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๐ŸงŠthermodynamics ii review

Converging-Diverging Nozzle

Written by the Fiveable Content Team โ€ข Last updated September 2025
Written by the Fiveable Content Team โ€ข Last updated September 2025

Definition

A converging-diverging nozzle is a type of nozzle designed to accelerate fluid flow to supersonic speeds by first converging and then diverging the flow path. This design is crucial in applications such as rocket engines and supersonic jets, where achieving high velocities and efficient mass flow rates are necessary for performance.

5 Must Know Facts For Your Next Test

  1. In a converging-diverging nozzle, the cross-sectional area decreases in the converging section until it reaches a minimum at the throat, followed by an increase in area in the diverging section.
  2. Flow can only reach supersonic speeds after passing through the throat of a converging-diverging nozzle; this is where choked flow occurs.
  3. The design of these nozzles allows for maximum efficiency in converting pressure energy into kinetic energy, making them essential for high-speed applications.
  4. As the flow expands in the diverging section, it can reach velocities greater than Mach 1, which is critical for applications like supersonic aircraft and space propulsion.
  5. The performance of converging-diverging nozzles can be significantly affected by factors such as inlet conditions, back pressure, and the specific heat ratio of the fluid.

Review Questions

  • How does a converging-diverging nozzle achieve supersonic speeds, and what role does choked flow play in this process?
    • A converging-diverging nozzle accelerates fluid flow to supersonic speeds by first decreasing the cross-sectional area, which increases velocity until it reaches the speed of sound at the throat. This condition is known as choked flow; once this point is reached, any further decrease in pressure upstream causes an increase in mass flow rate without affecting velocity. After passing through the throat, fluid continues into the diverging section, where it further accelerates beyond Mach 1 due to a decrease in pressure.
  • Explain how the design of a converging-diverging nozzle impacts its efficiency in converting pressure energy into kinetic energy.
    • The design of a converging-diverging nozzle plays a vital role in its efficiency by optimizing how pressure energy is transformed into kinetic energy. The converging section compresses and accelerates the fluid up to sonic speeds, while the diverging section allows for continued expansion and acceleration to supersonic speeds. This careful control of area change ensures that most of the input energy is effectively used to increase fluid velocity rather than being lost to turbulence or heat losses.
  • Evaluate the significance of specific heat ratio on the performance of a converging-diverging nozzle and its implications for different working fluids.
    • The specific heat ratio (ฮณ) significantly affects the performance of a converging-diverging nozzle by influencing both the Mach number achieved and the thermodynamic properties during expansion. For gases with higher specific heat ratios, such as helium or hydrogen, nozzles can achieve higher velocities compared to those with lower ratios like air. This variation has practical implications; for instance, rockets using liquid hydrogen can be designed for greater efficiency due to their favorable thermodynamic characteristics, leading to enhanced thrust and performance in space propulsion.