Combustion Chambers Questions and Answers

Answer: C

Correct answer: False, it is roughly four times leaner

The overall air-fuel ratio is about 60 to 1 by mass, against a stoichiometric ratio near 15 to 1, so the chamber as a whole is about four times leaner than a flame will sustain. The whole architecture of zones exists to resolve this: a fifth of the air makes the primary zone near stoichiometric, and the rest is added downstream.

Answer: D

Thermal formation of nitrogen oxides rises steeply with flame temperature, so it is at its worst at take-off. The products of incomplete combustion behave in exactly the opposite way, which is what makes the trade so awkward.

Answer: A

Correct answer: No, every serious design exceeds ninety nine per cent there

With all designs above 99 per cent, the differences lie in the last fraction of a per cent and are lost in measurement scatter, so the parameter ranks nothing. It is an essential requirement rather than a discriminator, and the real comparisons are made on pressure loss, pattern factor, stability and emissions.

Answer: B

Correct answer: Raising the velocity through the flame tube

Flame stability depends on the flow being slow enough for the flame to hold its position, so raising the velocity works directly against it and eventually blows the flame out. The other three all help: the diffuser slows the air, the swirler creates recirculation, and that recirculation carries hot products back to reignite the incoming mixture continuously.

Answer: C

A single can could be rigged and run cheaply with improvements then applied to the others. Testing a full annulus needs far more air and instrumentation, which is the price paid for its lower weight and better exit pattern.

Answer: D

Correct answer: The weak extinction limit

The weak extinction limit is a property of the primary zone - of the local mixture strength, the recirculation and the fuel preparation - and dilution holes are far downstream of it. What they do control is everything about the exit: the temperature pattern, its peak, and hence the pattern factor, which is why they are the feature adjusted during rig development.

Answer: A

Jets need momentum to penetrate and mix, and that momentum is bought with total pressure. The design balances mixing quality against the pressure it costs, which is why three to six per cent is typical rather than any smaller figure.

Answer: B

Correct answer: An improvement in exit temperature pattern

A light-around failure means the flame does not spread to every part of the chamber at start, so some sectors remain unlit while others burn. The exit temperature is then severely distorted, not improved, and the turbine may be damaged by the resulting hot streak. Interconnectors between tubes, or the continuous annulus, exist to make light-around reliable.

Answer: C

Thermal nitrogen oxide formation depends on time spent at high temperature as well as on the temperature itself. Longer residence helps completeness at idle and worsens nitrogen oxides at power, which is the same trade in another form.

Answer: D

Higher pressure raises reaction rates and makes ignition easier, which is why relight is straightforward on the ground. The other three all describe the conditions that make a high altitude restart the hardest ignition case of all.