FQs and PQs Exam 2 Flashcards
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
b) go down
For Pr»_space; 1 (e.g., a polymer melt), how do the thermal and momentum boundary-layer thicknesses compare?
a) dT «_space;d
b) dT ~ d
c) dT»_space; d
a) dT «_space;d
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]
c) Pr[air] < Pr[water]
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
b) Clouds impede the heat transfer, decreasing daytime temperatures and increasing nighttime temperatures.
Do you think most bodies reflect in a specular or diffuse manner?
Diffuse
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
d) all of the above
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.
a) It’s really expensive.
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).
b) Because, for gray bodies only, ε and a are independent of the external environment (e.g., temperature and λ).
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
c) All of the above
What temperature should the properties be calculated at to solve for the radiative heat-transfer coefficient?
a) Ti
b) To
c) Tav
c) Tav
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
a) F12 = F21 = 1
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.
True
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
a) Wearing extra socks when it is cold
b) Double paned windows
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?
Sink – As T decreases, density increases
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?
Yes – h varies with thermal boundary layer thickness
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
b) The brick plate will cool faster
** copper is more dense
Which heat-transfer medium on the outside of a tube gives approximately uniform wall temperature?
a) Cold water
b) Hot air
c) Condensing steam
c) Condensing steam
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
e) b and c, but not a
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.
a) q, the rate of heat transfer from the hot fluid = the rate of heat transfer to the cold fluid
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.
b) We only need to consider the convective resistance in the hot fluid.
True or false: cocurrent heat exchangers are sometimes referred to as concurrent heat exchangers.
True
Which of the following is generally the more efficient heat exchanger design?
a) Cocurrent
b) Countercurrent
b) Countercurrent
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]
c) h[boil/cond] > h[forced] > h[free]
[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
c) 4 tube passes and 2 shell passes
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
d) All of the above
True or False: The NTU and LTMD methods are essentially equivalent, since they were derived from the same basic equations.
True
If the temperature is increased, the gas-phase diffusivity will
a) Increase
b) Decrease
c) Stay the same
a) Increase
If the pressure is increased, the gas-phase diffusivity will
a) Increase
b) Decrease
c) Stay the same
b) Decrease
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
a) Instantaneous reaction rate
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
a) Baking a casserole
c) Burning yourself on the stove
What are “real world” examples of transient cooling?
a) Cooking a steak
b) Quenching hot metal
c) Putting hot soup in the freezer
b) Quenching hot metal
c) Putting hot soup in the freezer
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).
b) Regime III, because high heat fluxes can be achieved at only moderate ∆𝑇 (𝑇𝑤 ― 𝑇𝑠𝑎𝑡).
(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.
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.
- What is the value of ∑𝐽𝑖, where 𝐽𝑖 is the one-dimensional diffusive flux of species 𝑖?
a) 0
b) 1
c) ∞
a) 0
