Week 9 Flashcards
What is heat transfer?
Transit of thermal energy due to a spatial
temperature difference.
What is conduction?
Heat transfer through a solid or stationary fluid. Net effect is a diffusion of
energy down the temperature gradient.
What is convection?
Heat transfer from a surface to a moving fluid.
Includes energy transfer due to diffusion as well as bulk motion of the fluid.
What is radiation?
Matter which is above absolute zero temperature
emit energy. Emission is in the form of electromagnetic waves.
What is heat flux?
Heat transfer rate in the x direction per unit area perpendicular to the
direction of transfer.
Why is there a negative sign in Fourier’s law?
The negative sign ensures that heat flows from hot to cold.
What is the unit of heat flux?
W/m^2
What is the unit of thermal conductivity?
W/m . K
What is the difference between heat transfer rate and heat flux?
Heat transfer rate: Energy transfer per unit time.
Heat flux: Heat transfer rate per unit area (energy transfer per area per time).
What are units of heat transfer rate and heat flux?
Heat transfer rate: J/s
Heat flux: J/s . m^2
What does Newton’s law of cooling refer to?
Describes the convection
heat transfer process.
Using Newton’s law of cooling describe positive/negative heat fluxes.
Positive heat flux
describes the heat transfer from the surface to the fluid.
Negative heat flux describes the heat transfer from the fluid to the surface.
Define all the variables in Newton’s law of cooling.
Convective heat transfer coefficient (h)
Surface temperature (T_s)
Far field temperature (T_infinity)
What are the units of convective heat transfer?
W/m^2 . K
What is laten heat exchange?
Associated with a phase
change between liquid and vapour states (i.e. boiling,
condensation) during which the temperature
remains constant.
What is the main difference between conduction, convection and radiation?
Unlike conduction and convection, radiation
does not require a material medium to occur. Radiation transfer most efficient in a vaccuum.
What is irradiation (G)?
The rate at which radiation is incident
on a unit area.
What is surface emissive power, E? Units?
The rate of energy released per unit area is
despite being called a ‘power’.
Units: W/m^2
What law is gives the upper limit of surface emissive power, E?
Stefan-Boltzmann law
What is a real surface?
Non-ideal radiator
What is emissivity, what values can it take, and what does it depend on?
Radiative property of the surface, takes values 0 to 1 and is
non-dimensional.
Its value depends strongly on the
surface material and finish.
What is the equation for the rate at which thermal radiation is absorbed by a surface?
G_abs = alpha x G
Absorptivity is dependent on
the nature of irradiation (e.g. solar vs furnace) and
the surface properties.
What is absorptivity, alpha dependent on?
Nature of irradiance and surface properties.
What is first law of thermo?
The total energy of the system is
conserved.
