MEN 310 Final with Solution, Spring 2001

Notre Dame University Faculty of Engineering Mechanical Engincering Dep. Seo . 4 ge FINAL — MEN310 (Heat Transfer) ee an June 25, 2001 ” 1 An incandescent light bulb is-an inexpensive but highly inefficient device that converts . _ - “ =) electrical energy into light. It converts about 10% of the electrical energy it consumes” into light while converting the remaining.90% into heat. The glass bulb of a lamp heats up very quickly as a result of absorbing all that heat and dissipating ito the surroundings ° by convection and radiation. Consider a 60-W light bulb of 8-cm diameter (approximated by asphere) ina room at 25°C. Determine the equilibrium temperature of the glass bulb. Assume the interior surface of the room to be at room temperature. The emissivity of the glass is 0.9. es: A 2-m-inner-diameter double-walled staintess- steel spherical tank (fig.P2) is used to store iced water_at 0°C, Each wall is 0.5-cm-thick, and the \5-cm-thick air space between the two walls of the tank is evacuated in order to minimize heat transfer. The surfaces surrounding the evacuated space - are polished so that each surface has an emissivity of 0.15. The tank is placed in an ambient at 2C. Assuming that the heat transfer coefficient on the iced water side is very. large so that the inner wall temperature is at O°C, determine the rate of heat transfer to the iced water in the tank. By how much did the evacuation of the air space improve the insulation of the tank? Fig. P2 Spherical Tack. . & cross-flow heat exchanger consists of 40 thin walled copper tubes of 1-cm diameter located, as a single row across the flow, in a duct of Imxim cross-section. There are no ..fins attached to the tubes. Cold water enters the tubes at 18°C with an average velocity of 1 ms, while hot air enters the channel at 130°C and an average velocity of 1 mvs. If the overall eat transfer coefficient. is 80 W/m’°C, determine the exit temperatures ofboth . fluids and the rate of heat transfer, What can be said about such a desiga of heat exchanger-and how may the things be improved? Points: 1 (30%), 2 (50%), and 3 (20%). PS. Free convection in a spherical annulus can be described by Nu, =0.228Rae™* = for :10° < Rag <10” where 8 is the annulus thickness. A / | ! | —_ —-—— ps { ~ ay . ~~ C09 cle g om. oma : : Too = ASC Us Toor 2ST. i \ foun _ ; t . Ae a By Unaciictin iE, T (To -T) Eu, me 2 = Eb, Ooi Ae SAA " i-%, , y umwtdion ~ he A (Tr ~Tee) =A, af FOF = Tt caeition Fas! — Cp = Tus Veo) dL? - Grp gh Ce) L, B= ~e L Sr Re 1680994.01 Pa 18.0222 «107 wr foec Vy Pee 0.40254 L , Nap = a+ 0-5 (Ge e) RK. 9.O799734 Woe v4 : why ie BR frso.s (oP) |. 6.9944 i ra cL . - 34 4 08) {hv-4 _— Gox0.9-= ut (0 .0%).«9 9x54 xl0 (7-25 )+ G.99F14 x ATCO 03} (6 — Two WAS. 4tPC. . TEL.: 950415 - TELEX: 20049 AQUALI LE - BP. 80106 -8EYROUTH - LIBAN 3 ‘Te = AS.44 4.25 _ 49.285° - 345.235 K . a ~ aS KX. - \ -_ ° 345 1 oy Dz QOL ATPHLOM — P.. 0.6981 VG ly Nag = Aaod (Gr?) — Qe q. o84 | Whe °c tawaath — 6040.5 — UT (0-08) +. -- 2. Dome A > \Tos = 103.24 € | We cad “Gio JOA —aaao iF Note ret EW 3/0 7 s it Votre Ret, . ate pene as I ig Sphsacesl @ Teo 2 AS Hy. WY CONS Coin, = 3 ° Qe L.00S em Be, 4 TsiOC. tee LOR — Rey = & hy AC Ce (oas~ reutien t wee fk Ts ac Tes 250 OA 0A eine tlt R |. a4 ) S_S& Ly 09323 lemeeainaes Cbg AA > 2 / att x{¢~> : ark lor 4 —% 4 _ oy RET GSR) = AGOGO Ga | RB 1 ‘ ay 7 “ha” Ja}: Lee (h-ypaye ENE Ea Re z= +G-') (4) = 4645596.61 urtnes Rae E_ + E, (%) ee Pe 0.30996. —» GrPr. 3999T5C228 [7489 1999 WY, 6 Qa o.589 QM hela we [is (0-869 722) M0] 5 - W-ty = 234. F6( Walls > 4: ne \q= 234 wi | _ ty . Ras | > 4: Wet — [Ria ges |> Ty = 284-995 iC Rage 0, 8654 fr ec — Beat Oo + iT Leaer) Taz A¥3.00¢ 2 143.2099 — 9: ao —| a= 0-00 439°C | a= AB.oo¥fiK. _, y : ee TEL: 950415 - TELEX: 20049 AQUALI LE - B.P 80106 -BEYROUTH- LIBAN Reem CLEA [ We | (S425 25 — AIT K 2 of Ly Pe 1S. 2082 x10 wife fe. 0.0258033 Ysc, P.. 0.40954. Gor Pe. COED ORIR GIS © S2EE20 L, Tae 0-90 4276c. a 2T3 2o47Pie va Ly» wreck = 9. O8SIND / 4 a b 4 > ll. 20F Git fogs . 2-3 . ‘ Syl en tnat i vee encemntered a unt dre. — Ge cceuso os Tye Ly Pe 13-38 Wxto® 5 Reo. OMT | Pe. 0. HINGE ~» Geie= 7 Nag z BS - en 0.229 Rag q =k, = 0.028% +» — F h b Reqs Ly pan | i O° = qe =16 99 234 |g heesh ‘ 4st = 280 Se re / < / : sete 28,2 a], vac o~ host oty fea . 130 >| a 3] . osalie (See. AN Co, ATa = My Cy ATu. A\Si ue % l= faavc | - “yabes) + PVA C. BTas thy Cu BTa= BV AL C0 OTe. (Uery “pf > ~ aus = Oe 26x ( xO 99 x laly x 80 TW, s 20.028 °C WAPx lx Co xx 0.cos% 48 —_ Cwane anca B86. Voie - [30 -leo = O.264H5 OV ney 130 — 1% . . - Ra — Maing Peouns loH> , @agxaey Pe Oe Fal ac Peake - d : EAI owe NTU pt C2026 4 Co OR. 9 0646°5N- 0-45 ara . Cone Un ewe Ly Tu ce AU > he 443m. TEL.: 950415 - TELEX: 20049 AQUALI LE-B.P. 80106 -BEYROUTH - LIBAN | Nie aeeddy h te eee ee” et. 25, 00 | Qe DAAT.. = me Ata = we Si, . bn ( Th “el 30-1 :) ue —» BSS (120 —Tu.) - 1310S (th, - 8) — “os Cc, = izloS 99 (80 -Th,) + Bie Sx: 5 ya act Cee] =O we ete (eo) So \ . — . | _ - J _\iosx 18 _ &8C(i3o-tmh vohoar h _\e - Ty x (Slo {2i0y = 13991, ~ aslote er { BloS = | 1. 069Th, - 26.489 Gowns | L068 T= 26.901 | _ gaa (ia0 Tn) 4o((! 068 Ty. AY | (Bie Th. 93-S310H43 °C Lo Tare 20.465 °C . 4 323i. $34 wolls |