ME 300 Exam 1 October 9, :30 p.m. to 7:30 p.m.

Transcription

1 CIRCLE YOUR LECTURE BELOW: First Na Last Na 10:0 a.. 1:0 p.. Naik Gor ME 00 Exa 1 Octobr 9, 014 6:0 p.. to 7:0 p.. INSTRUCTIONS 1. This is a closd book and closd nots xaination. You ar providd with an quation sht and all th ndd proprty tabls.. Do not hsitat to ask th instructor if you do not undrstand a probl statnt.. Start ach probl on th sa pag as th probl statnt. Writ on only on sid of th pag. Matrials on th back sid of th pag will not b gradd. 4. Put only on probl on a pag. Anothr probl on th sa pag will not b gradd. 5. Idntify your syst(s), list rlvant assuptions, and start with basic quations. 6. If you giv ultipl solutions, you will rciv only a partial crdit although on of th solutions ight b corrct. Dlt th solution you do not want gradd. 7. For your own bnfit, plas writ clarly and lgibly. Maxiu crdit for ach probl is indicatd blow. 8. Aftr you hav ltd th xa, at your sat put your paprs in ordr. This ay an that you hav to rov th stapl and r-stapl. Do not turn in loos pags. 9. Onc ti is calld you will hav thr inuts to turn in your xa. Points will b subtractd for xas turnd in aftr ths thr inuts. Probl Possibl Scor Total 100 1

2 Probl 1 (5 points) Givn A piston-cylindr dvic initially contains 0.1 of saturatd watr vapor at 10 bar (Stat 1). Sta at 0 bar and 500ºC in a supply lin is supplid to th cylindr through a valv during a procss in which prssur insid th cylindr rains constant and th tpratur rachs 0ºC (Stat ). Th ass of sta ntring th cylindr fro th supply lin is Find (a) Calculat th work () during th procss. Indicat whthr it is don by th syst or on th syst. (b) Calculat th hat transfr () during th procss. Indicat whthr it is into th syst or out of th syst. Idntify your syst(s), list assuptions, and start with basic quations. Assuptions - Quasi-quilibriu procss - Nglct friction - Ignor KE and PE ffcts - Unifor flow at th supply lin inlt Basic Equations d i dt i Intgrating: 1 i L

3 Probl 1 (continud) de d i U KE PE Q W i h i gz i h gz dt dt i Intgrating: U U U1 u u 1 1 QW Lh + L Solution Stat 1: P 1 = 10 bar, saturatd vapor + + Using Tabl A-: v1 vg P , u1 ug P Stat : P = 10 bar, T = 0C Using Tabl A-: Tsat P179.9 C T SHV + Using Tabl A-4: v 0.678, u Supply Lin: P L = 0 bar, T L = 500C Using Tabl A-: Tsat PL 1.4 C TL SHV Using Tabl A-4: hl (a) Considring ass balanc, final ass in th cylindr: 1 L V1 v V v Nglcting friction, boundary work during th quasi-quilibriu xpansion procss: W W PdV P P V V kpa b W 94.5 ; W 0 work don by th sta on th surroundings (b) Considring nrgy balanc, hat transfr during th procss: Qu u 1 1W LhL Q 90 ; Q 0 hat rjctd fro th cylindr to th surroundings

4 Probl (45 points) Givn: Air ntrs an insulatd, stady-stat rssor with a ass flow rat of /s at 1.4 bar and 17ºC with a vlocity of 70 /s (Stat 1) and xits at 4. bar and 147ºC with a vlocity of 110 /s (Stat ). Considr that th rssor oprats in a surrounding of 1 bar and 17ºC. Find: (a) Calculat th powr (kw) rquird to oprat th rssor. (b) Find th rat of xrgy dstruction (kw) for th rssor. (c) Dtrin th xrgtic ( nd law) fficincy (%) for th rssor. Idntify your syst(s), list assuptions, and start with basic quations. 1 s Assuptions - Stady stat - On-dinsional flow - Ignor PE changs - Insulatd Q 0 - Air bhavs as an idal gas Basic Equations d 1 dt i i air 4

5 Probl (continud) de i Q W i hi gzi h gz dt i 1 W air h1h ds Q s i is airs s dt T b i 1 Solution Stat 1: P 1 = 1.4 bar, T 1 = 17C = 90 K 0 Using Tabl A-: h , s K Stat : P = 4. bar, T = 147C = 40 K 0 Using Tabl A-: h 41.6, s K (a) Considring nrgy balanc, powr rquird for th rssor: W air h1h s 1000 W 69.4 kw ; W 0 powr input rquird to driv th rssor (b) Considring ntropy balanc, rat of ntropy gnration for th rssor: 0 0 P 8.14 R air s s1 air s s1 Rair ln ; R kol-k air 0.87 P1 M air K kol 0 0 P 4. bar air s s1air s s1 Rair ln ln P1 s 1.4 bar -K kw K Rat of xrgy dstruction for th rssor: kw E d, T K K E d,.7 kw 5

6 Probl (continud) W (c) Exrgtic ( nd rvrsibl law) fficincy of th rssor: II, W actual Rvrsibl powr input: W W E d, kw rvrsibl actual 5.7 kw II, II, 87.5% 69.4 kw 6

7 Probl (0 points) On a hot sur day in Las Vgas, th outsid air is at 45C and rlativ huidity 0%. For cofort rasons, th tpratur insid th casinos ust b aintaind at C. (a) Can vaporativ cooling b usd for cooling th outsid air to C? You ust show th procss and provid justification using th psychrotric chart. (10 points) Ys No Insufficint Inforation Evaporativ cooling occurs at constant wt bulb tpratur (approxiatly constant nthalpy) Using th psychrotric chart, th iniu tpratur to which th outsid air can b coold with vaporativ cooling procss is: T1, wb 5 C (b) Can cooling coils, circulating chilld rfrigrant, b usd for cooling th outsid air to C without condnsation? You ust show th procss and provid justification using th psychrotric chart. (10 points) Ys No Insufficint Inforation Cooling without condnsation occurs at constant absolut huidity Using th psychrotric chart, th iniu tpratur to which th outsid air can b coold with cooling procss without condnsation is: T 1,dp 17 C ; with condnsation vn lowr tpratur can b achivd 7

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