CHAPTER 1 (BASICS OF HEAT TRANSFER) Flashcards
is the basic
science that deals with
the rate of transfer of
thermal energy.
Heat Transfer
Heat transfer plays a crucial role in
the _______of vehicles,
power plants,
refrigerators, electronic
devices, buildings,
bridges, etc.
design
a form of energy that can be transferred from one system to
another as a result of temperature difference.
Heat
- science that deals with the determination of the rates of
energy
Heat Transfer
The basic requirement for heat
transfer is the presence of
Temperature Difference
This is when the Heat was defined as the energy associated with the random motion
of atoms and molecules
middle of 19th century
theory proposed by the French chemist Antoine Lavoisier (1743–
1794) in 1789.
caloric theory
Caloric theory proposed by
French chemist Antoine Lavoisier
this theory states that heat is a fluid-like substance called the
caloric that is a massless, colorless, odorless, and tasteless substance that
can be poured from one body into another
caloric theory
In 1798, ___________ showed in his
papers that heat can be generated continuously through friction
Benjamin Thompson (Count Rumford) (1753–1814)
convinced the skeptics that heat was not a substance after all,
and put the caloric theory to rest.
James P. Joules (1843)
are designed primarily on the basis of heat
transfer analysis.
Heat transfer equipment
these are some of heat transfer equipments
1.heat exchangers,
2.boilers,
3.condensers,
4. radiators,
5.heaters,
6.furnaces,
7. refrigerators,
8. solar collectors
The heat transfer problems encountered in practice:
(1)rating and
(2)sizing problems.
deal with the determination of the heat transfer rate for an existing
system at a specified temperature difference.
rating problems
deal with the determination of the size of a system in order to
transfer heat at a specified rate for a specified temperature
difference.
sizing problems
can exist in numerous forms
energy
other forms of energy aside from heat
thermal, mechanical, kinetic, potential, electrical,
magnetic, chemical, nuclear, etc.
sum of all forms of energy of a system.
total energy
forms of energy related to the molecular
structure of a system and the degree of the
molecular activity.
microscopic energy
sum of all microscopic
forms of energy of a system
internal energy (U)
true or false.
We can get energy
from trash and
sewage.
true
SIunit of energy
Joule (J)
English unit of energy
British thermal unit (Btu)
is the energy needed to raise the temperature of 1 lbm of
water at 60°F by 1°F.
BTU
Another unit of energy is
calorie
is the energy needed to raise the temperature of 1
gram of water at 14.5°C by 1°C.
calorie
portion of the internal energy of a system associated with the
kinetic energy of the molecules
sensible energy or sensible heat
sum of the kinetic and potential energies of the molecules
internal energy
associated with the intermolecular forces between the molecules
of a system.
internal energy
internal energy associated with the phase of a system
latent energy or latent heat
internal energy associated with the atomic bonds in a molecule
chemical energy
internal energy associated with the bonds within the nucleus of the
atom itself
nuclear energy
energy needed to push a fluid and maintain flow
Flow energy (Pv)
At low P and high T,the ρ of a gas decreases and
the gas behaves like an
ideal gas
energy required to raise the temperature of a unit mass of a
substance by one degree
specific heat
The specific heat of a substance depends on
Temperature and Pressure
The specific heat of an ideal gas depends on
Temperature only
At low P, all real gases approach _____-_ , and therefore
their specific heats depend on temperature only.
ideal gas behavior
substance whose specific volume (or density) does not
change with temperature or pressure
incompressible substance
Energy can be transferred to or from a given mass by these two mechanisms,
heat (Q) and work (W).
work per unit time
power (W)
is defined as the measure of energy transfer that occurs
when an object is moved over a distance by an external force at least
part of which is applied in the direction of the displacement.
work
Examples of engineering systems that produce
work:
Steam Turbines
Hydraulic Turbines
Car engines
Examples of engineering systems that consume
work:
compressor
mixer
pumps
is the rate of heat transfer per unit area
normal to the direction of heat transfer.
heat flux
The first law of thermodynamics, also known as
conservation of energy principle
this states that
“Energy can neither be created nor destroyed; it can only
change forms.”
conservation of energy principle or the first law of thermodynamics
A large number of engineering devices such as water heaters and
car radiators involve mass flow in and out of a system, and are
modeled as _____
control volumes
The opposite of steady is unsteady or
transient
The total energy content of a control volume during a steady-flow
process remains
constant
the change in the total energy of the control volume
during such a process is
zero
The amount of mass flowing through a cross section of a flow device
per unit time is called the
mass flow rate
The volume of a fluid flowing through a pipe or duct per unit time
is called the
volume flow rate
The science that deals with the determination of the rates of energy
transfers is
heat transfer
TRUE OR FALSE
The transfer of energy as heat is ALWAYS from the higher - temperature
medium to the lower-temperature one, and heat transfer stops when
the two mediums reach the same temperature.
TRUE
Heat can be transferred in three different modes namely
conduction,
convection, and radiation
states that if two thermodynamic systems are both in thermal equilibrium with a third system, then the two systems are in thermal equilibrium
zeroth law of thermodynamics
is the transfer of energy from the more energetic
particles of a substance to the adjacent less energetic ones as a
result of interactions between the particles.
conduction
Conduction can take place in
solids, liquids, or gases
In gases and liquids, conduction is due to the _______ of the molecules during their random motion.
collisions and
diffusion
TRUE OR FALSE
In solids, it is due to the combination of vibrations of the molecules
in a lattice and the energy transport by free electrons.
TRUE
The rate of heat transfer by conduction through a medium depends on
the
- geometry of the medium,
- thickness,
-material of the medium, - temperature difference
Heat transfer by conduction is also known as the
Fourier’s law of heat conduction
measure of
the ability of a material to conduct
heat.
thermal conductivity (k)
- the rate of heat transfer through a unit thickness of the
material
per unit area per unit temperature difference.
thermal conductivity (k)
High value of thermal conductivity means the material is a
good heat conductor
Low value of thermal conductivity means the materials is
low heat conductor or insulator
Represents how fast heat diffuses through a material.
thermal diffusivity
Itis the ratio of the heat conducted through the material to
the heat stored per unit volume.
thermal diffusivity
TRUE OR FALSE
The larger the thermal diffusivity, the faster the propagation of
heat into the medium.
TRUE
T OR F
A small value of thermal diffusivity means that heat is mostly
absorbed by the material and a small amount of heat will be
conducted further.
T
It is the mode of energy transfer between a solid surface and
the adjacent liquid or gas that is in motion.
convection
Itinvolves the combined effects
of conduction and fluid motion.
convection
Itinvolves the combined effects
of conduction and fluid motion.
radiation
energy transfer by radia_____
and it suffers no attenuation in a vacuum. This is how the energy of the
sun reaches the earth.
radiation
The rate of heat transferby radiation (𝑸𝒆𝒎𝒊𝒕) is governed by ______ equation
Stefan–
Boltzmann
measure of how closely a
surface approximates a
blackbody for which ε =1
emissivity
the idealized surface that
emits radiation at this
maximum rate
blackbody
radiation emitted by a
blackbody
blackbody radiation
the fraction of the radiation energy incident on a surface that
is absorbed by the surface.
absorptivity
is essentially a
convection heat transfer coefficient modified to include the
effects of radiation.
combined heat transfer coefficient
T OR F
Not all three can exist simultaneously in a medium.
T
A _____may involve conduction and radiation but not convection
solid
a solid may involve heat transfer by ____ and/or
radiation on its surfaces exposed to a fluid or other surfaces.
convection
❑Heat transfer is
by ____ and ____ in a still fluid (no
bulk fluid motion)
conduction and possibly by radiation
heat transfer is by ____ and in a flowing fluid
convection and radiation
can be viewed as combined conduction and
fluid motion
convection
This is valid for both steady and transient conditions, and the _________ does not involve heat generation since a surface does not have a volume.
surface energy balance
It is necessary to keep track of the energy interactions at the surface, and this is done by applying the conservation of energy principle to the surface.
surface energy balance
