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According the first thermodynamic law the steam is accelerated in a convergent divergent nozzle at the expense of enthalpy.
What does that mean? I don't understand how the fall of enthalpy and not just the pressure accelerate the fluid
Any intuitive description will be of great help
Consider these simple experiments:
Blow air from your mouth slowly with the lips in the shape of a big "O": the air comes out warm, basically at the temperature of the body.
Now blow air from your mouth faster with the lips in the shape of a small "o": the air comes our colder! What is happening is that the air accelerates as it is forced to go through the small opening, so its kinetic energy increases. Conservation of energy says that this energy must come from somewhere. Well, it comes from two sources: (i) the internal energy of the air (temperature drops) and (ii) from the lower pressure outside the mouth. The two contributions together amount to the difference in $\Delta H$. Mathematically, $$ \Delta U + \Delta (P V) + \Delta\left(\frac{v^2}{2}\right) = 0 $$
A microscopic explanation of this behavior is that when molecules in a gas are forced to move faster without the input of external work (i.e., without using a fan to accelerate the gas), they must convert some part of the fluctuating component of the velocity as ordered velocity in the direction of the flow. As a result temperature, which is associated with fluctuations of velocity, decreases.
They are presumably citing Bernoulli's equation that says, for steady inviscid flow, the quantity $$ \frac 12 |{\bf v}|^2+ h $$ is constant along streamlines. Here $h$ is the specific enthalpy i.e the enthalpy $H=U+PV$ per unit mass. This is the usual expression one would use in rocket engine nozzle. It is easily dervived from Euler's equation for fluid flow, and imples that $h$ can be traded for velocity.
