Download - Intro Ht
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Chapter 1 Introduction to Heat Transfer
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Introduction
Heat is the form of energy that can be transferred from one system to another as a
result of temperature difference.
The science that deals with the determination of the rates of such energy
transfers is heat transfer.
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Cont
The science of thermodynamics deals with the amount of heat transfer as a system undergoes a process from one equilibrium state to another.
The science of heat transfer deals with the rate of heat transfer, which is the main quantity of interest in design and evaluation of heat transfer equipment.
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Application Areas of Heat Transfer
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Energy Transfer
The amount of heat transferred per unit during the process is denoted by Q. Unit : Joule (J)
The rate of heat transferred per unit time is is denoted by Unit: Joule per second (J/s) or Watt (W)
Q!
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Cont
The total amount of heat transfer Q during a time interval t can be determined from
The rate of heat transfer per unit area normal
to the direction of heat transfer is called heat flux.
=t
dtQQ0
!
)/(, 2mWAQq!
! =
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The First Law of Thermodynamics
Conservation of energy principle states that energy can neither be created nor destroyed; it can only change forms.
Total energy entering
the system
Total energy leaving
the system
Change in the total energy of
the system
_ =
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Energy balance for closed systems (fixed mass) Stationary closed system: Ein Eout = U = mCvT (J)
when the system involves heat transfer only and no work interactions across the boundary.
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Energy Balance for steady flow systems Mass flow rate and the volume flow rate of a fluid flowing in a pipe can be expressed as
When the changes in kinetic and potential energies are negligible, the energy balance for the system reduces to:
cVAm =! mVAV c!! ==
!Q = !mh = !mCpT(kJ / s)
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Heat Transfer Mechanisms
Heat can be transferred by: Conduction Convection Radiation All modes of heat transfer require the existence of a temperature difference. Heat transfer are from high temperature medium to a lower one.
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Basic theory of heat conduction
Transfer of energy from energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles.
Take place in solids, liquids, gases. In gases and liquids: due to collisions and
diffusions of the molecules during random motion.
In solids: due to combination of vibrations of the molecules and the energy transported by free electrons.
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Cont
The rate at which heat is conducted through a material is proportional to the area normal to the heat flow and to the temperature gradient along the heat flow path.
It depends on the geometry of the medium,
its thickness, and the material of the medium, as well as the temperature different across the medium.
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For a one dimensional, steady state heat flow the rate is expressed by Fouriers equation:
where: k = thermal conductivity, W/m-K = rate of heat flow, W A = contact area L = plane layer of thickness T = T2 - T1 = temperature difference
)(
)(
WLTkAQ
WdxdTkAQ
cond
cond
=
=
!
!
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Thermal conductivity, k Material ability to conduct heat. Unit: W/m-K or W/moC Definition: The rate of heat transfer through a
unit of thickness of the material per unit area per unit temperature difference.
It is a measure of how fast heat will flow in that material.
A large value of thermal conductivity indicates that the material is a good conductor, and a low value indicates the material is a poor heat conductor or insulator.
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Basic theory of heat convection
Made of energy transfer between a solid surface and the adjacent liquid or gas that is in motion.
There are two types of convection: 1. forced convection - fluid is forced to flow over the surface by
external mean, such as fan. 2. natural convection - caused by buoyancy forces that are
induced by density difference due to variation of temperature in the fluid.
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Example
Hot metal
Hot metal
Forced Convection Natural Convection
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The rate of convection heat transfer is expressed by Newtons Law of Cooling as
= hAs (Ts-T) (W)
h = heat transfer coefficient (W/m2.oC)
A = surface area thro which convection HT takes place
Ts = surface temperature
T = temperature of the fluid
convQ!
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Basic theory of heat radiation
Energy emitted by matter in the form of electromagnetic waves as a result of the changes in the electronic configurations of the atoms/molecules.
Does not require any medium. Energy transfer by radiation is the fastest (at
the speed of light) and it suffers no attenuation in a vacuum.
This is exactly how the energy from the sun reaches the earth.
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The maximum rate of radiation that can be emitted from a surface at an absolute temperature Ts is given by Stefan Boltzmann Law : = As Ts4 (W) = 5.67 x 10-8 W/m2.K4 = 0.1714 x 10-8 Btu/h.ft2.R4 = Stefan Boltzmann constant
max,emitQ!
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When a surface of emissivity and a surface A at an absolute temperature Ts is completely enclosed by a much larger (or black) surface at absolute temperature Tsurr separated by a gas (such as air), the rate of radiation HT: = As (Ts4 Tsurr4) (W)
= emissivity = 0 1 blackbody, = 1
radQ!
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