7/23/2019 2014 August Heat Trans Qualifier_v2 FINAL

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Code Number :..............

HEAT TRANSFER QUALIFYING EXAM

August 2014

One book allowed

Answer all questions

All questions have equal weight

TIME: 3.0 hrs

Prepared by : Profs. A. Bnard and N. Wright

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Question #1)

Scientists have recently found that hairs on elephants actually help these large mammals loose heat instead of

providing insulation. You are asked to assess the rate of additional heat loss through the hairs by answering thequestions below. The density of elephant hairs on the skin is about 1500 hairs/m2and their skin total surface

area is roughly 85 m2. The diameter of elephant hair is 0.5 mm and their conductivity is about 0.5 W/mK

Assume that the hairs are very long, cylindrical and that a light breeze provides a heat transfer coefficient of 50W/m2K for one hair. The elephant skin temperature is at 35 C and the environment is at 27 C.

a) What is the rate of heat transfer through one elephant hair?

b)

How much heat is lost through the hairs for one elephant? What is the fin effectiveness of one elephanthair? If the heat transfer coefficient on the elephant skin is 10 W/m 2K (not the hairs), what is the

percentage increase in the rate of heat loss?

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Question #2)

Wood composite posts or lumber are produced by mixing wood, PVC, and talc powder and extruding the

lumber through a die. The extrudate is cooled by convection by passing the cylindrical post through a waterbath attached to the die. Neglecting longitudinal conduction (in the long direction of the lumber), find the time

it will take for the lumber to have a core temperature that drops from 200 C to 60 C (temperature at which the

lumber has sufficient rigidity to be handled). Use the diagram below to obtain the relevant dimensions for thecross section. The properties of the composite mixture are: 10 W/mKk = , 3800 kg/m = , 900 J/kgK

pc = .

The heat transfer coefficient needs to be estimated if the water is at 10oC. If the post is extruded at 1 m/s, how

long should the water bath be? Assume that water flows perpendicular to the composite post.

15 cm

Composite

post

Water

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Question #3)

Water enters a pipe at 5 C. The volumetric flow rate of the water is 2 gpm (gallons per minute). The inside

diameter of the pipe is 2 cm and outside diameter is 2.5 cm, with = 2W m-1K-1. The outside surface of

the pipe is exposed to air at = 15C with a convective heat transfer coefficient of 5 W m-2K-1. The

distant surrounding walls are also at -15 C. Assume an emissivity for the pipe surface of = 0.2.The

density of water is 62.4 lb ft-3

and 1 g = 0.13368 ft3

. Calculate the temperature of the water after it hasflowed 10 m in the pipe.

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Question #4)

1. Consider two concentric spheres. The inner sphere has diameter = 10cm and the outer one has

diameter = 20cm.a. Find all of the view factors for the outer surface of the inner sphere and the inner surface of the

outer sphere.b. If the surfaces are at = 400 K and = 300K and the surfaces have emissivities of = 0.3,calculate the net radiation heat transfer.

c. If a radiation shield, with = 0.05on each surface, is placed between the two spheres (= 15cm), calculate the net radiation heat transfer if the conditions are otherwise the same as in part(b).

d. For the conditions given in part (c), calculate the temperature of the radiation shield.