A nozzle converts fluid enthalpy into directed kinetic energy, increasing speed. A diffuser does the reverse, slowing a stream while typically increasing its pressure or enthalpy.

For steady, adiabatic flow with no shaft work and negligible elevation change,

$$h_1+\frac{V_1^2}{2}=h_2+\frac{V_2^2}{2}.$$

Therefore a large velocity increase requires an enthalpy decrease.

For example, if an adiabatic nozzle receives a gas with negligible inlet speed and the enthalpy decreases by $50\ \mathrm{kJ/kg}$,

$$\frac{V_2^2}{2}=50,000\ \mathrm{J/kg},$$

so

$$V_2=\sqrt{100,000}\approx316\ \mathrm{m/s}.$$

Real nozzles also have frictional and other flow losses, but the steady-flow energy equation still applies. The ideal adiabatic model establishes the energy conversion that those losses modify.