工程热力学第一章基本概念.ppt

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1、第一章 基本概念CHAPTER 1.BASIC CONCEPTSCONTENTSl1.1 Thermodynamic System 热力系统热力系统l1.2 Properties of A System 系统的状态参数系统的状态参数l1.3 State and Equilibrium 状态及平衡状态状态及平衡状态l1.4 Process and Cycles 过程与循环过程与循环CompanyLogoTERMINOLOGY(术语）closed system 闭口系统闭口系统 open system 开开口系统口系统adiabatic system 绝热系统绝热系统 isolated syste

2、m 孤立系统孤立系统boundary 边界边界 surrounding 环境环境equilibrium state 平衡状态平衡状态 process 过程过程interaction 相互作用相互作用 Turbine 汽轮机汽轮机 piston cylinder device 活塞活塞-缸装置缸装置 temperature 温度温度pressure 压力压力 absolute pressure 绝对压力绝对压力relative pressure 相对压力相对压力 gage pressure 表表压力压力vacuum pressure 真空度真空度 atmospheric pressure 大气压

3、力大气压力reciprocal 倒数倒数 mass 质量，物质质量，物质1.1 热力系统 THERMODYNAMIC SYSTEM 1.System（热力系统）（热力系统）系统就是指被选做研究对象的物体或空间。（Aquantityofmatteroraregioninspacechosenforstudyis calledasystem.）For example:气缸气缸-活塞装置活塞装置Cylinder-pistondevices汽轮机汽轮机SteamturbineWork Output BoundarySystemHeat AbsorptionWorking MediumHeat reje

4、ctionWork OutputSystemWorking Medium(1)Surroundings（外界）Themassorregionoutsidethesystemiscalledsurroundings.(2)Boundary（边界）Therealorimaginarysurfacethatseparatesthesystemfromitssurroundingsiscalledboundary.(3)Interactionsbetweensystemanditssurroundings(系统与外界间的相互作用）Therearemassorenergyexchangebetweens

5、ystemanditssurroundings.Andtheymustcrosstheboundaries.系统与外界之间有质量及能量的交换，且都必须穿越边界2.System,Surroundings and Boundary 系统、外界和边界系统、外界和边界SystemSurroundingsMassexchangeEnergyexchangeMassenteringorleavingthesystemEnergyenteringorleavingthesystemEnergy can cross the boundary,in the form of heat and work.(4)Ch

6、aracteristics of boundaries Boundaries are selected subjectively 边界是人为选定的边界是人为选定的.nBoundaries can be fixed or movable.边界可以是固定的，也可以是可移动边界可以是固定的，也可以是可移动nBoundaries can be real or imaginary.边界可以是真实的，也可以是假想的边界可以是真实的，也可以是假想的 BOUNDARIES ARE SELECTED SUBJECTIVELY热力系统选取的人为性：锅锅炉炉boiler汽轮机汽轮机(turbine)发电机发电机(g

7、enerator)给水泵给水泵(pump)凝汽器凝汽器(condenser)过热器过热器(reheater)OnlyworkexchangeOnlyheatexchangeWorkandheatexchange真实、虚构固定、活动固定、活动fixed、movablereal、imaginary(1)Intermsoftheinteractionbetweensystemanditssurroundings依据系统和外界之间的相互作用依据系统和外界之间的相互作用YesNoMass?openclosedHeat?non-adiabaticadiabaticWork?withworkwithoutw

8、orkMass,heatorwork?non-isolatedisolated3.Types of systems 热力系统分类热力系统分类 A closed system(a control mass 控制质量)consists of a fixed amount of mass,and no mass can cross its boundary.That is,no mass enters or leave a closed system.闭口系与外界无物质交换 such as,Piston-cylinder device(汽缸-活塞装置）An open system(or a cont

9、rol volume 控制体积）is a properly selected region in space.Both mass and energy can cross the boundary of a control volume.开口开口系与外界有物质交换系与外界有物质交换 such as,A Water heater,a turbine and a compressor,etc A.Closed system and Open system 闭口系统和开口系统闭口系统和开口系统 Adiabatic system is that no heat cross the boundary o

10、r heat is negligible compared with the work cross the boundary 绝热系统与外界之间无热量交换绝热系统与外界之间无热量交换 Isolated system is a special case that no mass and energy cross the boundary.孤立系统与外界之间无任何物质和能量交换孤立系统与外界之间无任何物质和能量交换 B.Adiabatic system and Isolated system 绝热系统和孤立系统绝热系统和孤立系统1234mQW1opensystem非孤立系相关外界非孤立系相关外界孤

11、立系孤立系1+2closedsystem1+2+3adiabaticclosedsystem1+2+3+4isolatedsystem（2）IN TERMS OF INTERNAL CONDITION OF THE SYSTEM （依据系统的内部状况）（依据系统的内部状况）其它分类方式其它分类方式PhysicalandChemicalProperties(物理化学性质物理化学性质)EvenSystem均匀系均匀系UnevenSystem非均匀系非均匀系NumberofWorkingmedium工质种类工质种类multisubstancesystem多元系多元系Singlesubstancesy

12、stem单元系单元系Phase相态相态MultiplePhase多相多相SinglePhase单相单相1.2 STATE AND STATE PROPERTIES 状态及状态参数1.State The state reveals the physical condition of a system.状态：某一瞬间热力系所呈现的宏观状况状态：某一瞬间热力系所呈现的宏观状况2.Property（状态参数）（状态参数）（1）Properties are used to depict any characteristics of a system.状态参数：描述系统宏观物理状况的物理量状态参数：描述系

13、统宏观物理状况的物理量,简称参数简称参数 such as Pressure P（压力），），temperature T（温度），），volume V（体积），），mass m（质量），），internal energy U（热力学能），），enthalpy（焓），），entropy（熵），），viscosity（粘度），），thermal conductivity（导热系数）。）。（2）CHARACTERISTICS OF STATE PROPERTIES 状态参数的特征Propertiesofastatearedeterminedbythestate.Ifthestateisspecifie

14、d,itspropertiesarefixed,orviseversa.状态确定，则状态参数也确定，反之亦然状态确定，则状态参数也确定，反之亦然Themagnitudeofthechangeinpropertyisindependentofthepath(route),butjustdependontheinitialandfinalstates.状态参数的积分特征状态参数的积分特征：状态参数的变化量与路径无关，只与初终态有关状态参数的变化量与路径无关，只与初终态有关Propertiesarefunctionsofstate,thereexisttotaldifferentialforprop

15、erties状态参数的微分特征状态参数的微分特征：全微分：全微分Afteraseriesofchange,thestatereturnstotheoriginalone,thenthechangesinitspropertiesare0Assumingz=z(x,y)dz z是全微分是全微分充要条件：充要条件：是否是状态参数是否是状态参数？Totaldifferentials Pressure p、temperature T、Specific volume v canbemeasureddirectly(1)Density and Specific Volume密度和比容Densityis d

16、efined as mass per unit volume.密度指单位体积所含物质的多少。3.Basic State Properties 基本状态参数基本状态参数Specific volumeSpecific volumeisthereciprocalofdensityandisdefinedasvolumeperunitmass.单位质量的物质所占有的体积，用v表示。比体积是表示物质内部分子疏密程度的状态参数。Definition:Itisdefinedasthe force exerted by a fluid vertically on a surface of unit area.

17、垂直作用于单位面积上的力 (for solid is stress:person stand on foot)Unit of pressure It has the unit of Newton per square meter（N/m2)1 Pa=1 N/m2 1 kPa=103 Pa 1 MPa=106 PaSI Unit:(2)Pressure 压力压力绝对压力(absolutepressure)1 bar=105 Pa=100 kPa=0.1 MPa标准大气压 Standard atmosphere 1 atm=101325 Pa=101.325 kPa工程大气压 Engineer a

18、tmosphere 1 at=1 kgf/cm2 =9.807 N/cm2 =9.807*104 Pa液柱高度 Height of liquid column 1 atm=760 mmHg 1 at=10 mH2O Other units 其他单位其他单位 1 mmHg=1gh=133.3 PaAbsolute pressureis the actual pressure at a given position,denotedas .绝对压力指系统中工质的真实压力工质的真实压力。Relative pressureindicates the difference between the abso

19、lute pressure and the local atmospheric pressure.相对压力反映绝对压力与当地大气压力的差值绝对压力与当地大气压力的差值。Absolute pressure and relative pressure 绝对压力和相对压力绝对压力和相对压力 Gage Pressure and Vacuum Pressure 表压与真空度表压与真空度 Gage pressure（表压力）（表压力）is denoted as Pg表压力：当高于大气压时，压力计显示的绝对压力超出大气压力表压力：当高于大气压时，压力计显示的绝对压力超出大气压力的部分。的部分。表压力表压力=

20、绝对压力绝对压力-大气压力大气压力 真空度表示真空度表示绝对压力低于大气压力的量值绝对压力低于大气压力的量值Vacuum pressure（真空度）：（真空度）：Pressures below atmosphere pressure.It is the pressure difference between atmospheric and system pressure when system pressure is lower than atmospheric And it is denoted as PV or H.真空度真空度=大气压力大气压力-绝对压力绝对压力绝对压力与相对压力Ifp

21、pbIfp 有足够时间恢复新平衡有足够时间恢复新平衡准静态过程准静态过程RelaxationtimeAquasi-equilibriumprocesscanbeviewedasasufficientlyslowprocess.准静态过程的工程应用CompanyLogoForexample：活塞式内燃机活塞式内燃机2000转转/分分曲柄曲柄2冲程冲程/转，转，0.15米米/冲程冲程活塞运动速度活塞运动速度=2000 2 0.15/60=10m/s压力波恢复平衡速度（声速）压力波恢复平衡速度（声速）350m/s破坏平衡所需时间破坏平衡所需时间（外部作用时间）（外部作用时间）恢复平衡所需时间恢复平衡

22、所需时间（驰豫时间）（驰豫时间）一般的工程过程都可认为是一般的工程过程都可认为是准静态过程准静态过程具体工程问题具体分析。具体工程问题具体分析。“突然突然”“缓慢缓慢”2.REVERSIBLE PROCESS（可逆过程（可逆过程）CompanyLogo系统经历某一过程后，如果能使系统经历某一过程后，如果能使系统系统与与外界外界同时同时恢复到初始状态，而不恢复到初始状态，而不留下任何痕迹，则此过程为留下任何痕迹，则此过程为可逆过程可逆过程。A process that can reversed without leaving any trace on the surroundings.That

23、is,both the system and the surroundings are returned to their initial states at the end of the reverse process.注意注意可逆过程只是指可能性，并不可逆过程只是指可能性，并不是指必须要回到初态的过程。是指必须要回到初态的过程。（1）Definition在可逆过程结束时，在可逆过程结束时，系统和外界都回复到原来的状态系统和外界都回复到原来的状态Boththesystemandthesurroundings are returned to their initial states at th

24、e end of the reverse process.系统与环境组合系统的净热量与净功量之和为零系统与环境组合系统的净热量与净功量之和为零 Thenetheatandnetworkexchangebetweenthesystemandthesurroundingsiszeroforthecombined（originalandreverse）CompanyLogo For example,suppose we have a thermally insulated cylinder that holds an ideal gas.The gas is contained by a therm

25、ally insulated massless piston with a stack of many small weights on top of it.Initially the system is in mechanical and thermal equilibrium.Consider the following three processes:A.Alloftheweightsareremovedfromthepistoninstantaneouslyandthegasexpandsuntilitsvolumeisincreasedbyafactoroffour(afreeexp

26、ansion).B.Halfoftheweightisremovedfromthepistoninstantaneously,thesystemisallowedtodoubleinvolume,andthentheremaininghalfoftheweightisinstantaneouslyremovedfromthepistonandthegasisallowedtoexpanduntilitsvolumeisagaindoubled.C.Eachsmallweightisremovedfromthepistononeatatime,sothatthepressureinsidethe

27、cylinderisalwaysinequilibriumwiththeweightontopofthepiston.Whenthelastweightisremoved,thevolumehasincreasedbyafactoroffour.(2)如何实现可逆过程？如何实现可逆过程？How to fulfill reversible process?l工质与环境之间的温差和压差必须无限小工质与环境之间的温差和压差必须无限小Thetemperatureandpressuredifferencebetween the working fluid and its surroundingsshou

28、ldbeinfinitelysmall.l没有内部或机械摩擦损失没有耗散效应没有内部或机械摩擦损失没有耗散效应Nointernalormechanicalfrictionisallowed.-nodissipativeeffect.This requires that the working fluid goes through a continuous series of equilibrium states.A reversible process should satisfy the following criteria:可逆过程的实现准静态过程准静态过程 +无耗散效应无耗散效应 =可逆

29、过程可逆过程无不平衡势差无不平衡势差通过摩擦使功通过摩擦使功变热的效应变热的效应(摩阻，电阻，摩阻，电阻，非弹性变性，非弹性变性，磁阻等）磁阻等）不平衡势差不平衡势差不可逆根源不可逆根源 耗散效应耗散效应 耗散效应耗散效应irreversibilityDissipativeeffect耗散效应耗散效应DissipativeeffectThe factors that cause a process to be irreversible are called irreversibilities.（导致过程不可逆的因素称为不可逆性（导致过程不可逆的因素称为不可逆性）Such as heat tra

30、nsfer across a finite temperature difference （温差传热）（温差传热）Temperaturedifferenceisthedrivingforceofheattransfer.However,aheattransferprocessbecomeslessandlessirreversibleasthetemperaturedifferencebetweenthetwobodiesapproacheszero.friction（摩擦）（摩擦）Themorefrictionforceinvolved,themoreirreversibilitythepr

31、ocessis.（过程所涉及的摩擦阻力越大，过程的不可逆越强过程所涉及的摩擦阻力越大，过程的不可逆越强.）Frictioncanoccurbetweentwosolidbodies,andalsobetweensolidandafluid,evenbetweenthelayersofafluidmovingatdifferentvelocities.（3）Irreversibilities（不可逆性）（不可逆性）FREQUENTLY ENCOUNTERED IRREVERSIBILITIES常见的不可逆过程CompanyLogoHeattransferHeat transfer through

32、 finite temperature difference 不等温传热不等温传热T1T2T1T2QThrottling Process 节流过程节流过程 (阀门）阀门）p1p2p1p2ThrottlingValveFREQUENTLY ENCOUNTERED IRREVERSIBILITIES常见的不可逆过程混合过程混合过程自由膨胀自由膨胀真空真空UnrestrainedexpansionMixingprocesselectricresistance（电阻）inelasticdeformationofsolid（固体的塑性变形）chemicalreactions（化学反应）.lQuasi-e

33、quilibriumprocessisaninternallyreversibleprocess，whilereversibleprocessisatotallyreversibleprocess.准静态过程准静态过程是一个内部不可逆过程，而可逆过程是完全不可逆过程。是一个内部不可逆过程，而可逆过程是完全不可逆过程。lDissipativeeffectsarenotallowedinreversibleprocess.可逆过程可逆过程中不存在耗散效应。中不存在耗散效应。lReversibleprocessmustbequasi-equilibriumprocess.However，quasi-

34、equilibriumprocessisnotdefinitelyreversibleprocess.可逆过程可逆过程必然是准静态过程，但准静态过程不一定是可逆过程必然是准静态过程，但准静态过程不一定是可逆过程 lInbrief,reversibleprocessisaquasi-equilibriumprocesswithoutdissipativeprocess.简单简单地说，可逆过程就是没有耗散效应的准静态过程地说，可逆过程就是没有耗散效应的准静态过程（4）准静态过程与可逆过程的区别）准静态过程与可逆过程的区别 The distinction between quasi-equilibr

35、ium and reversible process 1-5 THERMODYNAMIC CYCLE 热力循环热力循环要实现要实现连续连续作功，必须构成作功，必须构成循环循环Definition：Asystemissaidtohaveundergoneacycleifitreturnstoitsinitialstateattheendoftheprocess.热力系统经过一系列变化回到初态，这一系列变化过程称为热力系统经过一系列变化回到初态，这一系列变化过程称为热力循环。热力循环。CYCLE AND PROCESS循环和过程Cycleisconsistedofprocesses.(循环由过程构

36、成循环由过程构成)IrreversibleCycleReversibleProcessIrreversibleCycleReversibleCycle(1)正循环正循环pVTS净效应：净效应：对外作功对外作功Neteffect:outputwork净效应：净效应：吸热吸热Neteffect:absorbheat顺时针方向顺时针方向(Clockwisecycle)2112动力循环动力循环Powercyclep T T v s 12345678Clockwise cycle(正向循环正向循环)The cycle proceeds clockwise (循环按顺时针方向进行)1-2-3，expand

37、 and do work on the surrounding 3-4-1,compressed and work is done on the system(在P-v图上：1-2-3，对外界作膨胀功，3-4-1，外界压缩系统消耗功。)The net work is positive and heat is converted to work.(循环净功wnet=0),把热能转换为功。在T-s图上:5-6-7，absorb heat from the high temperature reservoir(从高温热源吸热)，7-8-5，reject heat to low temperature

38、 reservoir(向低温热源放热)The net heat flow(净热量)is qnet=qq-qq,and wnet=qn 逆循环pVTSNeteffect:workconsumption净效应：净效应：对内作功对内作功Neteffect:heatrejection净效应：净效应：放热放热逆时针方向逆时针方向(Counter-clockwiseCycle)2112制冷循环制冷循环Refrigerationcycle热力循环的评价指标正循环：净效应（对外作功，吸热）正循环：净效应（对外作功，吸热）WT1Q1Q2T2动力循环动力循环Powercycle：热效率热效率ThermalEffi

39、ciency逆循环：净效应（对内作功，放热）逆循环：净效应（对内作功，放热）WT0Q1Q2T2制冷循环制冷循环Refrigerationcycle：制冷系数制冷系数CoefficientofPerformance制热循环制热循环Heatpumpcycle:制热系数制热系数CoefficientofPerformanceSUMMARYCompanyLogo 热力系热力系 System平衡态平衡态 Equilibrium State 准静态、可逆准静态、可逆 Quasi-static process Reversible process过程量、状态量、状态参数过程量、状态量、状态参数功量、热量、熵功量、热量、熵 Work,heat and Entropy p-V图、图、T-S图图 p-V and T-S Diagram 循环、评价指标循环、评价指标 CycleTHANK YOU