材料热力学习题答案2

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1、Theproblemsofthesecondlaw1Thesolarenergyfluxisabout4Jcm2/min.innofocusingcollectorthesurfacetemperaturecanreachavalueofabout900°C・Ifweoperateaheatengineusingthecollectorastheheatsourceandalowtemperaturereservoirat25°C，calculatetheareaofcollectorneedediftheheatengineistoproduce1ho

2、rsepower.Assumetheengineoperatesatmaximumefficiency・(2.1)SolutionP=一=746(W)5=0.664(m2)2Arefrigeratorisoperatedby0.25hpmotor.Iftheinterioroftheboxistobemaintainedat-20°Cganisteramaximumexteriortemperatureof35°C,whatthemaximumheatleak(inwatts)intotheboxthatcanbetoleratedifthemotorr

3、unscontinuously?Assumethecoefficientofperformanceis75%ofthevalueforareversibleengine・(2.2)4273-20^3X^X°-25X746=1，44(W)3supposeanelectricalmotorsuppliestheworktooperateaCarnotrefrigerato匚Theinterioroftherefrigeratorisat0°C.Liquidwateristakeninat0°Candconvertedtoiceat0°C.Toconvert1

4、goficeto1gliquid・AH=334J/gisrequired.Ifthetemperatureoutsidetheboxis20°C,whatmassoficecanbeproducedinoneminutebya0.25hpmotorrunningcontinuously?Assumethattherefrigeratorisperfectlyinsulatedandthattheefficienciesinvolvedhavetheirlargestpossiblevalue.(2.3)M=60m=457(g)4under1atmpres

5、sure,heliumboilsat4.126K・Theheatofvaporizationis84J/molwhatsizemotor(inhp)isneededtorunarefrigeratorthatmustcondense2molofgaseousheliumat4.126ktoliquidatthesametemperatureinoneminute?Assumethattheambienttemperatureis300Kandthatthecoefficientofperformanceoftherefrigeratoris50%ofth

6、emaximumpossible.(2.4)W=5wJh~TlQ.tllSolution:393(W)=0.52(切)1ifafossilfuelpowerplantoperatingbetween540and50°Cprovidestheelectricalpowertorunaheatpumpthatworksbetween25and5°C,whatistheamountofheatpumpedintothehouseperunitamountofheatextractedfromthepowerplantboiler.(a)assumethatth

7、eefficienciesareequaltothetheoreticalmaximumvalues(b)assumethepowerplantefficiencyis70%ofmaximumandthatcoefficientofperformaneeoftheheatpumpis10%ofmaximum(c)ifafurnacecanuse80%oftheenergyinfossilfoetoheatthehousewoulditbemoreeconomicalintermsofoverallfissilefuelconsumptiontouseah

8、eatpumporafurnace?dothecalculationsforca