Post on 08-Oct-2020
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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