ME 4102
Name:_______________________________
Homework #4 (due October 4, 2010)
Homework Format
1.
Use this page as a cover sheet.
2.
One problem per page.
3.
Always state what is given, your assumptions, provide a sketch, and analysis.
Circle your answer.
Include units.
4.
Write legibly.
5.
No late homework accepted.
6.
Work independently.
7.
Use paper with four straight edges.
8.
Staple together your work.
1.
A type K ChromelAlumel thermocouple is quickly placed into a hot air flow to measure its temperature. In
particular, it is desired to know the response time of the thermocouple to attain 99% of the initial
temperature difference between the hot air and the initial temperature of the thermocouple junction. The
thermocouple junction can be modeled as an isolated sphere with a diameter of 0.6 mm and the properties:
k
solid
= 35 W/mK,
ρ
= 8500 kg/m
3
, and
c
= 320 J/kgK. The convection heat transfer coefficient,
h
, between
the flow and the sphere is 65 W/m
2
K.
i)
What is the Biot number associated with this problem?
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 Spring '11
 Heat, Heat Transfer, convection heat transfer, thermocouple junction, initial temperature difference

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