13 CHAPTER 1 The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discon- 02 1atm,20℃ tinuities.This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules.This is the case in practically all problems,except some specialized ones.The continuum 3×1016 molecules/mm3 idealization is implicit in many statements we make,such as "the density of water in a glass is the same at any point." To have a sense of the distance involved at the molecular level,con- sider a container filled with oxygen at atmospheric conditions.The VOID diameter of the oxygen molecule is about 3 x 10-10 m and its mass is 5.3 x 10-26 kg.Also,the mean free path of oxygen at 1 atm pressure and 20C is 6.3 X 10-8 m.That is,an oxygen molecule travels,on average,a distance of 6.3 x 10-8 m (about 200 times of its diameter)before it col- FIGURE 1-24 lides with another molecule. Also,there are about 3 x 1016 molecules of oxygen in the tiny volume Despite the relatively large gaps between molecules,a gas can usually of 1 mm3 at 1 atm pressure and 20C (Fig.1-24).The continuum model be treated as a continuum because of is applicable as long as the characteristic length of the system(such as its the very large number of molecules diameter)is much larger than the mean free path of the molecules.At very even in an extremely small volume. high vacuums or very high elevations,the mean free path may become large (for example,it is about 0.1 m for atmospheric air at an elevation of 100 km).For such cases the rarefied gas flow theory should be used, and the impact of individual molecules should be considered.In this text we will limit our consideration to substances that can be modeled as a continuum. 1-5.DENSITY AND SPECIFIC GRAVITY Density is defined as mass per unit volume (Fig.1-25). Density: p= (kg/m) (1-4) V=12m3 m=3kg The reciprocal of density is the specific volume v,which is defined as vol- ume per unit mass.That is, p=0.25kg如3 V= m p (1-5) e8@ For a differential volume element of mass 6m and volume 6V,density can FIGURE 1-25 be expressed as p =6m/6V. Density is mass per unit volume; The density of a substance,in general,depends on temperature and pres- specific volume is volume sure.The density of most gases is proportional to pressure and inversely per unit mass. proportional to temperature.Liquids and solids,on the other hand,are essentially incompressible substances,and the variation of their density with pressure is usually negligible.At 20C,for example,the density of water changes from 998 kg/m3 at 1 atm to 1003 kg/m3 at 100 atm,a change of just 0.5 percent.The density of liquids and solids depends more strongly on temperature than it does on pressure.At I atm,for example,the density of water changes from 998 kg/m3 at 20C to 975 kg/m3 at 75C,a change of 2.3 percent,which can still be neglected in many engineering analyses
13 CHAPTER 1 The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discontinuities. This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules. This is the case in practically all problems, except some specialized ones. The continuum idealization is implicit in many statements we make, such as “the density of water in a glass is the same at any point.” To have a sense of the distance involved at the molecular level, consider a container filled with oxygen at atmospheric conditions. The diameter of the oxygen molecule is about 3 3 10210 m and its mass is 5.3 3 10226 kg. Also, the mean free path of oxygen at 1 atm pressure and 20°C is 6.3 3 1028 m. That is, an oxygen molecule travels, on average, a distance of 6.3 3 1028 m (about 200 times of its diameter) before it collides with another molecule. Also, there are about 3 3 1016 molecules of oxygen in the tiny volume of 1 mm3 at 1 atm pressure and 20°C (Fig. 1–24). The continuum model is applicable as long as the characteristic length of the system (such as its diameter) is much larger than the mean free path of the molecules. At very high vacuums or very high elevations, the mean free path may become large (for example, it is about 0.1 m for atmospheric air at an elevation of 100 km). For such cases the rarefied gas flow theory should be used, and the impact of individual molecules should be considered. In this text we will limit our consideration to substances that can be modeled as a continuum. 1–5 ■ DENSITY AND SPECIFIC GRAVITY Density is defined as mass per unit volume (Fig. 1–25). Density: r 5 m V (kg/m3 ) (1–4) The reciprocal of density is the specific volume v, which is defined as volume per unit mass. That is, v 5 V m 5 1 r (1–5) For a differential volume element of mass dm and volume dV, density can be expressed as r 5 dm/dV. The density of a substance, in general, depends on temperature and pressure. The density of most gases is proportional to pressure and inversely proportional to temperature. Liquids and solids, on the other hand, are essentially incompressible substances, and the variation of their density with pressure is usually negligible. At 20°C, for example, the density of water changes from 998 kg/m3 at 1 atm to 1003 kg/m3 at 100 atm, a change of just 0.5 percent. The density of liquids and solids depends more strongly on temperature than it does on pressure. At 1 atm, for example, the density of water changes from 998 kg/m3 at 20°C to 975 kg/m3 at 75°C, a change of 2.3 percent, which can still be neglected in many engineering analyses. FIGURE 1–24 Despite the relatively large gaps between molecules, a gas can usually be treated as a continuum because of the very large number of molecules even in an extremely small volume. VOID 1 atm, 20°C O2 3 ´ 1016 molecules/mm3 FIGURE 1–25 Density is mass per unit volume; specific volume is volume per unit mass. 3 = 12 m m = 3 kg 3 3 /kg = 0.25 kg/m = = 4 m 1 v V r r – cen98179_ch01_001-050.indd 13 11/28/13 3:14 PM
14 INTRODUCTION AND BASIC CONCEPTS TABLE 1-3 Sometimes the density of a substance is given relative to the density of a well-known substance.Then it is called specific gravity,or relative den- Specific gravities of some sity,and is defined as the ratio of the density of a substance to the den- substances at 0C sity of some standard substance at a specified temperature (usually water at Substance SG 4C,for which Po=1000 kg/m3).That is, Water 1.0 Blood 1.05 1.025 Specific gravity: SG=P (1-6) Seawater PH.o Gasoline 0.7 Ethyl alcohol 0.79 Mercury 13.6 Note that the specific gravity of a substance is a dimensionless quantity. Wood 0.3-0.9 However,in SI units,the numerical value of the specific gravity of a sub- Gold 19.2 stance is exactly equal to its density in g/cm3 or kg/L (or 0.001 times the Bones 1.7-2.0 density in kg/m3)since the density of water at 4C is 1 g/cm3 I kg/L Ice 0.92 1000 kg/m3.The specific gravity of mercury at 0C,for example,is 13.6. Air (at 1 atm) 0.0013 Therefore,its density at 0C is 13.6 g/cm3 13.6 kg/L 13,600 kg/m3. The specific gravities of some substances at 0C are given in Table 1-3. Note that substances with specific gravities less than I are lighter than water,and thus they would float on water. The weight of a unit volume of a substance is called specific weight and is expressed as Specific weight: y,=pg (N/m) (1-7) where g is the gravitational acceleration. The densities of liquids are essentially constant,and thus they can often be approximated as being incompressible substances during most processes without sacrificing much in accuracy. m=2kg m=2 kg T2-20℃ 1-6.STATE AND EQUILIBRIUM T1=20℃ 4=2.5m3 V-1.5m3 Consider a system not undergoing any change.At this point,all the prop- erties can be measured or calculated throughout the entire system,which gives us a set of properties that completely describes the condition,or (a)State 1 (b)State 2 the state,of the system.At a given state,all the properties of a system FIGURE 1-26 have fixed values.If the value of even one property changes,the state will A system at two different states. change to a different one.In Fig.1-26 a system is shown at two different states. Thermodynamics deals with equilibrium states.The word equilibrium implies a state of balance.In an equilibrium state there are no unbalanced potentials (or driving forces)within the system.A system in equilibrium 20°C 23℃ 32℃ 32℃ experiences no changes when it is isolated from its surroundings. 30C 32°C There are many types of equilibrium,and a system is not in thermody- namic equilibrium unless the conditions of all the relevant types of equi- 35°C 40°℃ 32C 32°C librium are satisfied.For example,a system is in thermal equilibrium 42°C 32C if the temperature is the same throughout the entire system,as shown in (a)Before (b)After Fig.1-27.That is,the system involves no temperature differential,which is the driving force for heat flow.Mechanical equilibrium is related to pres- FIGURE 1-27 sure,and a system is in mechanical equilibrium if there is no change in A closed system reaching thermal pressure at any point of the system with time.However,the pressure may equilibrium. vary within the system with elevation as a result of gravitational effects
14 INTRODUCTION AND BASIC CONCEPTS Sometimes the density of a substance is given relative to the density of a well-known substance. Then it is called specific gravity, or relative density, and is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C, for which rH2O 5 1000 kg/m3 ). That is, Specific gravity: SG 5 r rH2O (1–6) Note that the specific gravity of a substance is a dimensionless quantity. However, in SI units, the numerical value of the specific gravity of a substance is exactly equal to its density in g/cm3 or kg/L (or 0.001 times the density in kg/m3 ) since the density of water at 4°C is 1 g/cm3 5 1 kg/L 5 1000 kg/m3 . The specific gravity of mercury at 0°C, for example, is 13.6. Therefore, its density at 0°C is 13.6 g/cm3 5 13.6 kg/L 5 13,600 kg/m3 . The specific gravities of some substances at 0°C are given in Table 1–3. Note that substances with specific gravities less than 1 are lighter than water, and thus they would float on water. The weight of a unit volume of a substance is called specific weight and is expressed as Specific weight: gs 5 rg (N/m3 ) (1–7) where g is the gravitational acceleration. The densities of liquids are essentially constant, and thus they can often be approximated as being incompressible substances during most processes without sacrificing much in accuracy. 1–6 ■ STATE AND EQUILIBRIUM Consider a system not undergoing any change. At this point, all the properties can be measured or calculated throughout the entire system, which gives us a set of properties that completely describes the condition, or the state, of the system. At a given state, all the properties of a system have fixed values. If the value of even one property changes, the state will change to a different one. In Fig. 1–26 a system is shown at two different states. Thermodynamics deals with equilibrium states. The word equilibrium implies a state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. A system in equilibrium experiences no changes when it is isolated from its surroundings. There are many types of equilibrium, and a system is not in thermodynamic equilibrium unless the conditions of all the relevant types of equilibrium are satisfied. For example, a system is in thermal equilibrium if the temperature is the same throughout the entire system, as shown in Fig. 1–27. That is, the system involves no temperature differential, which is the driving force for heat flow. Mechanical equilibrium is related to pressure, and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time. However, the pressure may vary within the system with elevation as a result of gravitational effects. TABLE 1–3 Specific gravities of some substances at 0°C Substance SG Water 1.0 Blood 1.05 Seawater 1.025 Gasoline 0.7 Ethyl alcohol 0.79 Mercury 13.6 Wood 0.3–0.9 Gold 19.2 Bones 1.7–2.0 Ice 0.92 Air (at 1 atm) 0.0013 FIGURE 1–26 A system at two different states. m = 2 kg T2 = 20°C V2 = 2.5 m3 (a) State 1 m = 2 kg T1 = 20°C V1 = 1.5 m3 (b) State 2 FIGURE 1–27 A closed system reaching thermal equilibrium. 20°C (a) Before (b) After 23°C 35°C 40°C 30°C 42°C 32°C 32°C 32°C 32°C 32°C 32°C cen98179_ch01_001-050.indd 14 11/28/13 3:14 PM
15 CHAPTER 1 For example,the higher pressure at a bottom layer is balanced by the extra weight it must carry,and,therefore,there is no imbalance of forces.The variation of pressure as a result of gravity in most thermodynamic systems is relatively small and usually disregarded.If a system involves two phases, it is in phase equilibrium when the mass of each phase reaches an equi- librium level and stays there.Finally,a system is in chemical equilibrium if its chemical composition does not change with time,that is,no chemical reactions occur.A system will not be in equilibrium unless all the relevant equilibrium criteria are satisfied. The State Postulate As noted earlier,the state of a system is described by its properties.But we know from experience that we do not need to specify all the properties in order to fix a state.Once a sufficient number of properties are speci- fied,the rest of the properties assume certain values automatically.That is, specifying a certain number of properties is sufficient to fix a state.The number of properties required to fix the state of a system is given by the state postulate: The state of a simple compressible system is completely specified by two independent,intensive properties. A system is called a simple compressible system in the absence of elec- trical,magnetic,gravitational,motion,and surface tension effects.These effects are due to external force fields and are negligible for most engineer- Nitrogen ing problems.Otherwise,an additional property needs to be specified for T-25℃ each effect that is significant.If the gravitational effects are to be consid- v=0.9 m3/kg ered,for example,the elevation z needs to be specified in addition to the two properties necessary to fix the state. The state postulate requires that the two properties specified be indepen- FIGURE 1-28 dent to fix the state.Two properties are independent if one property can be The state of nitrogen is fixed by two varied while the other one is held constant.Temperature and specific vol- independent,intensive properties. ume,for example,are always independent properties,and together they can fix the state of a simple compressible system (Fig.1-28).Temperature and pressure,however,are independent properties for single-phase systems,but are dependent properties for multiphase systems.At sea level(P=1 atm), water boils at 100C,but on a mountaintop where the pressure is lower, water boils at a lower temperature.That is,T=f(P)during a phase-change Property A process;thus,temperature and pressure are not sufficient to fix the state of a two-phase system.Phase-change processes are discussed in detail in State 2 Chap.3. 1-7 PROCESSES AND CYCLES Process path Any change that a system undergoes from one equilibrium state to another State 1 is called a process,and the series of states through which a system passes Property B during a process is called the path of the process (Fig.1-29).To describe a process completely,one should specify the initial and final states of FIGURE 1-29 the process,as well as the path it follows,and the interactions with the A process between states 1 and 2 and surroundings. the process path
15 CHAPTER 1 For example, the higher pressure at a bottom layer is balanced by the extra weight it must carry, and, therefore, there is no imbalance of forces. The variation of pressure as a result of gravity in most thermodynamic systems is relatively small and usually disregarded. If a system involves two phases, it is in phase equilibrium when the mass of each phase reaches an equilibrium level and stays there. Finally, a system is in chemical equilibrium if its chemical composition does not change with time, that is, no chemical reactions occur. A system will not be in equilibrium unless all the relevant equilibrium criteria are satisfied. The State Postulate As noted earlier, the state of a system is described by its properties. But we know from experience that we do not need to specify all the properties in order to fix a state. Once a sufficient number of properties are specified, the rest of the properties assume certain values automatically. That is, specifying a certain number of properties is sufficient to fix a state. The number of properties required to fix the state of a system is given by the state postulate: The state of a simple compressible system is completely specified by two independent, intensive properties. A system is called a simple compressible system in the absence of electrical, magnetic, gravitational, motion, and surface tension effects. These effects are due to external force fields and are negligible for most engineering problems. Otherwise, an additional property needs to be specified for each effect that is significant. If the gravitational effects are to be considered, for example, the elevation z needs to be specified in addition to the two properties necessary to fix the state. The state postulate requires that the two properties specified be independent to fix the state. Two properties are independent if one property can be varied while the other one is held constant. Temperature and specific volume, for example, are always independent properties, and together they can fix the state of a simple compressible system (Fig. 1–28). Temperature and pressure, however, are independent properties for single-phase systems, but are dependent properties for multiphase systems. At sea level (P 5 1 atm), water boils at 100°C, but on a mountaintop where the pressure is lower, water boils at a lower temperature. That is, T 5 f(P) during a phase-change process; thus, temperature and pressure are not sufficient to fix the state of a two-phase system. Phase-change processes are discussed in detail in Chap. 3. 1–7 ■ PROCESSES AND CYCLES Any change that a system undergoes from one equilibrium state to another is called a process, and the series of states through which a system passes during a process is called the path of the process (Fig. 1–29). To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings. FIGURE 1–28 The state of nitrogen is fixed by two independent, intensive properties. Nitrogen T = 25°C v = 0.9 m3/kg FIGURE 1–29 A process between states 1 and 2 and the process path. State 1 State 2 Process path Property B Property A cen98179_ch01_001-050.indd 15 11/28/13 3:14 PM
16 INTRODUCTION AND BASIC CONCEPTS When a process proceeds in such a manner that the system remains infini- tesimally close to an equilibrium state at all times,it is called a quasi-static, or quasi-equilibrium,process.A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally (a)Slow compression so that properties in one part of the system do not change any faster than (quasi-equilibrium) those at other parts. This is illustrated in Fig.1-30.When a gas in a piston-cylinder device is compressed suddenly,the molecules near the face of the piston will not have enough time to escape and they will have to pile up in a small region in front of the piston,thus creating a high-pressure region there.Because of (b)Very fast compression this pressure difference,the system can no longer be said to be in equilib- (nonquasi-equilibrium) rium,and this makes the entire process nonquasi-equilibrium.However,if the piston is moved slowly,the molecules will have sufficient time to redis- FIGURE 1-30 tribute and there will not be a molecule pileup in front of the piston.As a Quasi-equilibrium and nonquasi- result,the pressure inside the cylinder will always be nearly uniform and equilibrium compression processes. will rise at the same rate at all locations.Since equilibrium is maintained at all times,this is a quasi-equilibrium process. It should be pointed out that a quasi-equilibrium process is an ideal- ized process and is not a true representation of an actual process.But many actual processes closely approximate it,and they can be modeled as quasi-equilibrium with negligible error.Engineers are interested in quasi- equilibrium processes for two reasons.First,they are easy to analyze;sec- Final state ond,work-producing devices deliver the most work when they operate on 2 quasi-equilibrium processes.Therefore,quasi-equilibrium processes serve Process path as standards to which actual processes can be compared. Initial Process diagrams plotted by employing thermodynamic properties as state coordinates are very useful in visualizing the processes.Some common 41 properties that are used as coordinates are temperature T,pressure p and volume V(or specific volume v).Figure 1-31 shows the P-V diagram of a compression process of a gas. V Note that the process path indicates a series of equilibrium states through which the system passes during a process and has significance for quasi- equilibrium processes only.For nonquasi-equilibrium processes,we are not able to characterize the entire system by a single state,and thus we cannot System speak of a process path for a system as a whole.A nonquasi-equilibrium process is denoted by a dashed line between the initial and final states (2) (1) instead of a solid line. The prefix iso-is often used to designate a process for which a particular FIGURE 1-31 property remains constant.An isothermal process,for example,is a process The P-V diagram of a compression during which the temperature T remains constant;an isobaric process is a process. process during which the pressure P remains constant;and an isochoric (or isometric)process is a process during which the specific volume v remains constant. A system is said to have undergone a cycle if it returns to its initial state at the end of the process.That is,for a cycle the initial and final states are identical. The Steady-Flow Process The terms steady and uniform are used frequently in engineering,and thus it is important to have a clear understanding of their meanings.The term
16 INTRODUCTION AND BASIC CONCEPTS When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasi-static, or quasi-equilibrium, process. A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts. This is illustrated in Fig. 1–30. When a gas in a piston-cylinder device is compressed suddenly, the molecules near the face of the piston will not have enough time to escape and they will have to pile up in a small region in front of the piston, thus creating a high-pressure region there. Because of this pressure difference, the system can no longer be said to be in equilibrium, and this makes the entire process nonquasi-equilibrium. However, if the piston is moved slowly, the molecules will have sufficient time to redistribute and there will not be a molecule pileup in front of the piston. As a result, the pressure inside the cylinder will always be nearly uniform and will rise at the same rate at all locations. Since equilibrium is maintained at all times, this is a quasi-equilibrium process. It should be pointed out that a quasi-equilibrium process is an idealized process and is not a true representation of an actual process. But many actual processes closely approximate it, and they can be modeled as quasi-equilibrium with negligible error. Engineers are interested in quasiequilibrium processes for two reasons. First, they are easy to analyze; second, work-producing devices deliver the most work when they operate on quasi-equilibrium processes. Therefore, quasi-equilibrium processes serve as standards to which actual processes can be compared. Process diagrams plotted by employing thermodynamic properties as coordinates are very useful in visualizing the processes. Some common properties that are used as coordinates are temperature T, pressure P, and volume V (or specific volume v). Figure 1–31 shows the P-V diagram of a compression process of a gas. Note that the process path indicates a series of equilibrium states through which the system passes during a process and has significance for quasiequilibrium processes only. For nonquasi-equilibrium processes, we are not able to characterize the entire system by a single state, and thus we cannot speak of a process path for a system as a whole. A nonquasi-equilibrium process is denoted by a dashed line between the initial and final states instead of a solid line. The prefix iso- is often used to designate a process for which a particular property remains constant. An isothermal process, for example, is a process during which the temperature T remains constant; an isobaric process is a process during which the pressure P remains constant; and an isochoric (or isometric) process is a process during which the specific volume v remains constant. A system is said to have undergone a cycle if it returns to its initial state at the end of the process. That is, for a cycle the initial and final states are identical. The Steady-Flow Process The terms steady and uniform are used frequently in engineering, and thus it is important to have a clear understanding of their meanings. The term FIGURE 1–30 Quasi-equilibrium and nonquasiequilibrium compression processes. (a) Slow compression (quasi-equilibrium) (b) Very fast compression (nonquasi-equilibrium) FIGURE 1–31 The P-V diagram of a compression process. Initial state Final state Process path 2 1 P V2 V1 V (2) System (1) cen98179_ch01_001-050.indd 16 11/28/13 3:14 PM
17 CHAPTER 1 steady implies no change with time.The opposite of steady is unsteady,or transient.The term uniform,however,implies no change with location over Mass 300℃ 250°℃ a specified region.These meanings are consistent with their everyday use (steady girlfriend,uniform properties,etc.). Control volume A large number of engineering devices operate for long periods of time 225℃ under the same conditions,and they are classified as steady-flow devices. Mass out Processes involving such devices can be represented reasonably well by a 200C150°℃ somewhat idealized process,called the steady-flow process,which can be Time:1 PM defined as a process during which a fluid flows through a control volume steadily (Fig.1-32).That is,the fluid properties can change from point to Mass point within the control volume,but at any fixed point they remain the same 300C 250°C during the entire process.Therefore,the volume V,the mass m,and the total Control volume energy content E of the control volume remain constant during a steady- 225C flow process (Fig.1-33). Mass Steady-flow conditions can be closely approximated by devices that are 200°℃150℃ out intended for continuous operation such as turbines,pumps,boilers,con- Time:3 PM densers,and heat exchangers or power plants or refrigeration systems.Some cyclic devices,such as reciprocating engines or compressors,do not sat- FIGURE 1-32 isfy any of the conditions stated above since the flow at the inlets and the During a steady-flow process,fluid exits will be pulsating and not steady.However,the fluid properties vary properties within the control volume with time in a periodic manner,and the flow through these devices can still may change with position but not with be analyzed as a steady-flow process by using time-averaged values for the time. properties. 1-8.TEMPERATURE AND THE ZEROTH LAW OF Mass THERMODYNAMICS Control volume Although we are familiar with temperature as a measure of "hotness"or mcy const "coldness,"it is not easy to give an exact definition for it.Based on our Mass Ecy=const. physiological sensations,we express the level of temperature qualitatively out with words like freezing cold,cold,warm,hot,and red-hot.However,we cannot assign numerical values to temperatures based on our sensations FIGURE 1-33 alone.Furthermore,our senses may be misleading.A metal chair,for exam- Under steady-flow conditions,the ple,will feel much colder than a wooden one even when both are at the mass and energy contents of a control same temperature. volume remain constant. Fortunately,several properties of materials change with temperature in a repeatable and predictable way,and this forms the basis for accurate temperature measurement.The commonly used mercury-in-glass thermo- 00 meter,for example,is based on the expansion of mercury with temperature. Iron Iron Temperature is also measured by using several other temperature-dependent 150C 60C properties. It is a common experience that a cup of hot coffee left on the table even- Copper Copper tually cools off and a cold drink eventually warms up.That is,when a body 20°C 60°℃ is brought into contact with another body that is at a different tempera- 2222 ture,heat is transferred from the body at higher temperature to the one at lower temperature until both bodies attain the same temperature (Fig.1-34). FIGURE 1-34 At that point,the heat transfer stops,and the two bodies are said to have Two bodies reaching thermal reached thermal equilibrium.The equality of temperature is the only equilibrium after being brought into requirement for thermal equilibrium. contact in an isolated enclosure
17 CHAPTER 1 steady implies no change with time. The opposite of steady is unsteady, or transient. The term uniform, however, implies no change with location over a specified region. These meanings are consistent with their everyday use (steady girlfriend, uniform properties, etc.). A large number of engineering devices operate for long periods of time under the same conditions, and they are classified as steady-flow devices. Processes involving such devices can be represented reasonably well by a somewhat idealized process, called the steady-flow process, which can be defined as a process during which a fluid flows through a control volume steadily (Fig. 1–32). That is, the fluid properties can change from point to point within the control volume, but at any fixed point they remain the same during the entire process. Therefore, the volume V, the mass m, and the total energy content E of the control volume remain constant during a steadyflow process (Fig. 1–33). Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as turbines, pumps, boilers, condensers, and heat exchangers or power plants or refrigeration systems. Some cyclic devices, such as reciprocating engines or compressors, do not satisfy any of the conditions stated above since the flow at the inlets and the exits will be pulsating and not steady. However, the fluid properties vary with time in a periodic manner, and the flow through these devices can still be analyzed as a steady-flow process by using time-averaged values for the properties. 1–8 ■ TEMPERATURE AND THE ZEROTH LAW OF THERMODYNAMICS Although we are familiar with temperature as a measure of “hotness” or “coldness,” it is not easy to give an exact definition for it. Based on our physiological sensations, we express the level of temperature qualitatively with words like freezing cold, cold, warm, hot, and red-hot. However, we cannot assign numerical values to temperatures based on our sensations alone. Furthermore, our senses may be misleading. A metal chair, for example, will feel much colder than a wooden one even when both are at the same temperature. Fortunately, several properties of materials change with temperature in a repeatable and predictable way, and this forms the basis for accurate temperature measurement. The commonly used mercury-in-glass thermometer, for example, is based on the expansion of mercury with temperature. Temperature is also measured by using several other temperature-dependent properties. It is a common experience that a cup of hot coffee left on the table eventually cools off and a cold drink eventually warms up. That is, when a body is brought into contact with another body that is at a different temperature, heat is transferred from the body at higher temperature to the one at lower temperature until both bodies attain the same temperature (Fig. 1–34). At that point, the heat transfer stops, and the two bodies are said to have reached thermal equilibrium. The equality of temperature is the only requirement for thermal equilibrium. FIGURE 1–32 During a steady-flow process, fluid properties within the control volume may change with position but not with time. 300°C 250°C 200°C 150°C Control volume 225°C Mass in Time: 1 PM Mass out 300°C 250°C 200°C 150°C Control volume 225°C Mass in Time: 3 PM Mass out FIGURE 1–33 Under steady-flow conditions, the mass and energy contents of a control volume remain constant. Control volume mCV = const. ECV = const. Mass in Mass out FIGURE 1–34 Two bodies reaching thermal equilibrium after being brought into contact in an isolated enclosure. 150°C Iron 20°C Copper 60°C Iron 60°C Copper cen98179_ch01_001-050.indd 17 11/28/13 3:14 PM