Heat Transfer is the transport of thermal energy due to a temperature difference across a mediums mediums gas liquid solid liquidgas solidgas solidliquid solidsolid etc Thermal Energy ID: 425215
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Slide1
Heat Transfer
Heat Transfer
is the transport of
thermal energy
due to a temperature difference across a
medium(s
)
mediums: gas, liquid, solid, liquid-gas, solid-gas, solid-liquid, solid-solid, etc.
Thermal Energy
is simply the kinetic energy (
i.e
. motion) of atoms and molecules in the
medium(s
)
Atoms/molecules in matter occupy different states
translation, rotation, vibration, electronic
the
statistics
of these individual
molecular-level
activities will give us the thermal energy which is approximated by
temperature
Heat Transfer, Thermal Energy,
and
Temperature
are
DIFFERENT. DO NOT confuse them.
Heat generation (electrical, chemical, nuclear, etc.) are not forms of heat transfer
Q
but forms of work
W
Q
is the transfer of heat across the
boundary of the system
due to a temperature differenceSlide2
Definitions
Thermal Energy
Temperature
Heat Transfer
Energy associated with molecular behavior of matter
U
[J] – extensive property
u [J/kg] – intensive property
Means of indirectly assessing the amount of thermal energy stored in matter
Quantity
Meaning
Symbol/Units
T [K] or [°C]
Thermal energy transport due to a temperature gradient (difference)
various
Heat
Heat Rate/Heat Flow
Heat Flux
Thermal energy transferred over a time interval (
Δt > 0)
Thermal energy transferred per unit time
Thermal energy transferred per unit time per unit surface area
Heat TransferSlide3
Modes of Heat Transfer
Conduction & convection require a temperature difference
across a medium
(the interactions of atoms/molecules)
Radiation transport can occur
across a vacuumSlide4
Conservation of Energy: Fundamental Law
time instant
time basis
time interval
First law of thermodynamics: Conservation of Energy
Heat transfer perspective:
for a
Volume
Heat transfer perspective: for a
Surface
Work!
heat transfer across boundary & mass transfer across boundary (advection)Slide5
Transient Conduction: Lumped Capacitance
General Transient Problem: Special Case
negligible radiation, heat flux & heat generation
Define:
thermal time constant
We can plot the
normalized
solution to the general problem
Notes:
The
change in thermal energy storage
due to the transient process is: