Convection
Contributions by:
Janet deGrazia, Stephanie Bryant
Department of Chemical and Biological Engineering
University of Colorado
Boulder, CO 80309-0424
A hot plate is being cooled by flowing air. The heat
transfer coefficient for turbulent air flow
is______________ that for laminar air flow.
A. less than
B. greater than
Turbulent Flow
C. the same as
Laminar Flow
For cold liquid metal flowing over a hot flat plate in
laminar flow, the thermal boundary layer will be
____________ the hydrodynamic boundary layer.
A. larger than
B. less than
C. the same as
For cold liquid metal flowing over a hot flat plate in
laminar flow, the onset of fully-developed flow will take
____________ that of water.
A. longer than
B. shorter than
C. the same as
Hot water flows over a cold isothermal flat plate. If the
water temperature increases, the heat transfer
coefficient _________________ and the transition flow
length ____________. The water is incompressible.
A. increases, increases
B. decreases, decreases
C. stays the same, stays the same
D. increases, decreases
E. decreases, increases
A hot cylinder is cooled by turbulent cross flowing air.
Which graph could represent the convective heat
transfer coefficient (h) as a function of the angular
position around the cylinder (θ)?
h
h
A.
B.
0
C.
θ
180
h
0
D.
0
θ
0
180
θ
180
h
0
θ
180
θ
180
Flowing cold air cools an aligned 4 x 3 hot tube bank.
The air is laminar before entering the bank. The
convective heat transfer coefficient in row 3 will be
_____________ that in row 1.
1
A. greater than
B. less than
C. the same as
Cross flow air
2
3
Laminar cross flow air enters an aligned tube bank.
Converting the tube bank from an aligned arrangement
into a staggered arrangement would ______________
the heat transfer rate.
Cross flow air
A. increase
B. decrease
C. not change
Aligned
Staggered
Turbulent cold air flows over an aligned 3 x 3 hot tube
bank. Doubling the number of tubes in the _________
direction, while maintaining the same tube spacing,
would have the hottest outlet air temperature.
Longitudinal
B. transverse
C. either
Cross flow fluid
Transverse
A. longitudinal
Laminar cold air flows over two aligned hot tube banks
each with 8 tubes. One bank has 4 rows of 2 tubes
each. The other has 2 rows of 4 tubes each. Which
tube bank will have the greater outlet air temperature?
B.
A.
Cross flow fluid
Cross flow fluid
C. Both will have the same
outlet temperature
D. Not enough information
For incompressible flow within a cylinder, which statements
are true?
I. The mean velocity is best used to determine the
Reynold’s number.
II. When flow is fully developed, the velocity profile does
not change with distance along the tube.
III. Transition to turbulent flow has a lower Reynold’s
number than for external flow.
IV. Flow becomes fully developed faster for fluids with a
high Prandtl number.
A. I, II
B. I, III
C. I, II, IV
E. All the statements are true
D. I, II, III
Water flows through a tube of length L with an mean
inlet temperature (Ti). The tube is held at a uniform
surface temperature (Ts) greater than Ti. Which
temperature profile best represents the mean
temperature of the water as if flows through the tube?
T
Ts
Water
A. Ts
Ti
L
Ti
T
B.
T
Tube
C.
T
D.
Water enters a tube of length L with an mean inlet
temperature (Ti). The water is heated with a uniform
heat flux through the tube. Which temperature profile
best represents the mean temperature of the water as
if flows through the tube?
Uniform heat flux
T
Water
A.
Ti
L
Ti
T
B.
Ti
Tube
T
C.
Ti
T
D.
Ti
Water flows through a tube of length L with an mean
inlet temperature (Ti). The tube is held at a uniform
surface temperature (Ts) greater than Ti. Assuming
fully developed, laminar flow, the outlet temperature
(To) _______ when the mass flow rate is doubled.
A. stays the same
B. increases
Water
Ts
C. decreases
Ti
L
To
Water flows through a tube of length L with an mean
inlet temperature (Ti). The tube is held at a uniform
surface temperature (Ts) greater than Ti. Assuming
fully developed, laminar flow, the heat flow _______
when the mass flow rate is doubled.
A. stays the same
B. increases
C. decreases
Water
Ts
Ti
L
A spacecraft contains a freshly hard boiled egg. The
atmosphere inside the spacecraft is air at room
temperature and atmospheric pressure. While the
spacecraft is in space, the rate of cooling is
___________ that while the spacecraft is on earth.
A. greater than
B. less than
C. equal to
A suspended spherical block of dry ice at -79°C is
exposed to atmospheric air at 30°C. The air close to
the dry ice moves _________.
A. up
B. down
Atmospheric air
30 °C
C. circular around the sphere
D. nowhere
Dry Ice
-79 ° C
A thin square plate is in a room held at an ambient
temperature of 25 °C. One side of the plate is
insulated while the other is kept at a constant 100 °C.
Orienting the plate _________ will result in the most
free convection heat transfer.
Vertical
A. vertically
B. horizontally
Horizontal
C. either way
Insulated
A thin plate has one side insulated and the other is
kept at a constant temperature. The ambient
temperature is less than the temperature of the plate.
Which orientation would result in the highest heat
transfer rate?
A.
B.
C.
D.
E. A and B will have the same heat transfer rate
An array of fins and cold air are cooling an electronic
chip. If the spacing of the fins is barely widened, while
keeping the same individual fin geometry, the rate of
heat transfer will _________________.
A. increase
B. decrease
C. stay the same
D. Not enough information
vs.
An array of fins and cold air are cooling an electronic
chip. If the number of the fins is halved, while keeping
the same individual fin geometry and spacing, the rate
of heat transfer will _________________.
A. increase
B. decrease
C. stay the same
D. Not enough information
vs.
Which fluid would be better at removing heat from an
array of fins? Assume all the fluids to be at ambient
temperature and are quiescent.
A. Air
B. Ethylene glycol
C. Hydrogen
Laminar air flows over an isothermal plate. The local
heat transfer rate _________ as the position in the x
direction increases, because the thermal boundary
layer ___________.
A. decreases, decreases
Air
B. increases, decreases
C. decreases, increases
D. increases, increases
x
E. stays the same, stays the same
Engine oil flows through a uniformly heated pipe. The
entrance length for thermal effects will be _________
the entrance length for hydrodynamic effects.
A. longer than
B. shorter than
C. the same as
Steam flows through a uniformly heated pipe. The
entrance length for thermal effects will be _________
the entrance length for hydrodynamic effects. Consider
steam to have the properties of water vapor.
A. longer than
B. shorter than
C. About the same as
Cold air flows between two very long parallel plates.
The air enters at 25°C . The top plate is held at 70°C.
The bottom plate is, initially, 25°C and insulated on
one side. After some time, the bottom plate could
reach _______________.
70 °C, constant
A. 10 °C
B. 25 °C
C. 35 °C
D. 70 °C
25 °C, initially
Cold Air
25°C at entrance