6 - Fundamentals of Convection

Question 1

Is the rate of heat transfer through a fluid higher through conduction or convection?

Answer 1

Convection due to fluid motion.

Question 2

What does convection heat transfer depend on?

Answer 2

Dynamic viscosity
Thermal conductivity
Density
Specific heat
Fluid velocity
Geometry of solid surface
Roughness of solid surface
Type of fluid flow

Question 3

Define convection heat transfer coefficient (h)

Answer 3

The rate of heat transfer between a solid surface and a fluid per unit surface area per unit temperature difference.

Question 4

What is a no-slip condition?

Answer 4

A fluid in direct contact with a solid ‘sticks’ to the surface due to viscous effects, and there is no slip.

Question 5

What is a boundary layer?

Answer 5

The flow region adjacent to the wall in which the viscous effects (and thus the velocity gradients) are significant.

Question 6

What is an implication of the no-slip condition?

Answer 6

Heat transfer from the solid surface to the fluid layer adjacent to the surface is by pure conduction since the fluid layer is motionless.

Question 7

What does the Nusselt number represent?

Answer 7

Dimensionless convection heat transfer coefficient.

Ratio of heat transfer by convection to heat transfer by conduction.

Question 8

What does a larger Nusselt number indicate?

Answer 8

More effective convection.

Question 9

What does a Nusselt number of 1 indicate?

Answer 9

Heat transfer by pure conduction.

Question 10

Define viscous flows

Answer 10

Flows in which the frictional effects are significant.

Question 11

Define inviscid flow regions

Answer 11

Regions (typically not close to solid surfaces) where viscous forces are negligibly small compared to inertial or pressure forces.

Question 12

Define external flow

Answer 12

The flow of an unbounded fluid over a surface such as a plate, a wire, or a pipe.

Question 13

Define internal flow

Answer 13

The flow in a pipe or duct if the fluid is completely bounded by solid surfaces.

Question 14

Define open-channel flow

Answer 14

Used for the flow of liquids in a duct if the duct is only partially filled with the liquid and there is a free surface.

Question 15

Define laminar flow

Answer 15

The highly ordered fluid motion characterised by smooth layers of fluid (typical of high-viscosity fluids at low velocities).

Question 16

Define turbulent flow

Answer 16

The highly disordered fluid motion that typically occurs at high velocities and is characterised by velocity fluctuations (typical of low-velocity fluids at high velocities).

Question 17

Define transitional flow

Answer 17

A flow that alternates between being laminar and turbulent.

Question 18

Define forced flow

Answer 18

A fluid is forced to flow over a surface or in a pipe by external means such as a pump or a fan.

Question 19

Define natural flow

Answer 19

Fluid motion is due to natural means such as the buoyancy effect, which manifests itself as the rise of warmer (and thus lighter) fluid and the fall of cooler (and thus denser) fluid.

Question 20

Define steady flow

Answer 20

Implies no change at a point with time.

Question 21

Define uniform flow

Answer 21

Implies no change with location over a specified region.

Question 22

Define periodic

Answer 22

Refers to the kind of unsteady flow in which the flow oscillates about a steady mean.

Question 23

What is a velocity boundary layer?

Answer 23

The region of the flow above the plate bounded by delta in which the effects of the viscous shearing forces caused by fluid viscosity are felt.

Question 24

How is the velocity boundary layer thickness (delta) typically defined?

Answer 24

The distance y from the surface at which u = 0.99V.

Question 25

What occurs in the boundary layer region?

Answer 25

The viscous effects and the velocity changes are significant.

Question 26

What occurs in the irrotational flow region?

Answer 26

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

Question 27

What is a thermal boundary layer?

Answer 27

The flow region over the surface in which the temperature variation in the direction normal to the surface is significant.
Develops when a fluid at a specified temperature flows over a surface that is at a different temperature.

Question 28

How is the thermal velocity boundary layer thickness (delta) typically defined?

Answer 28

The distance from the surface at which the temperature difference T - Ts = 0.99(Tinfinity - Ts).

Question 29

What does the Prandtl number represent?

Answer 29

The relative thickness of the velocity and thermal boundary layers.

Question 30

How does the Prandtl number differ between materials?

Answer 30

Pr &laquo_space;1 in liquid metals (i.e. heat diffuses very quickly relative to momentum).
P&raquo_space; 1 in oil (i.e. heat diffuses very slowly relative to momentum).

Question 31

How does the Prandtl number affect the thermal boundary layer?

Answer 31

Smaller Pr means thicker thermal boundary layer.

Larger Pr mean thinner thermal boundary layer.