工程热力学(英文版)第10单元ppt课件.ppt

1,Chap9 Summary,Basic considerations:actual cycle,ideal cycle,carnot cycle,P-V,T-S,Reciprocating engines,Assumptions of gas power cycles,TDC,air standard assumptions(空气标准假设):1)air=ideal gas,Cv=const;2)internal reversible process;3)combustionheat addition process;4)exhaust heat rejection process,SI engines-Otto cycle,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),CI engine-Diesel Cycle,The ideal Diesel cycle:1-2 isentropic compression(等熵压缩);2-3 constant P heat addition(定压吸热);3-4 isentropic expansion(等熵膨胀);4-1 constant V heat rejection(定容放热),Reciprocating engines are classified as spark-ignition(SI)engines(点燃式内燃机)compression-ignition(CI)engines(压燃式内燃机),BDC,Stroke,Bore,Intake valve,Exhause valve,MEP,The ideal Otto 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),2,Chap10 Summary,Carnot cycle is not a suitable model for vapor power cycle,Rankine Cycle,Ways to Increase of Rankine Cycle,Rankin cycles(朗肯循环),Rankine cycle:1-2 Isentropic compression(pump);2-3 Const P heat addition(boiler);3-4 Isentropic expansion(turbine);4-1 Const P heat rejection(condenser),Reheat rankine cycle（再热循环）,Reheat cycle:Expand the steam in the turbine in two stages and reheat it in between.,regenerative rankine cycle(回热循环),Regenerative cycle:extracting steam from the turbine to heat the feedwater before boiler,Basic idea:increase Thigh,avg for Qin,and decrease Tlow,avg for Qout,1,Lowering the condenser pressure(lower Tlow,avg),2,Superheating the steam to higher T(increase Thigh,avg),3,Increase the boiler pressure(increase Thigh,avg),Cogeneration(热电合供循环),Cogeneration is the production of more than one useful form of energy(such as process heat and from the same energy source,3,Chapter 10 Vapor Power Cycles(蒸汽动力循环),4,Steam is the most common working fluid used in vapor power cycles because of its many desirable characteristics:low cost,availability,and high hfg.This chapter is mostly devoted to the discussion of steam power plants.Steam power plants are commonly referred to as coal plants,nuclear plants,or natural gas plants,depending on the type of fuel used to supply heat to the steam.The steam goes through the same basic cycle in all of them.,5,10-1 The Carnot Vapor Cycle,Carnot cycle is the most efficient cycle operating between two specified temperature limits;It would be perfect if we make it as the ideal cycle for vapor power plants.However,it is not a suitable cycle for power cycles.Consider a steady-flow carnot cycle within the saturation dome of a pure substance.,Dome,Duomo,7,1-2:heated in a boiler.Isothermal expansion(等温膨胀)2-3:expanded in a turbine.Adiabatic expansion(定熵膨胀)；3-4:condensed in a condenser.Isothermal compression(等温压缩).4-1:compressed in a compressor.Adiabatic compression(绝热压缩).,Several impracticalities in this cycle(a)For 1-2 and 3-4,isothermal process are easy to achieve by keep them a constant P.But,this is only applicable for two phase systems,and MAX TH is 373.95 for water.2-3 isentropic is close to a well-designed turbine.But quality of steam is decreasing during 2-3.Low quality(high moisture is bad for turbine blades,because of erosion problem)4-1 involves compression of liquid-vapor to a saturated liquid.It is not practical to design a compressor that handles two phases.,8,For a carnot cycle like(b),problems are:4-1 Isentropic compression to extremely high pressures1-2 isothermal heat transfer at variable pressures.,So,we conclude that:the Carnot cycle cannot be approximated in actual devices and is not a realistic model for vapor power cycles.,9,10-2 Rankine cycle:the ideal cycle for vapor power cycles,Rankine cycle（朗肯循环）is the ideal cycle for vapor power cycles.It does not involve any internal irreversibilities and consists of four processes.,1-2 Isentropic compression in a pump2-3 Constant pressure heat addition in a boiler3-4 Isentropic expansion in a turbine4-1 Constant pressure heat rejection in a condenser,10,1-2 Isentropic compression in a pump(泵，等熵压缩)2-3 Constant P heat addition in a boiler(锅炉，定压加热)3-4 Isentropic expansion in a turbine(汽轮机，等熵膨胀)4-1 Constant P heat rejection in a condenser(冷凝器，定压放热),饱和水,过冷水,过热蒸汽,湿饱和蒸汽,11,Energy Analysis of the Ideal Rankine Cycle,All four components associated with the Rankine cycle(the pump,boiler,turbine,and condenser)are steady-flow devicesRankine cycle can be analyzed as steady-flow processes.The kinetic and potential energy changes of the steam are usually small and can be neglected.So,steady-flow energy equation per unit mass of the steam is,12,1-2:Pump(q=0)2-3:Boiler(w=0)3-4:Turbine(q=0)4-1:condenser(w=0)Thermal efficiency,Energy Analysis of the Ideal Rankine Cycle,13,State 1:P1,sat liquid h1,v1State 2:P2,s2=s1 h2State 3:P3,T3 h3,s3State 4:P4,sat Mix,s4=s3 h4,Known:P1=P4=75kPa;P2=P3=3MPa,T3=350,=0.26,14,10-4 How can we increase the efficiency of the rankine cycle?,The basic idea of all the modifications to increase the thermal efficiency of a power cycle is the same:Increase the average temperature at which heat is transferred to the working fluid in the boiler(heat addition),or decrease the average temperature at which heat is rejected from the working fluid in the condenser(heat rejection).,15,10-4 How can we increase the efficiency of the rankine cycle?,1.Lowering the condenser pressure(lower Tlow,avg),Lowering the condenser pressure lowers the temperature of steam,and thus the temperature at which heat is rejected.,16,10-4 How can we increase the efficiency of the rankine cycle?,1.Lowering the condenser pressure(lower Tlow,avg),Effects:(red for positive,blue for negative)Increase the net work output.Area1-4-4-1-2-2-1.Increase heat input also.Area under 2-2.Thermal efficiency of cycle is increased.,Increase the moisture content of steam in turbine,which would erodes the blades.x4x4(can be corrected)There is a lower limit for this P:the Psat corresponding to T of the cooling medium.PPsat(Tsat=Triver+T).E.g:Triver=15,T=10,P Psat(Tsat=25)=3.2KPaAir leakage into the condenser.,17,10-4 How can we increase the efficiency of the rankine cycle?,2.Superheating the steam to higher T(increase Thigh,avg),Superheating the steam to high T increase the T of steam,and thus the temperature at which heat is absorbed.,18,10-4 How can we increase the efficiency of the rankine cycle?,2.Superheating the steam to higher T(increase Thigh,avg),Effects:(red for positive,blue for negative)Increase the net work output.Area3-3-4-4-3.Increase heat input also.Area under 3-3Thermal efficiency of cycle is increased as overall effect,decrease the moisture content of steam in turbine.x4 x4There is a limit for this T,because of metallurgical problem.Current,the highest Tsteam at turbine inlet is 620.To improve or replace the materials is one way to increase this Tsteam.Like using ceramics.,19,10-4 How can we increase the efficiency of the rankine cycle?,3.Increase the boiler pressure(increase Thigh,avg),Increasing the operating pressure of the boiler is another way of increasing the average temperature during the heat addition process.,20,10-4 How can we increase the efficiency of the rankine cycle?,3.Increase the boiler pressure(increase Thigh,avg),Effects:(red for positive,blue for negative)Increase and decrease of net work output.Area2-2-3-2 and area 3-4-4-3Increase and decrease heat input.Thermal efficiency is increased as Thigh,avg increased.,increase moisture of steam in turbine,but can be corrected by reheating.Many steam power plants operate at supercritical pressure(P22.06MPa)and have thermal efficiencis of 40%.,21,22,10-5 the ideal reheat rankine cycle(再热循环),Then,a question is:How can we take advantage of the increased efficiencies at higher boiler pressures without facing the problem of excessive moisture at the final stages of the turbine?,“3.Increase the boiler pressure(increase Thigh,avg)”increases moisture of steam in turbine as well as thermal efficiency,23,10-5 the ideal reheat rankine cycle(再热循环),Two possibilities come to mind:1.Superheat the steam to very high temperatures before it enters the turbine.But this is not a viable solution,since it requires raising the steam temperature to metallurgically unsafe levels.2.Expand the steam in the turbine in two stages,and reheat it in between.In other words,modify the simple ideal Rankine cycle with a reheat process.it is a practical solution to the moisture problem in turbines,and is commonly used in modern steam power plants,24,10-5 the ideal reheat rankine cycle(再热循环),In the ideal reheat rankine cycle,the expansion process takes place in two stagesFirst stage(high pressure turbine),steam is expanded isentropically to an intermediate pressure and sent back to the boiler to be reheated at constant pressure,until reach the inlet T of first turbine stage.Second stage(low pressure turbine)steam is expanded isentropically to the condenser pressure.,25,10-5 the ideal reheat rankine cycle(再热循环),Effects:the incorporation of single reheat in a power plant improves the cycle efficiency by 4-5%.(Thigh,avg is increased)Decrease the moisture of the steam in turbine.More than 2 reheat is not practical because:Efficiency improvement gained by 2nd reheat is only half of 1st one;but cause Pturbine decrease and superheated exhaust then Tlow,avg increase.Added cost and complexityDouble reheat is only used on supercritical pressure power plants.If materials could withstand sufficiently high T,reheat cycle is not necessary.,26,10-6 the ideal regenerative rankine cycle(回热循环),In T-s diagram of the Rankine cycle,heat transfer during process 2-2 is found at a relatively low T,so lower cycle efficiency;raise T of the liquid leaving the pump before the boiler is one solution;A practical process(regeneration,回热)is accomplished by extracting steam from the turbine at various points and heat the feedwater before the boiler.The device where the feedwater is heated by regeneration is called a regenerator(回热器),or feedwater heater（给水加热器）.,27,10-6 the ideal regenerative rankine cycle(回热循环),Effects of regeneration:Improve cycle efficiency.Provide a convenient means of deaerating(除气)the feedwater and prevent corrosion in the boiler.Control the large volume flow rate of the steam at the final stages of the turbine(due to large specific volumes at low pressures)Regeneration has been used in all modern steam power plants since 1920s.the feedwater could be heated by mixing with steam(open feedwater heaters)or without mixing(closed feedwater heaters),28,10-6 the ideal regenerative rankine cycle(回热循环),An open(or direct-contact)feedwater heater is basically a mixing chamber,where the steam extracted from the turbine mixes with the feedwater exiting the pump.,29,10-6 the ideal regenerative rankine cycle(回热循环),In the closed feedwater heater,heat is transferred from the extracted steam to the feedwater without any mixing taking place.The two streams can be at different pressures,since they do not mix.,30,10-6 the ideal regenerative rankin cycle(回热循环),Comparison Open feedwater heaters are simple and inexpensive have good heat transfer characteristics.They bring the feedwater to the saturation state.For each heater,however,a pump is required to handle the feedwater.The closed feedwater heaters more complex because of the internal tubing network,and more expensive.Heat transfer is less effective since the two streams are not allowed to be in direct contact.However,closed feedwater heaters do not require a separate pump for each heater since the extracted steam and the feedwater can be at different P,10-6 the ideal regenerative rankine cycle(回热循环),Most steam power plants use a combination of open and closed feedwater heaters.,31,32,10-8 Cogeneration(热电合共循环),In general,cogeneration is the production of more than one useful form of energy(such as process heat and electric power)from the same energy source.An ideal cogeneration plant:120KW energy is input to the boiler,20KW power is produced in turbine,100KW heat is supplied as process heat.,Utilization factor(热量利用系数):,33,10-8 Cogeneration(热电合共循环),An ideal cogeneration plant is not practical since the power and process-heat is not adjustable.And a more practical cogeneration plant is shown in fig10-22.,Cogeneration plants(热电厂)have proved to be economically very attractive.More and more have been installed and are being installed.Especially in district heating(区域集中供热).,10-310-710-9自学,35,Chap10 Summary,Carnot cycle is not a suitable model for vapor power cycle,Rankine Cycle,Ways to Increase of Rankine Cycle,Rankine cycles(朗肯循环),Rankine cycle:1-2 Isentropic compression(pump);2-3 Const P heat addition(boiler);3-4 Isentropic expansion(turbine);4-1 Const P heat rejection(condenser),Reheat rankine cycle（再热循环）,Reheat cycle:Expand the steam in the turbine in two stages and reheat it in between.,regenerative rankine cycle(回热循环),Regenerative cycle:extracting steam from the turbine to heat the feedwater before boiler,Basic idea:increase Thigh,avg for Qin,and decrease Tlow,avg for Qout,1,Lowering the condenser pressure(lower Tlow,avg),2,Superheating the steam to higher T(increase Thigh,avg),3,Increase the boiler pressure(increase Thigh,avg),Cogeneration(热电合供循环),Cogeneration is the production of more than one useful form of energy(such as process heat and from the same energy source,36,10-1C10-2C10-7C10-8C10-9C10-10C10-12C 10-29C10-30C10-39C10-40C,