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FUNDAMENTALS OF HEAT TRANSFER
By
Dr. P. KARTHIKEYAN
Faculty from Department of Automobile Engineering
DEPARTMENT OF MECHANICAL ENGINEERING
PSG COLLEGE OF TECHNOLOGY
COIMBATORE – 641 0004, INDIA.
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THERMODYNAMICS AND HEAT TRANSFER ?
Both are deals with the transfer of energy at thermal equilibrium
Thermodynamics deals with transfer of energy in terms of heat
and work transfer - Conservation of Energy
Heat Transfer: It is the form of energy that can be transferred
from one system to another as a result of temperature difference
Driving Force - Temperature difference
Modes of heat transfer (Mechanisms) and Rate of heat transfer
(Time taken)
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TRANSFER OF ENERGY ?
How does the energy move from a hotter to a colderobject?
Three mechanisms:
Conduction
Convection
Radiation
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METHODS OF HEAT TRANSFER
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CONDUCTION
Particles in a solid are always vibrating.
If the solid gets hotter, the particlesvibrate more.
Note: the particles don't swap places, ormove around they just vibrate more on the
spot.
Solids are better at conducting heat than ………………. and
…………. because their particles are closer together. If the particles
are too spaced out it makes it ……………… for the energy to pass
along.Having said that, some solids conduct heat more than others. Metals
are ……….. conductors. Non-metals are …………… conductors
(insulators).
Heat energy
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Conduction Con …
Conduction is the process whereby heat is transferred directly
through a material, any bulk motion of the material playing no rolein 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 excellentthermal conductivity of metals.
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Conduction of Heat Through a Material
Rate of heat transfer by conduction, Q/t through the length, L across
the cross-sectional area, A is given by the following equation, where
k is the thermal conductivity and ΔT is the temperature difference
between the two ends. Based on Fourier Law of Heat Conduction,
L
T kAQ
SI Unit of Thermal Conductivity: W/( m · K)
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Q T kAt d
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Isotropic and Anisotropic Materials
In some material, the thermal conductivity varies with direction ofheat flow. Isotropic materials have the same thermal conductivityin all directions. Materials which show different thermalconductivities in different directions are known as Anisotropic
materials. Examples for anisotropic materials are fiber-reinforced polymers and woods.
Thermal conductivity for many materials is approximately as alinear function of temperature for limited temperature ranges
k(T) = ko (1 + k T)
Q =1 2( )
avk A
T T L
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Substance Thermal Conductivity, k [J/(s · m · C°)]
Metals Aluminum 240
Brass 110
Copper 390
Iron 79
Lead 35
Silver 420
Steel (stainless) 14
Gases
Air 0.0256
Hydrogen (H2) 0.180
Nitrogen (N2) 0.0258
High conductivity
High conductivity
High conductivity
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CONVECTION
When a liquid is heated it becomes less
dense (lighter) so it rises, cold liquid
takes its place.
The same thing happens when air is
heated. The hot air rises and cold air takes its place.
This heat transfer can happen through liquidsand gases.
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CONVECTION
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Explains why breezes come from the ocean
in the day and from the land at night
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Convective Heat Transfer Evaluating processes where there is convective heat transferred
to/from a solid surface
External or Internal
Gas or liquid
Two Major Types
Forced Convection: Flow is driven by external force – Pump,
Blower and Fan etc.. (Viscous Effect)
Natural Convection: Flow is driven by density difference due to
temperature gradient in the fluid (Buoyancy Effect)
Convective Heat Transfer
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Types of Flow – Convection
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General Approach to Calculating “h”
Determine Natural or Forced Convection
Collect the appropriate physical data of the fluid:
Thermal conductivity, Viscosity, Prandtl Number
Calculate appropriate dimensionless numbers
Use proper correlations to determine Nu
Calculate “h” from Nu
Evaluating Physical Properties
Film Temperature - Average between Tsur and Tamb - Used for forcedconvection, and natural convection
Average Temperature - Average between the two surface temperature
of an enclosure
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Nusselt Number = Convective Heat Transfer / Conductive Heat Transfer
Reynolds Number = Inertia Force / Viscous Force
Prandtl Number = Viscous Effect / Diffusion Effect
Forced Convection
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Internal Flows
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Combined convection and conduction and overall
heat transfer coefficient
In many engineering systems, heat transfer takes place between two fluidsseparated by a wall and the combined heat transfer coefficient is known as overall
heat transfer coefficient.
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Combined heat transfer equation is written as
Q = UA (Th – Tc)
The overall heat transfer coefficient
U =1
1 1
h c
L
h k h
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The radiation heat transfer between two black bodies at
temperatures T1 and T2 is given as Q1-2 = A1 F1-2 (T14 – T24)
Where F1-2 is the shape or view or configuration factorwhich accounts for the fraction of the total radiationleaving gray surface 1 and reaching the gray surface 2.
For two grey bodies this factor is given by
F12 =
Where 1 and 2 are the emissivities of the two bodies ofsurface area A1 and A2 and F12b is the view factor oftwo similar black bodies.
As per the reciprocating theorem, A1F12 = A2F21
1
1 12 2 2
1
1 1 1
1 1b
A
F A
Shape Factor
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Properties of Radiation Thermal radiation is an important concept in
thermodynamics as it is partially responsible for heatexchange between objects, as warmer bodies radiate moreheat than colder ones.
(Other factors are convection and conduction.) Theinterplay of energy exchange is characterized by the
following equation: α + ρ + Ψ =1 Here, represents spectral absorption factor, spectral
reflection factor and spectral transmission factor. All theseelements depend also on the wavelength. The spectralabsorption factor is equal to the emissivity ; this relation isknown as Kirchhoff's law of thermal radiation.
http://en.wikipedia.org/wiki/Thermodynamicshttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Physical_bodyhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Heat_conductionhttp://en.wikipedia.org/wiki/Emissivityhttp://en.wikipedia.org/wiki/Kirchhoff's_law_of_thermal_radiationhttp://en.wikipedia.org/wiki/Kirchhoff's_law_of_thermal_radiationhttp://en.wikipedia.org/wiki/Emissivityhttp://en.wikipedia.org/wiki/Heat_conductionhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Physical_bodyhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Thermodynamics
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A warm or hot object emit (gives off) infrared radiation as
heat waves, which can be absorbed (taken in) by anotherobject, heating it up
The weird thing is that the surface colour of an object
makes a difference
Radiation
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Radiation
Energy carried by electromagnetic waves
Light, microwaves, radio waves, x-rays
Wavelength is related to vibration frequency
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Radiation
average frequency absolute temperature
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Black Body
A mater ial that is a good absorber, l ikelampblack, is also a good emitter, and a
mater ial that is a poor absorber, l ike
polished silver, is also a poor emitter.
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Absorption of Radiation
Matt dark surfaces are betterabsorbers of infrared radiation thanshiny light surfaces.
Which metal block will heat up quicker?
If you lived in a hot country, would you paint your
house a light colour or a dark colour?
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Emission of Radiation
Matt dark surfaces are betteremitters of infrared radiation thanshiny light surfaces.
Hot soup62ºC
Hot soup62ºC
Which hot soup cup will lose heat quicker?If you had a food home delivery business, would youdeliver the hot food in dark coloured containers orAluminium foil?
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Summer Clothing
Q: People are uncomfortable wearing dark clothes during the
summer. Why?
A: Dark clothes absorb a large fraction of the sun's radiation andthen reemit it in all directions. About one-half of the emitted
radiation is directed inward toward the body and creates the
sensation of warmth. Light-colored clothes, in contrast, are cooler
to wear, since they absorb and reemit relatively little of theincident radiation.
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A White Sifaka Lemur
To warm up in the morning, they turn their dark bellies toward
the sun.
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Why is the mother
shielding her cub?
Haven’t we seen
this before?
Ratio of the surface area
of a cub to its volume ismuch larger than for its
mother.
To cool food, we cut it into smaller pieces, why?
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Thermos Bottle
A thermos bottle minimizes energy transfer
due to convection, conduction, and radiation.
Stopper- minimize conduction.
Double-walled glass vessel with the space
between the walls is evacuated to minimize
energy losses due to conduction and
convection.
The silvered surfaces reflect most of the
radiant energy that would otherwise enter or
leave the liquid in the thermos.
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Halogen Cooktop
In a halogen cooktop, quartz-iodine lamps emit a large amount of
electromagnetic energy that is absorbed directly by a pot or pan.
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Highly reflective metal foil covering this satellite minimizes heat
transfer by radiation.
Metal foil
HEAT EXCHANGERS
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HEAT EXCHANGERS
Classification of heat exchangers
Heat exchanger is a device, whose primary purpose is the transfer of heatenergy between two fluids. Heat exchangers are classified into regenerators,open-type exchangers and closed-type exchangers or recuperators. Theregenerators are heat exchangers in which the hot and cold fluids flowalternately through the same space with as little physical mixing between thetwo streams as possible.
The Open-type heat exchangers are devices wherein physical mixing of the twofluid streams actually occurs. Hot and cold fluids enter open-type heatexchangers and leave as a single stream. The recuperator is a type in which thehot and cold fluid streams do not come into direct contact with each other butare separated by a tube wall or a surface.
Heat exchangers are also classified according to the relative directions of thetwo streams, parallel flow if the fluids flow in the same direction, counter flowif the fluids flow in opposite directions and cross flow if the two fluids flow atright angles to one another.
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Indirect contact parallel flow
Indirect contact counter flow
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Indirect contact cross flow
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Direct contact heat exchanger
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Overall heat transfer coefficients for heat exchangers
Types of heat exchanger U[W/(m2K)
Gas-to-gas
Water-to-air in finned tubes (water in tubes)
Water-to-oil
Water-to-gasoline or kerosene
Water-to-water
Feedwater heaters
Steam-to-air in finned tubes (steam in tubes)
Steam-to-light fuel oil
Steam-to-heavy fuel oil
Steam condenser
Freon condenser (water cooled)
Ammonia condenser (water cooled)
Alcohol condensers (water cooled)
10 – 40
30 – 60
100 – 350
300 – 1000
850 – 1700
1000 – 8500
30 – 300
200 – 400
50 – 200
1000 – 6000
300 – 1000
800 – 1400
250 – 700
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Temperature profiles in parallel flow heat exchanger
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Temperature profiles in counter flow heat exchanger
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Log mean temperature difference (LMTD)
The temperatures of hot and cold fluids in a heat exchanger aregenerally not constant and vary from entry to exit due totransfer of heat from the hotter to the colder fluid. Due to thevariation of temperature difference at various sections, the rateof heat flow vary along the exchanger even for a constantthermal resistance. Neglecting the heat loss to the surroundings
and potential and kinetic energy change, the general heattransfer equation for a heat exchanger is
Heat lost by the hot fluid (Qh) = Heat transfer rate in heat
exchanger = Heat gained by the cold fluid (Qc)
mh Cph (Thi- Tho) = UA LMTD = mc Cpc (Tco – Tci)
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