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hw3 - ME 114 Homework Set 3 Fall 2008 Due'September 18 A...

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Unformatted text preview: ME 114 Homework Set 3 Fall 2008 Due'September 18 A thermopane window consists of two pieces of . glass 7 mm thick that enclose an air space 7 mm ;/ . p ‘ thick. The window separates room air at 20°C from f} V15 j 61) 6Q? : 94 CI _ {E ‘14) outside ambient air at -lO°C. The convection co- efficient associated with the inner (room-side) sur- face is 10 W/m2 ' K. (a) If the convection coefficient associated with the outer (ambient) air is 710 = 80 W/m2 - K, what is the heat loss through a Window that is 0.8 m long by 0.5 rn wide? Neglect radiation, and assume the air enclosed between the panes to be stagnant. E3] Compute and plot the effect of ha on the heat loss for 10 S 11,, S 100 W/rn2 - K. Repeat this calculation for a triple-pane construction in which a third pane and a second air space of equivalent thickness are added. Consider a composite wall that includes an 8-min- thiclc hardwood sidinc, 40—min by 130-mm hard- 5 {5 p . _ . . _ ° JAMS» g+a+zalig§4LZW wood studs on 0.65-ni centers with glass fiber in- sulation (paper faced, 28 leg/m3), and a 12-min layer of gypsum (vermiculite) wall board. Wood siding ' Stud Insulation Wall board 4O mm—vi i+— What is the thermal resistance associated with a wall that is 2.5 m high by 6.5 m wide (having 10 studs, each 2.5 in high)? ’1 lirethgine (1/; = 0.026 W/In - K) is used to insulate t e si ewa and the mp and bottom of a 0* lindrical ’1 ' ' ‘ ’ ‘ " ’ hot water tank. The insulation is 40 mm {hick and V15 " (99? Li q: / d 6U» is sandwiched between sheet metal of thin-wall J construction. The height and inside diameter of the tank are 2 In and 0.80 m, respectively, and the tank is in ambient air for which Tn =, 10°C and h = 10 W/m: ' K. If the hot water maintains the inner sur- face at 55°C and energy costs amount to $0.15/ltWh, what is the daily cost to maintain the water in stor- age? l . it? A .1 —Six identical power transistors with aluminum casing are attached on one side of a 1.2-cm—thick ZO-cm >< 30-c1n copper plate (it = 386 W/rn - ”C) by screws that exert an aver- age pressure of 10 MPa. The base area of each transistor is 9 cm2, and each transistor is placed at the center of a .lO-cm >< 10-cm section of the plate. The interface roughness is estimated to be about 1.4 ,um. All transistors are covered by a thick Plex- iglas layer, which is a poor conductor of heat, and thus all the heat generated at the junction of the transistor must be dissi- pated to the ambient at 23°C through the back surface of the Plexiglas .. = ‘ cover Lappu \ Transistor plate [.2 cm copper plate. The combined convection/radiation heat transfer coefficient at the back surface can be taken to be 30 W/rn2 . °C. If the case temperature of the transistor is not to exceed 75°C, determine the maximum power each transistor can dissipate safely, and the temperature jump at the case-plate interface. ’4 Mg: ATMCr’face : 0 J 35—3/le a? A 4-n1m-diameter and 10—crn-long aluminum (l: = 237 W/ni - °C) is attached to a surface. If the heat transfe coefficient is 12 W/m2 - “C, determine the percent error in rate of heat transfer from the fin when the infinitely long fin sumption is used. instead of the adiabatic fin tip assumptl i , Ma A alO-W power transistor is to be cooled by attaching it» to one of the commercially available heat sinks shown in la 3—6. Select a heat sink that will allow the case temnerature he tran51stor not to exceed 90°C in the ambient air at "30°C ...
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