!1!AREN!2110!Spring!2011!!EXTRA&CREDIT&OPPORTUNITY:&Due&end&of&d a y,&Friday,&April&29&!The!purpose!of!these!extra!credit!problems!is!to!give!you!structured!practice!in!preparing!for!the!final.!It!is!also!an!opportunity!to!improve!your!test!scores!(or!be!an!“insurance!policy”!for!your!final!exam!grade).!!There!are!20!multiple!choice!or!short!answer!questions!in!the!following!4!pages,!each!worth!1!point!(no!partial!credit).!The!points!for!all!correct!answers!will!be!added!directly!to!your!total!test!score!(comprised!of!the!two!midterms!and!the!final).!!Each!midterm!was!50!points,!and!the!final!will!be!75!points,!for!a!total!of!175!points!for!tests.!(Homework!grades!will!be!scaled!to!count!for!another!75!points.)!For!example,!if!you!received!scores!of!30/50!on!midterm!1,!40/50!on!midterm!2,!and!55/75!on!the!final!exam!and!answered!15!extra!credit!questions!correctly,!your!total!point!score!for!the!exam!component!of!your!grade!would!be:!30!+!40!+!55!+!15!=!140/175,!for!a!test!average!of!80%.!This!would!be!a!significant!improvement!over!your!total!test!score!without!extra!credit!=!125/175,!average!=!71.4%.!!The!application!of!the!extra!credit!will!have!a!ceiling!so!that!no!one!gets!an!average!of!over!100%!for!the!three!tests.!Of!course,!if!you!are!satisfied!with!your!test!score!and!preparation!you!do!not!have!to!do!any!of!these!problems.!!INSTRUCTIONS:!the!questions!are!open!book!and!notes,!but!you&must&do&them&on&your&own&–&no&consulting&with&other&students,&TA&or&former&students.!Show!all!your!work.!!Questions&involve&some&computation&or&explanation&and&you&will&not&receive&credit&for&an&answer&if&you&do&not&show&your&work,&i.e.,&there&is&no&credit&for&guessing.!You!are!encouraged!to!do!your!work!on!separate!sheets!and!attach!when!you!hand!in.!If!you!have!questions,!see!Prof.!Silverstein.!!Please!sign!the!honor!code!statement!below!before!turning!in!your!answers,!indicating!that!you!have!not!received!any!unauthorized!assistance.!!!Name!(print)_____________________________________________________________!!I!have!read,!understand,!and!agree!to!abide!by!the!University!of!Colorado!honor!code! in!this!test!context:!!I&have&neither&given&nor&received&unauthorized&assistance&during&this&examination.&&&Signed:________________________________________________&&&&Score:&____________/20&points&possible&&!2!1.!A!refrigerator!supplies!5!kw!cooling!for!a!cold!space!maintained!at!]5!oC!and!rejects!heat!at!a!rate!of!5.5!kw!to!a!room!kept!at!25!oC.!The!compressor!motor!provides!500!w!of!power.!The!refrigerator!satisfies:!!a)!1st!Law!only! ! ! ! ! b)!2nd!Law!only!!c)!1st!and!2nd!Laws! ! ! ! ! c)!Neither!1st!or!2nd!Law!!!For!problems!2]4,!a!Carnot!heat!engine!operates!with!high!and!low!temperature!reservoirs!at!300!oC!and!15!oC,!respectively.!The!change!in!entropy!of!the! working!fluid!in!the!isothermal!expansion!process!is!2.4511!kJ/kg]K!!2.!The!cycle!efficiency!is!!a)!0.25!! ! ! ! ! ! b)!0.50!!c)!0.75!! ! ! ! ! ! d)!0.95!!!3.!!Heat!is!rejected!at!a!rate!of!!a)!38!kJ/kg! ! ! ! ! ! b)!288!kJ/kg!!c)!706!kJ/kg! ! ! ! ! ! d)!1,404!kJ/kg!!!4.!The!net!work!output!from!the!heat!engine!is!!a)!699!kJ/kg! ! ! ! ! ! b)!706!kJ/kg!!b)!1,383!kJ/kg! ! ! ! ! d)!1,404!kJ/kg!!!5.!Entropy!is!“conserved:”!!a)!Never! ! ! ! ! ! b)!In!a!cycle!only!!b)!In!a!reversible!process! ! ! ! d)!In!an!isothermal!process!!!6.!Air!is!expanded!in!an!isentropic!turbine.!The!inlet!pressure!is!2!MPa!and!the!outlet!pressure!is!100!kPa.!!The!outlet!temperature!is!17!oC.!!The!inlet!temperature!is:!!a)!40!oC! ! ! ! ! ! b)!340!oC!!c)!410!oC! ! ! ! ! ! d)!5530!oC!!3!7.!A!steady!flow!of!5!kg/s!of!saturated!refrigerant!vapor!is!condensed!in!a n!internally!reversible!process!to!saturated!liquid!in!an!isobaric!heat!exchanger!at!900!kPa.!Heat!is!rejected!to!the!surroundings!at!15!oC.!The!entropy!generated!in!the!universe!is:!!a)!5.6!kw/K! ! ! ! ! ! b)!2.7!kw/K!!c)!]0.2!kw/K! ! ! ! ! ! d)!Zero!!For!questions!8!–!11.!!The!T]s!diagram!below!represents!an!ideal!Rankine!power!generation!cycle!with!a!boiler!pressure!=!8.6!MPa!(rounded!value)!and!a!condenser!pressure!=!12.4!kPa!(rounded!value).!Water!at!the!turbine!inlet!is!saturated!vapor!and!saturated!liquid!at!the!condenser!outlet.!!!!8.!The!quality!of!the!steam!at!the!turbine!outlet!is!! _____________________.!!!9.!The!cycle!efficiency!is!! ! ! ! ! _____________________.!!!10.!A!100!MW!plant!is!to!be!designed.!The!required!mass!flow!rate!of!steam!is:!!! ! ! ! ! ! ! ! _____________________.!!11.!The!required!mass!flow!rate!of!condenser!cooling!water!with!a!maximum!temperature!increase!of!7!oC!is:!!! ! ! ! ! ! ! ! _____________________.!0 100 200 300 400 500 600 0 1 2 3 4 5 6 7 8 9 10 T (oC) s (kJ/kg*K) T-s DIAGRAM FOR WATER!4!12.!Water!is!pumped!to!the!boiler!of!a!power!plant!in!an!adiabatic!and!reversible!process.!!State!properties!that!DO!NOT!CHANGE!from!inlet!to!outlet!are:!!a)!s! ! ! ! ! ! ! b)!s!and!h!!c)!s!and!T! ! ! ! ! ! d)!s,!T,!and!v!!!13.Which!of!the!following!conditions!will!improve!the!coefficient!of!performance!of!a!heat!pump?!!a)!Increasing!TL! ! ! ! ! b)!Increasing!TH!!c)!Increasing!power!input! ! ! ! d)!Lowering!evaporator!pressure.!!!14.!A!throttling!valve!is!used!to!reduce!the!pressure!of!saturated!liquid!R]134a!from!1.2!MPa!to!120!kPa.!During!the!process,!25!kJ/kg!heat!is!lost!to!the!surroundings!at!20!oC.!The!entropy!of!the!R]134a!at!the!outlet!is:!!a)!0.373!kJ/kg]K! ! ! ! ! ! b)!0.424!kJ/kg]K!!c)!0.469!kJ/kg]K! ! ! ! ! ! d)!0.572!kJ/kg]K!!!15.!The!velocity!of!helium!increases!from!30!to!500!m/s!as!it!flows!through!a!nozzle!in!an!isothermal!process.!!The!pressure!of!the!helium!at!the!inlet!is!100!kPa!and!85!kPa!at!the!outlet.!This!process!is:!!a)!adiabatic! ! ! ! ! ! ! b)!isentropic!!c)!reversible! ! ! ! ! ! ! d)!impossible!!!16.!Steam!at!200!kPa!and!200!oC!is!mixed!adiabatically!with!cold!water!at!the!same!pressure!and!25!oC!to!produce!1!kg/s!saturated!liquid!water!at!200!kPa.!The!rate!of!entropy!generation!in!the!surroundings!is:!!a)!0.055!kw/K! ! ! ! ! ! b)!0.128!kw/K!!c)!0.222!kw/K! ! ! ! ! ! d)!0.277!kw/K!!!17.!What!is!the!minimum!heat!load!to!a!room!maintained!at!21!oC!from!a!refrigerator!keeping!the!cold!box!at!0!oC!by!removing!heat!at!a!rate!of!12!kw?!!a)!11!kw! ! ! ! ! ! ! b)!12!kw!!c)!13!kw! ! ! ! ! ! !