heat transfer
TRANSCRIPT
Dr. i
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Dr. i
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Heat vs. Temperature
Heat is the actual energy…
Temperature is the measure of average kinetic energy of particles in
a substance.
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Heat is NOT Temperature
Dr. i
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What is Heat transfer?
This is the movement of thermal energy from a substance at a higher
temperature to another at a lower temperature.
Dr. i
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Heat Transfer
Heat always moves from a warmer place to a cooler place.
Hot objects in a cooler room will cool to room temperature.
Cold objects in a warmer room will heat up to room temperature.
Dr. i
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Heat transferheat transfer in three method
Heat transfer
conduction
convection
radiation
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Heat Transfer Modes
Conduction
• transfer of heat due to random molecular or atomic motion within a material (aka diffusion)
• most important in solids
Convection
• transfer of heat between a solid surface and fluid due to combined mechanisms of a) diffusion at surface; b) bulk fluid flow within boundary layer
Radiation
• transfer of heat due to emission of electromagnetic waves, usually between surfaces separated by a gas or vacuum
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Conduction Conduction heat transfer is the flowing of heat energy from a high-temperature object to a lower-temperature object.
Dr. i
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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
Dr. i
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Metals are different
The outer electrons of metal atoms drift, and are free to move.
When the metal is heated, this ‘sea of electrons’ gain kinetics energy and transfer it throughout the metal.
Insulators, such as wood and plastic, do not have this ‘sea of electrons’ which is why they do not conduct heat as well as metals.
Dr. i
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Material Thermal conductivity k (W.m-1.K-1)
diamond 2450
Cu 385
Al 205
Brick 0.2
Glass 0.8
Body fat 0.2
Water 0.6
Wood 0.2
Styrofoam
0.01
Air 0.024
Thermal conductivity, k property of the material
kdiamond very high: perfect heat sink, e.g. for high power laser diodes
khuman low: core temp relatively constant (37 oC)
kair very low: good insulator * home insulation * woolen clothing * windows double glazing
Metals – good conductors: electrons transfer energy from hot to cold
Dr. i
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Why does metal feel colder than wood, if they are both at the same temperature?
Metal is a conductor, wood is an insulator. Metal conducts the heat away from your hands. Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.
Dr. i
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For conduction between two plane surfaces (eg heat loss through the wall of a house) the rate of heat transfer is
energy transferred
through slabQ
THTC
L
H CQ T Tk At L
dQ dTk Adt dx
Thermal conductivity k (W.m-1.K-1)
steady-state
heat current H = dQ/dt
Q QA
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Heat Transfer Modes - Conduction
Steady-state heat conduction through a plane wall:
LTTkq
LTT
dxdT
dxdTk
dxdTkq
x
x
2112 ,
constant thenconstant, if
constant
L
T1 T2
x
q (T1>T2)
k
Dr. i
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Heat Transfer Modes - Conduction
Example: What thickness of plate glass would yield the same heat flux as 3.5 of glass-fiber insulation with the same S-S temperature difference (T1-T2) ?
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ConvectionConvection takes place when heated
molecules move from one place to another, taking the heat with them. Convection is
common in both the atmosphere, as well as in the oceans.
Convection is the primary way that heat moves through gases and liquids.
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Convection
Dr. i
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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.What is a fluid?A liquid or gas.
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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.
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Water movement
Hot water rises
Cooler water sinks
Convection current
Cools at the surface
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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.
Dr. i
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http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html
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Sea & Land Breezes, Monsoons
http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html
35 oC 20 oC 17 oC11 oC
What is the role of heat capacity, c of water and soil?
Dr. i
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Heat Transfer Modes - Convection
Rate equation (Newton, 1700) is known as Newton’s law of “cooling”:
where q” = heat flux normal to surface q = heat rate from or to surface As
Ts = surface temperature T = freestream fluid temperature As = surface area exposed to fluid h = convection heat transfer coefficient (W/m2-K)
)(or )( TThAqTThq sss
Fluid flow, T AsTs (>T)
q
Dr. i
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Heat Transfer Modes - Convection
The convection heat transfer coefficient (h)
• is not a material property
• is a complicated function of the many parameters that influence convection such as fluid velocity, fluid properties, and surface geometry
• is often determined by experiment rather than theory
• will be given in most HW problems until we reach Chapter 6
Dr. i
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Heat Transfer Modes - Convection
Types of Convection
• Forced convection: flow caused by an external source such as a fan, pump, or atmospheric wind
• Free (or natural) convection: flow induced by buoyancy forces such as that from a heated plate
• Phase change convection: flow and latent heat exchange associated with boiling or condensation
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Radiation
Dr. i
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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
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RADIATION Energy transferred by electromagnetic waves
All materials radiate thermal energy in amounts determined by their temperature, where the energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum.
Thermal radiation wavelength ranges: IR ~ 100 - 0. 8 mVisible ~ 0.8 - 0.4 m 800 – 400 nmUV ~0.4 - 0.1 m
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Ludwig Boltzmann (1844-1906)
All objects above absolute zero emit radiant energy and the rate of emission increases and the peak wavelength decreases as the temperature of object increases
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Power absorbed by surface of an object
A, a
Qabs
4absabs s
dQP Aa Tdt
Ts
Absorption & Stefan-Boltzmann Law
Incident radiation (INTENSITY I - energy passing through a square metre every second Iinc = P / A Iabs = a Iinc
• Surface Area, A• Absorption coefficient, a = 0 to 1• Stefan-Boltzmann constant
σ = 5.67 x 10-8 W.m-2.K-4
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Power radiated from the surface of an object
A, e, T Qrad
4radrad
dQP Ae Tdt
net rad absPP P
Emission & Stefan-Boltzmann Law
Pnet > 0 net heat transfer out of system
• Surface Area, A• Emissivity, e = 0 to 1• Stefan-Boltzmann constant
σ = 5.67 x 10-8 W.m-2.K-4
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A blackbody absorbs all the radiation incident upon it and emits the max possible radiation at all wavelengths (e = a = 1)
A graybody is a surface that absorbs a certain proportion of the energy of a blackbody, the constant being constant over the entire band of wavelengths(0 e = a < 1)
emissivity e
absorption coefficient (absorptivity) a
Stefan-Boltzmann constant = 5.6710-8 W.m-2.K-4