In a converging-diverging channel (often referred to as a nozzle), the behavior of the flow can be analyzed using principles from fluid dynamics and thermodynamics.
If the flow velocity at the entrance is 150 m/s and the area of the channel decreases by about 20%, the flow will accelerate as it moves through the converging section. According to the principles of conservation of mass (continuity equation) and Bernoulli's equation, the velocity of the fluid increases as the cross-sectional area decreases.
In this case, if we assume that the flow is incompressible (which is generally a safe assumption for low-speed flows, but not for high-speed, compressible flows), the pressure will decrease as the velocity increases. As the kinetic energy of the fluid increases, the potential energy in the form of pressure decreases, potentially leading to a lower temperature if we consider energy conservation in an isentropic process.
However, in compressible flow (which is more likely at such high velocities), the behavior is different. In a converging-diverging nozzle, if the flow reaches supersonic speeds, then the temperature may increase due to conversion of internal energy into kinetic energy, particularly if the flow expands in the divergent section.
Thus, without additional information about whether the flow is compressible or incompressible and the nature of the fluid, it is indeed difficult to definitively state what will happen to the temperature. Therefore, the most accurate choice is:
It is impossible to say what happens to the temperature without additional information.