Page 52 - Modelling in Transport Phenomena A Conceptual Approach

2.4. TOTAL FLUX 33

0 By inlet and/or outlet streams,

0 By exchange of energy between the system and its surroundings through the
boundaries of the system in the form of heat and work.
When energy enters and/or leaves the system by a conduit(s), the characteristic
velocity is taken as the average velocity of the flowing stream and it is usually large
enough to neglect the molecular flux compared to the convective flux, i.e., PeH >> 1.
Therefore, J3q. (2.415) simplifies to
= (Energy) ( Average ) ( Flow )
Volume velocity area (2.416)

Energy per unit volume, on the other hand, is expressed as the product of energy
per unit mass, e, and mass per unit volume, i.e., density, such that Eq. (2.416)
becomes
(2.417)

Y
Mass flow rate

NOTATION

A area, m2
CP heat capacity at constant pressure, kJ/ kg. K
C total concentration, kmol/ m3
ci concentration of species i, kmoI/ m3
DAB diffusion coefficient for system AB, m2/ s
E rate of energy, W
e total energy flux, W/ m2
F force, N
J' molecular molar flux, kmol/ m2. s
j molecular mass flux, kg/ m2. s
k thermal conductivity, W/ m. K
m total mass flow rate, kg/ s
mi mass flow rate of species i, kg/ s
N total molar flux, kmol/ m2. s
n total molar flow rate, kmol/ s
ni molar flow rate of species i, kmol/ s
P pressure, Pa
Q heat transfer rate, W
Q volumetric flow rate, m3/ s
4 heat flux, W/m2
T temperature, "Cor K