We assume the steam nozzle to be a passage of varying cross-section by means of which the energy of steam is converted into kinetic energy. The nozzle is so shaped that it will perform this conversion of energy with minimum loss. One may also define a nozzle as an opening through which steam is passed from a region of high pressure to one of lower pressure so as to derive additional velocity. It is chiefly used for producing a large velocity steam jet. In other words, its chief use is to produce a jet of steam for the purpose of driving steam turbines. The function of a nozzle in an impulse turbine is to admit steam to the active or moving parts of the turbine. In a reaction turbine the stationary nozzles admit steam to the moving parts which are also of nozzle shape and guide the steam from them.

The steam expanding, its velocity and specific volume will both increase; there will be condensation which will vary the degree of steam dryness. All these changes are found to affect the design of the nozzle. The weight of steam per second passing any nozzle section must be constant; hence, the nozzle cross-section varies according to the velocity and the specific volume.

At first the nozzle cross-section tapers to a smaller section to allow for these changes. On reaching this small diameter, it will diverge to a larger one. We know the throat to be the smallest section of the nozzle.

A nozzle which first converges to a throat and then diverges is known to be a converging-diverging nozzle; in this type the greatest diameter is at the exit end.

Some forms of nozzles end at the throat, and no diverging portion is fitted; this type is known as a converging nozzle and has its exit at the throat.

The flow of steam through a nozzle may be regarded, in its simplest form, as being an adiabatic flow. The steam enters the nozzle with a relatively small velocity and a high initial pressure, the initial velocity being so small compared with the final velocity that it may be neglected. As the steam expands, the velocity will increase, the heat energy of the steam being converted to kinetic energy. During the expansion of the steam through the nozzle no heat is supplied or rejected and, although no external work is done on a piston, work is done by increasing the kinetic energy of steam.

As the steam loses its pressure in passing through the nozzle, it is also losing its total heat; the change of total heat of the steam must, therefore, equal the increase in kinetic energy. Hence, the work done is equivalent to the heat drop.

It should be noticed that the expansion of steam through a nozzle is not a free expansion, and the steam is not throttled because of its having a large velocity at the end of the expansion. Work is done by the expanding steam in producing this kinetic energy.

In practice, there is friction produced between the steam and the sides of the nozzle; this friction causes a resistance to the flow which is converted into heat. The heat formed tends drying the steam. The effect of this friction in resisting the flow and in drying the steam must be taken into account in the design of the nozzle, as it makes a great difference to the results obtained.

A longitudinal sectional view of a converging-diverging steam nozzle is shown in the figure, a and that of a converging nozzle in the figure, b.