Mec203 + Flashcards
(49 cards)
What is heat?
The form of energy that can be transferred from one system to another as a result of temperature difference
What is specific heat capacity?
The energy required to raise the temperature of a unit mass of a substance by one degree
What is an incompressible substance?
A substance whose specific volume does not change with temperature or pressure.
What is heat transfer rate?
The amount of heat transferred per unit time
What is heat flux?
The rate of heat transfer per unit area normal to the direction of heat transfer
What is the first law of thermodynamics?
The first law of thermodynamics states that energy can neither be created or destroyed during a process, it can only change forms.
A surface contains……
no body or mass and thus no energy
What is conduction?
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
Fourier’s law of heat conduction?
Q = -kA dT/dx
What is thermal conductivity, k?
a measure of the ability of a material to conduct heat
What is thermal diffusivity?
α = k/ρcp how fast heat diffuses through a material
Heat conduction/Heat stored
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 fluid motion the greater the convection
Absence of any fluid motion heat transfer between surface and gas is pure conduction
Forced convection?
if the fluid is forced to flow over the surface by external means such as a fan, wind or pump.
Natural convection?
If the fluid motion is caused by the buoyancy forces that are induced by density difference due to the variation of temperature in a fluid.
Newtons law of cooling?
Q = hA(Ts - Tinf)
What does the convection heat transfer coefficient depend on?
- surface geometry
- the nature of the fluid motion
- the properties of the fluid
- the bulk fluid velocity
What is radiation?
The energy emitted by matter in the form of electromagnetic waves as a result of the change in the electronic configurations of the atoms or molecules
All bodies above 0K emit radiation
It is a volumetric phenomenon but usually considered a surface phenomenon for solids
Stephen-Boltzmann Law
Q = σAT^4
Radiation emitted by real surfaces: Q = σεAT^4
Define a lumped system
Variation with time but not with position, the temperature of the medium changes uniformly with time
Heat generation ……….
is a volumetric phenomenon
E = eV (w)
What is a boundary condition?
a mathematical expression of the thermal conditions at the boundaries
Thermal resistance concept …….
rate of heat transfer —-> electric current
thermal resistance —–> electrical resistance
temperature difference ——> voltage differnce
thermal resistance depends on the geometry and the thermal properties of the medium
Thermal resistance network for two parallel layers
1/Rtotal = 1/R1 +1/R2
When Ts and Tinf are fixed the two ways of increasing heat transfer is………………….
- increase convection heat transfer coefficient by installing a fan
- increase the surface areas by attaching fins made of highly conduction material (Al)
