Study Guide 1 Flashcards
Heat Transfer
thermal energy in transit due to a spatial temperature difference
Heat flux
HT rate in the 1D direction per unit area perpendicular to the direction of transfer: W/m^2
Mass Transfer
Movement of mass due to a species/[ ] gradient in the mixture
Mass flux
MT rate in the 1D direction per unit area perpendicular to the direction of transfer: kg of A /m^2 s
Conduction
HT through a solid material or stagnant fluid: electron movement (HT)
Diffusion
MT through a solid material or stagnant fluid: random particle movements
Convection (Heat or Mass)
HT or MT between a surface and an adjacent fluid: movement of heat/mass by both conduction/diffusion and advection (bulk fluid motion)
Radiation
HT transfer by emission and absorption of incident electromagnetic radiation
Thermal conductivity
Transport property that is characteristic of the wall material: k = W/mK
Binary diffusion coefficient
D,AB used in Fick’s Law nd is a function of T/P: D,AB = m^2/s
Mass diffusivity
Another name for the binary diffusion coefficient
Fourier’s law
q” = -k (dT/dz)
Fick’s law
j, Az = -D,AB(dpa/dz)
Stationary medium approximation
1) small concentration
2) no advection/bulk fluid movement
What allows us to say that j = N for Fick’s Law and Diffusion
Newton’s law of cooling
For Convection, q” = h(Ts - T-infinity)
Heat transfer coefficient
h = W/m^2 K
Parameter NOT a property: depends on how fast the movement is
Mass Transfer coefficient
kc = m/s
Used in the mass equivalent of the Newton’s Law of Cooling Equation (n,a” = kc(ps - p-infinity) where p = density
Forced convection
when the flow is caused by external means like a fan, pump, or atmospheric winds
Free convection
When the flow is induced by buoyancy forces which are due to density differences caused by temperature variations in the fluid
Stefan - Boltzmann law
There is an upper limit to a surface’s emissivity power
Constant: 5.67 x 10^-8 W/m^2 K^4
q” net = Eo(Ts^4 - Tsurr^4)
E = epsilon, o = sigma
0 <= E <= 1
Blackbody
Where the surface emissivity is 1:
Eb = q” emitted = oTs^4
Gray body
q” rad = EoTs^4 - aG = Eo(Ts^4 - Tsurr^4)
where a = alpha; absorptivity (0-1)
Irradiation
The rate at which all such radiation is incident on a unit area of the surface (G)
Absorptivity
A surface radiative property based on the opaqueness of a surface:
Opaque: <1
Emissivity
(Epsilon): property that provides a measure of how efficiently a surface emits energy relative to a blackbod
Opaque surface
Where the absorptivity is <1 and portions of irradiation are reflected
Hydrodynamic boundary layer
A region in the fluid through which the velocity varies from zero at the surface to a finite value (u-infinity) associated with the flow
Thermal boundary layer
A region of the fluid through which the temperature varies from Ts at y = 0 to T-infinity in the outer flow
Sensible energy
The energy that is being transferred (internal thermal energy)
Latent energy
Associated with the phase change between the liquid and vapor states of the fluid (convection process)
Heat or thermal energy generation
Associated with the conversion from some other form of internal energy to thermal or mechanical energy: it is a volumetric phenomenon (E.g)
Thermal energy storage
Est the energy stored in the surface
