Conduction Conceptests

Contributions by:Janet deGrazia, Stephanie Bryant

Department of Chemical and Biological EngineeringUniversity of Colorado

Boulder, CO 80309-0424

If the thickness of a plane wall is doubled from x to 2x while maintaining the same temperatures on each side of the wall, the conductive heat transfer rate (q) ___________.

A. increases

B. decreases

C. stays the same

q

x

2x

A cone frustum is conducting heat at steady state with no generation. If the frustum is insulated as shown, the heat flux in is ______________ the heat flux out.

A. equal to

B. greater than

C. less than

heat flux in

heat flux out

Insulation

A block of wood and a block of steel are placed flush against each other on one side. If temperature 1 (T1) is

greater than temperature 2 (T2) during steady state,

what temperature profile would result?

A. B.

C. D.

T1

T2

T1

T2

T1

T2

T1

T2 Key:wood

steel

A block of glass and a block of steel are placed flush against each other on one side. If Temperature 1 (T1)

is greater than Temperature 2 (T2) during steady state,

what temperature profile would result?

A. B.

C. D.

T1

T2

T1

T2

T1

T2

T1

T2 Key:glass

steel

Air is cooling the top and bottom of an electronic chip uniformly generating heat. At steady state, what temperature profile will result within the chip?

A.

B.

C.

D. None of the above

XT

Airfl

ow

XT

XT

An electronic chip is uniformly generating heat. The top of the chip is being cooled by flowing air while the base is insulated. If the chip has a height of L, the maximum temperature is at height _______________.

A. 0

B. L

C. L/2

D. 2L

insulation

Air

0

L

A pipe carrying hot water experiences an external cross flow of cold air. Given steady state conditions and no heat generation, the heat flux in to the pipe wall is _______________ the heat flux out of the pipe wall.

A. equal to

B. greater than

C. less than

D. unrelated toHot

Water

Cold Air

Heat flux in

Heat flux out

A concrete wall is insulated on one side and exposed to ambient air at 25°C on the other. If the air is constant and there is no heat generation in the wall, after an extended period of time, which of the following shows the temperature distribution?

A. B.

C. D.

xy 25°C

Insulation

Concrete wall

A concrete wall is insulated on one side and initially at a temperature of 25°C. Given no generation and an ambient temperature of 100°C, the temperature distribution will look like ____________.

A. B.

C. D.

A concrete wall is insulated on one side and initially at a temperature of 25°C. Given no generation and an ambient temperature of 0°C, the temperature distribution will look like ____________.

A. B.

C. D.

Two windows are of equal overall depth, but one is single pane and the other a double pane with an air gap. If the interior temperature is greater than the exterior, the interior window surface temperature of the single pane will be __________ than for the double pane.

A. lower than

B. the same as

C. higher than

Single pane

Double pane

Interior

Exterior

Air gap

A composite wall consists of wood and steel. The bond between the wood and steel is not perfect, resulting in small air gaps at the interface. If temperature 1 (T1) is

greater than temperature 2 (T2), at steady state what

temperature distribution could result?

A. B. C.

D. E.

woodsteel

T1

T2

T1

T2

T1

T2

T1

T2

T1 T2

Fin tip

Fin base

A rectangular fin is dissipating heat, where the temperature of the base, Tbase, is greater than the

ambient temperature, Tsurroundings. The maximum heat

transfer from a fin occurs when the temperature at the tip of the fin is equal to ______________.

A. Tbase

B. ½ Tbase

C. T surroundings

D. ½ Tsurroundings

In designing an effective fin to remove heat, which statement is correct?

A. Thermal conductivity of the fin material should be as low as possible.

B. Ratio of cross-sectional area to perimeter of the fin should be as high as possible.

C. The length of the fin should be very long.

D. Fins should be used when convective heat transfer coefficients are high.

E. None of the above.

To approximate a fin as infinitely long, it must be at __________ of maximum heat transfer.

A. 100%

B. 75%

C. 99%

D. 90%

A nuclear fuel rod is uniformly generating heat inside a cylinder. If metal fins are added to the outer surface for cooling and the system is at steady state, which statement is correct?

A. Heat flux at the outer surface

will increase.

B. Temperature at the center of

the fuel will decrease

C. Heat transfer rate will increase

D. Both B and C are correct

E. None of the above

Gen

erat

ed H

eat Removed H

eat

Added fin

Two finned surfaces are geometrically identical. Surface 1 has a convective heat transfer coefficient (h1) twice that of Surface 2 (h2). Surface 1 is

__________ efficient and _________ effective than Surface 2.

A. more, more

B. more, less

C. less, more

D. less, less

E. equally, equally

h1 = 2 h2

Surface 1 Surface 2

A warm stainless steel plate of uniform temperature is suddenly exposed to a quiescent fluid at room temperature. If heat loss is due to free convection, what could the temperature distribution be after a short period of time?

A. B.

C. D.

A hot sphere is placed in a cold water bath. The temperature at the center of the sphere will be close to the temperature at the surface of the sphere when the material has a _____________________.

A. high thermal conductivity

B. high density

C. low heat capacity

D. large surface area to volume ratio

sphere

bath

Object A and object B are initially at the same temperature and suddenly placed in identical cooler environments. At the same instant in time, the two objects have the following temperature distributions. If the convective heat transfer coefficient is the same for both, object ___ has a higher Fourier number.

A. B. C. Not enough

information

Consider two solid copper spheres, 1 and 2. Sphere 1 has twice the diameter of sphere 2. If both spheres are initially at 0 °C and placed in quiescent air at 25 °C, which sphere will reach air temperature first?

A. Sphere 1

B. Sphere 2

C. Both spheres will reach temperature at the same time