Objective 05: 3 Modes of Heat Transfer

Question 1

What are the 3 basic modes of heat transfer?

Answer 1

1. Convection
2. Conduction
3. Radiation

Question 2

Heat Transfer by Convection

Answer 2

The transfer of heat through a fluid by movement of heated particles of the fluid

Question 3

Why can Convection only take place in a fluid?

Answer 3

It requires movement of a fluid

Question 4

How are convection currents often caused?

Answer 4

Variations in density due to the temperature changes

Question 5

How does convection work in a tank with a heated bottom surface?

Answer 5

1. The portion of the fluid adjacent to the surface will be heated and expand, thereby decreasing in density
2. The heated fluid is less dense than the colder fluid above it and will rise to the top of the tank and spread out
3. The cooler, denser fluid will move in and fill up the space vacated by the hot liquid
4. This cooler fluid is then heated and displaced, thus establishing circulation

Question 6

Natural Convection

Answer 6

Movement of fluid without the use of mechanical devices

Question 7

Forced Convection

Answer 7

Movement of fluid with the use of mechanical devices such as pumps or fans

Question 8

Heat Transfer by Conduction

Answer 8

The transfer of heat from one part of a substance to another part of the same substance, or from one substance to another substance in direct physical contact, without significant movement of molecules

Question 9

What is the mechanism of conduction?

Answer 9

1. When the temperature of a substance is increased by the addition of heat, the additional energy causes molecular vibration to increase
2. As individual molecules are affected, each one will transfer some energy to adjacent molecules
3. The vibration of the adjacent molecules will then increase until all the molecules have the same energy and intensity of vibration

Question 10

What factors does the quantity of heat transferred by conduction directly vary with?

Answer 10

1. Surface Area
2. Temperature Difference
3. Time

Question 11

What factor does the quantity of heat transferred by conduction inversely vary with?

Answer 11

Thickness

Question 12

What is the constant used to calculate Heat Flow by Conductivity?

Answer 12

The thermal conductivity constant/coefficient of thermal conductivity (λ), measured in W/m⁰C

Question 13

What is measured by the Thermal Conductivity Constant (λ)?

Answer 13

The ability of a substance to conduct heat, which is directly related to the composition of the substance, e.g. number of air pockets

Question 14

Are metals generally good conductors?

Answer 14

Yes

Question 15

Is temperature drops across metal elements of an insulated pipe normally considered and why?

Answer 15

No, due to the relatively high coefficient of thermal conductivity

Question 16

Heat Transfer by Radiation

Answer 16

The transfer of heat energy from one body to another through space by rays of electromagnetic waves

Question 17

How does Radiation differ from Conduction and Convection?

Answer 17

Radiated heat transfer does not require a material medium

Question 18

Can radiative heat pass through a medium such as air or glass?

Answer 18

Yes

Question 19

What factors does the rate at which an object radiates or absorbs heat depend on?

Answer 19

1. Temperature

2. Surface

Question 20

What happens to the molecules of an object that radiates heat?

Answer 20

They lose energy and the object begins to cool

Question 21

What happens to the molecules of an object that absorbs radiation?

Answer 21

They increase in internal energy and temperature

Question 22

What is a general way to identify a good absorber of radiation?

Answer 22

An object whose surface is black and rough

Question 23

Why must a good absorber of radiation also be a good emitter?

Answer 23

Otherwise the object would absorb more energy than it emits and its energy and temperature would increase uncontrolled until it eventually melts or blows up

Question 24

How can on object subjected to radiation maintain a constant temperature and internal energy?

Answer 24

The object emits and absorbs radiation at the same rate

Question 25

Blackbody Radiator

Answer 25

An object that perfectly absorbs and emits all radiation

Question 26

What is Emissivity (ε) in the Stefan-Boltzmann law?

Answer 26

The ratio of heat emitted by the body to the heat emitted by a perfect black body for the same surface area, temperature, and time