东北大学级《工程热力学》期末试题星期四shbao.docx
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东北大学级《工程热力学》期末试题星期四shbao
东北大学2002级《工程热力学》期末试题(2004年7月22日星期四)
QuestionsofSemesterExaminationaboutEngineeringThermodynamics
答题在答题纸上进行,写清题号,不必抄题。
答题语言随意。
1.WriteouttheFollowingConceptsandLaws:
(1)Theisolatedsystem;(4%)
(2)Regenerationcycle;(4%)
(3)TheClausiusstatementofthesecondlawofthermodynamics;(4%)
(4)Theconverging-divergingnozzle;(4%)
(5)Thefirstlawofthermodynamics;(4%)
(6)Thecoefficientofperformance.(4%)
2.Example2-13DifferentMethodsofEvaluatingGasPressure(p97,12%)
PredictthepressureofnitrogengasatT=175Kandv=0.00375m3/kgonthebasisof(a)theideal-gasequationofstate,(b)thevanderWaalsequationofstate.Comparethevaluesobtainedtotheexperimentallydeterminedvalueof10000kPa.(Critical-pointpropertiesofnitrogenare126.2Kand3.39MPa.)
3.Example16-9AirFlowthroughaConverging-DivergingNozzle(p796,12%)
Airentersaconverging-divergingnozzleat1.0MPaand800Kwithanegligiblevelocity.Theflowissteady,one-dimensional,andisentropicwithk=1.4.foranexitMachnumberofM=2andathroatareaof20cm3,determine(a)thethroatconditions,(b)theexitplaneconditions,includingtheexitarea,and(c)themassflowratethroughthenozzle.
4.Example4-19CoolingofBananasinaColdStorageRoom(12%)
Atypicalone-halfcarlotcapacitybananaroomcontains18palletsofbananas.Eachpalletconsistsof24boxes,andthustheroomstores432boxesofbananas.Aboxholdsanaverageof19kgofbananasandismadeof2.3kgoffiberboard.Thespecificheatsofbananaandthefiberboardare3.55kJ/(kg℃)。
and1.7kJ/(kg℃),respectively.Thepeakheatofrespirationofbananasis0.3W/kg.thebananasarecooledatarateof0.2℃/h.Disregardinganyheatgainthroughthewallsorothersurfaces,determinetherequiredrateofheattransferfromthebananaroom.
5.Example4-20FreezingofChickeninaBox(12%)
Asupplyof50kgofchickenat6℃containedinaboxistobefrozento-18℃inafreezer.Determinetheamountofheatthatneedstoberemoved.Thelatentheatofthechickenis247kJ/kg,anditsspecificheatis3.32kJ/(kg℃)abovefreezingand1.77kJ/(kg℃)belowfreezing.Thecontainerboxis1.5kg,andthespecificheatoftheboxmaterialis1.4kJ/(kg℃).Also,thefreezingtemperatureofchickenis–2.8℃.
6.Example6-8EconomicsofreplacingaValvebyaTurbine(14%)
Acryogenicmanufacturingfacilityhandlesliquidmethaneat115Kand5MPaatarateof0.280m3/s.Aprocessrequiresdroppingthepressureofliquidmethaneto1MPa,whichisdonebythrottlingtheliquidmethanebypassingitthroughaflowresistancesuchasavalve.Arecentlyhiredengineerproposestoreplacethethrottlingvalvebyaturbineinordertoproducepowerwhiledroppingthepressureto1MPa.UsingdatafromTable6-1,determinethemaximumamountofpowerthatcanbeproducedbysuchaturbine.Also,determinehowmuchthisturbinewillsavethefacilityfromelectricityusagecostsperyeariftheturbineoperatescontinuously(8760h/yr)andthefacilitypays$0.075/kWhforelectricity.(Known:
Formethane,whenp=5MPaandT=115K,h=232.2kJ/kg,s=4.9945kJ/(kgK),and=422.15kg/m3;whenp=1Mpaands=4.9945kJ/(kgK),h=222.8kJ/kg.)
7.Example10-1TheIdealVapor-CompressionRefrigerationCycle(14%)
Arefrigeratorusesrefrigerant-134aastheworkingfluidandoperatesonanidealvapor-compressionrefrigerationcyclebetween0.14and0.8MPa.Ifthemassflowrateoftherefrigerantis0.05kg/s,determine(a)therateofheatremovalfromtherefrigeratedspaceandthepowerinputtothecompressor,(b)therateofheatrejectiontotheenvironment,and(c)theCOPoftherefrigerator.(Known:
Forrefrigerant-134a,whenp=0.14MPa,h"=236.04kJ/kgands"=0.9322kJ/(kgK);whenp=0.8MPaands=0.9322kJ/(kgK),h=272.05kJ/kg;whenp=0.8MPa,h'=93.42kJ/kg.)
Answer:
1.
(1)Ifevenenergyisnotallowedtocrosstheboundary,thatsystemiscalledanisolatedsystem.
(2)Therefrigerationisthetransferofheatfromalowertemperatureregiontoahighertemperatureone.Thecycleonwhichrefrigeratorsoperatearecalledrefrigerationcycles.
(3)Itisimpossibletoconstructadevicethatoperatesinacycleandproducesnoeffectotherthanthetransferofheatfromalower-temperaturebodytoahigher-temperaturebody.
(4)Acceleratingafluidtosupersonicvelocitiescanbeaccomplishedonlybyattachingadivergingflowsectiontothesubsonicnozzleatthethroat.Theresultingcombinedflowsectionisaconverging-divergingnozzle,whichisstandardequipmentinsupersonicaircraft.
(5)Energycanbeneithercreatednordestroyed;itcanonlychangeforms.
(6)TheCOPsofallreversiblerefrigeratorsorheatpumpscanbedeterminedbyreplacingheattransferratiosintheaboverelationsbytheratiosoftheabsolutetemperatureofthehigh-andlow-temperaturereservoirs.
2.R=
kJ/(kgK)
pideal=
=13851kPa
whichisinerrorby38.5%.
a=
=0.175(m6kPa)/kg2
b=
=0.00138m3/kg
pvon=
=9471kPa
whichisinerrorby5.3%.
3.v0=
=0.2296m3/kg,0=
=4.355kg/m3
pcr=crp0=0.5281.0=0.528MPa
Tcr=
=666.6K
vcr=
=0.3624m3/kg,cr=
=2.761kg/m3
ccr=
=517.5m/s
M2=2=
T2=
=444.44K
p2=
=0.1278MPa
v2=
=0.998m3/kg
2=v2–1=0.998–1=1.002kg/m3
c2=
=845.17m/s
=2.7612010–4517.5=2.858kg/s
A2=
=33.75cm2
4.mbanana=19432=8208kg
mbox=2.3432=993.6kg
Erespiration=mbananaerespiration=82080.3=2462W
Ubanana=(mCT/)banana=82083.55(–0.2)=–5828kJ/h=–1619W
Ubox=(mCT/)box=993.61.7(–0.2)=–338kJ/h=–94W
So,Qout=Erespiration–Ubanana–Ubox=2462–1619+94=4175kW
5.Ufreshchicken=mCT=503.32(–2.8–6)=–1461kJ
Ufreezing=mulatent=50(–247)=–12350kJ
Ufrozenchicken=mCT=501.77(–18–(–2.8))=–1345kJ
Ubox=(mCT)box=1.501.4(–18–6)=–50kJ
Qout=–Uchicken–Ubox=–(Ufreshchicken+Ufreezing+Ufrozenchicken)–Ubox
=1461+12350+1345+50=15206kJ
6.Themassflowrateofliquidmethaneis
=422.150.280=118.2kg/s
Thepoweroutputoftheturbine
=118.2(232.3–222.8)=1123kW
Annualpowerproduction=
=11238760=0.9837107kWh/yr
Annualpowersavings=(Annualpowerproduction)(Unitcostofpower)
=0.9837107$0.075=$737800/yr
7.
(h1–h4)=0.05(236.04–93.42)=7.13kW
(h2–h1)=0.05(272.05–236.04)=1.80kW
(h2–h3)=0.05(272.05–93.42)=8.93kW
COPR–
=3.96
东北大学2002级《工程热力学》期末试题
(2004年7月22日星期四,小语种)
答题时不必抄题,要写清题号。
一.问答题(每题4分,共60分)
1.三九严冬,气温在零下20℃,夜里向窗外望去,只见大雾弥漫。
到了后半夜,天气转晴。
第二天清晨,我们会发现什么?
在相图(p-T图)上是个什么过程?
2.已知水蒸气的气体常数461.5J/(kgK),0.1MPa、150℃时的定压比热为1.98kJ/(kgK),那么此时它的定容比热为1.5185kJ/(kgK)吗?
为什么?
3.已知新型制冷工质HFC134a(氟里昂134a,CH2F-CF3,属于不对大气臭氧层具有破坏作用的环保型制冷工质。
)的分子量为102.031,临界温度为101.15℃,临界压力为4.064MPa,试求它的临界比容。
(提示:
请教一下范·德瓦尔老先生!
)
4.前一题较难,现在轻松一下。
请说出两种不可逆因素。
5.分别用100℃的蒸汽和100℃的热水快速(比如1秒钟)接触皮肤,准会被烫伤。
但是,那种情况烫得更厉害?
为什么?
6.一台气体压缩机,余隙容积占总容积的15%。
那么,由于余隙容积的影响,压缩机耗功增加了百分之几?
7.发电厂汽轮机排汽温度为什么设计成32℃左右,如果设计成0℃,再用制冷机创造低温环境,是不是可以提高循环热效率?
8.干球温度、湿球温度、湿空气温度和露点温度的关系怎样?
假定是一般的室内空气。
9.技术功包括哪几部分?
10.请写出卡诺定理的内容。
11.孤立系统轴功输出大致有多大?
12.可逆绝热过程的熵不变,不可逆绝热过程的熵增大;可逆循环的熵不变,不可逆循环的熵也不变。
这话有什么毛病没有?
13.水是实际气体,在蒸汽动力循环中是否可以当作理想气体处理?
在干燥过程中呢?
14.理论上跨音速喷管与跨音速扩压管有什么区别?
它们各应是什么形状?
15.蒸汽压缩制冷系统中,节流阀起什么作用?
二.计算题
16.定量空气,V1=2m3,初始温度t1=15℃,在定压下加热4187kJ,使容积变为V2=8m3。
求过程终了时气体的温度、所作的功及内能(热力学能)和焓的变化,并表示在p-v图和T-s图上。
设比热容为定值。
(13分)
17.将初态压力为98066.5Pa、温度为30℃的1kg质量的空气,在汽缸内定温压缩至原来容积的1/15。
若比热容为定值,求压缩所需的过程功;压缩时传递的热量;内能(热力学能)变化量;压缩后的压力、温度;并表示在p-v图和T-s图上。
(13分)
18.蒸汽进行理论的朗肯循环,最高压力为16700000Pa,最低压力为4700Pa,锅炉的过热器出口蒸汽温度为535℃,忽略泵功,试计算该循环的热效率。
(14分)
附:
饱和水与饱和蒸汽表(按压力排列)
p,MPa
t,℃
h',kJ/kg
h",kJ/kg
s',kJ/(kgK)
s",kJ/(kgK)
0.004
28.981
121.41
2554.1
0.4224
8.4747
0.0045
31.034
129.98
2557.8
0.4507
8.4327
0.005
32.90
137.77
2561.2
0.4762
8.3952
0.0055
34.60
144.91
2564.2
0.4995
8.3613
0.006
36.18
151.50
2567.1
0.5209
8.3305
未饱和水和过热蒸汽表
t,℃
p=16MPa
p=17MPa
v,m3
h,kJ/kg
s,kJ/(kgK)
v,m3
h,kJ/kg
s,kJ/(kgK)
530
0.02053
3385.4
6.4131
0.01916
3370.6
6.3738
540
0.02093
3410.4
6.4477
0.01954
3399.1
6.4090
东北大学2002级《工程热力学》期末试题(2004年7月23日星期四)
QuestionsofSemesterExaminationaboutEngineeringThermodynamics
答题在答题纸上进行,写清题号,不必抄题。
答题语言随意。
1.WriteouttheFollowingConceptsandLaws:
(1)opensystem;(5%)
(2)Carnotcycle;(5%)
(3)TheKelvin-Planckstatementofthesecondlawofthermodynamics;(5%)
(4)TheLavalnozzle;(5%)
(5)Idealgas;(5%)
(6)Thethermalefficiencyofaheatengine.(5%)
2.
Example1-10Thepistonofaverticalpiston-cylinderdevicecontainingagashasamassof60kgandacross-sectionalareaof0.04m2,asshowninFig.1-66.Thelocalatmospherepressureis0.97bar(0.097MPa),andthegravitationalaccelerationis9.81m/s2.(a)Determinethepressureinsidethecylinder.(b)Ifsomeheatistransferredtothegasanditsvolumeisdoubled,doyouexpectthepressureinsidethecylindertochange?
(10%)
3.Example2-10MassofAirinaRoom
Determinethemassoftheairinaroomwhosedimensionsare4m5m6mat100kPaand25℃.(10%)
4.Example4-9DecelerationofAirinaDiffuser
Airat10℃and80kPaentersthediffuserofajetenginesteadilywithavelocityof200m/s.Theinletareaofthediffuseris0.4m2.Theairleavesthediffuserwithavelocitythatisverysmallcomparedwiththeinletvelocity.Determine(a)themassflowrateoftheairand(b)thetemperatureoftheairleavingthediffuser.(15%)
5.Example5-5HeatingaHousebyaHeatPump
Aheatpumpisusedtomeettheheatingrequirementsofahouseandmaintainitat20℃.Onadaywhentheoutdoorairtemperaturedropsto-2℃,thehouseisestimatedtoloseheatatarateof80,000kJ/h.IftheheatpumpundertheseconditionshasaCOP(CoefficientofPerformance)of2.5,determine(a)thepowerconsumedbytheheatpump,and(b)therateatwhichheatisabsorbedfromthecoldoutdoorair,and(c)Ifwewe