Lecture 17 - Heat Transfer

3 main types of heat transfer

• conduction

• convection

• radiation

what is conduction

• within solids, liquids and gases

• heat transfer via molecular collisions

formula for conduction

delta Q/ delta t = (kA(Th - Tc))/L

• delta Q = change in heat transferred

• delta t = change in time

• k = thermal conductivity

• T = temperature

• L = length (m)

small k =

good insulator

large k =

good conductor

how to further reduce heat loss from glass window?

use 2 panes of glass separated by an air gap, this will decrease heat loss more than just a thicker window

R =

and replaces

• thermal resistance

• k

1 Btu (British Thermal Unit) =

1.054 × 10³ J

the larger the R =

better the insulator

Convert Btu to SI units

1 Btu = 1.054 × 10³ J

therefore = (1.054 × 10³)(Btu’s calc’d) /60×60

= J/s

what is convection

• heat transferred via mass movement of molecules from one place to another, carrying heat with them

• only occurs for fluids = gases and liquids that move freely

natural convection

caused by changes in density of the heated fluid near the object

• warm air is less dense and therefore rises

• cold air is more dense and therefore sinks

ocean example of convection currents

• at night time, water is warmed than the land and therefore land breeze occurs as the cool air from the land moves towards the sea

• opposite during the day (sea breeze) → cool air from the sea moves towards the land

what is forced convection

• produced by forcing a fluid past the hot object

• eg. car radiator/cooling system = air con and fans

• cool water is forced around the hot engine to remove heat

what is radiation

• no medium required to transmit waves

• all objects above 0K (absolute 0) radiate and absorb energy as electromagnetic radiation

Stefan-Boltzmann equation =

delta Q/delta t = eσAT⁴

• e = emissivity of the surface

• σ = Stefan Boltzmann Constant = 5.67 × 10⁻⁸ W/m²K⁴

• A = surface area

• T = absolute temperature in Kelvin

What is emissivity

a measure of a materials radiating efficiency

• e = energy radiated/energy radiated by perfect emitter

e = 0

very poor radiator

e = 1

perfect radiator
AKA black body

bodies emit and absorb…

radiation simultaneously

delta Q/delta t = eσA(T1⁴ - T2⁴)

net flow rate of heat radiation

• if T1>T2, body cools

• if T1<T2, body heats up