Page 12 - Entrophy Analysis in Thermal Engineering Systems

2 Entropy Analysis in Thermal Engineering Systems

this book, also available [2]. EES has been the primary tool for calculating the
properties and modeling of most systems discussed in the present book. This
will be reiterated in the forthcoming chapters where appropriate.

1.2 Conservation of mass
The principle of mass conservation states that matter is neither created
nor destroyed. This principle like many physics laws is empirical; that is, its
validity rests on experimental observations. In every process, it is necessary to
obey the law of mass conservation. The total amount of matter in a given
process is fixed, but it may change from one form to another. For example,
consider condensation of steam. In this process, water is initially in gas phase,
but then it undergoes a condensation process. At the final state, water is in
liquid phase. The conservation of mass requires that the mass of water at its
initial state (steam) be equal to the mass of liquid water.
Mass is also conserved in chemical reactions. For example, consider
oxidation of hydrogen. The product of reaction is water. The reactants
(O 2 and H 2 ) no longer exist after the reaction and a new product (H 2 O)
is formed. The conservation of mass requires that the sum of the masses
of oxygen and hydrogen be equal to the mass of water. In general, the total
mass of reactants should equal the total mass of products in a chemical reac-
tion in accordance with the law of mass conservation.
In mathematical form, the conservation of mass applied to an open sys-
tem undergoing a steady-state process with n inlet and m outlet ports is writ-
ten as follows.

n m
X X
_ m i ¼ _ m j (1.1)
i¼1 j¼1
where _m denotes mass flow per unit of time, or mass flowrate.
If the system undergoes an unsteady (transient) process, the conservation
of mass is expressed as

n m
X X
Δm sys ¼ m i m j (1.2)
i¼1 j¼1
Eq. (1.2) states that in a transient process taking place over a given time
period, t, the net change in the mass of the system, Δm sys , is equal to the
sum of the masses entering the system through n inlet port minus the
sum of the masses leaving the system through m outlet ports.