NOZZLE EFFICIENCY (OR) EFFECT OF FRICITION IN A NOZZLE :

When the steam flows through a nozzle the final velocity of steam for a given pressure drop is reduced due to the following reasons.

1. Due to the friction between the nozzle surface and steam.

2. Due to Internal fluid friction in the steam.

3. Due to shock losses.

Most of these frictional losses occur between the throat and exit in convergent-divergent nozzle. The effects of these frictional losses are listed below :

1. The expansion is no more isentropic and enthalpy drop is reduced resulting in lower exit velocity

2. The final dryness fraction of the steam is increased as the part of the kinetic energy gets converted into the heat due to the friction and is absorbed by steam with the increase in enthalpy.

3. The specific volume of steam is increased as the steam becomes drier due to this frictional reheating.

This can be best understood with the help of h-s diagram or mollier chart.

The point A represents the initial condition of steam. It is a point, where the saturation line meets the initial pressure (p1) line.

If the friction is neglected, the expansion of steam from entry to throat is represented by the vertical line AB. This is done, as the flow through the nozzle is isentropic.

The enthalpy (h1-h2) is known as isentropic enthalpy drop.

Due to friction in nozzle, the actual enthalpy drop in the steam will be less than (h1-h2). This enthalpy drop is shown as AC instead of AB. Final Condition of steam is obtained by drawing a horizontal line through C to meet the final pressure (p2) line at B'. Now, the actual expansion of steam in the nozzle is expressed by the curve AB' ( Adiabatic expansion) instead of AB (Isentropic expansion). So, the actual enthalpy drop is (h1- h3).

Co-efficient of Nozzle or Nozzle efficiency is defined as the ratio of actual enthalpy drop to the isentropic enthalpy drop.

n = (Actual enthalpy drop / Isentropic drop enthalpy) = (AC / AB)

= (h1 - h3) / (h1 - h2)

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