# Gas Dynamics Questions and Answers – Diffusers

This set of Gas Dynamics Multiple Choice Questions & Answers (MCQs) focuses on “Diffusers”.

1. The purpose of the diffuser is to slow down the flow to lower velocity with ________
a) Minimum loss of total temperature
b) Minimum loss of temperature
c) Minimum loss of total pressure
d) Minimum loss of pressure

Explanation: A diffuser is a duct designed to decrease the velocity of an incoming fluid at the exit. However, in doing so the total pressure of the fluid is reduced by some amount, which in turn results in inefficiency or a loss of capability to do a required amount of work. Hence the purpose of the diffuser is to minimize the total pressure loss while slowing down the fluid.

2. What is the minimum pressure ratio required to obtain a supersonic flow in the nozzle test section?
a) A*1/A*2
b) A*2/A*1
c) A*1/Ae
d) A*2/A*e

Explanation: Since the actual supersonic diffuser have two throat section to reduce the flow velocity, we can say that by the continuity equation,
ρ*1V*1A*1 = ρ*2V*2A*2
Now at throat V*1 = V*2 = a*
A*1/A*2 = ρ*2/ρ*1 = p*2/p*1     (As T*1 = T*2)
And from isentropic relation at M = 1,
$$\frac {p}{p_0} = (\frac {γ + 1}{2} )$$-γ/(γ-1)
Hence p*2/p*02 = p*1/p*01
Therefore A*1/A*2 = p02/p01

3. A supersonic oblique shock diffuser has a better pressure recovery than a normal shock diffuser.
a) True
b) False

Explanation: From the basic understanding of shocks, it is evident that the flow will experience more pressure loss across the normal shock compared to the oblique shock as the normal shock is the strongest one. And since the function of the diffuser is to minimize the pressure loss while slowing down fluid, the oblique shock diffuser provides better efficiency than normal shock diffuser.

4. If the diameter of the first throat is 0.43 m and the pressure ratio across the shock is 0.82 bar, then what will be the required diameter of second throat for the flow to reach to sonic speed inside the nozzle?
a) 0.427
b) 0.430
c) 0.462
d) 0.474

Explanation: Since the required pressure ratio for a flow to reach supersonic velocity is given by;
A*1/A*2 = p02/p01
Then, for given throat area and pressure ratio,
$$\frac {d_1^2}{d_2^2}$$ = 0.82
$$\frac {0.43^2}{d_2^2}$$ = 0.82
Hence, d2 = 0.474 m

5. What should be the ratio of diffuser throat to the nozzle throat area, if a nozzle is to be designed for Mach number = 2?
a) 1.387
b) 2.682
c) 1.765
d) 2. 593

Explanation: If we assume the normal shock wave at the inlet of the diffuser, then as per isentropic flow properties the total pressure ratio across the shock is,
p01/p02 = 0.7209
And since throat area ratio is; A*1/A*2 = p02/p01
A*1/A*2 = 1/0.7209 = 1.387

6. In a supersonic diffuser, the First throat must always be larger than the second throat.
a) True
b) False

Explanation: Since the flow always passes through the shock wave inside the diffuser, the total pressure always decreases across the shock wave. Hence the p02 < p01 always. And if the second throat is smaller than the first throat the diffuser will chock and it will not be able to pass the mass flow coming from the isentropic, supersonic expansion through the nozzle.

7. The supersonic wind tunnel having undesired Mach number in the test section is termed as __________
a) Imbalanced
b) Imperfect
c) Unstarted
d) Ideal

Explanation: When the ratio of diffuser throat to the nozzle throat area is less than unity, the diffuser chocks, that causes the flow to be subsonic and also results in the non – uniform flow. Hence the Mach number inside the test section becomes undesired and the tunnel is termed as unstarted.

8. Which is the desired angle for a diffuser or diffuser walls?
a) 5°
b) 6°
c) 7°
d) 8°

Explanation: A supersonic wind tunnel should be designed to have a small angle diffuser, with an angle of about 5° between opposite walls. As a result of this diffuser length increases which allows incorporating the devices that help to prevent flow separation.

9. Which of the following expression defines the diffuser efficiency?
a) η = Δp0/Δp
b) η = Δq/Δp
c) η = Δp/Δp0
d) η = Δp/Δq

Explanation: The diffuser efficiency is defined as, “the ratio of the rise in the static pressure to the loss in the dynamic pressure.”
Hence, η = $$\frac {2(p_2 – p_1)}{\rho V_1^2 – \rho v_2^2}$$
i.e. η = Δp/Δq

10. In a diffuser the flow is _________
a) Non – isentropic
b) Isobaric
c) Isentropic
d) Isothermal

Explanation: The flow inside the diffuser is always non – isentropic as there is an occurrence of the normal or an oblique shock wave and interaction of shock waves with boundary layers which results in large entropy gradient.

Sanfoundry Global Education & Learning Series – Gas Dynamics.

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