HEAT TRANSFER

Instructor: M.T. Esfidani

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Heat transfer

• Heat transfer is the exchange of thermal energy between physical systems, depending on the temperature and pressure, by dissipating heat.

• The fundamental modes of heat transfer are:• conduction 

• convection • radiation.

• Heat transfer always occurs from a region of high temperature to another region of lower temperature.

 • Thermal equilibrium is reached when all involved bodies and

the surroundings reach the same temperature.

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Definitions Mechanisms of Heat (Thermal Energy) Transfer:

Conduction: transmission of heat across matter, due to direct physical contact, e.g. in solids, liquids and gases.

Radiation: heat transfer due to electromagnetic radiation across a space, even in a vacuum.

Convection: heat transfer by “currents” in a gas or liquid, due to temperature differences or forced flow, an important mechanism of energy transfer between a solid surface and a liquid or a gas.

(Source of illustrations: http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html)

All mechanisms may be involved in practical heat transfer, but the dominant mechanism differs in different cases. 3

Definitions

Heat CAPACITY (of a system): Quantity of ENERGY required to heat a whole system by 1 K (Kelvin). Unit: J·K-1

Concepts & Terminologies:

SPECIFIC HEAT (or SPECIFIC HEAT CAPACITY): Quantity of ENERGY required to heat a unit mass of a system by 1 K. Unit: J·kg-1·K-1

Kelvin = Degree Celsius (oC) + 273

J= joule4

Definitions

( note: 1W = 1 J s-1)

THERMAL CONDUCTIVITY, k : a measurement of heat flow through a body. It is the heat transmitted in unit time, in a direction normal to a surface of unit area, through a distance, d, across a unit temperature difference over the distance.

Unit: Wm-1K-1 (or Wm-1 °C-1) (error in handout “m-3” )

(analogous to electrical conductivity or hydraulic permeability)

T1

T2

Q A

d

Temperature profile

(Eq. 6a)(Eq. 6a)Q, heat flow per unit time (Js-1) × d, distance (m)

A, area (m2) × (T1-T2) temperature difference (K)

k =

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Definitions

Thermal conductivity of other common materials:

Material Thermal conductivity (298 K), W·m-1·K-1

Diamond 895-2300 ( the highest k)

Carbon Nanotubes

1400

Silver 429

Copper 386

Gold 317

Aluminium 237

Iron 80.2

BrickWood WoolStyrofoam(for building

insulation)

0.15 – 0.60.12 -0.040.04 (very low k)0.01

Why are diamonds so “cool”?

Why dose wool feel so “warm”?

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conversion

Thermal conductivity of other common materials:

Home Work: capacity of your heater and cooler?

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Phase change

Thermal conductivity of other common materials:

T-H Diagram

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