Question

Why is the diffuser throat area greater than the nozzle throat area?

(a) To reduce speed

(b) To maintain constant mass flow rate

(c) Rise in entropy within diffuser

(d) Rise in entropy within nozzle

I had been asked this question in a job interview.

Enquiry is from Quasi-One-Dimensional Flow Diffusers topic in chapter Quasi-One-Dimensional Flow of Aerodynamics

Answer 1

The correct answer is (c) Rise in entropy within diffuser

Easiest explanation: In a supersonic wind tunnel, there are two throats present. One is that of the nozzle which is known as the first throat which has sonic speed with corresponding area as A*. The second throat is that of the diffuser which is always greater than the first throat because there’s an increase in entropy due to the presence of shock waves.