Gas Dynamics

Question 1

What is the difference between static and stagnation enthalpy?

Answer 1

Stagnation enthalpy is also known as total enthalpy, it represents the total energy of a flowing fluid stream per unit mass, which remain constant in an adiabatic situation. Static enthalpy only represents the potential energy of fluid stream per unit mass, which doesn’t include the kinetic energy part. However, if kinetic energy is zero, stagnation enthalpy will equal to static enthalpy

Question 2

On an enthalpy-entropy diagram, how does the actual stagnation state differ from the isentropic stagnation state?

Answer 2

See Page 245 of the Mega PDF for Diagram

Notice the stagnation enthalpy of the fluid is the same for both cases.
However the actual stagnation pressure is lower than the isentropic stagnation pressure since the entropy increases due to fluid friction

Question 3

Explain the principle of a converging-diverging nozzle.

Answer 3

When air enters the converging nozzle, its pressure, temperature and velocity of
sound decrease while the fluid velocity and Mach number increase in the flow direction. The density decreases slowly at first and rapidly later as the fluid velocity increases. The Mach number is unity and pressure reach critical
condition at the location of the smallest flow area, called the throat. After that, the air reached the divergent nozzle where the velocity of the fluid keeps increasing with further reductions in pressure, although the flow area increases
rapidly in that region. This increase in velocity past the throat is due to the rapid decrease in the fluid density.