For steady one-dimensional flow, the equation of continuity is
Differentiating(after taking log), we get
 | (39.6) |
Energy Equation
Consider a control volume within the duct shown by dotted lines in Fig. 39.3. The first law of thermodynamics for a control volume fixed in space is
 | (39.7) |
where 
is the kinetic energy per unit mass.
Let us discuss the various terms from above equation:
- The first term on the left hand side signifies the rate of change of energy (internal + kinetic) within the control volume
- The second term depicts the flux of energy out of control surface.
- The first term on the right hand side represents the work done on the control surface
- The second term on the right means the heat transferred through the control surface.
It is to be noted that dA is directed along the outward normal.
Assuming steady state, the first term on the left hand side of Eq. (39.7) is zero. Writing 
(where the subscripts are for Sections 1 and 2), the second term on the left of Eq. (39.7) yields
The work done on the control surfaces is
The rate of heat transfer to the control volume is
where Q is the heat added per unit mass (in J/kg).
Invoking all the aforesaid relations in Eq. (39.7) and dividing by 
, we get
 |
(39.8)
|
We know that the density 
is given by /VA , hence the first term on the right may be expressed in terms of 
(specific volume=1/ρ).
Equation (39.8) can be rewritten as
 | (39.9) |
NOTE:- 
is the work done (per unit mass) by the surrounding in pushing fluid into the control volume. Following a similar argument, is the 
work done by the fluid inside the control volume on the surroundings in pushing fluid out of the control volume.
Since 
Eq. (39.9) gets reduced to
 | (39.10) |
This is energy equation, which is valid even in the presence of friction or non-equilibrium conditions between secs 1 and 2.
It is evident that the sum of enthalpy and kinetic energy remains constant in an adiabatic flow. Enthalpy performs a similar role that internal energy performs in a nonflowing system. The difference between the two types of systems is the flow work p required to push the fluid through a section.
