Question

What is the diffuser efficiency for supersonic flow?

(a) ηD = 1

(b) ηD > 1

(c) ηD < 1

(d) ηD = 1/2

I got this question in exam.

Question is taken from Quasi-One-Dimensional Flow Diffusers topic in portion Quasi-One-Dimensional Flow of Aerodynamics

Answer 1

Correct answer is (b) ηD > 1

For explanation: The diffuser efficiency is given by the ratio of actual total pressure ratio across the diffuser to the total pressure ratio of a hypothetical normal shock wave at test section Mach number. For a supersonic test section Mach number, the diffusers perform better than the normal shock, thus the numerator of the ratio is greater than the denominator, hence ηD > 1.