1. Enthalpy & First Law for CVs
P M V Subbarao
Professor
Mechanical Engineering Department
Discovery of new Form of Energy Driving the Engineering Systems…..

2. The Family of Steam Engines
A Direct Hardware Creations to the Essential Need …..

3. Piston-Cylinder Systems : Constant Pressure Process

4. The issue of Increasing unit Temperature of A Pure Substance

5. Microscopic Energy of A Flowing Fluid
Unlike control mass, control volumes involve mass flow across their boundaries.
The substance inside a control volume will be at some pressure, temperature…..
The fluid entering the control volume is pushing itself against the pressure of the control volume.
Some work transfer is involved in pushing this mass into the control volume.
This is an internal work.
This is called flow work or flow energy.

6. Visualization of Flow Workp L
F= pA will be a driving force responsible for the pushing of the fluid into the CV.
This force will perform a work transfer of F.L to push the fluid.
Therefore, the flow work = F ×L = p × A × L = p × V
This can exists even when there is a fluid pushed out of the CV.

7. Flow work
It is interesting to note that unlike other work quantities, the flow work is expressed in terms of state variables.
This is also a state variable, point function and hence a thermodynamic property.
This is also called flow energy, convected energy or transport energy.
The total energy of non flowing fluid: E = m(u + ½V2 + gz)
The total energy of flowing fluid: Q =m(u + pv + ½V2 + gz).
This total energy is also called as Methalpy.
The term u + pv is called as specific Enthalpy, h.

8. Energy transport by Moving fluid
Amount of energy transport by a moving fluid of mass m:
Q = mθ = m ( h + ½V2 + gz )
Rate of Energy Transport:

9. Internal Energy & Enthalpy of Wet Mixtures
x is the dryness fraction.
U = (1-x) Uf + x Ug
Specific Internal energy: Internal energy per unit mass ; u
u = (1-x) uf + x ug
Specific enthalpy
h = (1-x) hf+ x hg T
ufg uf u ug

10. Rate Equations for Laws of Nature : Control Mass
Conservation of Mass :
Conservation of Momentum :
First law of thermodynamics :

11. First Law for A Control Volume
Conservation of mass:
Conservation of momentum:
Conservation of energy: