Please help me overall in this homework. I did number 1 but the others I do not get 

1. Read sections 16.2 ? 16.4 in the book.

Write the following equations for ? , in [kJ/s], using the variables at right.

A. Conduction heat transfer

B. Convection heat transfer

C. Radiation heat transfer A = area

h = heat transfer coefficient

? = emissivity

k = thermal conductivity

? = Stefan-Boltzmann constant

L = distance

T1, T2 = temperatures

?T = temperature difference 2. A wall of area A is made of 20-mm thick decorative panel (k=1.0 W/m°C) and 200-mm thick brick

(k=2.0 W/m°C). On a winter day, the inner room air temperature was maintained at 21°C, external

air temperature was -19°C, the convective heat transfer coefficient was 25 W/m2K on the inside and

100W/m2K on the outside, and the total heat loss was found to be 5.2 kW. Neglect radiation.

A.

B.

C.

D. Draw the thermal resistance circuit showing all thermal resistors between the inner and

outer temperature.

Write an equation for total thermal resistance. Which of the walls, brick or panel, will

have a greater temperature gradient?

Calculate the area A of the wall in m2.

Calculate the temperatures of the left, middle, and outer surfaces, assuming the wall

area A = 22m2 and other conditions similar to above. 3. Three friends spend an hour trick-or-treating outside on a windy 0°C Halloween night. The costumes:

A:

B:

C: Iron-man (2mm-thick, uninsulated, molded Aluminum)

Captain Jack Sparrow (2mm-thick felt fabric, k = 0.08 W/mK)

Giant banana (2mm-thick polyethylene plastic, (k = 0.45 W/mK) How many candy bars must each eat to recover the heat energy lost during the time spent outside?

Assumptions: neglect radiation heat transfer

a person can be modeled as a vertical cylinder 0.3m in diameter, 1.5m tall

costumes fully contact the skin (constant skin surface temp = 32°C)

candy bars contain 250 Calories. (1 Cal = 1 kcal = 4.1868 kJ)

convection heat transfer coefficient, h = 14W/m2K

Hint: use the thermal resistance method.

4. NASA?s New Horizons spacecraft flew by the planet Pluto this past summer. Suppose the outer

surface of the spacecraft has an emissivity of 0.8 and a solar absorptivity of 0.3. Consider the

following two conditions:

A.

B. At launch, solar radiation is incident on the craft at a rate of 1320 W/m2.

At its current position near Pluto, solar radiation is only 1.5 W/m2. Determine the surface temperature of the spacecraft in each of the two cases, when the radiation

emitted equals the solar energy absorbed.

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Sep 13, 2020

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