Conduction

Question 1

What is conduction?

Answer 1

Conduction is the flow of heat through solids or stationary liquids.

Question 2

What is thermal conductivity?

Answer 2

Thermal conductivity is the rate at which a substance transfers thermal energy through a unit of thickness.

Question 3

What is the thermal resistance of a plane wall dependent on?

Answer 3

For a plane wall, the thermal resistance depends on the thickness (path length), thermal conductivity and the cross sectional area.

Question 4

What is the total thermal resistance for heat transfer through multiple layers?

Answer 4

For heat transfer through multiple layers, the sum of the individual thermal resistances is the total thermal resistance.

Question 5

What is the thermal resistance of a cylindrical wall dependent on?

Answer 5

For a cylindrical wall, the thermal resistance depends on the length of the cylinder, the thermal conductivity, and the inner and outer radiuses.

Question 6

What is a film?

Answer 6

A film, is a fluid wall formed from the contact between a moving fluid and a wall.

Question 7

What is the film coefficient?

Answer 7

Also known as the convective heat transfer coefficient. It is used because the film thickness is not easily determined.

Question 8

What is the thermal resistance of a film dependent on?

Answer 8

For a film, the thermal resistance depends on the surface area and the convective heat transfer coefficient.

Question 9

Fourier’s Law of Conduction

Answer 9

Is the equation for the steady state heat transfer by conduction, from a high temperature point to a lower temperature point.

Question 10

What is Fourier’s equation equal to?

Answer 10

Fourier’s law is equal to the temperature difference between points. Divided by the resistance through the type of wall, heat transfer is going through.

Question 11

What is the difference between q and Q?

Answer 11

Q is the heat transfer while q is the heat transfer per unit area.

Question 12

What is the procedure to find the temperature at an intermediate area?

Answer 12

if the heat transfer is known the procedure is to calculate the thermal resistance up to the point of unknown temperature and then to solve for the temperature difference.

Question 13

What is the logarithmic mean area?

Answer 13

The logarithmic mean area is used in place of the normal area when heat flow is through an area that is not constant.

Question 14

What is the critical insulation radius and how is it measured?

Answer 14

It is the addition of insulation to a bare pipe or wire and it is measured from the center of the pipe or wire.

Question 15

How does critical insulation radius work?

Answer 15

Only adding insulation past the critical thickness will decrease heat loss. Adding insulation up to the critical thickness may increase the heat loss.

Question 16

Lumped Capacitance Method

Answer 16

Approximates the transient heat flow. If the internal thermal resistance of a body is small in comparison to the external thermal resistance.

Question 17

How is heat flow effected by a transient condition?

Answer 17

The rate of heat transfer varies with time in a transient condition,

Question 18

What is the biot number and what are its limits?

Answer 18

It’s a comparison of the internal thermal resistance to the external resistance of a body. The biot number cannot be greater than 1.

Question 19

What happens if the biot number is less than 0.1?

Answer 19

The Lumped Capacitance Method can be used.

Question 20

What is the Lumped Capacitance Method used for?

Answer 20

Finding the instantaneous heat transfer at a particular moment and finding the temperature of a body as a function of time.

Question 21

How is the total heat transfer found in a transient condition?

Answer 21

The energy change can be determined from the starting and ending conditions, independent of the duration of time.

Question 22

What’s a fin?

Answer 22

Fins are extended surfaces or features that receive and move thermal energy by conduction along their lengths and widths prior to convective and radiative heat removal.

Question 23

What is the heat transfer through a fin dependent on?

Answer 23

The heat transfer through a fin is dependent on the temperature of the base and of the environment. The cross sectional area, perimeter and its length.

Question 24

What is the difference between an adiabatic and non adiabatic fin?

Answer 24

For an adiabatic fin the actual fin length is used. For a non adiabatic fin a corrected length is used.