between the flame and the body), or convection (since convection flows are always directed upwards). Here, the third type of heat transfer takes place - radiant heat transfer.
Take a small, smoked on one side, flask.
Insert a glass tube bent at a right angle through the cork into it. In this tube, which has a narrow channel, we introduce a colored liquid. Having fixed the scale on the tube, we get the device - thermoscope. This device allows you to detect even a slight heating of the air in a smoked flask.
If a piece of metal heated to high temperature, then the liquid column will move to the right. Obviously, the air in the flask heated up and expanded. The rapid heating of air in a thermoscope can only be explained by the transfer of energy from a heated body to it. As in the case of a fire, the energy here was transferred not by thermal conductivity and not by convective heat transfer. Energy in this case transmitted by invisible rays emitted by a heated body. These rays are called thermal radiation.
Radiant heat transfer can take place in a complete vacuum. This distinguishes it from other types of heat transfer.
All bodies radiate energy: both strongly heated and weakly, for example, the human body, a stove, an electric light bulb. But the higher the temperature of the body, the stronger its thermal radiation. The radiated energy, having reached other bodies, is partially absorbed by them, and partially reflected. When absorbed energy thermal radiation turns into internal energy of bodies, and they heat up.
Light and dark surfaces absorb energy differently. So, if in an experiment with a thermoscope, turn the flask to a heated body, first smoked, and then bright side, then the liquid column in the first case will move to greater distance than in the second (see the figure above). It follows from this that bodies with a dark surface absorb energy better (and therefore heat up more) than bodies with a light or specular surface.
Bodies with a dark surface not only absorb better, but also radiate energy better.
The ability to absorb radiation energy in different ways finds wide application in tech. For example, Balloons and the wings of airplanes are often painted silver so that they are less heated by the sun's rays.
If you need to use solar energy(for example, to heat some appliances installed on artificial satellites), then these devices are painted in a dark color.
MINISTRY OF EDUCATION AND SCIENCE OF THE RUSSIAN FEDERATION
federal state autonomous educational institution higher professional education
"Northern (Arctic) federal university named after M.V. Lomonosov"
Institute of Oil and Gas | |
Department of Heat Engineering | |
131000.62 "Oil and gas business" | |
(code and name of the direction of training / specialty) | |
by discipline "Thermodynamics and heat transfer" | |
Lecture 1. Subject and method of thermodynamics .............................................. ......................... | |
Thermodynamic system .................................................................. ............................... | |
Thermodynamic parameters of the state ............................................................... ...... | |
Equation of state .................................................................. ......................................... | |
Thermodynamic process .................................................................. ......................... | |
Heat capacity of gases .................................................................. ............................................ | |
Lecture 2. Mixtures of ideal gases............................................... ................................................ | |
Analytical expression of the first law of thermodynamics .............................................. | |
Internal energy........................................................................................... | |
Extension operation .................................................................. ......................................... | |
Heat................................................. ................................................. ............. | |
Enthalpy................................................. ................................................. ........... | |
Entropy................................................. ................................................. ........... | |
Lecture 3. General formulation of the second law.............................................. ................... | |
Direct Carnot Cycle .............................................................. ............................................... | |
Reverse Carnot cycle .................................................................. ......................................... | |
Change of entropy in non-equilibrium processes .............................................. | |
Lecture 4. Thermodynamic processes of ideal gases in closed systems.......... | |
Lecture 5. Thermodynamic processes of real gases .............................................. ..... | |
Equation of state of real gases............................................................... ................. | |
Lecture 6 | |
Converging nozzle outflow .................................................................. .................... | |
Main regularities of gas flow in nozzles and diffusers.................................................. |
Calculation of the expiration process with h-s diagrams .................................. | |
Throttling of gases and vapors .............................................. ............................... | |
Lecture 7. Thermodynamic Efficiency of cycles of thermal power plants.......... | |
Reciprocating engine cycles internal combustion.................................... | |
Cycles of gas turbine plants .......................................................... ......................... | |
Cycles of steam turbine plants .......................................................... ................... | |
Rankine cycle on superheated steam .............................................. ......................... | |
Thermal efficiency of the cycle ............................................................... ................................................ | |
Heat supply .................................................................. ................................................. .... | |
General characteristics of refrigeration units …………………………….. | |
Lecture 8 ................................... | |
Basic concepts and definitions ……………………………………………. | |
Theory of thermal conductivity. Fourier's law .................................................. ............ | |
flat wall | |
Cylindrical wall ................................................................ .................................... | |
Lecture 9. Heat transfer ............................................... ................................................. .... | |
Flat wall................................................... ................................................. .. | |
Cylindrical wall ................................................................ ...................................... | |
Heat transfer enhancement .................................................................. ....................... | |
Thermal insulation .................................................................. ............................................... | |
Lecture 10 Convective heat transfer. Basic law of convective heat transfer. | |
Boundary layer .............................................................. ............................................... | |
Similarity numbers .................................................. ................................................. ... | |
Lecture 11. Particular cases of convective heat transfer. Cross flow | |
single pipe and pipe bundle .............................................................. ............................... | |
Coolant flow inside pipes .................................................................. ................... | |
Heat transfer during natural convection .............................................................. .......... | |
Approximate values of heat transfer coefficients .............................................. | |
Lecture 12. Description of the radiation process. Basic definitions…………………………………………. | |
Heat transfer by radiation of a system of bodies in a transparent medium .................................... | |
Transfer of radiant energy in an absorbing and radiating medium ............................... | |
Lecture 13 ................................... | |
Types of heat exchangers ............................................................... ......................... | |
Fundamentals of thermal calculation of heat exchangers .............................................. |
Subjectmethod of thermodynamics
Thermodynamics studies the laws of energy conversion in various
processes occurring in macroscopic systems and accompanied by thermal
mi effects. A macroscopic system is any material object
ect, consisting of a large number particles. The sizes of macroscopic systems do not correspond
measurable more sizes molecules and atoms.
Depending on the objectives of the study, technical or chemical
thermodynamics, thermodynamics biological systems etc. Technical thermodynamicsstudies the regularities of the mutual transformation of thermal and mechanical energy and the properties of the bodies participating in these transformations. Together with the theory of heat transfer, it is the theoretical foundation of heat engineering. On its basis, the calculation and design of all heat engines, as well as all kinds of technological equipment, are carried out.
Considering only macroscopic systems, thermodynamics studies the
regularities of the thermal form of the motion of matter, due to the presence of a huge
number of continuously moving and interacting micro-
structural particles (molecules, atoms, ions).
The physical properties of macroscopic systems are studied by statistical thermodynamic methods. Statistical Method based on the use of theo-
rii of probabilities and certain models of the structure of these systems and represents
attraction of model ideas about the structure of matter and is a phenomenal
logical (i.e., considers "phenomena" - phenomena as a whole).
At the same time, all the main conclusions of thermodynamics can be made using only two main empirical law thermodynamics.
In the future, based on the thermodynamic method, we will
ability to use molecular-kinetic ideas about the structure of matter.
Thermodynamic system
THERMO DYNAMIC SYSTEM is a set of material bodies that are in mechanical and thermal interactions with each other and with external bodies surrounding the system("external environment").
The choice of the system is arbitrary and dictated by the conditions of the problem being solved. Bodies not included in the system are called environment . The system is separated from the environment
living environment control surface(shell). So, for example, for the simplest system- gas contained in the cylinder under the piston, the external environment
doi is ambient air, and the walls of the ci-
lindra and piston.
Mechanical and thermal interactions of the thermodynamic system
pass through the control surfaces. During mechanical interaction by the system itself or on the system, work is done. (AT general case electric, magnetic and other forces can also act on the system, under the influence of which the system will perform work. These types of work can also be taken into account in the framework of thermodynamics, but we will not consider them further). In our example, mechanical work is performed when the piston is moved and accompanied by
given by the change in volume. Thermal interaction consists in the transfer of heat
between the individual bodies of the system and between the system and the environment. AT
In this example, heat can be supplied to the gas through the walls of the cylinder.
In the most general case, the system can exchange with the medium and matter
(mass transfer interaction). Such a system is called open. Gas or steam flows in turbines and pipelines - examples open systems. If the thing-
property does not pass through the boundaries of the system, then it is called closed. In the far-
We will consider closed systems, unless otherwise specified.
A thermodynamic system that cannot exchange heat with an
environment is called thermally insulated or adiabatic. Note-
rum of the adiabatic system is a gas in a vessel, the walls of which are covered
you are an ideal thermal insulation, excluding heat exchange between the enclosed
vessel with gas and surrounding bodies. Such an insulating shell is called adiabatic. A system that does not exchange energy or matter with the environment
society is called isolated (or closed).
The simplest thermodynamic system is working body, osu-
the mutual transformation of heat and work. In an internal combustion engine, for example, the working fluid is the go-
combustible mixture of air and gasoline vapors.
Thermodynamic parameters of the state
The properties of each system are characterized by a number of quantities, which are usually called thermodynamic parameters. Let us consider some of them, using the molecular-kinetic concepts of an ideal gas known from the course of physics as a collection of molecules that have an disappearing
very small sizes, are in chaotic thermal motion and interact
interact with each other only in collisions.
The pressure is due to the interaction of the molecules of the working fluid with
surface and is numerically equal to the force acting per unit area of the surface of the body along the normal to the latter. In accordance with the molecular kinetic theory, the gas pressure is determined by the relation
where n is the number of molecules per unit volume;
m - mass of the molecule; s 2 - root mean square velocity forward movement molecules.
AT international system units (SI) pressure is expressed in pascals
(1Pa=1 N/m2). Since this unit is small, it is more convenient to use 1 kPa = 1000 Pa and
1 MPa=106 Pa.
Pressure is measured using pressure gauges, barometers and vacuum gauges.
Liquid and spring pressure gauges measure excess pressure, pre-
which is the difference between the total or absolute pressure p measured
medium and atmospheric pressure p atm , i.e. p el p atm p
Devices for measuring pressures below atmospheric are called vacuum
meters; their readings give the value of vacuum (or vacuum):
r in r atm r, i.e. excess atmospheric pressure over the absolute.
Note that the state parameter is absolute pressure.
This is what enters into the thermodynamic equations.
The temperature is called physical quantity characterizing the
body heat stump. The concept of temperature follows from the following statement:
if two systems are in thermal contact, then in case of inequality of their temperature
temperatures, they will exchange heat with each other, if their temperatures are equal to
If so, then there will be no heat transfer.
From the point of view of molecular kinetic concepts, temperature is a measure of intensity thermal motion molecules. Her numerical value associated with
where k is Boltzmann's constant equal to 1.380662 10ˉ23 J/K. temperature T,
defined in this way is called absolute.
In the SI system, the unit of temperature is the kelvin (K); in practice, widely
the degree Celsius (°C) applies. The ratio between absolute T and stogra-
temperature t has the form
T t 273.15.
AT industrial and laboratory conditions, the temperature is measured using liquid thermometers, pyrometers, thermocouples and other instruments.
Specific volume v is the volume of a unit mass of a substance. If one
a foreign body of mass M occupies a volume v, then by definition
v=V/M.
In the SI system, the unit of specific volume is 1 m3 / kg. There is an obvious relationship between the specific volume of a substance and its density:
To compare the quantities characterizing systems in the same states,
introduced the concept of "normal physical conditions": p \u003d 760 mm Hg \u003d 101.325 kPa; T \u003d 273.15K.
AT different industries technology and different countries introduce their own, somewhat different
from the given " normal conditions”, for example, “technical” (p = 735.6 mm
Hg = 98 kPa, t = 15˚C) or normal conditions for assessing the performance of compressors (p = 101.325 kPa, t = 20˚C), etc.
If all thermodynamic parameters are constant in time and the same at all points of the system, then this state of the system is called equilibrium.
If between different points there are temperature differences in the system
tour, pressure and other parameters, then it is non-equilibrium . In such a system, under the influence of gradients of parameters, flows of heat, substances, and others arise, tending to return it to a state of equilibrium. Experience shows that
An isolated system always comes to a state of equilibrium over time and can never get out of it spontaneously. In classical thermodynamics, only equilibrium systems are considered.
State equation
For an equilibrium thermodynamic system, there is functional connection between the state parameters, which is called the equation
standing. Experience shows that the specific volume, temperature and pressure
the simplest systems, which are gases, vapors or liquids, are connected thermal equation states of the form f (p ,v ,T ) 0.
The equation of state can be given another form: p f 1 (v ,T );v f 2 (p ,T );
T f 3 (p, v);
These equations show that of the three main parameters that determine the state of the system, any two are independent.
To solve problems by thermodynamic methods, it is absolutely necessary to know the equation of state. However, it cannot be obtained within the framework of thermodynamics and must be found either experimentally or by methods of statistical physics.
ki. specific kind the equation of state depends on the individual properties of the thing
Equation of state for ideal gases
Equations (1.1) and (1.2) imply that p nkT .
Consider 1 kg of gas. Considering that it contains N molecules and, consequently,
The constant value Nk, referred to 1 kg of gas, is denoted by the letter R and
call gas constant. That's why
The resulting relation is the Clapeyron equation.
Multiplying (3) by M, we obtain the equation of state for an arbitrary gas mass
pV MRT . |
The Clapeyron equation can be given a universal form if we attribute the
call constant to 1 kmole of gas, i.e., to the amount of gas, the mass of which is in kilo-
grams is numerically equal to molecular weightμ. Putting in (1.4) М= μ and V=V μ , semi-
chim for one mole the equation of Clapeyron - Mendeleev:
pV RT .
Here V is the volume of a kilomole of gas, and R is the universal gas constant.
In accordance with the law of Avogadro (1811), the volume of 1 kmole, the same in one
them and the same conditions for all ideal gases, under normal physical conditions
wiah is equal to 22.4136 m3, therefore
The gas constant of 1 kg of gas is | ||||||||
thermodynamic process
The change in the state of a thermodynamic system with time is called
thermodynamic process. So, when the piston moves in the cylinder, the volume, and with it the pressure and temperature of the gas inside, will change,
the process of expanding or compressing the gas will take place.
As already noted, the system, brought out of equilibrium, and pre-
delivered at constant environmental parameters to itself, through
which time will come again equilibrium state corresponding to these para-
meters. Such a spontaneous (without external influence) return of the system to a state of equilibrium
is called relaxation, and the period of time during which the system
ma returns to a state of equilibrium, called relaxation time.
It is different for different processes: if it is always required to establish an equilibrium pressure in a gas, then to equalize the temperature in the volume of the same gas, it is necessary
us ten; minutes, and in the volume of the heated solid - sometimes several hours.
A thermodynamic process is called equilibriumif all para-
the meters of the system during its flow change rather slowly compared to the corresponding relaxation process. In this case, the system is actually in a state of equilibrium with the environment all the time, which determines the name of the process.
For the process to be equilibrium, the rate of change of the system parameters dA d must satisfy the relation
dA d c relay A relay
where A is the parameter that changes most rapidly in the considered pro-
cess; with rel - the rate of change of this parameter in the relaxation process; τ rel -
relaxation time.
Consider, for example, the process of compressing a gas in a cylinder. If the displacement time of the piston from one position to another significantly exceeds the relaxation time,
then in the process of moving the piston, the pressure and temperature will have time to equalize according to
the entire volume of the cylinder.
This alignment is provided by the continuous collision of molecules, in
as a result of which the energy supplied from the piston to the gas is fast enough and equal to
numbered among them. If subsequent displacements of the piston will occur in a similar way, then the state of the system at each moment of time will be practically equilibrium. In this way, equilibrium process is continuous series successive states of equilibrium, therefore, at each of its points, the state of the thermodynamic system can be described by the equation of state of the given working fluid. That is why classical thermodynamics in its research operates only with equilibrium processes. They are a convenient idealization real processes, which in many cases significantly simplifies the solution of the problem. This idealization is quite justified, since the condition
(1.8) is fulfilled quite often in practice. Since mechanical perturbations
vibrations propagate in gases at the speed of sound, the process of gas compression and cylinder
pe will be in equilibrium if the speed of the piston is much less than the speed of sound.
Processes that do not satisfy the condition dAd rel D A rel , proceed with imbalance, i.e. are nonequilibrium . If, for example, it rapidly increases the ambient temperature, then the gas in the cylinder will gradually
heat through its walls, relaxing to a state of equilibrium corresponding to new environmental parameters. In the process of relaxation, the gas is not in equilibrium with the environment and cannot be characterized by the equation of state
niya, if only because in different points volume of gas temperature has different values.
LECTURE #1
DEFINITION OF ENERGY AND ITS TYPES.
THERMODYNAMICS AND ITS METHODS.
THERMODYNAMIC SYSTEMS.
Heat engineering - general technical discipline that studies the methods of obtaining, converting, transferring and using heat, as well as the principles of operation and design features heat and steam generators, heat engines, apparatuses and devices.
Thermodynamics ( component heat engineering) studies the laws of energy conversion in various physical and chemical processes occurring in macroscopic systems and accompanied by thermal effects.
known different kinds energy: thermal, electrical, chemical, magnetic, etc. The tasks of research can be different - this is the thermodynamics of biosystems, technical thermodynamics, etc. We are interested in technical thermodynamics, which studies the patterns of mutual transformation of heat and mechanical energy(together with the theory of heat transfer) and therefore is the theoretical foundation of heat engineering. Without this theoretical foundation, it is impossible to calculate and design a heat engine.
The thermodynamic method is phenomenological. The phenomenon is considered as a whole. The relationship between the macroscopic parameters that determine the behavior of the system is established by the two principles of thermodynamics. Thermodynamic system is a set of material bodies that are in mechanical and thermal interaction with each other and with external bodies surrounding the system.
Thermodynamic state body (for example, gas) is characterized by its mass, molar mass μ, pressure, volume, temperature (and possibly other quantities, for example, defining it chemical composition). All these quantities are called thermodynamic parameters of the body. However, as will be seen from what follows, such parameters as , have meaning only when the body is, at least approximately, in the so-called state of thermodynamic equilibrium (t.d.r.). This is the name of the state in which all thermodynamic parameters remain constant over time (to this we should add the condition of the absence of stationary flows). If, for example, the gas is rapidly heated, as shown in Fig. 9.1, the temperature of the directly heated part of the vessel A will be higher than the temperature of part B. The pressures in parts A and B will not be equal either. In this case, the concept of temperature or pressure of the entire gas does not make sense. Another example is to let a beam of fast molecules into a gas. It is clear that it makes no sense to talk about the temperature of the gas until fast molecules, due to a series of collisions with others, acquire velocities of the order average speed other molecules, in other words, until the system reaches the state of s.f.r.
In a state of etc. for each substance, the thermodynamic parameters are interconnected by the so-called equation of state:
Here R=8.31 J/(molK) is the universal gas constant, μ - molar mass. For carbon (C), the value of μ is 12g, for hydrogen (H 2) - 2g, for oxygen (O 2) - 32g, for water (H 2 O) - 18g, etc.
A mole of any substance contains the same number of molecules N 0, called the Avogadro number:
The ratio of the universal gas constant R to the Avogadro number (i.e. the universal gas constant per molecule) is called Boltzmann constant:
An ideal gas is a gas so rarefied that it obeys equation (1.2) or (1.6). The meaning of this definition is, obviously, that in order to obey equation (1.6), the gas must be sufficiently rarefied. If the gas, on the other hand, is compressed to a sufficient high densities(so-called real gas), then instead of (1.6) we have
The choice of the thermodynamic system is arbitrary. The choice is dictated by the conditions of the problem being solved. The bodies that are not included in the system are the environment. The separation of the thermodynamic system and the environment is carried out by the control surface. So, for example, for the simplest thermodynamic system cylinder-gas-piston, external environment ambient air, and the control surface is the cylinder shell and piston. The mechanical and thermal interaction of the thermodynamic system is carried out through the control surfaces.
During the mechanical interaction of the system itself or on it, work is done. It should be noted that work can be done under the influence of other power- electric, magnetic.
Considering the example with the cylinder-piston system, we can note the following: mechanical work is performed when the piston moves and is accompanied by a change in volume. Thermal interaction consists in the transfer of heat between the individual bodies of the system and between the system and the environment. In the example under consideration, heat can be supplied to the gas through the walls of the cylinder. For an open thermodynamic system, the exchange takes place with the environment and matter (mass transfer processes). In what follows, we will consider closed thermodynamic systems. If the system is thermally insulated, then we call it adiabatic, for example, a gas in a vessel with ideal thermal insulation. Such a system does not exchange either heat or matter with the environment and is called closed (isolated).
The transformation of heat into work and vice versa work into heat is carried out by systems representing gases and vapors, they are called working bodies.
In the development of thermodynamics as a science huge contribution made by Russian scientists: M.V. Lomonosov - defined the essence of heat as internal movement matter, in addition, he determined the essence of the subsequently developed laws of thermodynamics, a hundred years before Clausius (1850), gave the content of the second law of thermodynamics, quantification was given by Lomonosov in two of his works of 1750 and 1760. We can mention G.G. Hess (1840), who established a law on thermal effect chemical reaction, prof. Schiller N.N. (Kyiv University) - gave more than rigorous justification second law of thermodynamics, prof. Afanas'eva-Ehrenfest T.A. for the first time showed the expediency of a separate interpretation of the second law of thermodynamics for equilibrium and non-equilibrium processes. Research in applied and theoretical terms was carried out by the scientists of Moscow Higher Technical School Grinevetsky V.I., Kirsh K.V., Mertsalov N.I., Ramzin L.K., Oshurkov B.M. The first Soviet textbook on thermodynamics was written by Oshurkov B.M. Scientists VTI, MPEI Vukalovich M.P., Kirillin V.A., Novikov I.I., Timrot D.A., Vargaftik N.B. conducted extensive research to obtain new data on thermophysical properties a number of new working bodies. From foreign scientists huge contribution Sadi Carnot, R. Stirling, R. Mayer, Clausius, Helmholtz, Joule, Thomson, Reynolds and others contributed to the development of thermodynamics. By the way, R. Stirling 8 years before S. Carnot in 1816 patented a machine that does work due to heated air.
1 DK 536.7(07) + 536.24 Reviewers: Department of Heat Engineering and Thermal Power Plants of St. Petersburg state university means of communication (Doctor of Technical Sciences, Prof. I.G. Kiselev), Professor B.S. Fokin (JSC NPO "TsKTI named after I.I. Polzunov") Sapozhnikov S.Z., Kitanin E.L. Technical thermodynamics and heat transfer: Textbook for universities. St. Petersburg: Publishing house of St. Petersburg State Technical University, 1999. 319 p. ISBN 5-7422-0098-6 The Basics Outlined technical thermodynamics and heat transfer. The principles of thermodynamics, methods for calculating thermodynamic processes with an ideal gas and with real working fluids, cycles of power plants, refrigeration machines and heat pumps are presented. The processes of stationary and non-stationary heat conduction, convective heat transfer, and heat transfer by radiation are described. The basics of thermal calculation of heat exchangers are given. Designed for bachelors in the direction 551400 “Terrestrial transport systems ". I8BN 5-7422-0098-6 St. Petersburg State Technical University, 1999 Sapozhnikov S.Z., Kitanin E.L., 1999 2 CONTENTS Foreword................................. ................................................. .... 1. TECHNICAL THERMODYNAMICS ............... 1.1. The subject and method of technical thermodynamics ....... 1.2. Basic concepts of thermodynamics ........................ 1.2.1. Thermodynamic system and thermodynamic parameters .............................................................. .............. 1.2.2. Thermodynamic equilibrium and equilibrium thermodynamic process .................................................. 1.2.3. Thermal equation of state. Thermodynamic surface and state diagrams…………………………………………………. 1.2.4. Mixtures of ideal gases............................................... 1.2.5. Energy, work, heat ....................................... 1.2.6. Heat capacity................................................. ........ 1.3. The first law of thermodynamics .............................................. 1.3.1. Equation of the first beginning .............................. 1.3.2. Internal energy as a function of state .............................................................. ............................... 1.3.3. Enthalpy and its properties .............................................. 1.3.4. Equation of the first law for an ideal gas .............................................................. ............................................... 1.4. Analysis of processes with an ideal gas ....................... 1.4.1. Isobaric process.............................................. 1.4. 2. Isochoric process.................................................... 1.4 .3. Isothermal process.............................................. 1.4.4. Adiabatic process.................................................... 1.4.5 . Polytropic processes ........................................ 1.4.6. Compression of gas in a reciprocating compressor ............... 1.5. The second law of thermodynamics .............................................. 1.5.1. Reversible and irreversible processes ................. 1.5.2. Cycles and their efficiency ....................................................... ...... 1.5.3. Statements of the second law .............................. 1.5.4. Carnot cycle. Carnot's theorem.............................. 3 1.5.5. Entropy, its change in reversible and irreversible processes .............................................................. ......................... 1.5.6. T–s state diagram. Entropy change in ideal gas processes....................... ................................................. ................... 1.5.7. Thermodynamic temperature scale ............... 1.6. Cycles of reciprocating internal combustion engines .............................................................. .................................... 1.6.1. Cycle with isochoric heat supply (Otto cycle) 1.6.2. Cycle with isobaric heat supply (Diesel cycle) .............................................................. ................................................. ................ 1.6.3. Comparison of the efficiency of internal combustion engine cycles .............. 1.7. Cycles of gas turbine plants.............................................. 1.7.1. Scheme and cycle with isobaric heat supply. 1.7.2. Thermal efficiency of the Brayton cycle.............................. 1.7.3. GTU regenerative cycle .............................................. 1.7.4. Efficiency of real cycles................... 1.8. Thermodynamics of real working bodies.............................. 1.8.1. Equations of state of real gases ............... 1.8.2. Change state of aggregation substances .... 1.8.3. State Diagrams and Tables .................................. 1.9. Cycles of steam power plants .................................. 1.9.1. Steam Carnot cycle .................................................. 1.9.2. Rankine Cycle .................................................. ..... 1.10. Cycles of refrigeration machines and heat pumps 1.10.1. Reverse Carnot cycle .................................................. 1.10 .2. Vapour-compression refrigeration cycle with steam superheat and throttling .............................. 1.10.3. Heat pump cycle.............................................. 1.11. Wet air................................................ .......... 1.11.1 Basic concepts and definitions .................. 1.11.2. h–d-diagram of humid air............... 2. HEAT TRANSFER....................... ................................... 4 2.1. General representations about heat transfer ........................ 2.2. Thermal conductivity................................................. ....... 2.2.1. Basic concepts and definitions ............... 2.2.2. Hypothesis Bio-Fourier .................................. 2.2.3. Differential equation of heat conduction. …………………………………………………………… 2.2.4. Conditions for uniqueness .............................. 2.2.5. Models of bodies in problems of heat conduction .............. 2.3. Stationary thermal conductivity ........................................ 2.3.1. Thermal conductivity of plates and shells ......... 2.3.2. Thermal conductivity of ribbed surfaces. 2.4. Non-stationary thermal conductivity .............................. 2.4.1. Thermal conductivity of thermally thin bodies....... 2.4.2. Thermal conductivity of a semi-infinite body and rod .............................................................. .......... 2.4.3. Heating and cooling of plate, cylinder and ball. 2.4.4. Heating and cooling of bodies of finite dimensions…….. 2.4.5. Regular thermal regime ............................... 2.5. Approximate methods of the theory of heat conduction. 2.5.1. Electrothermal analogy .............................. 2.5.2. Graphical method ........................................ 2.5.3. Finite difference method ........................................ 2.6. Physical foundations convective heat transfer.. 2.6.1. Basic concepts and definitions .................. 2.6.2. Differential equations of convective heat transfer .................. ......................................... 2.7. Fundamentals of the theory of similarity .............................................. 2.7.1. Similarity of physical phenomena ............................... 2.7.2. Similarity theorems.................................................... 2.7.3 . Similarity equations .................................................. 2.7.4. Modeling Rules .................................. 2.8. Convective heat transfer in a single-phase medium..... 2.8.1. Flow regimes of liquids and gases ............... 5 2.8.2. Boundary layer.............................................. 2.8.3. Heat transfer in a laminar boundary layer on a flat surface .............................................................. ....... 2.8.4. Heat transfer in a turbulent boundary layer on a flat surface .............................................................. ... 2.8.5. Heat transfer during forced convection in pipes and channels .................................................. 2.8.6. Heat transfer in a stabilized flow section. Integral Lyon................................... 2.8.7. Heat transfer in laminar flow in pipes ……………………………………………………….. 2.8.8. Heat transfer at turbulent flow in pipes... 2.8.9. Heat transfer in the flow around pipes and tube bundles .............................................................. ............................... 2.8.10. Heat transfer with free convection ........ 2.8.11. Heat transfer in fluidized media ....... 2.9. Convective heat transfer during boiling and condensation .............................................................. ............................... 2.9.1. Boiling heat exchange ......................................... 2.9.2. Condensing Heat Transfer .......................................... 2.9.3. Heat pipes ................................................................ 2.10. Heat exchange by radiation .............................................. 2.10.1. Physical bases of radiation.............................. 2.10.2. Calculation of heat transfer by radiation ............... 2.10.3. Solar radiation .................................................. 2.10.4. Complex heat transfer ........................................ 2.11. Heat exchangers ................................................................ ......... 2.11.1 Classification and purpose .......... 2.11.2. Fundamentals of thermal calculation .............................. 2.11.3. Efficiency of heat exchangers. Actual heat transfer coefficients .............................................. 2.11.4. Hydraulic calculation of heat exchangers ... References .................................................. ................... 6 FOREWORD “Technical thermodynamics and heat transfer” is one of the main courses given to bachelors in the direction “Land transport systems”. It is saturated with information and compressed in terms of study time to 1–2 semesters, so most fundamental textbooks will not help students much: they are too detailed, not focused on the range of tasks associated with transport systems, and, finally, are simply designed for much larger courses. For transport engineers, the main thing is to understand the subject and basic ideas of thermodynamics and heat transfer, to master the established terminology of these sciences. It is absolutely necessary to remember 10-15 basic formulas(such as the equation of state ideal gas, the formula for calculating heat transfer through a multilayer plate, the Stefan-Boltzmann law, etc.). The rest of the information, for all its importance, you just need to understand, present physically, connect with examples from various fields of life and technology. Therefore, the authors tried to pay the main attention to the physical side of the phenomena under consideration, and left a worthy, but modest place to the mathematical apparatus. The authors express their deep gratitude to the reviewers - the department "Heat engineering and thermal power plants" of the St. Petersburg State University of Railways represented by Dr. Sciences prof. I. G. Kiseleva and Ph.D. tech. Sciences Assoc. V. I. Krylov, as well as Dr. tech. Sciences prof. B. S. Fokin for valuable remarks, which made it possible to improve the original text. Special thanks - Cand. tech. Sciences G. G. Gavre for great help in preparing the manuscript; she came up with the idea to compare N, ε - a method for calculating heat exchangers with a traditional calculation scheme. And, of course, the help in the design of the book of the employees of the department was very valuable. Theoretical basis Heat Engineering” of St. Petersburg State Technical University 7 E. O. Vvedenskaya, R. M. Groznoy, graduate students Yu. V. Burtseva and E. M. Rotinyan. S. Sapozhnikov E. Kitanin 8 1. TECHNICAL THERMODYNAMICS 1.1. SUBJECT AND METHOD OF TECHNICAL THERMODYNAMICS Thermodynamics - the science of energy transformations - is fundamental for a power engineering engineer. The birth of thermodynamics coincides in time with the appearance of the first steam engines. In 1824, the French engineer S. Carnot considered energy interaction water and steam with various parts of the engine and with the environment, he owns the first efficiency rating steam engine. Since then, processes in power machines, aggregate transformations of substances, physicochemical, plasma and other processes have become the subject of study of thermodynamics. These studies are based on thermodynamic method: the object of study can be any bodies included in the so-called thermodynamic system. This system should be: sufficiently extensive and complex so that statistical regularities are observed in it (the movement of molecules of a substance in a certain volume, heating and cooling of particles of a solid material in a backfill, etc.); closed, i.e., have limits in all spatial directions and consist of a finite number of particles. There are no other restrictions for the thermodynamic system. Objects material world, not included in the thermodynamic system, is called the environment. Returning to the works of S. Carnot, we note that water and the steam obtained from it are a thermodynamic system. By tracing the energy interaction of water and steam with the surrounding bodies, it is possible to evaluate the efficiency of converting the heat supplied to the machine into work. But modern power machines do not always use water to convert energy. We agree to call any medium that is used to convert energy a working body. 9 Thus, the subject of technical thermodynamics is the laws of energy conversion in the processes of interaction of working bodies with elements of power machines and with the environment, analysis of the perfection of power machines, as well as the study of the properties of working bodies and their changes in the processes of interaction. Unlike statistical physics, which studies the physical model of a system with clear patterns of interaction between microparticles, thermodynamics is not connected in its conclusions with any structure of the body and with certain forms of connection between the elements of this structure. Thermodynamics uses the laws universal character, i.e., valid for all bodies, regardless of their structure. These laws form the basis of all thermodynamic reasoning and are called the principles of thermodynamics. The first principle expresses the law of conservation of energy - the universal law of nature. It determines the balance of energy in interactions within the thermodynamic system, as well as between the thermodynamic system and the environment. The second law determines the direction of energy transformations and significantly expands the possibilities of the thermodynamic method. Both principles are of an experimental nature and are applicable to all thermodynamic systems. Based on these two principles, presented in mathematical form, it is possible to express the parameters of energy exchange at various interactions, establish connections between the properties of substances, etc. However, in order to bring the results to specific numbers, the "internal resources" of thermodynamics alone are not enough. It is necessary to use experimental or theoretical results that take into account the nature of the working fluid in a real thermodynamic system. If, for example, one uses experimental data on the density of a substance, then with the help of thermodynamic analysis one can calculate its heat capacity, etc. 10 Thus, thermodynamic studies are based on the fundamental laws of nature. At the same time, engineering calculations in thermodynamics are impossible without the use of experimental data or the results of theoretical studies. physical properties working bodies. 1.2. BASIC CONCEPTS OF THERMODYNAMICS 1.2.1. Thermodynamic system and thermodynamic parameters We have called a thermodynamic system any body or system of bodies interacting with each other and (or) with the environment (such a system may, in particular, include the working bodies of power machines). The definition does not specify what exactly is considered a thermodynamic system, and what is considered an environment. It is possible, for example, to consider the working fluid itself as a thermodynamic system, and to consider “everything else” as the environment; it is possible to single out only a part of the body, and consider the rest of the body and all other bodies as the environment. It is possible, on the contrary, to expand the thermodynamic system - to include in it, besides the first body, several others, and consider all other bodies as the environment. Such an expansion or narrowing of the circle of objects that make up a thermodynamic system allows us to find out important features working bodies and energy interactions between them. It is known that the same substance can be in a liquid, gaseous or solid state. In this case, naturally, the properties of this substance, this thermodynamic system, will also be different, for example, density, coefficient of volumetric expansion, magnetic permeability, sound speed, etc. All these, as well as other quantities characterizing the state of a thermodynamic system, are called thermodynamic parameters states. There are a lot of them; traditionally allocate
Thermodynamic calculation of the heating cycle
Thermodynamic fundamentals of district heating
As you know, heat engines, by their very definition, are designed to convert a chaotic form of energy transfer (in the form of heat) into an ordered form ( mechanical movement, electricity, etc.). However, in addition to an ordered form of energy, humanity also needs warmth in its activities, in particular for heating and for the implementation of all kinds of technological processes(cooking, drying, chemical Technology, metallurgy, etc.).
At first glance, it may seem that the problem of economic improvement of heat supply to technical thermodynamics as a science of improving heat engines has no direct relationship, However, it is not. The fact is that heat, as one of the forms of energy transfer, in addition to the quantity measured in joules, also has a quality, namely, potential, i.e. temperature. In fact, few people are interested a large number of heat supplied to a dwelling at a temperature of 10 ... 12 ° C. On the other hand, the combustion temperature of most of the fossil fuels, whether it be firewood, coal, gas, oil, etc., is too high to be directly used for heating purposes, or for other technological processes. Technical thermodynamics points to one of possible ways rational use“thermal energy” (note that this phrase, which is well-established in everyday life, is not correct from the point of view of thermodynamics; it should be borne in mind that we should be talking about the transfer of energy in the form of heat). Since the heat potential (temperature) commonly used for heating purposes is 50 ... 150 ° C (330 ... 430 K), and the fuel combustion temperature (torch temperature) is about 1500 ... 2000 ° C (1800 ... 2300 K), it seems very rational to carry out between these temperature levels (potentials) a cycle of some heat engine, thereby reducing exergy losses, i.e. losses associated with irreversible heat exchange between the heated room and the heat source. Such a joint production of an ordered form of energy (usually electrical) and heat for industrial needs and space heating is called district heating.
Let us show that the joint generation of electrical and thermal energy (cogeneration) is always more economical from a thermodynamic point of view than separate generation. To do this, consider the diagram on which we conditionally depict the temperature levels for various processes supply and removal of heat (Fig. III.27). The dots above the values in the diagram represent the total time derivative, i.e. we will compare the powers various schemes thermal and electrical energy. In this case, we will not take into account the inevitable losses in such installations, since taking them into account will not affect the course of reasoning, although it will significantly complicate the analysis.
Separate generation of thermal and electrical energy is shown in fig. III.27 diagrams and . In a heating boiler house, the products of fuel combustion give off heat in the process in an amount to the coolant (usually water), which, through heating network is supplied to the consumer, providing the heat load (excluding losses). Electrical load N is provided by a steam power plant operating according to the Rankine cycle with heat discharge to the cooling water in the condenser. This setup is called condensation.
The total heat consumption in the boiler house and in the condensing unit for given thermal and electrical loads will then be determined by the sum
With the joint production of the same amounts of thermal and electrical energy, the thermal power of the steam generator will be equal (also without taking into account losses)
The difference between expressions and gives heat savings (and hence fuel)
Heat supply is widely used in thermal and nuclear power plants supplying electricity and heat to large settlements and large energy-intensive industries. At the same time, in energy practice two schemes of heating cycles are used - with backpressure and with steam extraction for heating.
Thermodynamic calculation of the heating cycle
With back pressure
Schematic diagram of a backpressure heating plant and diagram T-s cycles are shown in fig. III.28.
The design of a backpressure cogeneration plant is not structurally different from that of a conventional condensing plant, except that in a backpressure plant, the pressure of the exhaust steam at the outlet of the turbine is maintained sufficiently high (hence the name back pressure), so that the temperature of the exhaust steam is 150 ... 180 ° C (the saturation pressure is 5 ... 10 bar). For this reason, in a backpressure installation, the condenser is replaced by a less bulky heat exchanger called boiler ( English boiler – boiler,boiler,evaporator).
Let us present an algorithm for the thermodynamic calculation of a heating cycle with back pressure, taking into account losses in the steam generator, turbine, mechanical and electrical losses and losses in thermal networks. All these losses are numerically estimated using the coefficients η pg, , η mech, η el, η ts.
Using a chart h–s or using tables thermodynamic properties water and water vapor we find in the standard way the specific enthalpies h 1 , h 2 , h 3 . Further, based on the definition of the relative internal efficiency of the turbine, we find the actual value of the specific enthalpy of the exhaust steam
Assuming the boiler is ideally insulated, from its heat balance we find the mass flow rate of steam in the installation, providing a given heat load,
The power of the installation, taking into account the listed losses, will be
The heat supplied in the steam generator to the working fluid
and the thermal power of the steam generator, taking into account losses η pg, will be equal to
which makes it possible to calculate the fuel consumption at known value its calorific value
