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1、First Law of ThermodynamicsObjectiveObjective First Law of Thermodynamics Energy Forms and Energy TransferWork, Heat and Mass Energy Balance for Closed System Energy Balance for Open System Energy Balance for Steady-Flow SystemFirst Law of ThermodynamicsConservation of Energy Principle Energy can be
2、 neither created nor destroyed ;It can only change forms. Energy can change many forms, but its total value keeps constant. The perpetual-motion machine of the first kind can never be made.First Law of ThermodynamicsIn 1843, at the age of 25, James Prescott Joule did a series of careful experiments
3、to prove the equivalence of heat and work.Forms of Energy TransferThere are three forms of energy transfer: Work:caused by a force acting through a distancevolume work: moving boundary work Heat: caused by temperature difference between the system and surroundings. Mass flow: mass flow in or out of
4、the system serves as an energy transfer.Energy Entering CV Energy Leaving CV=Energy Accumulating in CVEnergy Transfer by Work Definitions Energy transfer associated with force acting through a distance Energy crossing the boundary of a closed system that is not heat must be work系统与外界相互作用而传递的能量，其全部效果
5、为使外界物体改变宏观运动状态。 包含两个必要条件： 1) 确定有力作用在边界上，即系统与外界有相互作用，且力 的不平衡势差无限小； 2) 系统边界发生位移，即外界物体改变宏观运动状态。二者缺一不可Energy Transfer by Work例1：Vacuump拔掉销钉后，系统向真空膨胀， 系统是否做功？例2：pQ刚性容器，加热后系统内压力 升高，系统是否向外做功？Energy Transfer by Work Units or J; usually kJ in SI Work examples Boundary work (e.g., a gas working against a movi
6、ng piston) closed system Shaft work (e.g., a rotating crankshaft on a motor)open systemEnergy Transfer by WorkWorkProcess function功是过程量示功图Energy Transfer by Work Work per unit mass Sign Convention W 0: work done by the systemW 0: heat transfer to the system Q 0,wt0 work done by the system dp = 0, wt
7、=0对可逆过程Mechanical Energy ConservationFor Reversible ProcessFor Quasi-Equilibrium Process with Friction=0Bonulli EquationExample例2-2123 45空气压缩机换热器喷管气轮机q换热器吸热空气流量求:1)压缩机功率；2)喷管出口流速；3)气轮机功率；4)整套装置功率解：工质在整套装置内的流动为稳定流动，应用稳定流动能量方程求解。过程中忽略位能变化绝热压缩绝热膨胀1)压缩机功率2)喷管出口流速,流经换热器和喷管3)气轮机功率123 45空气压缩机换热器喷管气轮机q4)整套装
8、置功率或 将整套装置取为系统End of 1st Law of ThermodynamicsWell done!2.6 Second Law of Thermodynamics2nd Law is used to determine thedirectioncondition limitation of thermal process.Natural process is directive.* We will use the 2nd Law as a tool to evaluate whether a process is possible. *Spontaneous ProcessTra
9、nsferring heat to a paddle wheel will not cause it to rotate. Heat TransferA cup of hot coffee does not get hotter in a cooler room. Others: Gas Free Expansion Mixture Process Combustion and Reaction ProcessMechanicalHeat一切实际的热力过程都具有方向性，只能单独自动地朝一个方向进行，这类过程称为自发过程；而其逆方向地过程不能单独自动进行，这类过程称为非自发过程。若要非自发过程得
10、以实现，必须附加某些补充条件，付出一定的代价。2.6 Second Law of ThermodynamicsStatement of 2nd Law1.Clausius: 2. It is impossible to transfer energy from a cooler to a hotter body as a sole effect (requires a heat pump or other device, which needs energy input) 2. Kelvin-Planck:It is impossible to operate a thermodynamic
11、cycle to produce work with only heat transfer from a single reservoir (requires both heat addition and heat rejection)Statement of 2nd Law1.Clausius:Low-Temperature Reservoir TLHigh- Temperature Reservoir THReservoir: a “large” body that can absorb or supply heat without a “noticeable” temperature c
12、hangeStatement of 2nd Law2. Kelvin-PlanckHigh- Temperature Reservoir THHeat EngineStatement of 2nd Law3. 2nd perpetual-motion machine NEVER be madeSecond Law of Thermodynamics热力学第二定律的实质:自发过程是不可逆的；若要非自发过程得以实现，必须伴随一个适当的自发过程作为补充条件。 Heat TransferSpontaneous Process若要实现热量由低温向高温的非自发过程，必须消耗功，即配合功变热这个自发过程作为
13、补充条件。 MechanicalHeatMechanicalHeat自发过程非自发过程1.若要实现热转化功的非自发过程，必须配合向低温热源放热这一自发过程，因此热机的效率必定小于1。 2.“功可以全部转化成热，但热不能完全变为功”理想气体的等温膨胀3.热变功的最高效率Carnot CycleCarnot Cycle(Reversible) Isothermal Expansion Adiabatic Compression Isothermal Compression Adiabatic ExpansionTSTHTLS1S2Thermal EfficiencyThermal Efficien
14、cy=Desired Output Required Input=Net Work Output Total Heat Input=WO Qin=1Qout QinDefinition:Q1heat transfer between cycle device and high-temerature medium at TH Q2heat transfer between cycle device and Low-temerature medium at TLThermal Efficiency For Heat Engine(卡诺热机) Low-Temperature Reservoir TL
15、High- Temperature Reservoir THHeat Engine区别？Thermal Efficiency For Refrigerator(卡诺制冷机) Low-Temperature Reservoir TLHigh- Temperature Reservoir THRefrig- erator 制冷系数Thermal Efficiency For Heat Pump(卡诺热泵) Low-Temperature Reservoir TLHigh- Temperature Reservoir THHeat Pump 供暖系数Carnot Principles1. All r
16、eversible cyclesoperating between TLand TH have the sameefficiency (rev) .2. For same TL and TH,reversible cycles havehigher efficiencies thanany irreversible cycles.Conclusions of Carnot Principles1. Carnot 循环热效率只取决于高温热源和低温热源的温度，即工质吸热和放热的温度。提高T1或降低T2均可提高热效率。2. 任何循环的热效率均小于1。3. T1=T2时，循环热效率为零，说明只从单一热源吸热是不可能把热转变成功的。4. 要提高实际装置的热效率，必须尽可能减少摩擦等不可逆功损失。Carnot Cycle with Multi-rese
