Introduction of Heat Transfer

Prepared by: Nimesh GajjarGIT-MED

Difference between heat and temperature

Temperature is a measure of the amount of energy possessed by the molecules of a

substance. It manifests itself as a degree of hotness, and can be used to predict the

direction of heat transfer. The usual symbol for temperature is T. The scales for

measuring temperature in SI units are the Celsius and Kelvin temperature scales.

Heat, on the other hand, is energy in transit. Spontaneously, heat flows from a hotter

body to a colder one. The usual symbol for heat is Q. In the SI system, common units

for measuring heat are the Joule and calorie.

Thermodynamics tells us:

• how much heat is transferred (dQ)

• how much work is done (dW)

• final state of the system

Heat transfer tells us:

• how (with what modes) dQ is transferred

• at what rate dQ is transferred

• temperature distribution inside the body

Difference between thermodynamics and heat transfer

Definition of Heat Transfer

Heat transfer is energy in transit due to temperature difference . Whenever there

exists a temperature difference in a medium or between media, heat transfer

must occur. Basic requirement for heat transfer : presence of temperature

difference .

Note: There can be no net heat transfer between two mediums that are at the

same temperature. Heat flow occurs only in the direction of decreasing

temperature

The temperature difference is the driving force for heat transfer, just as the

voltage difference is the driving force for electric current flow and pressure

difference is the driving force for fluid flow.

Modes of Heat Transfer

Conduction: An energy transfer across a system boundary due to a temperature

difference by the mechanism of intermolecular interactions. Conduction needs matter

and does not require any bulk motion of matter.

Convection: An energy transfer across a system boundary due to a temperature

difference by the combined mechanisms of intermolecular interactions and bulk

transport. Convection needs fluid matter.

Radiation: Radiation heat transfer involves the transfer of heat by electromagnetic

radiation that arises due to the temperature of the body. Radiation does not need matter.

Thermal ConductivityThe heat transfer characteristics of a

solid material are measured by a

property called the thermal

conductivity (k) measured in W/m.K.

It is a measure of a substance’s ability

to transfer heat through a solid by

conduction. The thermal conductivity

of most liquids and solids varies with

temperature. For vapors, it depends

upon pressure.

Convective Heat Transfer CoefficientThe convective heat transfer coefficient (h), defines, in part, the heat transfer due to convection.

Common units used to measure the convective heat transfer coefficient are W/m2.K

Quantity Text Notation SI Unit English Unit heat Q Joule (J) Btu (heat transfer) heat rate q Watt (W) Btu/hr (heat transfer rate) (heat energy rate) (rate of heat flow) heat flux q” W/m2 Btu/hr-ft2 (heat rate per unit area) heat rate per unit length q’ W/m Btu/hr-ft volumetric heat generation q W/m3 Btu/hr-ft3

Heat Quantities

• When the handle of a spoon stirring a cup of hot chocolate gets hot, it’s because of conduction.

Conduction is the transfer of heat through a solid or from one solid to another.

• When the particles of a solid are heated they gain energy and vibrate more quickly. They bump into neighboring particles and transfer the energy to them.

• When a stove is hot, the particles in the element speed up. They bump against the particles in the pot so the pot particles gain energy and heat up!

Conduction

When you heat a metal strip at one end, the heat travels to the other end.

As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat. We call this? Conduction

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Conduction

HOT(lots of vibration)

COLD(not much vibration)

Heat travels along the rod

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Conduction is the process whereby heat is transferred directly through a material, any bulk motion of the

material playing no role in the transfer.Those materials that conduct heat well are called

thermal conductors, while those that conduct heat poorly are known as thermal insulators.

Most metals are excellent thermal conductors, while wood, glass, and most plastics are common thermal

insulators.The free electrons in metals are responsible for the

excellent thermal conductivity of metals.

Conduction

• The soup particles that are heated by

conduction have increased energy and

spread out. They rise and push the cooler

particles near the top toward the sides.

• These cold particles then sink to take the

place of the rising hot particles. When

these particles come in contact with the

bottom of the pot they heat up and the

cycle continues. Eventually all of the soup

is warm.

Convection

What happens to the particles in a liquid or a gas when you heat them?

The particles spread out and become less dense.

This effects fluid movement.

A liquid or gas.

Fluid movement

Cooler, more dense , fluids sink through warmer, less dense fluids.

In effect, warmer liquids and gases rise up.

Cooler liquids and gases sink.

Water movement

Hot water rises

Cooler water sinks

Convection current

Cools at the surface

Cold air sinks

Where is the freezer

compartment put in a fridge?

Freezer compartment

It is put at the top, because cool air

sinks, so it cools the food on the way

down.

It is warmer at the bottom, so this warmer air

rises and a convection

current is set up.

The third method of heat transfer

How does heat energy get from the Sun to the Earth?

There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection.

?RADIATION

Through the process of radiation, heat flows in the form of waves. Theradiation waves flow from hot objects and are absorbed by cooler objects. Thecooler objects heat up as they absorb the waves.

Radiation

Radiation travels in straight lines

True/False

Radiation can travel through a vacuum

True/False

Radiation requires particles to travel

True/False

Radiation travels at the speed of light

True/False

1. Which of the following is not a method of heat transfer?

A. RadiationB. InsulationC. Conduction

D. Convection

2. In which of the following are the particles closest together?

A. SolidB. LiquidC. Gas

D. Fluid

3. How does heat energy reach the Earth from the Sun?

A. RadiationB. ConductionC. Convection

D. Insulation

Why Do We Sweat?

Sweating is important for our body to stay

cool in hot weather.

When a liquid changes to a gas, energy is

absorbed from the liquid’s surroundings.

Heat removed from your body as your

perspiration evaporates, makes you cooler.

R. Shanthini 17 May 2010 25

Latent heat ConductionConvection

Radiation

Mechanisms of Heat Transfer

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