Homework1 - ME 114 Homework 1 Due 1 CD A 10-mm thick...

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Unformatted text preview: ME 114 Homework 1 Due January 31, 2008 1 CD A 10-mm thick Teflon sheet is perfectly bonded to a large 9-mrn thick. type 316 stainless steel sheet The interface is at 119 "C and the outer surface of thesteel sheet is at 120 ”C. In steady state. determine (a) The heat flux across the steel sheet. (13) The heat flux across the Teflon sheet. (a) The outer surface temperature of the Teflon sheet. FIGURE F131 @ The heat flux across slab A in the sketch is 60 Win11. What are the heat fluxes across slabs B. C. and D. TE: LE=2LA // ka=ka Tm 'Taa = Tm — Ts: TA: Ta: (b) L =L Tc: c A \ / ricc=ka ‘\. Tot 'Tcz z 2(TA1_TA2) Tm I‘D =1“ / kn=3kA T131- To: = TA] ' TA: (9) (d) Water enters a 2.5-cm ID. tube at a rate of 0.8 ltgfs at 10 EC and leaves at 30 “C. Determine (a) The average velocity of the water in the tube. (b) The heat transfer rate to the water. Water {18 [cg/s 10 clC 3 In Problem flag, at the location where the water temperature is 40 uC, the temper— anue of the tube surface is 90 “C, and the convective heat transfer coefficient is- 8000 Wins" “C. Determine the heat transfer rate per unit length of the tube at that location. Water as lag/5 —-—~— To determine the emissivity of the surface of a copper surface. a 4-cm diameter copper sphere with an embedded heating element was placed inside a large evac- ulated vacuum chamber. To maintain a surface temperature of 200 ”C, with the surroundings at 20 c'C, it was found that the power supply to the heating element was 1.2 W. Estimate the emissivity of the surface of the sphere. ‘ lulDI—l a; The roof of a house consists of a lS-cm—thick «$5 concrete slab (k = 2 With - “(3) that is 15 m wide and 20 m long. The emissivity of the outer surface of the roof is 0.9, and the convection heat transfer coefficient on that surface is estimated to he 15 \thn2 . “C. The inner surface of the roof is maintained at 15°C. On a clear winter night, the am- bient air is reported to be at 10°C while the night slq,r tempera- ture for radiation heat transfer is 255 K. Considering both radiation and convection heat transfer. determine the outer sur— face temperature and the rate of heat transfer through the roof. If the house is heated by a furnace burning natural gas with an eificienCy of 85 percent. and the unit cost of natural gas is $0.60/therm (1 therm = 105 .500 III of energy content)l determine the money lost through the roof that night during a 14-hour period. Fair Fri-’0‘ flow mag )MSI fill/a qutLr vat/ta} OLVISWUK‘. @ Consider a medium in which the heat conduction eq don is given in its simplest form as 63?" 3x3 Q" at =1 :1 (a) Is heat transfer steady or transient? (b) Is heat transfer one-. two-, or three-dimensional? (o) Is there heat generation in the medium? (:1) Is the thermal conductivity of the medium moment 01%4 variable? g ' Consider a medium in which the heat conduction equ-f tion is given in its simplest form as ‘ (a) Is heat transfer steady or transient? (b) Is heat transfer one-. two~ or three- dimensional? (o) Is there heat generation in the medium? (d) Is the thermal conductivity of the medium constant 0%” variable? - F.\ Consider a medium in which the heat conduction equa- @ fifinj's given in its simplest form as _1_ J“133“?" _1 6T 1' "dr 6r rt) Is heat transfer steady or transient? b) Is heat transfer one-, two-. or three-dimensional? c)-‘ Is there heat generation in the medium? ' d) Is the thermal conductivity of the medium constant or variable? ,-.\ Consider a medium in which the heat conduction equa~ 53611 is given in its simplest form as 9* (b) 15 heat transfer one-. two‘, or three-dimensional? :_ o) Is there heat generation in the medium? firm) Is the thermal conductivity of the medium constant or variable? ...
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