Introduction and basic concepts Flashcards
What is heat?
The form of energy that can be transferred from one system another as a result of temperature difference
Transfer of energy is
always from higher temperature to lower temperature
Heat transfer stops when
two mediums reach the same temperature
Three main types of heat transfer
conduction convection and radiation
What are rating problems in heat transfer
deal with determination of the heat transfer rate for an existing system at a specified temperature difference
What are sizing 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
Advantage of experimental approach
actual physical system, quantity determined by measurement within the limits of experimental error
Disadvantage of experimental approach
slow expensive and often impractical
analytical approach adv disadv
fast and inexpensive but results obtained are subject to accuracy of the assumptions
Sum of all microscopic forms of energy called
internal energy
what is enthalpy made up of
flow energy and internal energy (Pv + u)
What is the specific heat
energy required to raise the temperature of a unit mass of a substance by one degree
two types of specific heat
specific heat at constant volume and at constant pressure
What do real gasses approach ideal gas behaviour and what does this mean
at low pressure and therefore specific heats depend on temperature only
change in internal energy =
m * specific heat at constant volume * change in temperature
change in enthalpy
m * specific heat at constant pressure * change in temperature
heat transfer rate is
the amount of heat transferred per unit time
heat flux is the (units)
rate of heat transfer per unit area normal to the direction of heat transfer W/m^2
symbol of heal flux
little q with a dot
symbol of heat transfer
Q with a dot
relationship between heat flux and heat transfer
q. = Q. / A
Power =
the work done per unit time
What is the first law of thermodynamics also called
conservation of energy principal
What does the first law of thermodynamics state
energy can neither be created nor destroyed during a process it can only change forms
what does the first law of thermodynamics mean
Total energy enetring the system - Total energy leaving the system = change in total energy of the system
Ein - Eout = delta E
In steady operation the rate of energy transfer
to a system is equal to the rate of energy transfer from the system ie Ein = Eout
Typical heat balance equation for heat transfer problem
Q in - Q out + Egen = detla E
How is the conversion of nuclear, chemical, mechanical and electrical energies into thermal treated
as heat generation
What is a closed system
consists of a fixed mass
what does a closed system mean for heat transfer
Ein - Eout = delta U = m *specific heat transfer at constant volume * delta T
Why are steady flow systems important
can be used to analyse control volume which are used alot
What does steady mean
no change with time at a specified location
mass flow rate =
density * Velocity * Ac (cross sectional area)
volume flow rate =
Velocity * Ac
Heat/energy transfer in steady flow systems
m. * delta h = m. cp delta T
Surface energy balanace
E.in = E.out
Why is the surface energy balance so simple
as no volume or mass and thus no energy generations
Heat transfer for a surface
Q.in = Q.out
When does conduction occur
when there is no movement (solids)
when does convection occur
when you have a fluid moving (gas/liquid)
What is radiation and what does it not require
no median required
what do all modes of heat transfer require
existence of a temperature difference
What is conduction
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
What does fouriers law describe
heat transfer by conduction
What is fouriers law
Q. = - kA dT/dx
what is k in fouriers law
thermal conductivity
how can fouriers law be simplified in linear varition of temperature
dT/dx = change in T/thickness
why is there a negative sign in fouriers law
heat is conducted in the direction of decreasing temperature, and the temperature gradient becomes negative when temperature decreases with increasing x. The negative sign in the equation ensure that heat transfer in the positive x direction is a positive quantity
What is thermal conductivity
a measure of the ability of a material to conduct heat
defined as the rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference
What is delta T / delta x (fouriers law)
slope of the temperature curve on a T-x diagram
heat flux in fouriers law=
-k delta T/delta x
when does dT/dx = delta T / delta x
when your object is long and thin as assuing that it is constant in calculations
what does a high value of thermal conductivity indicate
that the material is a good heat conductor and a low value indicates the material is a good insulator
highest thermal conductivity materials
diamond graphite 1000W/mK
lowest thermal conductivity materials
air carbon dioxide 0.1 W/mK
symbol of thermal conductivity
k
units of k
W/m*K
What will k always be in this module
constant
heat capacity =
density * cp (J/m^3*K)
thermal diffusivity symbol =
alpha = heat conduction/heat storage = k/rho * cp (m^2/s)
what does thermal diffusivity represent
how fast heat diffuses through a material
larger the thermal diffusivity
the faster the propagation of heat into the medium
what does a small value of thermal diffusivity represent
heat is mostly abosrbed by the the material and a small amount of heat is conducted further
What is convection
the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion and it involves the combined effects of conduction and fluid motion
faster the fluid motion the
fast the convection heat transfer
in the absence of bulk fluid motion heat tranfer between a solid surface and the adjacent fluid is by
pure conduction
What is Tinf
temperature at dist where fluid does not feel affect of plate (can be very small)
What is forced convection
fluid is forced to flow over the surface by external means such as a fan, pump, or the wind
natural or free convection
if the fluid motion is caused by the buoyancy forces that are induced by density differences due to the variation of temperature in the fluid
what are heat transfer processes that involve a change of phase considered
convection because of the fluid motion induced during the process such as the rise of vapour bubbles during boiling or the fall of the liquid droplets during condenation
Equation of convection
Q. conv = h*As(Ts - Tinf)
what is h in convection
convection heat transfer coefficient W/m^2 *K
what is the h coefficient not
a property of the fluid it is an experimentally determined parameter whose value depends on all the variables influencing convection
what factors effect h coefficient
surface geom
nature of fluid motion
properties of the fluid
bulk fluid velocity
Radiation is
the energy emitted by matter in the form of electromagnetic waves as a result of the changes in the electronic configurations of the atoms or molecules
What does radiation not require
an intervening medium
Do all bodies above absolute zero do
emit thermal radiation
Radiation is what type of phenomenon
a volumetric phenomenon, however usually considered surface for solids
Equation of radiation =
Q.emit = emissivity * sigma * As * (Ts^4 - Tsurr^4)
what is sigma for equation of net radiation transfer
the stefan boltzman constant
what has an emissivity of 1
a blackbody, idealised surface that emits radiation at max rate
what is emissivity
a measure of how closely a surface approximates a blackbody between 0 and 1
What is net radiation transfer
difference between the rate of radiation emitted by the surface and the radiation absorbed
How does radiation compare to other heat transfer processes
significant relative to conduction or natural convection but negligible relative to forced convection
What is a combined heat transfer coefficient
it includes the effects of both convection and radiation
Physical method of heat transfer in solid liquid and gasses
solids conduction is due to combination of the vibrations of the molecules in a lattice and energy transported by free electrons, in gases and liquid it is due to the collisions of the molecules during their random motion
