Unit 6: THERMOACOUSTIC INSTABILITY - Practice Quiz

1 Thermoacoustics primarily studies the interaction between which two phenomena?

2 According to the Rayleigh criterion, acoustic oscillations are amplified when heat release is mainly:

3 What is an acoustic mode in a combustion chamber?

4 Which quantity commonly oscillates during thermoacoustic instability?

5 In a longitudinal thermoacoustic mode, the main acoustic variation occurs along the combustor's:

6 Which mode has pressure variations mainly around an annular combustor's circumference?

7 A transverse thermoacoustic mode primarily varies in a direction that is:

8 What is a common mechanical consequence of strong thermoacoustic instability?

9 How can thermoacoustic instability affect an aeroengine's operating range?

10 Which effect may result from unstable combustion in an aeroengine?

11 A one-dimensional thermoacoustic model usually assumes that variables change mainly along:

12 What is a major advantage of a one-dimensional thermoacoustic calculation?

13 What must be specified at the ends of a one-dimensional acoustic domain?

14 Why is a three-dimensional method useful for annular combustors?

15 In linear combustion instability analysis, disturbances are assumed to be:

16 What does a positive growth rate indicate in linear stability analysis?

17 Which device passively absorbs acoustic energy in a combustor?

18 What distinguishes active control from passive control?

19 Which action can help control thermoacoustic instability by changing flame response?

20 What is the main purpose of thermoacoustic instability control?

21 According to the Rayleigh criterion, which condition promotes growth of a thermoacoustic oscillation over one cycle?

22 A flame releases maximum unsteady heat when the local acoustic pressure is near its maximum. What is the most likely effect?

23 If the mean gas temperature rises while the combustor geometry remains unchanged, how will its acoustic resonance frequencies generally change?

24 Pressure measurements taken along a combustor axis show alternating axial nodes and antinodes, with little circumferential variation. Which mode is dominant?

25 An annular combustor exhibits a pressure pattern that travels continuously around its circumference. How should this instability be classified?

26 Two equal-amplitude azimuthal waves travel in opposite directions around an annular combustor. What pattern do they form?

27 Why can sustained high-amplitude thermoacoustic oscillations shorten combustor liner life?

28 A combustor oscillation causes periodic excursions of the local equivalence ratio toward very lean values. Which operational risk is most directly increased?

29 Which observation most strongly indicates that a combustor pressure oscillation is affecting turbine durability?

30 A uniform duct is closed at one end and open at the other. If and , what is its lowest acoustic frequency?

31 In a one-dimensional transfer-matrix model, what does each duct-element matrix normally relate?

32 In an – flame model, the heat-release fluctuation is written as . What is the main role of ?

33 Why is an impedance boundary condition used at the outlet of a one-dimensional combustor model?

34 What is the principal advantage of a three-dimensional linear analysis over a one-dimensional model for an annular combustor?

35 A three-dimensional eigenvalue calculation uses the convention with . Which result indicates an unstable mode?

36 A finite-element combustion-instability model predicts the passive acoustic modes accurately but gives incorrect growth rates. Which input should be checked first?

37 A Helmholtz resonator is installed on a combustor wall and tuned to the unstable frequency. How does it primarily suppress the instability?

38 An active controller commands a secondary fuel injector using a pressure-sensor signal. What phase should the controller seek at the unstable frequency?

39 Why can changing the fuel-injection location suppress a thermoacoustic mode even if the total mean fuel flow is unchanged?

40 A passive damper is effective at one operating condition but ineffective after engine power is increased. What is the most likely reason?

41 At a flame location, the acoustic pressure and unsteady heat-release rate are and . Neglecting acoustic losses, for which phase does the flame provide the maximum acoustic-energy input per cycle?

42 During each acoustic cycle, unsteady combustion adds energy equal to of the initial modal energy, while damping removes of that same initial energy. What is the approximate fractional increase in acoustic-pressure amplitude per cycle?

43 A flame transfer function is modeled as . At the flame, the velocity fluctuation leads the pressure fluctuation by phase . In the absence of damping, which condition gives maximum Rayleigh driving?

44 A combustor is approximated as a uniform duct closed at the injector and acoustically open at the nozzle. If and , which fundamental mode is expected?

45 In a perfectly axisymmetric annular combustor, the first azimuthal eigenfunctions are proportional to and . Which combination represents a wave spinning in one azimuthal direction?

46 Pressure probes equally spaced around an annular combustor record nearly equal amplitudes, while the measured phase increases approximately linearly through over one circumference. What type of mode is indicated?

47 A combustor exhibits large pressure oscillations but almost no first-order change in cycle-averaged heat release. Why can its mean thrust and efficiency nevertheless change?

48 The mean pressure drop across a fuel injector is only slightly larger than the amplitude of chamber-pressure oscillation. Which consequence is most likely?

49 Two unstable combustor modes have equal pressure amplitudes. Mode X is near a liner structural resonance, whereas Mode Y is far from all structural resonances. Which assessment is most defensible?

50 For a compact flame in a constant-area, low-Mach-number duct, with negligible mean-flow discontinuity, which acoustic jump model is appropriate?

51 A one-dimensional network model relates inlet and outlet acoustic states by . After homogeneous boundary conditions are applied, the system becomes . What determines a thermoacoustic eigenfrequency?

52 At a mode, velocity at the flame leads pressure by . The flame heat release follows velocity after a pure delay . What is the smallest positive delay producing maximum pressure–heat-release coupling?

53 A plane acoustic wave in a duct of characteristic impedance encounters a termination with impedance , both purely real. What is the pressure reflection coefficient?

54 A three-dimensional Helmholtz solver uses a flame response containing . Why is the resulting stability calculation a nonlinear eigenvalue problem?

55 A perfectly symmetric annular combustor has a degenerate pair of azimuthal modes. A small geometric asymmetry is introduced without changing the mean annulus length. What is the most likely linear effect?

56 In a non-self-adjoint three-dimensional thermoacoustic model, where does a small localized model change usually have the greatest first-order influence on an eigenvalue?

57 A solver assumes perturbations proportional to and reports . How should this mode be interpreted?

58 A Helmholtz resonator has neck area , effective neck length , cavity volume , and sound speed . Approximately what frequency is it tuned to?

59 Several installation points are available for a passive acoustic damper tuned to one unstable mode. Assuming comparable temperature and accessibility, where should it normally be installed for maximum coupling?

60 An active controller acts through a plant with negligible additional phase shift. To make its generated pressure exactly antiphase with a instability, what is the smallest positive total feedback delay?