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Unformatted text preview: ﬂ/w wax 5 Final Exam ME 3345 Heat Transfer Fall 2008 Hesketh
FINAL EXAM TLME ALLOWED 150 Matures
Name: Grade: Question. 1 (20 pts): multiple choice (circle the correct answer): 1.1 Total Hemispherical Emissivity is equal to Total Hemispherical Absorptivity: A — Always B For diffuse gray surfaces C — for diffuse surfaces 1.2 Parallel flow and counter flow heat exchangers have the following
characteristics: A « T he same heat transfer coefﬁcient.
3 ., l’arallel ﬂow are more compact that counter flow.
' Counter flow are more compact that parallel flow. l3 rl‘he angle factor or View factor can only he applied to problems with: Diffuse surfaces malform irradiation Steadystate 1.4 In pipe ﬂow the thermal flow development length is largest for: 1.5 The thermal equivalent circuit can M be applied to problems with: ”A _ radiation
.3“ heat generation
C — transient analysis, Page 1 Final Exam ME 3345 Heat Transfer Fall 2008 Hesketh Question 2 (Meats):
Tsun = 5800K Solar Irradiation , .
Solar lrra diatlon 0.9 0.2 D=10m Consider the sollar radiation furnace shown above. The cylinder of diameter 10cm,
iength fire. has a spectral ahsorptivity shown and is a diffuse surface. Determine
the nett heat ﬂow to the surface given that the surface temperature is 500K and the
soiar radiation heat ﬂux is 15th W/mzx and the ambient air is at a temperature of
300K. Assume the hemispherical reﬂector has zero absorptivity and that aii of the
incident radiation is reﬂected onto the cylinder. Also assume that the convection
coefficient can he represented by a ﬂat plate with the same projected area° Assume
uniform irradiation and neglect convection from the ends of the cylinderg = 9.8m2/s Properties air: it = one W/rnzK, v = 45 x 10" mzls , a = 66 x to" mzls , ri m ass AT:?,¢S’J’IV‘0: 7—7?0C7 , $_A; 0‘7? 4— «Q‘ka/ '
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\ % Combustion chamber 0.1m i1 eometty shown heiiow are attached to a combustion 0 WHITE. Thetmai conductivity of At is
convection coefﬁcient is 1100 ‘W/nt2 K.
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200 W/mK and steel is St] W/mK. The externai
Caicnllate the internai temperature of the wail under steady radiation, assume no contact resistance.
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7 20mm TS 150mm $9 A vertical array of square printed circuit boards of 150mm height is air cooled such
that the maximum temperature on the boards does not exceed 60 C with an ambient
temperature of 25 C. Determine the aiiowabie snrt‘ace heat ﬂux from the electrical
components under the following conditions: (a) Free convection oniy (h) Downward air ﬂow at velocity of 0.6nn/s (c) Upward air ﬂow at velocity of 0.3nm’s and free convection. Properties air; it = one iii/mac, v = is x to" nizls 9 a = 25 x to6 inzls , Pr = mo
g=9.8rnfs2 (Assume steady state conditions and. negiect radiation) Final Exam ME 3345 Heat Transfer Fall 2008 Hesketh Question 5 (20 pts) A sheﬂntube heat exchanger with one shetl pass and 20 tube passes used hot water
on the tube side heat oil on the shell side. The singte copper tube has inner and
outer diameter of 20 and 24mm and. a length per pass of 31m. The water enters at 87
C and 0.2Kg.s and leaves at 27 C. Inlet and outtet temperatures of the oili are '7 and
37 C. What is the average convection coefﬁcient for the tube outer surface. Properties water: k = 0.65 W/mzK, u = s x 10'4 mzls 9 Cp = moo J/kg K, Pr = 3
Thermal conductivity of copper = 400 W/mK. Page 5 00/4 hcﬂt Zn‘ﬂéL 4340
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 Summer '08
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 Heat Transfer

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