8 - Internal Forced Convection

Question 1

What tube shape gives the most heat transfer for the least pressure drop, withstanding large pressure differences between the inside and outside without significant distortion?

Answer 1

Circular

Question 2

What is the irrotational (core) flow region?

Answer 2

The frictional effects are negligible and the velocity remains essentially constant in the radial direction.

Question 3

What is the boundary layer region?

Answer 3

The viscous effects and the velocity changes are significant.

Question 4

What is the hydrodynamic entrance region?

Answer 4

The region from the pipe inlet to the point at which the velocity profile is fully developed.

Question 5

What is the hydrodynamic entry length?

Answer 5

The length of the hydrodynamic entrance region.

Question 6

What is the hydrodynamically fully developed region?

Answer 6

The region beyond the entrance region in which the velocity profile is fully developed and remains unchanged.

Question 7

What is the bulk mean fluid temperature?

Answer 7

The arithmetic average of the mean temperatures at the inlet and the exit.

Question 8

What is the thermal entrance region?

Answer 8

The region of flow over which the thermal boundary layer develops and reaches the tube centre.

Question 9

What is the thermal entry length?

Answer 9

The length of the thermal entrance region.

Question 10

What is thermally developing flow?

Answer 10

Flow in the thermal entrance region, where the temperature profile develops.

Question 11

What is the thermally fully developed region?

Answer 11

The region beyond the thermal entrance region in which the dimensionless temperature profile remains unchanged.

Question 12

Define fully developed flow

Answer 12

The region in which the flow is both hydrodynamically and thermally developed.

Question 13

What are the friction and convection coefficients in the fully developed region of a tube?

Answer 13

Constant (i.e. do not vary with x).

Question 14

What are the properties and effect of the entrance region?

Answer 14

Properties are higher pressure drop, heat flux, Nusselt number and h values.
Effect is to increase average friction factor and heat transfer coefficient for the whole tube.

Question 15

When can the flow be assumed to be fully developed?

Answer 15

When x > 10D, as the Nusselt number reaches a constant value at a distance of less than 10 diameters.

Question 16

When is the constant surface temperature condition realised?

Answer 16

When a phase change process occurs at the outer surface of a tube.

Question 17

When is the constant surface heat flux condition realised?

Answer 17

The tube is subjected to radiation or electric resistance heating uniformly from all directions.

Question 18

What are the two suitable ways of expressing the change in average temperature?

Answer 18

Arithmetic mean temperature difference

Logarithmic mean temperature difference

Question 19

Assumption of the arithmetic mean temperature difference

Answer 19

Mean fluid temperature varies linearly along the tube.

Question 20

Define NTU

Answer 20

Number of transfer units.

A measure of the effectiveness of the heat transfer systems.

Question 21

What does a small value of NTU indicate?

Answer 21

More opportunities for heat transfer.

Question 22

What does an NTU greater than 5 indicate?

Answer 22

The fluid flowing in the tube will reach the surface temperature at the exit regardless of the inlet temperature.

Question 23

What is fully developed laminar flow in a circular tube subjected to constant heat flux dependent on?

Answer 23

Nusselt number (constant)
No dependence on Reynolds or Prandtl numbers

Question 24

When is the Nusselt number constant?

Answer 24

Laminar flow
Fully developed flow
Confined flow