Heat Transfer

Question 1

What are the three mechanisms of heat transfer

Answer 1

• Conduction through a solid, liquid or gaseous medium
• Convection in a fluid medium
• Radiation which does not require any medium

Question 2

What is the temperature of Kelvin

Answer 2

Celsius + 273.15

Question 3

What is the definition of heat transfer

Answer 3

The movement of thermal energy from one body to another

Question 4

When does heat transfer occur

Answer 4

When there is a difference in temperature between bodies

Question 5

What is convective heat transfer

Answer 5

The transfer of heat between two bodies by currents of moving gas or fluids

Question 6

Why is convection an efficient way of heat transfer

Answer 6

It maintains a steep temperature gradient between the body and surrounding air or water

Question 7

What is free convection

Answer 7

When air or water moves away from the heated body as the warm air or water rises and is replaced by a cooler parcel of air or water

Question 8

What is forced convection

Answer 8

When air or water is forcibly moved across the body surface (such as in wind or wind-generated water currents) and efficiently removes heat from the body

Question 9

What is a conductor

Answer 9

A material that allows thermal energy to be transmitted through it easily

Question 10

Why are metals the best conductors of heat

Answer 10

Because they have loosely held outer electrons surrounding the atoms called free electrons

Question 11

When does conduction happen

Answer 11

When materials or objects are in direct contact with each other

Question 12

How does conduction happen

Answer 12

Warm molecules vibrate faster than the cooler ones, the faster vibrating molecules collide with the slower molecules, this makes the cooler molecules vibrate more quickly, and the object gets warmer

Question 13

How does heat transfer in metals occur

Answer 13

When one end of a metal bar is heated, the atoms at the end vibrate more rapidly. These vibrating atoms collide with neighbouring atoms, transferring their energy to them

Question 14

How is heat in central heating systems transferred

Answer 14

To radiators by conduction, due to the hot water flowing against the metal. The boiler or heat pump produces heat, which is transferred to the radiators through the circulating hot water

Question 15

What is a method of heat transfer that requires no particles

Answer 15

Infrared radiation

Question 16

What is infrared radiation

Answer 16

A type of electromagnetic radiation that involves waves

Question 17

Why are insulating materials unable to permit significant heat transfer through them

Answer 17

Because of a lack of free electrons

Question 18

Why are insulators used in situations where heat transfer must be prevented

Answer 18

For thermal comfort, energy efficiency or safety reasons

Question 19

What is Fourier’s law

Answer 19

Fourier’s law of heat conduction states that the rate of heat transfer through a material is proportional to the temperature gradient and the area through which the heat flows

Question 20

What happens to the thermal conductivity of a material when the density increases

Answer 20

The thermal conductivity increases

Question 21

What happens to the thermal conductivity of a material when it has increasing moisture

Answer 21

The thermal conductivity increases

Question 22

What happens to the thermal conductivity as porosity increases

Answer 22

The thermal conductivity decreases

Question 23

What is the definition of thermal resistance

Answer 23

How much temperature difference is required to transfer a unit of heat (in Watts) through an area of 1m^2 of the material

Question 24

What is the emissivity of a material

Answer 24

The relative ability of the surface of a material to emit energy by radiation. Its the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature

Question 25

What type of materials have a higher emissivity

Answer 25

Emissivity is higher for unpolished/matt materials, and for dark materials as opposed to white materials. The more reflective a material is the lower its emissivity

Question 26

What is heat reflectivity

Answer 26

A materials ability to reflect radiant heat. Materials with high reflectivity reflects a large portion of the radiant heat that strikes them

Question 27

What is the relationship between reflectivity and emissivity

Answer 27

An inverse relationship, As one increases the other decreases

Question 28

What are they key factors that affect heat loss within a building

Answer 28

-The building envelope: walls, roof, floor, windows and doors -Ventilation -Internal heat sources (e.g. appliances, people)

Question 29

What are the main ways to reduce heat loss in a building

Answer 29

-Improve the insulation of the building envelope -Seal air leaks in the building envelope -Use energy efficient windows and doors -Consider passive solar design for the building - Optimise ventilation systems

Question 30

What are the main types of insulation materials used in insulating houses

Answer 30

-Mineral Wool (Rock Wool and Glass Wool): common choices due to their high insulation values and fire resistance - Foam insulation: can be rigid or sprayed - Cellulose insulation: made from recycled paper and cardboard, it is a sustainable and effective insulation material

Question 31

Why are modern doors and windows better at preventing heat loss

Answer 31

They are double or triple glazed, significantly reducing the heat losses and thus improving the energy efficiency of the building

Question 32

How do mechanical ventilation with heat recovery systems work

Answer 32

Fresh air is drawn into the home via a low energy heat recovery unit, located in the attic. The ventilation unit filters the incoming air to remove pollutants and insects. Once passed through the heat exchanger within the ventilation power unit, the warm, clean, fresh filtered air is distributed around the home through a series of ducts which are run to each habitable room

Question 33

What is a heat bridge in a building

Answer 33

A heat bridge is a weak point in a building envelope that allows heat to flow more easily between the inside and outside

Question 34

When can a heat bridge occur

Answer 34

When a material with higher thermal conductivity than the surrounding materials creates a path for heat transfer

Question 35

What is the definition of thermal conductance

Answer 35

The heat in watts that will be transferred through 1m^2 of the construction when there is a difference of 1K between the inside and outside air temperatures