Mec203 + Flashcards
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)
Fin effectiveness
heat transfer rate from the fin of base area Ab / heat transfer rate from the surface of area Ab
Adiabatic
An adiabatic process occurs without the transfer of heat or mass of substances between a thermodynamic system and its surroundings.
Semi-infinite solid
an idealized body that has a single plane surface and extends to infinity in all directions
Limitations of analytical solution methods
limited to highly simplified problems in simple geometries
Convection heat transfer coefficient
the rate of heat transfer between a solid surface and a fluid per unit surface area per unit temperature difference
Viscous flow
flows in which the frictional effects are significant
inviscid flow regions
A region where viscous forces are negligibly small compared to inertial or pressure forces
Laminar flow
the highly ordered fluid motion characterised by smooth layers of fluid.
Turbulent flow
the highly disordered fluid motion that typically occurs at high velocity and is characterised by velocity fluctuations
Velocity boundary layer
the region of flow over a plate in which the effects of the vicious and shearing forces caused by the fluid’s viscosity are felt
irrotational flow
the frictional effects are negligible and the velocity remains essentially constant
free stream velocity
the velocity of the fluid relative to an immersed solid body
Drag
the force a flowing fluid exerts on a body in the flow direction
separated region
the low-pressure region behind the body, here recirculating and backflow occurs
What does the transition from laminar to turbulent depends on
- surface geometry
- surface roughness
- upstream velocity
- surface temperature
- type of fluid
In the fully developed region of the tube what remains constant?
friction and convection coefficients
what is the effect of the entrance region
to increase the average friction factor and heat transfer coefficients for the entire tube
Buoyancy force
the upward force exerted by a fluid on a body completely or partially immersed in it in a gravitational field.magnitude of buoyancy force is equal to the weight of the fluid displaced by the body
Archimedes principle
a body immersed in a fluid will experience a ‘weight loss’ in an amount equal to the weight of the fluid it displaces
Volume expansion coefficient
1/T
compact heat exchanger
it has a large heat transfer surface area per unit volume.
cross flow heat exchanger
in compact heat exchangers, the two fluids usually move perpendicular to each other. the cross-flow is further classified as unmixed and mixed flow
Fouling factor increases with
operating temperature, length of service and decreases with the velocity of fluids
irradiation
radiation flux incident on a surface, G
what is the second law of thermodynamics
The second law of thermodynamics states that the entropy of any isolated system always increases.
