A convergent-divergent nozzle is a specially designed duct or tube that is used to accelerate a compressible fluid (like a gas) to high velocities, often beyond the speed of sound. This type of nozzle is crucial in many applications, such as jet engines, rocket engines, and steam turbines, where efficient conversion of thermal energy into kinetic energy is required.

Nozzle Anatomy and Flow Dynamics

The design of a convergent-divergent nozzle consists of three distinct sections, each playing a vital role in controlling the fluid flow:

• Convergent Part: In this initial section, the cross-sectional area of the nozzle gradually decreases. As the fluid flows through this narrowing passage, its velocity increases, while its pressure and density decrease. This part is designed to accelerate the fluid from subsonic (less than the speed of sound) to sonic (at the speed of sound) conditions.
• Throat: This is the most critical section of the convergent-divergent nozzle. The throat is defined as the section with the smallest cross-sectional area. It is at this point that the fluid velocity typically reaches the local speed of sound (Mach 1) when the nozzle is operating under choked flow conditions. Beyond the throat, the flow can become supersonic.
• Divergent Part: After passing through the throat, the fluid enters the divergent section, where the cross-sectional area gradually increases. Counter-intuitively, for supersonic flow, an increasing area causes the fluid to accelerate further, reaching supersonic speeds. This part is designed to expand the high-velocity fluid, converting remaining pressure energy into kinetic energy.

Therefore, based on the design and function of a convergent-divergent nozzle, the section with the smallest cross-sectional area is known as the throat. It acts as the critical link between the subsonic convergent flow and the supersonic divergent flow.