FQs and PQs Exam 1

Question 1

How do heat transfer and thermodynamics differ?
a). They are the same
b). Thermodynamics deals with the dynamics of thermal processes, while heat transfer deals with equilibrium states of thermal processes
c). Heat transfer deals with the rate at which thermal energy is transferred from hot to cold, while thermodynamics deals with the relationships between different forms of energy, usually at equilibrium

Answer 1

c). Heat transfer deals with the rate at which thermal energy is transferred from hot to cold, while thermodynamics deals with the relationships between different forms of energy, usually at equilibrium

Question 2

If the T-profile was curved (such as would be the case at non-steady-state, non-constant k, or with internal heat generation), which expression would be correct?
a). qy = kA(T1-T2)/L
b). qy = -kAdT/dy

Answer 2

b). qy = -kAdT/dy

Question 3

Which of the following statements is true?
a). Temperature, T, is a scalar, and therefore has no direction associated with it.
b). The heat flux, q”, is a vector, and therefore has a direction associated with it.
c). Both (a) and (b) are true.

Answer 3

c). Both (a) and (b) are true.

Question 4

If you hold your hand an inch or two above a hot burner, what is the primary mode of heat transfer?
a). Conduction
b). Convection
c). Radiation

Answer 4

c). Radiation

Question 5

For stirred soup inside a metal pan or a hot burner, what is the primary mode of heat transfer within the soup?
a). Conduction
b). Convection
c). Radiation

Answer 5

b). Convection

Question 6

At the interface between two solids, which of the following is always true?
a). The rate of heat loss, qx, is the same for the two solids.
b). The thermal conductivities of the two solids are the same.
c). Both (a) and (b) are always true.

Answer 6

a). The rate of heat loss, qx, is the same for the two solids.

Question 7

True or false? The analogy between the overall resistance for multilayer heat transfer and current flow holds for more than two layers and if one or more layers are characterized by a heat-transfer coefficient.

Answer 7

True

Question 8

If you wanted to make the walls of your house more thermally resistant to heat loss, which of these options might be a reasonable design choice?
a). Increase the width, L, of the insulation.
b). Increase the thermal conductivity, k, of the insulation (e.g., change the insulation type).
c). Wick more heat away from the house on the outside (higher h).
d). (a) and (c) are both reasonable design choices!

Answer 8

a). Increase the width, L, of the insulation.

Question 9

The radial temperature profile across the pipe wall of a hollow cylinder is:
a). Linear
b). Non-linear

Answer 9

b). Non-linear

Question 10

Which of the following statements is true for heat transfer through a spherical shell?
a). The heat-transfer rate, qr, is constant, but the heat flux, qr “, is not necessarily constant (instead, it is r^2 * qr” that is constant).
b). Neither the heat-transfer rate, qr, nor the heat flux, qr “, is constant

Answer 10

a). The heat-transfer rate, qr, is constant, but the heat flux, qr “, is not necessarily constant (instead, it is r^2 * qr” that is constant).

Question 11

True or false: another term for uniform heat generation is homogeneous heat generation, which means is the same no matter how thin we make our differential slice.

Answer 11

True

Question 12

For the case of 1D steady-state heat conduction through a flat wall with uniform heat generation, how does the temperature profile, depend on the heat generation, q-dot?
a). The effect of q-dot on T(x) monotonically depends on where you are in x.
b). The effect of q-dot on T(x) parabolically depends on where you are in x.

Answer 12

b). The effect of q-dot on T(x) parabolically depends on where you are in x.

Question 13

If the fin length is increased, the fin efficiency…
a). increases
b). decreases
c). stays the same

Answer 13

b). decreases

Question 14

Through which of the following materials does heat penetrate the most quickly?
a). Copper
b). Brick
c). Insulation

Answer 14

a). Copper

Question 15

Which of the following can occur in a vacuum?
a). Radiation
b). Conduction
c). Convection
d). (a) and (b), but not (c)

Answer 15

a). Radiation

Question 16

Consider multi-layer heat transfer in the perpendicular (x) direction through multiple walls of finite thickness. At steady state, which of the following is always true?
a). The heat-transfer rate, q, is the same at the interface between the walls.
b). The heat flux, q/A, is the same at the interface between the walls.
c). The gradient of temperature with respect to the x-dir, dT/dx, is the same at the interface.
d). Both a and b, but not c.

Answer 16

d). Both a and b, but not c.

Question 17

How do the Nusselt # and Biot # differ?
a). They are the same
b). The Nusselt number contains the fluid conductivity while the Biot number contains the solid conductivity
c). The Nusselt number contains the solid conductivity while the Biot number contains the fluid conductivity

Answer 17

b). The Nusselt number contains the fluid conductivity while the Biot number contains the solid conductivity

Question 18

Which mechanism gives the highest heat-transfer coefficient?
a). Forced convection by air
b). Free convection in air
c). Condensation of steam from stagnant air

Answer 18

c). Condensation of steam from stagnant air

Question 19

For an incompressible fluid with constant density, what can we say about d vx / dx + d vy / dy?

Answer 19

d vx / dx + d vy / dy = 0

Question 20

True or false: The thermal diffusivity can be thought of as a material property proportional to the rate at which incoming heat diffuses through a material.

Answer 20

True

Question 21

Do you expect Re to be larger or smaller for water?

Answer 21

Larger

Question 22

What fluids have Pr ≈ 1?
a). Highly viscous fluids such as maple syrup
b). Water at room temperature
c). Most gases at room temperature
d). Liquid metals, such as mercury

Answer 22

c). Most gases at room temperature

Question 23

Assuming that the velocities are the same, would you expect water or air, under the same temperature and pressure conditions, to have a higher value for Nu?

Answer 23

Water

Question 24

What is the effect of turbulence on the heat-transfer rate in a fluid?
a). Increase
b). Decrease
c). Stay the same

Answer 24

a). Increase

Question 25

Consider a flat plate with a fluid flowing over the top of it. If the plate and the fluid are at the same temperature, will a thermal boundary layer still form?

Answer 25

No

Question 26

Consider a flat plate with a fluid flowing over the top of it. If the plate and the fluid are at the same temperature, will a momentum boundary layer still form?

Answer 26

Yes

Question 27

Which of the following numbers indicates the relative ability of a fluid to diffuse momentum and internal energy by molecular means?
a). Nusselt
b). Biot
c). Reynolds
d). Prandtl

Answer 27

d). Prandtl

Question 28

In the summer in New Orleans, the average temperature (including days and night) is 84 °F. Air-
conditioning costs for a typical house kept at 68 °F might be $400 per month. Besides going to California
for summer vacation (which is what my wife’s family did), which action would reduce the monthly cooling
bill the most? Explain your reasoning.
a). Raising the thermostat setting to 72 °F
b). Doubling the 𝑅-value of the house insulation and windows
c). Moving to a smaller home with half the square footage of floor space

Answer 28

b). Doubling the 𝑅-value of the house insulation and windows

Question 29

For heat transfer via convection from an extended surface (e.g., fin), what is the appropriate area for the
local heat transfer?
a). A(x), the cross-sectional area of the extended surface
b). S(x), the surface area of the extended surface
c). P(x), the perimeter of the extended surface

Answer 29

b). S(x), the surface area of the extended surface