工程热力学与传热学(双语)第9章习题.ppt

-Chapter9Conduction-,Tutorial,HeatTransfer,1.如图所示的墙壁，其导热系数为50W/(m.K)，厚度为50mm，在稳态情况下墙壁内一维温度分布为t=2002000 x2。式中x的单位为m。试求（1）墙壁两侧表面的热流密度；（2）壁内单位体积的内热源生成热。,2.有一锅炉围墙由三层平壁组成，内层是厚度1=0.23m，1=0.63W/(m.K)的耐火粘土砖；外层是厚度3=0.25m，3=0.56W/(m.K)的红砖层；两层中间填以厚度2=0.1m，2=0.08W/(m.K)的珍珠岩材料。炉墙内侧与温度为tf1=520的烟气接触，其表面传热系数h1=35W/(m2.K)，炉墙外侧空气温度tf2=22，空气侧表面传热系数h2=15W/(m2.K)。试求（1）通过该炉墙单位面积的散热损失；（2）炉墙内外表面温度及层与层交界面的温度；（3）画出炉墙内的温度分布曲线。,3.一建筑物墙壁由如图所示的空心砖砌成，空心砖尺寸300mm300mm300mm。设该混凝土导热系数为0.8W/(m.K)，空气当量导热系数为0.28W/(m.K)。设温度只沿墙壁厚度X方向发生变化，室内温度为25，表面传热系数为10W/(m2.K)，室外空气温度为-10，表面传热系数为20W/(m2.K)。求通过每块砖的导热量。,4.外径5cm的不锈钢管，外面包扎着厚度为6.4mm的石棉隔热层，导热系数为0.166W/(m.K)，再外面包扎着厚度为2.5mm的玻璃纤维隔热层，导热系数为0.0485W/(m.K)。不锈钢管外壁温度为315，隔热层外表面温度为38。试计算石棉-玻璃纤维交界面的温度。,5.蒸汽管道的外直径d1=30mm，准备包两层厚度都是15mm的不同材料的热绝缘层。a种材料的导热系数a=0.04W/(m.K)，b种材料的导热系数b=0.1W/(m.K)。若温差一定。试问从减少热损失的观点看下列两种方案：（1）a在里层，b在外层；（2）b在里层，a在外层。哪一种好，为什么？,6.一高为30cm铝制圆锥台，顶面直径为8.2cm，底面直径为13cm；底面和顶面温度各自均匀且恒定，分别为520和120，侧面（曲面）绝热。试确定通过此台的导热量（铝的导热系数取为100W/(m.K)。,7.图中所示为纯铝制作的圆锥形截面。其圆形截面直径为D=ax1/2，其中a=0.5m1/2。小端位于x1=25mm处，大端位于x2=125mm处，端部温度分别为T1=600K和T2=400K，周侧面隔热良好。（1）作一维假定，推导用符号形式表示的温度分布T(x)的表示式，画出温度分布的示意图。（2）计算传热热流量Q。,8.初始温度为30，壁厚为9mm的火箭发动机喷管，外壁绝热，内壁与温度为1750的高温燃气接触，燃气与壁面间的表面传热系数为1950W/(m2.K)。假定喷管壁可作为一维无限大平壁处理，材料物性如下：=8400kg/m3，c=560J/(kg.K)，=24.6W/(m.K)。试确定（1）为使喷管材料不超过材料允许温度（1000）而能允许的运行时间；（2）在允许时间的终了时刻，壁面中的平均温度梯度与最大温度梯度。,9.用热电偶测量气罐中气体的温度。热电偶的初始温度为20，与气体表面的表面传热系数为10W/(m2.K)。热电偶近似为球形，直径为0.2mm。试计算热电偶插入10s后，其过余温度为初始过余温度的百分之几？要使热电偶过余温度不大于初始过余温度的1%，至少需要多长时间？已知热电偶焊丝的=67W/(m.K)，=7310kg/m3,c=228J/(kg.K)。,10.一直径为0.5mm的热电偶，其材料的密度=8930kg/m3,比热c=400J/(kg.K)。初始温度为25，被突然放于表面传热系数为95W/(m2.K)，温度为120的气流中。试问热电偶的过余温度为初始过余温度的1%及0.1%时需要多少时间？这时热电偶指示的温度是多少？,11.Alargewindowglass0.5cmthick=0.78W/(m.)isexposedtowarmairat25overitsinnersurface,andtheheattransfercoefficientfortheinsideairis15W/(m2.K).Theoutsideairis-15,andtheheattransfercoefficientassociatedwiththeoutsidesurfaceis50W/(m2.K).Determinethetempoftheinnerandoutersurfaceoftheglass.,12.Asteeltube=15W/(m.)ofoutsidediameter7.6cmandthickness1.3cmiscoveredwithaninsulationmaterial=0.2W/(m.)ofthickness2cm.Ahotgasat320withaheattransfercoefficientof200W/(m2.K)flowsinsidethetube.Theoutersurfaceoftheinsulationisexposedtocoolerairat20withaheattransfercoefficientof50W/(m2.K)。Calculate(1)theheatlossfromthetubetotheairfora5-mlengthofthetube.(2)thetempdropsduetothethermalresistancesofthehotgasflow,thesteeltube,theinsulationlayer,andtheoutsideair.,13.ASteampipewithanoutsideradiusof4cmiscoveredwithalayerofasbestosinsulation=0.15W/(m.)1cmthick,whichisinturncoveredwithfiberglassinsulation=0.05W/(m.)3cmthick.Theoutsidesurfaceofthesteampipeisatatempof330,andtheoutsidesurfaceofthefiberglassinsulationisat30.Determinetheheattransferratepermeterlengthofthepipeandtheinterfacetempbetweentheasbestosandthefiberglassinsulation.,Tutorial,One-DimensionalUnsteadyStateConductionProblem,14.Asteelplatea=1.210-5m2/s,=43W/(m.),cp=465J/(kg.),=7833kg/m3ofthickness10cm,initiallyatauniformtempof240,issuddenlyimmersedinanoilbathat40.Theconvectionheattransfercoefficientbetweenthefluidandthesurfaceis600W/(m2.).(a)Howlongwillittakeforthecenter-planetocoolto100?(b)Whatisthetempatadepth3cmfromtheoutersurface?(c)Calculatetheenergyremovedfromtheplateduringthistime.,15.Usinglumpedsystemanalysis,determinethetimerequiredforasolidsteelballofradiusR=2.5cm,=54W/(m.),=7833kg/m3,andc=0.465kJ/(kg.)tocoolfrom850to250ifitisexposedtoanairstreamat50havingaheattransfercoefficienth=100W/(m2.).,16.ALargealuminumplate=204W/(m.),=2702kg/m3,andc=0.896kJ/(kg.)ofthicknessL=0.1m,thatisinitiallyatauniformtemperatureof250iscooledbyexposingittoanairstreamattemperature50.Usinglumpedsystemanalysis,determinethetimerequiredtocoolthealuminumplateto100iftheheattransfercoefficientbetweentheairstreamandtheplatesurfaceish=80W/(m2.).,17.A6-cm-diametersteelball=61W/(m.),=7865kg/m3,andc=0.46kJ/(kg.)isatatemperatureof800.Itistobehardenedbysuddenlydroppingitintoanoilbathatatemperatureof50.Ifthequenchingoccurswhentheballreachesatemperatureof100andtheheattransfercoefficientbetweentheoilandthesphereis500W/m2.,determinehowlongtheballshouldbekeptintheoilbath.If100ballsaretobequenchedperminute,determinetherateatwhichheatmustberemovedfromtheoilbathinordertomaintainthebathtemperatureat50.,18.Athermocoupleistobeusedtomeasurethetemperatureinagasstream.Thejunctionisapproximatedasaspherewiththermalconductivityof25W/(m.),densityof9000kg/m3,andspecificheatof0.35kJ/(kg.).Theheattransfercoefficientbetweenthejunctionandthegasis250W/(m2.).Calculatethediameterofthejunctionifthethermocoupleshouldmeasure95percentoftheappliedtemperaturedifferencein3s.,19.Thetemperatureofagassteamistobemeasuredbyathermocouplewhosejunctioncanbeapproximatedasa1-mm-diametersphere