Isentropic Efficiency of Nozzles

Nozzles increase the velocity of a fluid at the expense of pressure. Thus, since velocity (or kinetic energy) is the target, for isentropic efficiency we have:

$$\begin{array}{rl}{\eta }_N& =\frac{\text{Actual KE at nozzle exit}}{\text{Isentropic KE at nozzle exit}}=\frac{V_{2s}^2}{V_{2a}^2}\\ & \approx \frac{h_1-h_{2a}}{h_1-h_{2s}}\end{array}$$

Example