The CRC Handbook of Mechanical Engineering. Chapter 2. Engineering Thermodynamics (776125), страница 2
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In thermodynamic relationships, temperature is always in termsof the Kelvin or Rankine scale unless specifically stated otherwise.A degree of the same size as that on the Rankine scale is used in the Fahrenheit scale, but the zeropoint is shifted according to the relationT (°F) = T (°R) − 459.67(2.3)Substituting Equations 2.1 and 2.2 into Equation 2.3 givesT (°F) = 1.8T (°C) + 32(2.4)This equation shows that the Fahrenheit temperature of the ice point (0°C) is 32°F and of the steampoint (100°C) is 212°F. The 100 Celsius or Kelvin degrees between the ice point and steam pointcorresponds to 180 Fahrenheit or Rankine degrees.To provide a standard for temperature measurement taking into account both theoretical and practicalconsiderations, the International Temperature Scale of 1990 (ITS-90) is defined in such a way that thetemperature measured on it conforms with the thermodynamic temperature, the unit of which is thekelvin, to within the limits of accuracy of measurement obtainable in 1990.
Further discussion of ITS90 is provided by Preston-Thomas (1990).The First Law of Thermodynamics, EnergyEnergy is a fundamental concept of thermodynamics and one of the most significant aspects of engineering analysis. Energy can be stored within systems in various macroscopic forms: kinetic energy,gravitational potential energy, and internal energy. Energy can also be transformed from one form toanother and transferred between systems.
For closed systems, energy can be transferred by work andheat transfer. The total amount of energy is conserved in all transformations and transfers.WorkIn thermodynamics, the term work denotes a means for transferring energy. Work is an effect of onesystem on another that is identified and measured as follows: work is done by a system on its surroundingsif the sole effect on everything external to the system could have been the raising of a weight. The testof whether a work interaction has taken place is not that the elevation of a weight is actually changed,nor that a force actually acted through a distance, but that the sole effect could be the change in elevationof a mass.
The magnitude of the work is measured by the number of standard weights that could havebeen raised. Since the raising of a weight is in effect a force acting through a distance, the work conceptof mechanics is preserved. This definition includes work effects such as is associated with rotating shafts,displacement of the boundary, and the flow of electricity.Work done by a system is considered positive: W > 0. Work done on a system is considered negative:W < 0. The time rate of doing work, or power, is symbolized by Ẇ and adheres to the same signconvention.EnergyA closed system undergoing a process that involves only work interactions with its surroundingsexperiences an adiabatic process.
On the basis of experimental evidence, it can be postulated that when© 1999 by CRC Press LLC2-5Engineering Thermodynamicsa closed system is altered adiabatically, the amount of work is fixed by the end states of the system andis independent of the details of the process. This postulate, which is one way the first law of thermodynamics can be stated, can be made regardless of the type of work interaction involved, the type ofprocess, or the nature of the system.As the work in an adiabatic process of a closed system is fixed by the end states, an extensive propertycalled energy can be defined for the system such that its change between two states is the work in anadiabatic process that has these as the end states.
In engineering thermodynamics the change in theenergy of a system is considered to be made up of three macroscopic contributions: the change in kineticenergy, KE, associated with the motion of the system as a whole relative to an external coordinate frame,the change in gravitational potential energy, PE, associated with the position of the system as a wholein the Earth’s gravitational field, and the change in internal energy, U, which accounts for all otherenergy associated with the system. Like kinetic energy and gravitational potential energy, internal energyis an extensive property.In summary, the change in energy between two states of a closed system in terms of the work Wad ofan adiabatic process between these states is( KE2 − KE1 ) + ( PE2 − PE1 ) + (U2 − U1 ) = −Wad(2.5)where 1 and 2 denote the initial and final states, respectively, and the minus sign before the work termis in accordance with the previously stated sign convention for work.
Since any arbitrary value can beassigned to the energy of a system at a given state 1, no particular significance can be attached to thevalue of the energy at state 1 or at any other state. Only changes in the energy of a system havesignificance.The specific energy (energy per unit mass) is the sum of the specific internal energy, u, the specifickinetic energy, v2/2, and the specific gravitational potential energy, gz, such thatspecific energy = u +v2+ gz2(2.6)where the velocity v and the elevation z are each relative to specified datums (often the Earth’s surface)and g is the acceleration of gravity.A property related to internal energy u, pressure p, and specific volume v is enthalpy, defined byh = u + pv(2.7a)H = U + pV(2.7b)or on an extensive basisHeatClosed systems can also interact with their surroundings in a way that cannot be categorized as work,as, for example, a gas (or liquid) contained in a closed vessel undergoing a process while in contactwith a flame. This type of interaction is called a heat interaction, and the process is referred to asnonadiabatic.A fundamental aspect of the energy concept is that energy is conserved.
Thus, since a closed systemexperiences precisely the same energy change during a nonadiabatic process as during an adiabaticprocess between the same end states, it can be concluded that the net energy transfer to the system ineach of these processes must be the same. It follows that heat interactions also involve energy transfer.© 1999 by CRC Press LLC2-6Section 2Denoting the amount of energy transferred to a closed system in heat interactions by Q, these considerations can be summarized by the closed system energy balance:(U2 − U1 ) + ( KE2 − KE1 ) + ( PE2 − PE1 ) = Q − W(2.8)The closed system energy balance expresses the conservation of energy principle for closed systems ofall kinds.The quantity denoted by Q in Equation 2.8 accounts for the amount of energy transferred to a closedsystem during a process by means other than work.
On the basis of experiments it is known that suchan energy transfer is induced only as a result of a temperature difference between the system and itssurroundings and occurs only in the direction of decreasing temperature. This means of energy transferis called an energy transfer by heat. The following sign convention applies:Q > 0: heat transfer to the systemQ < 0: heat transfer from the systemThe time rate of heat transfer, denoted by Q̇ , adheres to the same sign convention.Methods based on experiment are available for evaluating energy transfer by heat.
These methodsrecognize two basic transfer mechanisms: conduction and thermal radiation. In addition, theoretical andempirical relationships are available for evaluating energy transfer involving combined modes such asconvection. Further discussion of heat transfer fundamentals is provided in Chapter 4.The quantities symbolized by W and Q account for transfers of energy. The terms work and heatdenote different means whereby energy is transferred and not what is transferred.
Work and heat are notproperties, and it is improper to speak of work or heat “contained” in a system. However, to achieveeconomy of expression in subsequent discussions, W and Q are often referred to simply as work andheat transfer, respectively. This less formal approach is commonly used in engineering practice.Power CyclesSince energy is a property, over each cycle there is no net change in energy.
Thus, Equation 2.8 readsfor any cycleQcycle = WcycleThat is, for any cycle the net amount of energy received through heat interactions is equal to the netenergy transferred out in work interactions. A power cycle, or heat engine, is one for which a net amountof energy is transferred out by work: Wcycle > 0. This equals the net amount of energy transferred in by heat.Power cycles are characterized both by addition of energy by heat transfer, QA, and inevitable rejectionsof energy by heat transfer, QR:Qcycle = QA − QRCombining the last two equations,Wcycle = QA − QRThe thermal efficiency of a heat engine is defined as the ratio of the net work developed to the totalenergy added by heat transfer:© 1999 by CRC Press LLC2-7Engineering Thermodynamicsη=WcycleQA= 1−QRQA(2.9)The thermal efficiency is strictly less than 100%.
That is, some portion of the energy QA supplied isinvariably rejected QR ≠ 0.The Second Law of Thermodynamics, EntropyMany statements of the second law of thermodynamics have been proposed. Each of these can be calleda statement of the second law or a corollary of the second law since, if one is invalid, all are invalid.In every instance where a consequence of the second law has been tested directly or indirectly byexperiment it has been verified.
Accordingly, the basis of the second law, like every other physical law,is experimental evidence.Kelvin-Planck StatementThe Kelvin-Plank statement of the second law of thermodynamics refers to a thermal reservoir. A thermalreservoir is a system that remains at a constant temperature even though energy is added or removed byheat transfer.
A reservoir is an idealization, of course, but such a system can be approximated in a numberof ways — by the Earth’s atmosphere, large bodies of water (lakes, oceans), and so on. Extensiveproperties of thermal reservoirs, such as internal energy, can change in interactions with other systemseven though the reservoir temperature remains constant, however.The Kelvin-Planck statement of the second law can be given as follows: It is impossible for any systemto operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundingswhile receiving energy by heat transfer from a single thermal reservoir. In other words, a perpetualmotion machine of the second kind is impossible.
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