工程热力学(英文版)第9单元课件.ppt

1、SHANDONG UNIVERSITYChap7 Summary1Clausius inequality:the cyclic integral of is always zeroIncrease of Entropy Principle(熵增原理熵增原理)Entropy(熵)1,Entropy change of pure substances:Entropy change of a closed system:Increase of entropy principle(孤立系统熵增原理，简称熵增原理孤立系统熵增原理，简称熵增原理)：the entropy of an isolated sy

2、stem during a process always increase or,in the limiting case of a reversible process remains constant.(孤立系统的熵可以增大，或保持不变，但不可能减少孤立系统的熵可以增大，或保持不变，但不可能减少)EntropySome remarksIso TReversible processIrreversible process 0sf at 0.01=0 kJ/kg.k2,Isentropic process0,adibatic0,Reversible3,T-S,h-s diagrams=4,Th

3、e 3rd law of thermodaynamics:The entropy of a pure crystalline substance at absolute zero temperature is zero 5,T ds relations:6,reversible work outputSHANDONG UNIVERSITYChap9 Summary2Basic considerations:actual cycle,ideal cycle,carnot cycle,P-V,T-SReciprocating enginesAssumptions of gas power cycl

4、esTDCair standard assumptions(空气标准假设空气标准假设):1)air=ideal gas,Cv=const;2)internal reversible process;3)combustionheat addition process;4)exhaust heat rejection processSI engines-Otto cycleFour strokes：compression stroke,expansion or power stroke,exhaust stroke,intake stroke.(压缩冲程、做功（燃烧、膨胀）冲程、排气冲程和吸气冲程

5、)CI engine-Diesel CycleThe ideal Diesel cycle:1-2 isentropic compression (等熵压缩等熵压缩);2-3 constant P heat addition (定压吸热定压吸热);3-4 isentropic expansion (等熵膨胀等熵膨胀);4-1 constant V heat rejection (定容放热定容放热)Reciprocating engines are classified as spark-ignition(SI)engines(点点燃式内燃机燃式内燃机)compression-ignition(

6、CI)engines(压燃式内燃机压燃式内燃机)BDCStrokeBoreIntake valveExhause valveMEPThe ideal Otto cycle:1-2 isentropic compression (等熵压缩等熵压缩);2-3 constant V heat addition (定容吸热定容吸热);3-4 isentropic expansion (等熵膨胀等熵膨胀);4-1 constant V heat rejection (定容放热定容放热)SHANDONG UNIVERSITYApplication of thermodynamics Two importa

7、nt areas:Power generation(engines-produce work)power cycles Chapter 9 and chapter 10 Refrigeration(refrigerators,air conditioners,heat pumps)refrigeration cycles chapter 11SHANDONG UNIVERSITYApplication of thermodynamics Thermodynamics cycles,according to different phase of work fluid Gas cycles(wor

8、king fluid remains in gaseous phase during the cycle)Vapor cycles(working fluid exists in vapor phase and liquid phase during different part of the cycle)Thermodynamics cycles can also be categorized as closed cycles and open cycles.SHANDONG UNIVERSITYApplication of thermodynamics Heat engines are c

9、ategorized as internal combustion(内燃机):like automobile engines.It is done by burning the fuel within the system boundaries.and external combustion engines(外燃机):like steam power plants.Heat is supplied to the working fluid from an external source such as a furnace,a geothermal well,a nuclear,or even

10、the sun.SHANDONG UNIVERSITYChapter 9 Gas Power Cycles(气体动力循环)SHANDONG UNIVERSITY9-1 basic considerations in the analysis of power cycles Most power-producing devices operate on cycles,and the study of powercycles is an exciting and important part of thermodynamics.The cycles encountered in actual de

11、vices are difficult to analyze.So,we Get rid of the internal irreversibilities and complexitiesUse ideal cycle:made up totally of internally reversible processes.The cycles discussed in this chapter are somewhat idealized,but they still retain the general characteristics of the actual cycles they re

12、present.The conclusions reached from the analysis of ideal cycles are also applicable to actual cycles.SHANDONG UNIVERSITY Heat engines:QW thermal efficiency,Wnetout/Q Thermal efficiency of reversible HE is highest.Most cycles encountered in practice differ significantly from the Carnot cycle,which

13、makes it unsuitable as a realistic model.Each ideal cycle discussed in this chapter is related to a specific work-producing device and is an idealized version of the actual cycle.SHANDONG UNIVERSITY The idealizations and simplifications can be summarized as follows:The cycle involves no any friction

14、.Therefore,the working fluid does not experience any pressure drop as it flows in pipes or devices such as heat exchangers.All expansion and compression processes take place in a quasiequilibrium manner.The pipes connecting the various components of a system are well insulated,and heat transfer thro

15、ugh them is negligible Neglecting the changes in kinetic and potential energies of the working fluid.except nozzles and diffusers,SHANDONG UNIVERSITY P-v and T-s diagrams serve as valuable aids in the analysis of thermodynamic processes.The area enclosed by the process curves of a cycle represents t

16、he net work produced during the cycle,also equivalent to the net heat transfera heat-addition process proceeds in the direction of increasing entropy;a heat-rejection process proceeds in the direction of decreasing entropy;an isentropic(internally reversible,adiabatic)process proceeds at constant en

17、tropySHANDONG UNIVERSITY9-2 the carnot cycle and its value in engineering The four reversible processes of Carnot cycle are isothermal expansion(等温膨胀)(定温吸热)；adiabatic expansion(绝热膨胀)(定熵膨胀)；isothermal compression(等温压缩)(定温放热)；adiabatic compression(绝热压缩)(定熵压缩)Carnot cylce can be executed in a closed sy

18、stem or in a steady-flow system.Either a gas or a vapor can be as the working fluidCarnot cylce is the most efficient cylce between given TL and THSHANDONG UNIVERSITYIt is not practical to build an engine operating on a cycle as Carnot cycleReversible isothermal heat transfer is very difficult to ac

19、hieve in reality because it would require very large heat exchangers and a very long time(a power cycle in a typical engine is completed in a fraction of a second).The real value of Carnot cycle is its being a standard against the actual cycles.Thermal efficicency increase with increase of TH and de

20、crease of TL.And we also know TH and TL are limited.TH is limited by the MAX temperature that components of Heat engines(piston,turbine blades)can withstand.TL is limited by the cooling medium of the cycle,such as a lake,a river,or the atmospheric air.SHANDONG UNIVERSITY?SHANDONG UNIVERSITY9-3 air s

21、tandard assumptions of gas power cycles A gas power cycles,the working fluid remains a gas throughout the entire cycle.E.g:spark-ignition engines(点燃式热机),diesel engines(狄塞尔热机)In all these devices,energy is provided by burning a fuel within the system boundaries,so they are called internal combustion

22、engines(内燃机内燃机)Internal combustion engines operate on a mechanical cycle.The working fluid:either air&fuel or combustion products.It does not undergo a complete thermodynamic cycle,since the exhaust gases is thrown out of the engine at some point in the cycle.Working on an open cycle is the characte

23、ristic of all internal combustion engines.SHANDONG UNIVERSITY9-3 air standard assumptions of gas power cycles The actual gas power cycles are rather complex.To reduce the analysis to a manageable level,we utilize the following approximations(air-standard assumptions 空空气标准假设气标准假设)1.The working fluid

24、is air,which continuously circulates in a closed loop and always behaves as an ideal gas.Air has constant specific heats.(工质为空气，作理想气体处理，比热容取定值)2.All the processes that make up the cycle are internally reversible.3.The combustion process is replaced by a heat-addition process from an external source.

25、(燃烧过程看成从高温热源的可逆吸热过程)4.The exhaust process is replaced by a heat-rejection process that restores the working fluid to its initial state.(排气过程看成从向低温热源的可逆放热过程)SHANDONG UNIVERSITY9-4 an overview of reciprocating engines(往复式内燃机)The basic components of a reciprocating engineTop dead center(TDC)(上止点上止点)：th

26、e position of the piston when it forms the smallest volume in the cylinderBottom dead center(BDC)(下止点下止点)：the position of the piston when it forms the largest volume in the cylinder.Stroke(冲程冲程)：The largest distance between the TDC and the BDCBore(缸径缸径):The diameter of the piston is called the bore.

27、Intake valve(进气阀进气阀)：The air or airfuel mixture is drawn into the cylinder through the intake valveExhaust valve(排气阀排气阀)：the combustion products are expelled from the cylinder through the exhaust valve.SHANDONG UNIVERSITY9-4 an overview of reciprocating engines(往复式内燃机)The minimum volume formed in th

28、e cylinder when the piston is at TDC is called the clearance volume(余隙容积余隙容积)The volume displaced by the piston as it moves between TDC and BDC is called the displacement volume(位移容积位移容积)The ratio of the maximum volume formed in the cylinder to the minimum(clearance)volume is called the compression

29、ratio(压缩比压缩比)r of the engine*The compression ratio is a volume ratio,not pressure ratio*SHANDONG UNIVERSITY9-4 an overview of reciprocating engines(往复式内燃机)Mean effective pressure(MEP)(平均有效压力).It is a fictitious pressure that would produce the same amount of net work as that produced during the actua

30、l cycle.MEP can be used as a parameter to compare the performances of reciprocating engines of equal size.The engine with a larger value of MEP delivers more net work per cycle and thus performs better.SHANDONG UNIVERSITY9-4 an overview of reciprocating engines(往复式内燃机)Depending on how the combustion

31、 process in the cylinder is initiated,reciprocating engines are classified as spark-ignition(SI)engines(点燃式内燃机点燃式内燃机)compression-ignition(CI)engines(压燃式内燃机压燃式内燃机)In SI engines,the combustion of the airfuel mixture is initiated by a spark plug(火花塞).Otto cycle is the ideal cycle for SI reciprocating e

32、ngines.In CI engines,the airfuel mixture is self-ignited as a result of compressing the mixture above its self-ignition temperature.Diesel cycle is the ideal cycle for CI reciprocating engines.SHANDONG UNIVERSITY9-5 Otto cycle:the ideal cycle for spark-ignition engines（点燃式内燃机）The Otto cycle is the i

33、deal cycle for spark-ignition reciprocating engines;It is named after Nikolaus A.Otto,who built a successful four-stroke engine in 1876 in Germany;In most spark-ignition engines,the piston executes four complete strokes(two cycles)within the cylinder,and the crankshaft completes two revolutions for

34、each thermodynamic cycle.These engines are called four-stroke internal combustion engines(四冲程内燃机).SHANDONG UNIVERSITYActual cycles in spark-ignition engines and P-v diagrams.Four strokes：compression stroke,expansion or power stroke,exhaust stroke,intake stroke.(压缩冲程、做功（燃烧、膨胀）冲程、排气冲程和吸气冲程)TDCBDCSHAND

35、ONG UNIVERSITYIdeal cycles in spark-ignition enginesThe thermodynamic analysis of the actual four stroke is not a simple task.But.the analysis can be simplified according to the air-standard assumptions.The ideal Otto cycle,consisting of four internal reversible processes:1-2 isentropic compression;

36、2-3 constant V heat addition;3-4 isentropic expansion;4-1 constant V heat rejection.TDCBDCSHANDONG UNIVERSITYThe ideal Otto cycle in P-v,T-s diagramsCarnot cycle？1-2 isentropic compression(等熵压缩);2-3 constant V heat addition(定容吸热);3-4 isentropic expansion(等熵膨胀);4-1 constant V heat rejection(定容放热).SHA

37、NDONG UNIVERSITY Energy balance of the Otto cycle:2-3 and 4-1:Const volume heat transfer process1-2 Isentropic3-4 Isentropicr is the compression ratio and k is the specific heat ratio cp/cvSHANDONG UNIVERSITY The thermal efficiency of ideal Otto cycle increases with both the compression ratio and th

38、e specific heat ratio;It is also true for actual spark-ignition internal combustion engines,but is less than that of an ideal Otto cycle because of the irreversibilities;The thermal efficiencies of actual spark-ignition engines range from about 25 to 30 percent.Too high compression ratio will cause

39、premature ignition of the fuel,called autoignition(自燃自燃).It will produce an audible noise,which is called engine knock(爆震爆震)and cannot be tolerated because it hurts performance and can cause engine damage.SHANDONG UNIVERSITY9-6 diesel cycle:the ideal cycle for compression-ignition engines(压燃式内燃机)The

40、 Diesel cycle is the ideal cycle for CI reciprocating engine，first proposed by Rudolph Diesel in the 1890s.In CI engines(called also diesel engines-柴油机),only air is compressed to a temperature that is above the autoignition temperature of the fuel,combustion starts as the fuel is injected into this

41、hot air.SHANDONG UNIVERSITY In spark-ignition engines（点燃式内燃机）（点燃式内燃机）(also known as gasoline engines（汽油机）汽油机）),the airfuel mixture is compressed to a temperature that is below the autoignition temperature of the fuel,and the combustion process is initiated by firing a spark plug（火花塞）（火花塞）；In compres

42、sion-ignition engines（压燃式内燃机）（压燃式内燃机）(also known as diesel engines（柴油机）（柴油机）),the air is compressed to a temperature that is above the autoignition temperature of the fuel,and combustion starts on contact as the fuel is injected into this hot air.Therefore,the spark plug and carburetor are replaced

43、by a fuel injector（喷油器）（喷油器）in diesel enginesSHANDONG UNIVERSITY In diesel engines only air is compressed during the compression stroke,eliminating the possibility of autoignition.Therefore,diesel engines can be designed to operate at much higher compression ratios,typically between 12 and 24.Not ha

44、ving to deal with the problem of autoignition has another benefit:many of the stringent requirements placed on the gasoline can now be removed,fuels that are less refined(thus less expensive)can be used in diesel engines.the combustion process takes place over a longer interval.So,the combustion pro

45、cess in the ideal Diesel cycle is approximated as a constant-pressure heat-addition process.And,this is the only process where the Otto and the Diesel cycles differ.SHANDONG UNIVERSITYOtto cycleDiesel cycle1-2 isentropic compression (等熵压缩等熵压缩);2-3 constant V heat addition (定容吸热定容吸热);3-4 isentropic e

46、xpansion (等熵膨胀等熵膨胀);4-1 constant V heat rejection (定容放热定容放热)1-2 isentropic compression (等熵压缩等熵压缩);2-3 constant P heat addition (定压吸热定压吸热);3-4 isentropic expansion (等熵膨胀等熵膨胀);4-1 constant V heat rejection (定容放热定容放热)SHANDONG UNIVERSITY1-2 isentropic compression(等熵压缩等熵压缩)：2-3 constant P heat addition(定

47、压吸热定压吸热)：3-4 isentropic expansion(等熵膨胀等熵膨胀):4-1 constant V heat rejection(定容放热定容放热):Cutoff ratio(切断比切断比)：ratio of the cylinder volumes after and before the combustion processCompression ratio:ratio of MAX volume formed in the cylinder to MIN volumeSHANDONG UNIVERSITYEfficiencies of Otto and Diesel c

48、ycleswhen both cycles operate on the same compression ratio.As the cutoff ratio rc decreases,the efficiency of the Diesel cycle increases.For limiting case of rc=1,the quantity in the brackets becomes unity and the efficiencies of the Otto and Diesel cycles become identical.But,diesel engines operat

49、e at much higher compression ratios and thus are usually more efficient than the spark-ignition(gasoline)engines;The diesel engines also burn the fuel more completely since they usually operate at lower revolutions per minute and the airfuel mass ratio is much higher than spark-ignition engines;Ther

50、mal efficiencies of large diesel engines range from about 35 to 40 percent.The higher efficiency and lower fuel costs of diesel engines make them attractive in applications requiring relatively large amounts of powerSHANDONG UNIVERSITY Approximating the combustion process in internal combustion engi