powerengineeringlecturesac1introductiontothermodynamics动力工程英文教材教程教案讲义

1、Power Engineering Lectures - AC1 Introduction toThermodynamics 动力工程英文教材教程教案讲义Introduction to Thermodynamics THRM 6006 1 Learning Outcome When you complete this module you will be able to: Explain the principles of thermodynamics, including the laws of thermodynamics and the modes of heat transfer. L

2、earning Objectives Here is what you will be able to do when you complete each objective: 1. Describe the laws of thermodynamics. 2. Explain the different temperature scales used in thermodynamics. 3. Define heat and specific heat and perform sensible heat calculations. 4. Describe the expansion of s

3、olids and liquids. 5. Describe the three modes of heat transfer.INTRODUCTION Thermodynamicsis a branch of physics that deals with the conversion of heat into other forms of energy, or other forms of energy into heat. An operator is involved in heat transfer in manyaspects of the job. For example, in

4、 boilers heat energy is transferred from burning fuel to water and steam. In internal combustion engines, fuel is burned with air in a cylinder to perform work on a piston. In refrigeration systems, heat is extracted from the substance to be cooled and transferred to other systems to be utilized or

5、disposed of. The efficient transfer and utilization of heat energy is therefore of major importance to the operator, and it is essential to have a good understanding of thermodynamics. To understand energy conversion it is necessary to understand the concepts of temperature, heat, expansion of liqui

6、ds and solids, pressures, the general laws of thermodynamics, and certain other topics which are fundamental to the study of thermodynamics. DEFINITIONS The following definitions are for terms used throughout this module. Absolute pressure: The pressure measured above a perfect vacuum. It is the sum

7、 of atmospheric pressure and gauge pressure and is expressed as kPa (abs) or psia. Absolute zero: The temperature at which all molecular vibrational motion ceases. Atmospheric pressure: The pressure exerted by the earth s atmosphere. This pressure can be expressed as follows: In SI units as kPa, mm

8、of mercury (mm Hg), or metres of water (m H2O). In Imperial units as psi, inches of mercury (in. Hg) or feet of water (ft H2O). The standard atmospheric pressure expressed in SI units is 101.325 kPa, 760 mm Hg or 10.33 m H2O. The equivalent units in the Imperial system are: 14.696 psi, 29.92 in. hg

9、or 33.39 ft H2O. The standard atmospheric pressure is the average atmospheric pressure at sea level. Actual atmospheric pressure varies from location to location on the earth s surface and from time to time at each specific location. THRM6006 2Change of state: The altering of a substance from solid

10、to liquid or liquid to gas or vice-versa. Conduction: The flow of heat from molecule to molecule within a substance or from molecules of one body to those of another body in direct contact with it. Convection: The transfer of heat within a fluid by movementof the fluid whereby warm fluid is displace

11、d by cooler fluid. Cooling: The process of removing heat from a substance, resulting in a decrease in temperature and/or a change in state. Density: The mass per unit volume of a substance expressed in kg/m3 or lbs/ft3. Enthalpy or heat content: The amount of heat expressed in kJ/kg or Btu/lb, conta

12、ined in a substance, relative to a base temperature at which the enthalpy is defined to be zero. For example, in calculating the enthalpy of steam, the base temperature of water is taken at0C (32 F) and the enthalpy of the steam is thesum of the heat required to raise a unit mass of the water to its

13、 boiling point (sensible heat) and the heat required to evaporate this water into steam (latent heat). Should the steam be superheated, the heat required to raise the temperature of the steam above the boiling point (sensible heat) must also be added to arrive at the enthalpy. The enthalpy of refrig

14、erants is determined in a similar way, however, the base temperature is taken at -40C ( - 40F) or at absolute zero. Gage pressure: Thepressure in a closed vessel asregistered on a pressure gauge. This is pressure above atmospheric pressure. It is expressed as kPa (gauge) or psig. Heat: This is a for

15、m of energy which when supplied to a body or substance, will increase the internal energy of that body or substance. Heating: The process of increasing the heat energy of a substance, resulting in a decrease in temperature and/or change of state. Heat transmission: The movement of heat. Heat always

16、flows from a warmer (higher temperature) substance to a colder (lower temperature) substance. Latent heat: Heat that causes a change of state of a substance without changing its temperature. Latent heat of evaporation: The amount of heat required to change a unit mass of a substance from liquid to v

17、apour without changing its temperature. Latent heat of fusion: The amount of heat required to change a unit mass of a substance from solid to liquid without changing its temperature. THRM 6006 3Pressure: Force per unit area. The unit of pressure in the SI system is the pascal (Pa) which equals 1 N/m

18、2. It is a force of 1 newton (N) acting uniformly over an area of 1 m2. Since the pascal is a very small unit, the kilopascal (kPa) is commonly used to express pressures. In the Imperial system, the unit of pressure is the pound per square inch (psi) written as 1 lb/sq in. 1 psi = 6.895 kPa. Radiati

19、on: The transfer of heat by electromagnetic waves causing a rise in temperature in the body that they strike by increasing the motion of the molecules of that body. Saturated steam: Steam that is fully saturated with latent heat and has no water particles present. Saturation temperature: The tempera

20、ture at whicha liquid reaches its boiling point. This temperature depends on the pressure on the surface of the liquid. For example, at atmospheric pressure at sea level (101.3 kPa absolute or 14.7 psia) the saturation temperature or boiling point of water is 100C (212 F). When thepressure is raised

21、 to 198.53 kPa absolute (28.8 psia), the boiling point rises to 120C(248 F). Sensible heat: Heat that causes a change in the temperature of a substance without changing its state. Specific heat: The amount of heat required to raise the temperature of a unit mass of a substance 1 C without changing t

22、he state of the substance. SpecificVolume: The volume of a unit mass of a substance expressed in m3/kg or ft3/lb. For gases, this will depend on the temperature and pressure of the gas. Superheat: The temperature increase of a gas or vapour above its saturation temperature after all the liquid has b

23、een evaporated. It is expressed in C or F, as in 50C of superheat. Temperature: A measure of the intensity or “hotness ” of heat in a body. Unit of heat energy: The unit used to measure heat energy. It is the joule (J) or kilojoule (kJ) in the SI system, and the BritishThermal Unit (Btu) in the temp

24、erature of 1 m3 of air 1 temperature of 1 lb of water 1 below atmospheric pressure.Imperial system. It takes approximately 1 kJ to raise the C at standard atmospheric pressure and 1 Btu to raise theF. 1 kJ = 0.9478 Btu. Vacuum: The reduction in pressureIt is usually expressed in mmHg or in inches Hg

25、. If atmosphericpressure on a particularday is 760 mmHg, the a perfect vacuum would be 760 mmHg of vacuum.THRM 6006 4Wet Steam: Steam that does not have its full quantity of latent heat energy and has water present. LAWS OF THERMODYNAMICS There are two laws of thermodynamics that the student must fu

26、lly understand. The First Law of Thermodynamics The first law of thermodynamics states that heat and work are mutually convertible. This law can be stated in the general form: Work = Heat In practice it is usually the case that only a part of the heat is converted to work, or only a part of the work

27、 generates heat. For example, when considering the expansion of a gas in a cylinder moving a piston (called a nonflow process), then the heat supplied does work in moving the piston and the internal energy of the gas also increases so that: Heat supplied = increase in internal energy + work done Q =

28、 ?U + W. D. In a steam boiler, the same relationship applies. As the water is being heated, the internal energy increases and work is being done in increasing the volume of the water. When steam is being produced, the internal energy increases and work is being done to increase the volume during the

29、 change from water to steam at constant temperature. The Second Law of Thermodynamics The second law of thermodynamics states that, unaided, heat will only flow from a hot substance to a colder substance. If it is required to transfer heat from a cold substance to a hotter substance (as in refrigera

30、tion) then external work must be supplied.THRM 6006 5TEMPERATURE The temperature of a body is a measure of the speed at which the bodysmolecules vibrate. A high temperature indicates an increased molecular velocity and a low temperature indicates a decreased molecular velocity. Also, a body at a hig

31、h temperature will have the ability to transfer heat to a lower temperature body, so the temperature determines the direction of the heat flow between a body and its surroundings. Suppose that a piece of metal is liftedfrom the heat of a forge and set on an anvil. Heat willflow fromthe red hot metal

32、 to the anvil. We say that the metal temperature is higher than that of its surroundings. Alternatively, suppose that an ice block is removed from a refrigerated locker and placed into an ice box. Heat will then flow from the contents of the ice box to the block of ice. The ice then is said to be at

33、 a lower temperature than that of its surroundings. Heat flows naturally from a point of high temperature toward a point of low temperature. The temperature at the points between gradually decreases. The rate of temperature change along these pointsis called a temperature gradient. Heat is a measure

34、of the quantity of internal vibrational energy in a substance. Temperature is a measure of the intensity or“hotness ” of heat in a body. For example a red hot rivet will containa small quantity of high grade (high temperature) heat as compared to the water in a low-pressure firetube boiler which cou

35、ld be said to contain a large quantity of low grade (low temperature) heat. Temperature is measured by means of a scaled instrument known as a thermometer. The scale on the thermometer is established with reference to two fixed points; namely, the boiling point of water and the melting point of ice.

36、 The space between these fixed points is divided into equal parts and each division is called a degree (). Thedivisions are also extended above and below the fixed points in order to measure temperatures above the boiling point of water and below the melting point of ice. Four different temperature

37、scales are in common use: Fahrenheit, Rankine, Celsius, and Kelvin. The Fahrenheit scale considers the boiling point of water as 212 and the melting point of ice as 32, giving 180 divisions or degrees between these points. The Rankine scale is based on Fahrenheit divisions and considers 0 as the low

38、est possib le temperature that may betheoretically achieved (absolute zero). This point is - 460 on the Fahrenheit scale. THRM 6006 6The Celsius scale (previously calledthe Centigrade scale) considers the boiling point ofwater as 100 and the melting point of ice as 0, giving 100 divisions or degrees

39、 between these points. The Kelvin scale is based upon degrees Celsius and like the Rankine scale considers 0 K as the lowest possible temperature (absolute zero). This point is -273 onthe Celsius scale. Note that the degree symbol is not used with the Kelvin scale. The absolute scales, Rankine and K

40、elvin, are used principally in thermodynamics. These four scales are shown in relation to each other in Fig. 1. THRM 6006 7 Figure 1 Temperature Scales The three fixed points used to compare the scales are the freezing point of water, the boiling point of water, and absolute zero. Note that the Cels

41、ius scale shows 100 degrees between freezing and boiling water, the Fahrenheit scale shows 180 degrees between the same points. Thus 180 Fahrenheit degrees = 100 Celsius degrees or 9 F = 5 C. The relationship between Fahrenheit and Celsius temperature can be expressed as: F = 9/5C + 32 C = 5/9 (F -

42、32) AC_1_0_1.gif GAbsolute Scales It is useful in engineering and in scientific work to have a range of temperatures beginning at absolute zero. This is the temperature at which all molecular motion is said to completely cease. These temperature scales beginning at absolute zero are called absolute

43、scales and are particularly useful in the calculation of the behaviour patterns of gases and vapours. The Fahrenheit absolute scale is called the Rankine scale and begins at 492 below the melting point of ice or 460 below zero on the Fahrenheitscale. The relationship between Fahrenheit and Rankine t

44、emperatures can be expressed as: R = F + 460 Note that the size of the degrees or divisions on both Fahrenheit and Rankine scales are the same. The Celsius absolute scale is called the Kelvin Scale and begins at 27 3 below the melting point of ice or 273 below zero on the Celsius scale.The relations

45、hip between Celsius and Kelvin temperatures can be expressed as: K =C +273 Note that the degrees or divisions on both Celsius and Kelvin scales are the same. From absolute zero to the melting point of ice (freezing point of water) measures a span of 492 Rankine degrees and 273 Kelvin. The ratio 492

46、to 273 is the same as 9 to 5. Hence 9 Rankine degrees denotes the same temperature difference as 5 K. The relationship between Rankine and Kelvin temperatures can be expressed as:R = 9/5 x K K = 5/9 xR MeasuringTemperature There is a considerable range of instruments available for the measurement of

47、 temperature. The particular type chosen depends upon the temperatures to be measured and the conditions under which the measurements will be made. THRM 6006 8 1. Liquid-in-Glass Thermometers The mercury or alcohol thermometer is constructed of a thick-walled glass tube with a small bore. A bulb is

48、on one end and the other end is sealed. Before sealing, the bulb and glass tube are completely filled with liquid at the highest temperature of the thermometer. The liquid cools, the level drops and a vacuum is created above the liquid. The thermometer is calibrated by immersing the bulb in melting

49、ice to obtain the freezing point of water, and then exposing the bulb to steam rising from boilingwater to obtain the boiling point. These points are marked on the stem or on a separate scaleand the space between is divided into100 or 180 equal divisions.The scale is then extended2. Bimetal Thermome

50、ters Bimetal fixed together. Due to their different strips will bend when subjected toby linkages or directly, to a pointerabove and below the freezing and boiling points. thermometers consist of two dissimilar metals rigidly coefficients of linear expansions these bimetal temperature changes. This

51、movementcan be transmitted, to read the temperature on a graduated scale. 3. Pyrometers The term pyrometer is used to describe instruments made for measuring temperatures in the range above that suitable for the mercury thermometer. There are several types using different principles of operation. Tw

52、o common types are the thermoelectric pyrometer and the optical pyrometer. (a)Thermoelectric Pyrometer The thermoelectric pyrometer makes use of a thermocouple that produces an electrical voltage which is proportional to temperature. A thermocouple consists of two wires each made of a different meta

53、l. The two wires are joined together ateach of their ends and if one of the joints is at a higher temperature than the other then a voltage is produced in the wires. The high temperature joint is called the hot junction and the other joint is called the cold junction. The voltage produced is proport

54、ional to the temperature difference between the junctions and this voltage is measured by means of a millivoltmeter which is calibrated to read in degrees of temperature. Fig. 2 is a simplified sketch of a thermoelectric pyrometer. THRM 6006 9 AC_1_0_2.gif G AC_1_0_3.jpg GTHRM6006 10 Figure 2 Thermo

55、electric Pyrometer (b) Optical Pyrometer This instrument is suitable for measuring extremely high temperatures. Its operation is based upon the fact that the intensity of light emitted from a hot surface will vary with the temperature of that surface. In the pyrometer, the brightness of the object t

56、o be measured is compared to the brightness of a filament. The filament brightness is produced by an electric current which heats the filament. The amount of current required to produce the necessary brightness is measured and will be proportional to the temperature of the object. HEAT Heat is a for

57、m of energy which may be transferred from one body to another by virtue of a difference in temperature. Heat can only betransferred from a hot body to a colder body, under normalcircumstances. If it is required to transfer heat from a cold body to a hot body (as in refrigeration) then external work

58、is required to makethe transfer. It should be noted that if there is no temperature difference between the heat source and the substance then there can be no heat transfer. Heat Units The joule is the basic unit of all energy, including heat. Units of heat are therefore expressed in joules or multip

59、les of the joule. 1 kilojoule (kJ) = 1000 J or 103 J 1 megajoule (MJ) = 1 000 000 J or 106 J AC_1_0_4.gif GThe unit used in the Imperial system is called the British Thermal Unit (Btu) and it isdefined as the amount of heat needed to raise the temperature of 1 lb of water by 1F. 1Btu = 1055.06 J Mechanical Equivalent of Heat It has been shown by experiment that one joule is equivalent to the work done by a force of one