Download Exam 2 Review Problems

4261: Air-Breathing Engines
Exam #2
Name: _______________________
REVIEW
Show as much work as possible and qualify any assumptions you make
Circle the best answer choice or multiple answers that are correct. (2 Points Each)
1) A convergent-divergent geometry is a/an (excellent / moderate / poor / depends on
operating regime) choice for use as a supersonic inlet.
2) Afterburners generate temperatures that are hotter than those in the primary
combustor because they are easier to cool and there are no downstream rotating
components. The pressure in the afterburner is (lower / higher / about the same as /
depends on whether the afterburner is ON or OFF) relative to the main combustor.
3) The primary zone of a modern aircraft engine operates (below / near / above /
depends on throttle setting) relative to stoichiometric.
4) As the pressure in the combustor is increased, dissociation is (increased / does not
vary / suppressed / depends strongly on the fuel type / dependent on flow velocity).
5) The efficiency of a combustor (increases / decreases / exhibits no dependence /
depends on the flight speed) as the operating altitude of the aircraft is increased.
6) For a turbofan engine, increasing the bypass ratio at fixed flight Mach number and
keeping all else constant, the propulsive efficiency will (increase / decrease / remain
the same / cannot answer unless the actual bypass ratio is specified).
7) Typical aviation fuels have a hydrogen-carbon ratio of around (1 / 2 / 4 / 10 / 100).
8) The required length of a combustor designed to operate at sea-level is (longer /
shorter / the same as) as compared with a combustor designed to operate at 30,000 ft.
9) Turbulent flame formation within the primary zone of a modern combustor is
(encouraged / suppressed / not controlled / destructive to the combustor structure).
10) For a fixed turbine inlet temperature and static temperature (t) and fixed bypass ratio
() there exists an optimal compressor (size / inlet temperature / pressure ratio / exit
velocity) to maximize thrust.
11) Doubling the pressure in the combustor will (halve / more than halve / double / more
than double / will not change) the characteristic chemical time scales.
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Practice Question 1:
Consider the design of a turbojet engine for a supersonic aircraft. Assume the engine uses
conventional hydrocarbon fuel, with a heating value of 42.8 MJ/kg.
As discussed in class, there is a temperature limit at the inlet to the turbine (Tt4) which
may range as high as 2000 K due to turbine cooling (operates above the melting point of
the material). Another materials limitation is the exit temperature of the compressor (Tt3),
which due to its small size is difficult to cool, so no active cooling is currently used. Take
the current limit on Tt3 to be 1000 K.
For a turbojet engine with a compressor pressure ratio of 25, and a turbine inlet
temperature of 1800 K, under what flight conditions (Mach number) would the
compressor inlet temperature limit be exceeded? Consider performance at sea-level
(Tatm=300 K) and at 10 km altitude (Tatm=215 K).
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Practice Question 2:
Below is a schematic of a turbine within a turbojet engine. Parameters describing the
flow into and out of the turbine are given. The changes in flow properties across the
turbine result in a force that is carried by the casing around the blades and main shaft.
What is the magnitude and direction of this force?
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Practice Question 3:
An ideal bypass turbofan has a bypass ratio of 8 and a maximum allowable turbine inlet
temperature of 1,000 °C. The bypass nozzle exhaust and core engine nozzle exhaust
velocities are identical. For these conditions, and using any assumptions that you feel are
justifiable, estimate the compressor pressure ratio that will maximize take-off thrust?
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