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Unit 4: Centrifugal Compressors - Practice Quiz

ASE202 — Propulsion-I 60 Questions
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1 What is the primary function of the impeller in a centrifugal compressor?

Principle of operation of centrifugal compressor Easy
A. To increase the fluid velocity
B. To remove inlet swirl
C. To cool the working fluid
D. To reduce the fluid pressure

2 In a centrifugal compressor, air usually enters the impeller in which direction?

Principle of operation of centrifugal compressor Easy
A. Radially through the diffuser
B. Circumferentially through the volute
C. Tangentially through the casing
D. Axially through the eye

3 Why does the fluid move outward through a centrifugal compressor impeller?

Principle of operation of centrifugal compressor Easy
A. Because of gravitational action
B. Because of nozzle contraction
C. Because of diffuser rotation
D. Because of centrifugal action

4 What is the main purpose of the diffuser in a centrifugal compressor?

Principle of operation of centrifugal compressor Easy
A. To convert velocity into pressure
B. To increase the shaft speed
C. To convert pressure into velocity
D. To supply power to the impeller

5 Which component supplies mechanical work to the fluid in a centrifugal compressor?

Work done and pressure rise Easy
A. The rotating impeller
B. The stationary diffuser
C. The fixed volute
D. The inlet casing

6 For a centrifugal compressor, Euler's specific work is represented by which expression?

Work done and pressure rise Easy
A.
B.
C.
D.

7 If there is no prewhirl at the impeller inlet, what is the usual value of ?

Work done and pressure rise Easy
A. Zero
B. Greater than
C. Equal to
D. Equal to

8 What generally happens to the static pressure as fluid passes through the diffuser?

Work done and pressure rise Easy
A. It becomes zero
B. It decreases
C. It increases
D. It remains unchanged

9 In a compressor velocity diagram, what does represent?

Velocity diagrams Easy
A. Fluid relative velocity
B. Fluid absolute velocity
C. Local acoustic velocity
D. Blade peripheral velocity

10 In a compressor velocity diagram, what does commonly represent?

Velocity diagrams Easy
A. Blade peripheral velocity
B. Absolute fluid velocity
C. Local acoustic velocity
D. Relative fluid velocity

11 In a compressor velocity diagram, what does commonly represent?

Velocity diagrams Easy
A. Absolute fluid velocity
B. Blade peripheral velocity
C. Local acoustic velocity
D. Relative fluid velocity

12 Which vector relation connects absolute velocity , blade velocity , and relative velocity ?

Velocity diagrams Easy
A.
B.
C.
D.

13 How should diffuser vanes generally be oriented at their inlet?

Diffuser vane design considerations Easy
A. Normal to the incoming flow direction
B. Opposite to the impeller rotation
C. Parallel to the compressor shaft
D. Along the incoming flow direction

14 Why should excessive divergence be avoided in a diffuser passage?

Diffuser vane design considerations Easy
A. It can cause flow separation
B. It can eliminate pressure rise
C. It can increase shaft speed
D. It can reverse impeller rotation

15 What is a main advantage of using vanes in a centrifugal compressor diffuser?

Diffuser vane design considerations Easy
A. More controlled pressure recovery
B. Complete removal of fluid friction
C. Greater impeller rotational speed
D. Lower inlet stagnation pressure

16 What is prewhirl in a centrifugal compressor?

Concept of prewhirl Easy
A. Reverse motion caused by the volute
B. Radial motion removed from outlet air
C. Axial motion produced in the diffuser
D. Tangential motion given to inlet air

17 Which device is commonly used to produce prewhirl?

Concept of prewhirl Easy
A. Diffuser throat vanes
B. Volute collector vanes
C. Impeller exit vanes
D. Inlet guide vanes

18 What is a common effect of prewhirl applied in the direction of impeller rotation?

Concept of prewhirl Easy
A. It eliminates diffuser losses
B. It reduces inlet relative velocity
C. It stops the impeller rotation
D. It reverses the axial inlet flow

19 What is rotating stall in a centrifugal compressor?

Rotation stall Easy
A. A complete stop of the compressor shaft
B. A steady increase in mass flow
C. A uniform rise in diffuser pressure
D. A rotating region of separated flow

20 Rotating stall is most likely to occur under which operating condition?

Rotation stall Easy
A. Constant shaft-speed operation
B. High inlet-pressure operation
C. Normal design-point operation
D. Low mass-flow operation

21 Which sequence best describes the energy conversion in a centrifugal compressor?

Principle of operation of centrifugal compressor Medium
A. The diffuser supplies shaft work, and the impeller reduces the fluid's angular momentum
B. The diffuser adds stagnation enthalpy, and the impeller converts pressure into kinetic energy
C. The impeller reduces absolute velocity, and the diffuser increases stagnation temperature
D. The impeller adds stagnation enthalpy, and the diffuser converts kinetic energy into static pressure

22 Why does static pressure normally rise as air passes through an ideal diffuser?

Principle of operation of centrifugal compressor Medium
A. The diffuser adds shaft work while keeping stagnation pressure constant
B. The diffuser decreases velocity and converts kinetic energy into enthalpy
C. The diffuser increases angular speed while keeping density constant
D. The diffuser decreases flow area and converts pressure into velocity

23 In a conventional centrifugal compressor, how does the main flow direction change through the impeller?

Principle of operation of centrifugal compressor Medium
A. It enters radially and leaves approximately tangentially
B. It enters tangentially and leaves approximately axially
C. It enters radially and leaves approximately axially
D. It enters approximately axially and leaves approximately radially

24 Which Euler compressor equation gives the stagnation enthalpy rise when heat transfer and mechanical losses are neglected?

Work done and pressure rise Medium
A.
B.
C.
D.

25 A compressor has no inlet whirl. If and , what is the ideal specific work input?

Work done and pressure rise Medium
A.
B.
C.
D.

26 Air enters a compressor at . The actual work input is , , compressor efficiency is , and . What is the approximate total-pressure ratio?

Work done and pressure rise Medium
A.
B.
C.
D.

27 In an ideal adiabatic diffuser, velocity decreases from to . What is the approximate static enthalpy rise?

Work done and pressure rise Medium
A.
B.
C.
D.

28 A radial-bladed impeller has , no inlet whirl, and slip factor . Assuming the ideal outlet whirl equals , what is the actual specific work?

Velocity diagrams Medium
A.
B.
C.
D.

29 At impeller exit, , , and the backward blade angle is measured from the tangential direction. Neglecting slip, what is ?

Velocity diagrams Medium
A.
B.
C.
D.

30 At the impeller outlet, and . What is the absolute flow angle measured from the radial direction?

Velocity diagrams Medium
A.
B.
C.
D.

31 A compressor has , , , and same-direction inlet prewhirl . What is the ideal specific work?

Concept of prewhirl Medium
A.
B.
C.
D.

32 At the impeller eye, and . If the blade inlet angle is from the tangential direction, what inlet whirl gives shockless entry?

Concept of prewhirl Medium
A.
B.
C.
D.

33 At the impeller eye, and . How does same-direction prewhirl of change the relative inlet speed compared with zero prewhirl?

Concept of prewhirl Medium
A. It increases from about to
B. It decreases from about to
C. It decreases from about to
D. It remains constant at about

34 For minimum incidence loss at the design condition, how should a diffuser vane's leading-edge angle be selected?

Diffuser vane design considerations Medium
A. It should remain tangential to the diffuser outer casing
B. It should remain radial for every impeller exit condition
C. It should match the absolute flow direction leaving the impeller
D. It should match the relative flow direction inside the impeller

35 What is the most likely result if the area expansion through a vaned diffuser is made excessively rapid?

Diffuser vane design considerations Medium
A. A strong adverse pressure gradient causes boundary-layer separation
B. A reduction in pressure recovery eliminates diffuser incidence
C. A rise in throat Mach number prevents all flow separation
D. A weak favorable pressure gradient causes excessive acceleration

36 If the effective throat area of a vaned diffuser is reduced while inlet stagnation conditions remain fixed, what happens to its choking mass-flow capacity?

Diffuser vane design considerations Medium
A. It remains unchanged because choking depends only on impeller speed
B. It increases because the diffusion ratio becomes larger
C. It decreases because the minimum flow area becomes smaller
D. It doubles because the throat static pressure becomes lower

37 Why is a short vaneless space commonly provided between the impeller exit and vaned diffuser?

Diffuser vane design considerations Medium
A. It eliminates slip by forcing the flow to rotate with the casing
B. It adds shaft work before flow reaches the diffuser throat
C. It smooths circumferential nonuniformities before flow enters the vanes
D. It maintains constant static pressure across the entire diffuser

38 Which observation most clearly indicates rotating stall in a centrifugal compressor?

Rotation stall Medium
A. One or more separated-flow cells travel around the annulus below rotor speed
B. All blade passages choke simultaneously at the design rotational speed
C. The discharge pressure rises steadily without any flow-field disturbance
D. The complete compressor flow reverses uniformly at every blade passage

39 How does rotating stall differ most directly from compressor surge?

Rotation stall Medium
A. Rotating stall occurs only at high flow, while surge occurs only at design flow
B. Rotating stall reverses shaft rotation, while surge increases shaft speed
C. Rotating stall is always axisymmetric, while surge affects one passage at a time
D. Rotating stall is circumferentially nonuniform, while surge is a system-wide oscillation

40 Why is rotating stall more likely when a centrifugal compressor operates well below its design mass flow?

Rotation stall Medium
A. Reduced impeller speed raises the diffuser throat area and causes choking
B. Reduced meridional velocity increases incidence and encourages flow separation
C. Increased inlet temperature aligns the flow with every blade passage
D. Increased meridional velocity removes the adverse pressure gradient

41 For an adiabatic centrifugal-compressor stage, station 1 is at the impeller eye and station 2 is at the impeller exit. Which statement correctly separates the roles of the impeller and an ideal stationary diffuser?

Principle of operation of centrifugal compressor Hard
A. The impeller raises stagnation enthalpy by , while the diffuser mainly converts kinetic energy into static enthalpy.
B. The impeller and diffuser each raise stagnation enthalpy by one-half of .
C. The diffuser raises stagnation enthalpy through torque, while the impeller only increases the absolute velocity.
D. The impeller raises static enthalpy without changing stagnation enthalpy, while the diffuser supplies the Euler work.

42 A centrifugal impeller has radial entry, , , backward-swept blades with measured from the tangential direction, and slip factor . Using , what is the specific Euler work?

Work done and pressure rise Hard
A.
B.
C.
D.

43 Air enters a compressor stage at . The actual specific work is , , , and the total-to-total isentropic efficiency is . Neglecting heat transfer, what total-pressure ratio is produced?

Work done and pressure rise Hard
A.
B.
C.
D.

44 Across an adiabatic impeller, the Euler work is . The absolute speeds are and . Neglecting potential-energy change, what is the static enthalpy rise within the impeller?

Work done and pressure rise Hard
A.
B.
C.
D.

45 At an impeller eye, and with no inlet whirl. If the relative inlet angle is measured from the tangential direction, which pair gives the relative speed and angle required for incidence-free entry?

Velocity diagrams Hard
A.
B.
C.
D.

46 At the impeller exit, the absolute speed is at from the tangential direction, while . What are the magnitude and direction angle from the backward tangential direction of the relative velocity?

Velocity diagrams Hard
A.
B.
C.
D.

47 An impeller inlet blade angle is fixed at from the tangential direction. At an off-design condition, and . What inlet whirl is required for zero incidence?

Velocity diagrams Hard
A.
B.
C.
D.

48 In an ideal incompressible vaneless diffuser of constant width, the radius doubles between inlet and exit. Assuming conservation of angular momentum and mass, what fraction of the inlet kinetic energy per unit mass is ideally converted into static enthalpy?

Diffuser vane design considerations Hard
A.
B.
C.
D.

49 A vaned diffuser receives flow with and . Ignoring blockage and boundary-layer deviation, what leading-edge metal angle measured from the tangential direction minimizes incidence?

Diffuser vane design considerations Hard
A.
B.
C.
D.

50 A vaned diffuser exhibits suction-surface separation because each passage imposes excessive diffusion. Which redesign offers the most defensible trade-off while retaining approximately the same overall area ratio?

Diffuser vane design considerations Hard
A. Increase vane solidity and reduce diffusion per passage, accepting higher friction and blockage.
B. Decrease vane solidity and increase passage divergence, accepting lower turning and blockage.
C. Increase leading-edge thickness and preserve passage divergence, accepting stronger incidence sensitivity.
D. Decrease throat area and preserve vane solidity, accepting an earlier choking boundary.

51 For fixed impeller speed and fixed exit whirl, positive inlet prewhirl is introduced in the direction of rotation. What simultaneous first-order effects follow from the Euler equation and inlet velocity triangle?

Concept of prewhirl Hard
A. Specific work increases, and inlet relative speed generally decreases.
B. Specific work decreases, and inlet relative speed generally decreases.
C. Specific work remains constant, and inlet relative speed generally decreases.
D. Specific work decreases, and inlet relative speed generally increases.

52 At an impeller eye, and . Introducing positive prewhirl of leaves unchanged. By approximately what percentage does the inlet relative speed decrease?

Concept of prewhirl Hard
A.
B.
C.
D.

53 A compressor has , , , and . What Euler work is transferred to the fluid, and how does it compare with zero prewhirl?

Concept of prewhirl Hard
A. , a decrease of
B. , an increase of
C. , a decrease of
D. , an increase of

54 A rotor runs at . Two equally spaced rotating-stall cells propagate in the direction of rotation at of rotor speed. What dominant stall-cell frequency should a stationary casing pressure probe detect?

Rotation stall Hard
A.
B.
C.
D.

55 Which observation most strongly identifies rotating stall rather than surge in a centrifugal compressor rig?

Rotation stall Hard
A. Circumferential probes detect phase-shifted pressure disturbances moving around the annulus.
B. The compressor operating point repeatedly traverses the entire unstable system characteristic.
C. The delivery duct exhibits periodic flow reversal synchronized across the full annulus.
D. The plenum pressure and total mass flow undergo nearly axisymmetric low-frequency oscillations.

56 As a fixed-speed centrifugal compressor is throttled below its design flow, which mechanism most directly promotes rotor rotating-stall inception?

Rotation stall Hard
A. The inlet velocity triangle departs from the blade angle, causing localized leading-edge separation.
B. The impeller tip speed decreases locally, causing alternate passages to lose centrifugal loading.
C. The diffuser stagnation temperature decreases, causing condensation at the rotor leading edge.
D. The Euler work necessarily becomes negative, causing uniform pressure collapse across every passage.

57 Static pressure can rise substantially inside a centrifugal impeller even when the absolute speed increases from inlet to exit. Which explanation is physically correct?

Principle of operation of centrifugal compressor Hard
A. Static pressure rises only because the downstream diffuser transmits pressure upstream into the impeller.
B. Coriolis acceleration supplies heat to the fluid without requiring torque from the impeller shaft.
C. Absolute acceleration alone raises static pressure according to the stationary-flow Bernoulli equation.
D. Shaft work raises stagnation enthalpy, while relative diffusion and centrifugal pressure gradients raise static enthalpy.

58 A radial-bladed impeller has zero inlet whirl, tip speed , and slip factor . Neglecting blockage, the slip model gives . What is the theoretical specific work transferred to the fluid?

Work done and pressure rise Hard
A.
B.
C.
D.

59 An adiabatic stationary diffuser reduces absolute velocity from to . If there are no losses or changes in potential energy, what static enthalpy rise is available?

Diffuser vane design considerations Hard
A.
B.
C.
D.

60 A vaned-diffuser throat is choked while upstream total pressure, upstream total temperature, gas properties, and discharge coefficient remain fixed. If the effective throat area is reduced by , what happens to maximum mass flow?

Diffuser vane design considerations Hard
A. It increases by , because the reduced area raises throat velocity.
B. It decreases by , and lowering downstream pressure cannot restore it.
C. It remains unchanged, because sonic velocity fixes mass flow independently of area.
D. It decreases by , and lowering downstream pressure fully restores it.
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