Page 129 - Mechanical Engineers' Handbook (Volume 4)

118 Exergy Analysis, Entropy Generation Minimization, and Constructal Theory

powerful technique for modeling and optimizing real systems and processes. The use of the
EGM method has expanded greatly. 4
The most recent development in thermodynamics is the focus on the generation of flow
system configuration (architecture, shape, structure). 1,5,6 Flow systems are thermodynamically
imperfect because of resistances for fluid, heat, electricity, etc. Resistances cannot be elim-
inated. At best, they can be distributed (balanced) such that their combined effect is mini-
mum. Distribution means configuration, drawing, design. The search for design is being
pursued on the basis of principle—the constructal law, which is the statement that as con-
figurations change on the designer’s table the ones that survive are those that offer greater
access (less resistance) to currents. The numerous observations that the flow configurations
generated by the constructal law (e.g., tree networks) also occur in nature have led to con-
structal theory, 1,5 which is the thought that natural flow structures can be predicted based on
the same principle. The constructal law bridges the gap between engineered and natural flow
systems, and elevates ‘‘design’’ to the rank of scientific method. Moreover, the evolutionary
search for the best design can be summarized in terms analogous to the classical energy
minimum principle, as shown in the concluding section of this chapter.
SYMBOLS AND UNITS
a specific nonflow availability, J/kg
A nonflow availability, J
A area, m 2
b specific flow availability, J/kg
B flow availability, J
B duty parameter for plate and cylinder
B s duty parameter for sphere
B 0 duty parameter for tube
˙
˙
˙
Be Bejan number, S /(S gen, T S )
gen, T
gen, P
c P specific heat at constant pressure, J/(kg K)
C specific heat of incompressible substance, J/(kg K)
C heat leak thermal conductance, W/K
C* time constraint constant, s/kg
D diameter, m
e specific energy, J/kg
E energy, J
e ch specific flow chemical exergy, J/kmol
e t specific total flow exergy, J/kmol
e x specific flow exergy, J/kg
e x specific flow exergy, J/kmol
E Q exergy transfer via heat transfer, J
˙
E W exergy transfer rate, W
E x flow exergy, J
EGM the method of entropy generation minimization
ƒ friction factor
F D drag force, N
g gravitational acceleration, m/s 2
G mass velocity, kg/(s m )
2
h specific enthalpy, J/kg
h heat transfer coefficient, W/(m K)
2
h total specific enthalpy, J/kg
H total enthalpy, J