Supersonic and Subsonic Flow in Nozzles and Diffusers

Introduction:
In fluid dynamics, a nozzle is a device that accelerates the flow of a fluid (usually a gas) by increasing its velocity. A diffuser, on the other hand, is a device that slows down the flow of a fluid by increasing its pressure. Both nozzles and diffusers can operate in either subsonic or supersonic flow conditions. Let us analyze the given statements and determine which options are correct.

Analysis:
1. Supersonic Nozzle: A supersonic nozzle is designed to accelerate the flow of a fluid to supersonic speeds. It is characterized by a converging-diverging shape. As the fluid passes through the converging section, its velocity increases, and it reaches its maximum velocity at the throat of the nozzle. After passing through the throat, the flow expands in the diverging section, maintaining supersonic speeds. Therefore, a tube with a section diverging in the direction of flow can be used as a supersonic nozzle. Hence, Statement 1 is correct.

2. Subsonic Nozzle: A subsonic nozzle is designed to accelerate the flow of a fluid to subsonic speeds. It is characterized by a converging shape. The fluid passes through the converging section, and its velocity increases. However, it does not reach supersonic speeds. Therefore, a tube with a section diverging in the direction of flow cannot be used as a subsonic nozzle. Hence, Statement 2 is incorrect.

3. Supersonic Diffuser: A supersonic diffuser is designed to slow down the flow of a supersonic fluid by increasing its pressure. It is characterized by a converging-diverging shape, similar to a supersonic nozzle but in the opposite direction. As the supersonic flow enters the diffuser, it expands in the diverging section, decreasing its velocity and increasing its pressure. Therefore, a tube with a section diverging in the direction of flow can be used as a supersonic diffuser. Hence, Statement 3 is incorrect.

4. Subsonic Diffuser: A subsonic diffuser is designed to slow down the flow of a subsonic fluid by increasing its pressure. It is characterized by a diverging shape. As the fluid enters the diffuser, it expands in the diverging section, decreasing its velocity and increasing its pressure. Therefore, a tube with a section diverging in the direction of flow can be used as a subsonic diffuser. Hence, Statement 4 is correct.

Conclusion:
From the analysis, we can conclude that a tube with a section diverging in the direction of flow can be used as a supersonic nozzle and a subsonic diffuser. Therefore, the correct answer is option 'C' (1 and 4).