Question

Why is a variable – geometry diffuser used?

(a) Ease in manufacturing

(b) Higher efficiency

(c) More mass flow rate

(d) Inexpensive

I had been asked this question by my college director while I was bunking the class.

The above asked question is from Quasi-One-Dimensional Flow Diffusers in division Quasi-One-Dimensional Flow of Aerodynamics

Answer 1

The correct answer is (b) Higher efficiency

Easiest explanation: In the case of fixed – geometry diffuser, the second throat area is kept larger than the first throat area so that there is no starting problem in the wind tunnel but it does not operate at its maximum efficiency.

Variable – geometry diffuser on the other hand can vary the throat area by hydraulic or mechanical means. The second throat area at the start of the operation is kept high enough so that the normal shock is able to pass through the diffuser and there is no starting problem. Once the wind tunnel starts working, its throat area is reduced so that it operates with a higher efficiency.