FQs and PQs Exam 2

Question 1

If a hot, horizontal surface was turned to face down instead of up, would you expect the rate of heat loss from free convection to
a) go up
b) go down
c) not change

Answer 1

b) go down

Question 2

For Pr&raquo_space; 1 (e.g., a polymer melt), how do the thermal and momentum boundary-layer thicknesses compare?
a) dT &laquo_space;d
b) dT ~ d
c) dT&raquo_space; d

Answer 2

a) dT &laquo_space;d

Question 3

How do the Prandtl numbers for air and water compare at standard temperature and pressure?
a) Pr[air] > Pr[water]
b) Pr[air] = Pr[water]
c) Pr[air] < Pr[water]

Answer 3

c) Pr[air] < Pr[water]

Question 4

What role do clouds play in radiant heat transfer between the earth and space?
a) Clouds promote the heat transfer, increasing daytime temperatures and decreasing nighttime temperatures.
b) Clouds impede the heat transfer, decreasing daytime temperatures and increasing nighttime temperatures.
c) Clouds have little-to-no impact on radiant heat transfer between earth/space

Answer 4

b) Clouds impede the heat transfer, decreasing daytime temperatures and increasing nighttime temperatures.

Question 5

Do you think most bodies reflect in a specular or diffuse manner?

Answer 5

Diffuse

Question 6

Following from Kirchhoff’s Law, which of the following can be said to be true for gray bodies? Hint: remember that gray bodies are opaque bodies!
a) tau = 0
b) rho = 1 - ε
c) rho = 1 - a
d) all of the above

Answer 6

d) all of the above

Question 7

Which of the following is a downside of radiant floor heating?
a) It’s really expensive.
b) It leads to uneven air distribution.
c) It stirs up dust and allergens.

Answer 7

a) It’s really expensive.

Question 8

Why, for gray bodies, does Kirchhoff’s Law apply whether or not the bodies (or surfaces) are in thermal equilibrium?
a) Because gray bodies are always at steady state.
b) Because, for gray bodies only, ε and a are independent of the external environment (e.g., temperature and λ).

Answer 8

b) Because, for gray bodies only, ε and a are independent of the external environment (e.g., temperature and λ).

Question 9

Considering the heat-transfer areas of the two bodies are now not the same, what might the implications be?
a) All radiation leaving the larger body will not land upon (or be incident upon) the smaller body.
b) We should formulate our equations in terms of rates, rather than flux
c) All of the above

Answer 9

c) All of the above

Question 10

What temperature should the properties be calculated at to solve for the radiative heat-transfer coefficient?
a) Ti
b) To
c) Tav

Answer 10

c) Tav

Question 11

What are the respective view factors for two very large parallel plates labeled surface 1 and surface 2?
a) F12 = F21 = 1
b) Depends on whether one or both of the plates are considered gray or black bodies

Answer 11

a) F12 = F21 = 1

Question 12

True or false: in general, for a hot object in a room, heat loss will occur by both radiation (to walls) and to the air by convection/conduction through the fluid film.

Answer 12

True

Question 13

What are “real world” examples of insulation? (1 or more)
a) Wearing extra socks when it is cold
b) Double paned windows
c) Leaving your cooler open on a hot da

Answer 13

a) Wearing extra socks when it is cold
b) Double paned windows

Question 14

Consider a wall separating inside and outside. The inside temp is greater than the outside temp. (see 15b for diagram)
Does the inside inside air rise or sink?

Answer 14

Sink – As T decreases, density increases

Question 15

Consider a wall separating inside and outside. The inside temp is greater than the outside temp. (see 15b for diagram)
Does the heat-transfer coefficient vary along the wall height?

Answer 15

Yes – h varies with thermal boundary layer thickness

Question 16

Would you expect a hot copper plate or a hot brick plate of the same size to cool faster when placed in a bath of cold water?
a) The copper plate will cool faster
b) The brick plate will cool faster
c) The brick and copper plates will cool at the same rate

Answer 16

b) The brick plate will cool faster
** copper is more dense

Question 17

Which heat-transfer medium on the outside of a tube gives approximately uniform wall temperature?
a) Cold water
b) Hot air
c) Condensing steam

Answer 17

c) Condensing steam

Question 18

The heat-transfer coefficient for a fluid adjacent to a hot solid is a strong function of the solid surface temperature (besides the dependence of physical properties on temperature) for
a) Forced convection
b) Free convection
c) Radiation
d) a and b, but not c
e) b and c, but not a

Answer 18

e) b and c, but not a

Question 19

In this double-tube heat exchanger scenario, since we are at steady-state and there is no heat loss to the environment, which of the following statements is true
a) q, the rate of heat transfer from the hot fluid = the rate of heat transfer to the cold fluid
b) There is no heat transfer from the hot to the cold fluid.

Answer 19

a) q, the rate of heat transfer from the hot fluid = the rate of heat transfer to the cold fluid

Question 20

Problem Statement from Example 1 in 17b
Based on the problem statement, which of these resistances do we really need to consider in our calculation of U?
a) ALL three resistances are likely important and must be considered
b) We only need to consider the convective resistance in the hot fluid.
c) We only need to consider the two convective resistances and can neglect the conductive resistance through the tube wall.

Answer 20

b) We only need to consider the convective resistance in the hot fluid.

Question 21

True or false: cocurrent heat exchangers are sometimes referred to as concurrent heat exchangers.

Answer 21

True

Question 22

Which of the following is generally the more efficient heat exchanger design?
a) Cocurrent
b) Countercurrent

Answer 22

b) Countercurrent

Question 23

How do you expect the heat-transfer coefficients for boiling and condensation situations to compare to those for forced and free convection?
a) h[boil/cond] ~ h[forced] > h[free]
b) h[forced] > h[free] > h[boil/cond]
c) h[boil/cond] > h[forced] > h[free]

Answer 23

c) h[boil/cond] > h[forced] > h[free]

Question 24

[See 18b for image]
How many tube passes and how many shell passes does this shell-and-tube heat exchanger have?
a) 2 tube passes and 1 shell pass
b) 2 tube passes and 2 shell passes
c) 4 tube passes and 2 shell passes

Answer 24

c) 4 tube passes and 2 shell passes

Question 25

When should NTU used over the LMTD method?
a) Not all inlet/outlet temperatures known
b) A given heat exchanger is used under conditions other than for which it was designed
c) When Prof. Davis and Prof. Sprenger say in your HW questions (:
d) All of the above

Answer 25

d) All of the above

Question 26

True or False: The NTU and LTMD methods are essentially equivalent, since they were derived from the same basic equations.

Answer 26

True

Question 27

If the temperature is increased, the gas-phase diffusivity will
a) Increase
b) Decrease
c) Stay the same

Answer 27

a) Increase

Question 28

If the pressure is increased, the gas-phase diffusivity will
a) Increase
b) Decrease
c) Stay the same

Answer 28

b) Decrease

Question 29

In the final example of lecture 21b, which situation will result in the faster rate of silica film formation?
a) Instantaneous reaction rate
b) Fast but non-instantaneous reaction rate

Answer 29

a) Instantaneous reaction rate

Question 30

What are “real world” examples of transient heating?
a) Baking a casserole
b) Leaving the door open in the winter while the heater is on in the house
c) Burning yourself on the stove

Answer 30

a) Baking a casserole
c) Burning yourself on the stove

Question 31

What are “real world” examples of transient cooling?
a) Cooking a steak
b) Quenching hot metal
c) Putting hot soup in the freezer

Answer 31

b) Quenching hot metal
c) Putting hot soup in the freezer

Question 32

Which pool boiling regime is of the highest engineering importance?
a) Regime VI, because the highest heat fluxes can be achieved there.
b) Regime III, because high heat fluxes can be achieved at only moderate ∆𝑇 (𝑇𝑤 ― 𝑇𝑠𝑎𝑡).

Answer 32

b) Regime III, because high heat fluxes can be achieved at only moderate ∆𝑇 (𝑇𝑤 ― 𝑇𝑠𝑎𝑡).

Question 33

(18b) In a shell and tube heat exchanger, why does it make sense that the area, A0, is smaller when we don’t factor in the correction factor, F (or in other words, when F=1)?
a) Because F=1 implies fully countercurrent flow, which would be ideal and thus would require
less area to achieve the same heat transfer rate.
b) Because F=1 would imply fewer passes or tubes/pass in our heat exchanger, both of which
would decrease the overall area for heat transfer.

Answer 33

a) Because F=1 implies fully countercurrent flow, which would be ideal and thus would require
less area to achieve the same heat transfer rate.

Question 34

1. What is the value of ∑𝐽𝑖, where 𝐽𝑖 is the one-dimensional diffusive flux of species 𝑖?
   a) 0
   b) 1
   c) ∞

Answer 34

a) 0