Steam Nozzle

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Steam Nozzles and Types

Nozzle is a duct by flowing through which the velocity of a fluid increases at the expense of pressure drop. if the fluid is steam, then the nozzle is called as Steam nozzle.
The flow of steam through nozzles may be takenas adiabatic expansion. The steam possessesa very high velocity at the end of the expansion, and the enthalpy decreases as expansion occurs. Friction exists between the steam and the sides of the nozzle; heat is produced as the result of the resistance to the flow. The phenomenon of super saturation occurs in the steam flow through nozzles. This is because of thetime lag in the condensation of the steam during the expansion.

The area of such duct having minimum cross-section is known as throat.

A fluid is calledcompressible if its density changes with the change in pressure brought about by the flow.

If the density changes very little or does not changes, the fluid is said to be incompressible. Generallythe gases and vapors are compressible, whereas liquids are incompressible.

Types of Nozzles:

There are three types of nozzles

Convergent nozzle

Divergent nozzle

Convergent-divergent nozzle.

Convergent Nozzle:

A typical convergent nozzle is shown in the Fig.1. In a convergent nozzle, the cross sectional area decreases continuously from its entrance to exit. It is used in a case where the back pressure is equal to or greater than the critical pressure ratio.

Fig 1. Convergent nozzle

Divergent nozzle:

The cross sectional area of divergent nozzle increases continuously from its entrance to exit. It is used in a case where the back pressure is less than the critical pressure ratio.

Fig 2. Divergent nozzle

Convergent – Divergent nozzle:

In this condition, the cross sectional area first decreases from its entrance to the throat and then again increases from throat to the exit. This case is used in the case where the back pressure is less than the critical pressure. Also, in present day application, it is widely used in many types of steam turbines.

Fig 3. Convergent-Divergent nozzle

Flow of steam Through Nozzle

Supersaturated flow or metastable flow of in Nozzles: As steam expands in the nozzle, the pressure and temperature in it drop, and it is likely that the steam start condensing when it strikes the saturation line. But this is not always the situation. Due to the high velocities, the time up to which the steam resides in the nozzle is small, and there may not be sufficient time for the needed heat transfer and the formation of liquid droplets due to condensation. As a result, the condensation of steam is delayed for a while. This phenomenon is known as super saturation, and the steam that remains in the wet region without holding any liquid is known as supersaturated steam. The locus of points where condensation occursregardless of the initial temperature and pressure at the entrance of the nozzle is called the Wilson line. The Wilson line generally lies between 4 and 5 percent moisture curves in the saturation region on the h-s diagram in case of steam, and is often taken as 4 percent moisture line. The phenomenon of super saturation is shown on the h-s chart below:

Fig 4. The h-s diagram for the expansion of steam in the nozzle

Effects of Supersaturation:

The following are the effects of supersaturation in a nozzle.

The temperature at which the steam becomes supersaturated will be less than the saturation temperature corresponding to that pressure. Therefore, supersaturated steam will havethe density more than that of equilibrium condition which results in the increase in the mass of steam discharged.

Supersaturation causesthe specific volume and entropy of the steam to increase.

Supersaturation reduces the heat drop. Thus the exit velocity of the steam is reduced.

Supersaturation increases the dryness fraction of the steam.

Effect of Friction on Nozzles:

Entropy is increased.

The energy available decreases.

Velocity of flow at the throat getdecreased.

Volume of flowing steam is decreased.

Throat area requiredto discharge a given mass of steam is increased

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