Question

In Rayleigh flow, what happens to the stagnation pressure when heat is added, regardless of whether the flow is subsonic or supersonic?

• A. It increases because energy is being added to the system
• B. It remains constant because the duct area is constant
• C. It decreases because heat addition is an irreversible process
• D. It only decreases if the flow is supersonic

Hint:

Rayleigh flow: \[ \boxed{ \begin{aligned} &\text{Constant-area duct} &\text{Heat transfer} &\text{No wall friction} \end{aligned} } \] Heat addition increases entropy, therefore \[ \boxed{ P_0\downarrow. } \]

Answer 1

The Correct Option is C

Step 1: Recall the characteristics of Rayleigh flow. Rayleigh flow is the flow through a constant-area duct with \[ \boxed{\text{heat transfer but no friction}.} \]

Step 2: Determine the effect of heat addition. Although heat is added to the fluid, \[ \boxed{ \text{the process is irreversible,} } \] which causes an increase in entropy. As a result, the stagnation pressure decreases. Therefore, \[ \boxed{\text{Stagnation pressure decreases due to irreversibility}.} \] Hence, \[ \boxed{(C)} \] is the correct answer.