ConvectionContributions by:

Janet deGrazia, Stephanie Bryant Department of Chemical and Biological Engineering

University of ColoradoBoulder, 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

C. the same as

Laminar Flow

Turbulent 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

θ

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)?

A. B.

C. D.

θ0 180

h

θ0 180

0 180θ

0 180

0 180

h

hh

θ

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.

A. greater than

B. less than

C. the same as Cross flow air

1 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.

A. increase

B. decrease

C. not change Aligned Staggered

Cross flow air

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.

A. longitudinal

B. transverse

C. either Cross flow fluid

Transverse

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?

A.

Cross flow fluidCross flow fluid

Both will have the same outlet temperature

B.

Not enough informationC. D.

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 D. I, II, III

E. All the statements are true

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?

A.

B. C. D.

Water

Ti

Ts

L

T

Ti

Ts

Tube T TT

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?

A.

B. C.

D.

Water

Ti L

Uniform heat flux

T

Ti

TubeT

Ti

T

Ti

T

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

C. decreases

Water

Ti

Ts

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

Ti

Ts

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

C. circular around the sphere

D. nowhere Dry Ice-79 ° C

Atmospheric air30 °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.

A. vertically

B. horizontally

C. either way

Insulated

Vertical

Horizontal

A. B. C. D.

E. A and B will have the same heat transfer rate

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?

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 informationvs.

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

B. increases, decreases

C. decreases, increases

D. increases, increases

E. stays the same, stays the same

Air

x

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 _______________.

A. 10 °C

B. 25 °C

C. 35 °C

D. 70 °C

70 °C, constant

25 °C, initiallyCold Air25°C at entrance