Chapter 1 heat trasfer
-
Upload
mohammed-al-saleh -
Category
Documents
-
view
240 -
download
0
Transcript of Chapter 1 heat trasfer
-
7/28/2019 Chapter 1 heat trasfer
1/27
Chapter 1: Introduction and
Basic Concepts
-
7/28/2019 Chapter 1 heat trasfer
2/27
ObjectivesWhen you finish studying this chapter, you should be able to:
Understand how thermodynamics and heat transfer are related toeach other,
Distinguish thermal energy from other forms of energy, and heattransfer from other forms of energy transfer,
Perform general energy balances as well as surface energy
balances, Understand the basic mechanisms of heat transfer, which are
conduction, convection, and radiation, and Fourier's law of heatconduction, Newton's law of cooling, and the StefanBoltzmannlaw of radiation,
Identify the mechanisms of heat transfer that occur simultaneouslyin practice,
Develop an awareness of the cost associated with heat losses, and
Solve various heat transfer problems encountered in practice.
-
7/28/2019 Chapter 1 heat trasfer
3/27
Thermodynamics and Heat Transfer
The science ofthermodynamics deals with theamount of heat transferas a system undergoes aprocess from one equilibrium state to another,
and makes no reference to how long the processwill take.
The science ofheat transfer deals
with the determination of the rates
of energy that can be transferred
from one system to another as a
result of temperature difference.
-
7/28/2019 Chapter 1 heat trasfer
4/27
Thermodynamics deals with equilibrium statesand changes from one equilibrium state toanother. Heat transfer, on the other hand, deals
with systems that lack thermal equilibrium, andthus it is a nonequilibrium phenomenon.
Therefore, the study of heat transfer cannot bebased on the principles of thermodynamics alone.
However, the laws of thermodynamics lay theframework for the science of heat transfer.
Thermodynamics and Heat Transfer
-
7/28/2019 Chapter 1 heat trasfer
5/27
Heat Transfer
The basic requirement for heat transferis the presence
of a temperature difference.
The second law requires that heat
be transferred in the direction of
decreasing temperature. The temperature difference is the driving force for heat
transfer.
The rate ofheat transferin a certain direction depends
on the magnitude of the temperature gradient in thatdirection.
The largerthe temperaturegradient, the higherthe rateofheat transfer.
-
7/28/2019 Chapter 1 heat trasfer
6/27
Application Areas of Heat Transfer
-
7/28/2019 Chapter 1 heat trasfer
7/27
Heat and Other Forms of Energy
Energy can exist in numerous forms such as: thermal,
mechanical,
kinetic,
potential,
electrical,
magnetic,
chemical, and
nuclear.
Their sum constitutes the total energyE(oreon aunit mass basis) of a system.
The sum of all microscopic forms of energy is calledthe internal energyof a system.
-
7/28/2019 Chapter 1 heat trasfer
8/27
Internal energy may be viewed as the sum ofthe kinetic and potential energies of themolecules.
The kinetic energy of the molecules is calledsensible heat.
The internal energy associated with thephase of
a system is called latent heat.
The internal energy associated with the atomicbonds in a molecule is called chemical(or
bond) energy. The internal energy associated with thebonds
within the nucleus of the atom itself is callednuclear energy.
-
7/28/2019 Chapter 1 heat trasfer
9/27
Internal Energy and Enthalpy
In the analysis of systemsthat involve fluid flow,we frequently encounterthe combination of
properties u andPv.
The combination isdefined as enthalpy(h=u+Pv).
The termPv representsthe flow energy of thefluid (also called the flowwork).
-
7/28/2019 Chapter 1 heat trasfer
10/27
Specific Heats of Gases, Liquids, and
Solids Specific heatis defined as the energy required to
raise the temperature of a unit mass of a substance byone degree.
Two kinds of specific heats: specific heat at constant volumecv, and specific heat at constant pressurecp.
The specific heats of a substance, in general, dependon two independent properties such as temperature
and pressure. For an ideal gas, however, they depend on
temperature only.
-
7/28/2019 Chapter 1 heat trasfer
11/27
Specific Heats
At low pressures all real gases approach ideal gasbehavior, and therefore their specific heats depend ontemperature only.
A substance whose specific volume (or density) does
not change with pressure is called an incompressiblesubstance.
The constant-volume and constant-pressure specificheats are identical for incompressible
substances. The specific heats of incompressible
substances depend on temperature
only.
-
7/28/2019 Chapter 1 heat trasfer
12/27
Energy Transfer
Energy can be transferred to or from a given mass by two
mechanisms: heat transfer, and
work.
The amount of heat transferred during a process is denoted by Q.
The amount of heat transferred per unit time is called heattransfer rate, and is denoted by Q.
The total amount of heat transferQduring a time interval Dtcanbe determined from
The rate of heat transfer per unit area normal to the direction ofheat transfer is called heat flux, and the average heat flux isexpressed as
0
(J)
t
Q Qdt
D
2
(W/m )
Q
q A
(1-6)
(1-8)
-
7/28/2019 Chapter 1 heat trasfer
13/27
The First Law of Thermodynamics
The first law of thermodynamics states that energycan neither be created nor destroyed during a process;
it can only change forms.
The energy balancefor any system undergoing any
process can be expressed as (in the rate form)
Total energy
entering the
system
Total energy
leaving the
system
Change in the
total energy of
the system
- =
(W)in out systemE E dE dt
Rate of net energy transfer
by heat, work, and mass
Rate of change in internal
kinetic, potential, etc., energies
(1-9)
(1-11)
-
7/28/2019 Chapter 1 heat trasfer
14/27
In heat transfer problems it is convenient to write
a heat balanceand to treat the conversion ofnuclear, chemical, mechanical, and electrical
energies into thermal energy as heat generation.
The energy balance in that case can be expressedas
, (J)in out gen thermal systemQ Q E E D
Net heat
transfer
Change in
thermal energy
of the system
Heat
generation
(1-13)
-
7/28/2019 Chapter 1 heat trasfer
15/27
Energy Balance
Closed systems
Stationary closed
system, no work:
Steady-Flow Systems
For system with one inlet and
one exit:
When kinetic and potentialenergies are negligible, and
there is no work interaction
(J)vQ mc T D (kg/s)in out m m m
(kJ/s)pQ m h mc T D D
(1-15)
(1-18)
-
7/28/2019 Chapter 1 heat trasfer
16/27
Heat Transfer Mechanisms
Heat can be transferred in three basic modes: conduction,
convection,
radiation.
All modes of heat
transfer require the
existence of a temperature difference.
All modes are from the high-temperature
medium to a lower-temperature one.
-
7/28/2019 Chapter 1 heat trasfer
17/27
Conduction Conduction is the transfer of energy from the more
energetic particles of a substance to the adjacent lessenergetic ones as a result of interactions between the
particles.
Conduction can take place in solids,
liquids, or gases
In gases and liquids conduction is due to
the collisions and diffusion of the
molecules during their random motion.
In solids conduction is due to the
combination ofvibrations of the
molecules in a lattice and the energy
transport by free electrons.
-
7/28/2019 Chapter 1 heat trasfer
18/27
Conduction
1 2 (W)condT T T
Q kA kAx x
D
D D
Area Temperature differenceRate of heat conductionThickness
where the constant of proportionality kis the
thermal conductivityof the material.
In differential form
which is called Fouriers law of heat conduction.
(1-21)
(1-22)(W)conddT
Q kAdx
-
7/28/2019 Chapter 1 heat trasfer
19/27
Thermal Conductivity
The thermal conductivity of a material is a
measure of the ability of the material to conduct
heat.
High value for thermal conductivity
good heat conductor
Low valuepoor heat conductororinsulator.
-
7/28/2019 Chapter 1 heat trasfer
20/27
Thermal Conductivities of Materials
The thermal conductivities
ofgases such as air vary by
a factor of104 from those
ofpure metals such ascopper.
Pure crystals and metals
have the highest thermal
conductivities, and gasesand insulating materials the
lowest.
-
7/28/2019 Chapter 1 heat trasfer
21/27
Thermal Conductivities and
Temperature
The thermal conductivities
of materials vary with
temperature.
The temperaturedependence of thermal
conductivity causes
considerable complexity in
conduction analysis.
A material is normally
assumed to be isotropic
(i.e., uniform in alldirections .
-
7/28/2019 Chapter 1 heat trasfer
22/27
Thermal diffusivity
The thermal diffusivity represents how fast heat
diffuses through a material.
Appears in the transient heat conduction analysis.
A material that has a high thermal conductivity or a
low heat capacity will have a large thermal diffusivity. The larger the thermal diffusivity, the fasterthe
propagation ofheat into the medium.
2Heat conducted ( m s)Heat stored p
k
c
(1-23)
-
7/28/2019 Chapter 1 heat trasfer
23/27
Convection
Convection is the mode of energy transfer between a
solid surface and the adjacent liquid or gas that is in
motion. Convection is commonly classified into three sub-
modes:
Forced convection,
Natural (or free) convection,
Change of phase (liquid/vapor,
solid/liquid, etc.)
Convection =Conduction+Advection
(fluid motion)
-
7/28/2019 Chapter 1 heat trasfer
24/27
Convection
The rate ofconvection heat transferis expressed byNewtons law of coolingas
his the convection heat transfer coefficientinW/m2C.
h depends on variables such as the
surface geometry, the nature of fluid
motion, the properties of the fluid,
and the bulk fluid velocity.
( ) (W)conv s sQ hA T T (1-24)
-
7/28/2019 Chapter 1 heat trasfer
25/27
Radiation
Radiation is the energy emitted by matter in the form ofelectromagnetic waves(orphotons) as a result of thechanges in the electronic configurations of the atoms ormolecules.
Heat transfer by radiation does not require the presence ofan intervening medium.
In heat transfer studies we are interested in thermalradiation (radiation emitted by bodies because of their
temperature). Radiation is a volumetric phenomenon. However, radiation
is usually considered to be asurface phenomenon forsolids that are opaque to thermal radiation.
-
7/28/2019 Chapter 1 heat trasfer
26/27
The maximum rate of radiation that can be emitted from a
surface at a thermodynamic temperature Ts(in K or R) is givenby the StefanBoltzmann lawas
s=5.670X108 W/m2K4 is the StefanBoltzmann constant.
The idealized surface that emits radiation at this maximum rate
is called a blackbody.
The radiation emitted by all real surfaces is less than the
radiation emitted by a blackbody at the same temperature, and
is expressed as
eis the emissivityof the surface.
4
,max (W)emit s sQ A Ts
4
,max (W)
0 1
emit s sQ A Tes
e
Radiation - Emission
(1-25)
(1-26)
-
7/28/2019 Chapter 1 heat trasfer
27/27
Radiation - Absorption
The fraction of the
radiation energy incident
on a surface that is
absorbed by the surface istermed the absorptivity .
Both eand of a surface depend on the temperatureand the wavelength of the radiation.
0 1