Gas Turbines Quiz | Technology and Computers

Quiz Answer Key and Fun Facts

1. First some conceptual questions: A thermodynamic cycle describes a unique series of pressure, temperature, entropy, and volume changes that a fluid (e.g. air, water vapor, etc.) will undergo in order to produce power. For example, the Diesel Cycle describes the operations performed on the air inside a Diesel engine cylinder. Which thermodynamic cycle describes the operation of a gas turbine?

Answer: Brayton Cycle

The Rankine Cycle describes the operation of a steam engine, in which the operating fluid is continuously flowing water/steam. The Otto Cycle describes the operation of a four-stroke gasoline engine, where the operating fluid is a trapped volume of gas such as air.

The Ericsson Cycle describes the operation of a largely theoretical external combustion engine, which uses a continuously flowing stream of gas as the operating fluid. The Brayton Cycle describes the compression, heat addition (or combustion), and expansion of a continuously flowing stream of gas as it passes through a gas turbine engine.

2. In the ideal Brayton Cycle, compression of the gas is assumed to be:

Answer: Isentropic (constant entropy)

In the ideal Brayton Cycle both the compressor and the turbine are assumed to operate isentropically (i.e. they do not change the amount of entropy of the gas). Gas pressure, temperature, and enthalpy (heat content) all increase as the gas is compressed. Of course, since no real compression process is isentropic, the ideal Brayton Cycle is only an approximation of the operation of a real engine.

3. In the ideal Brayton Cycle, heat addition (usually in the form of combustion) occurs:

Answer: Isobarically (at constant pressure)

In the ideal Brayton Cycle, heat addition is assumed to occur at a constant pressure. Unlike the ideal cycle's isentropic compression and expansion, this portion of the cycle more closely matches what happens in a real gas turbine.

4. In an axial-flow gas turbine's compressor (the type commonly seen on an airliner engine) the spinning blades are called:

Answer: Rotors

Stators are the stationary blades located between the rotating stages of the compressor. An impeller is the spinning portion of a centrifugal compressor. The only gas turbines found with a propeller are turboshafts (on helicopters) or turboprops (on airplanes), which spin the propeller through a series of shafts and gears.

The propeller does not participate in the thermodynamics of the gas turbine portion of the engine.

5. The MiG-15, made famous during the Korean War, was equipped with a different kind of compressor than that found on modern jet fighters and airliners. However, this type of compressor still sees use on some land-based gas turbines and some turboprop aircraft. What kind of compressor did the MiG-15 use?

Answer: Centrifugal

The MiG-15's engine was essentially a reverse-engineered Rolls-Royce Nene centrifugal-flow jet engine. Modern jet fighters and airliners use axial-flow compressors, which have a smaller frontal area than centrifugal-flow compressors and are capable of higher compression ratios. The F-86 Saber, the MiG's primary opponent, was equipped with an axial-flow compressor.
Many turboprop aircraft still use centrifugal compressors due to their relatively small size.

6. The United States' M1 Abrams main battle tank is powered by a Honeywell gas turbine. Which of the following is an inherent advantage of a gas turbine engine and thus one of the reasons why a gas turbine was chosen for use on the Abrams?

Answer: It can use a variety of fuels

Gas turbine engines are relatively inefficient while idling, are generally more expensive to produce than piston engines, and are not as responsive as piston engines. However, Gas turbines are more flexible than gasoline or Diesel engines when it comes to the kind of fuels they can consume.

The Abrams' gas turbine engine can burn gasoline, Diesel, kerosene, and jet fuel. When conditions on the battlefield may limit what fuel types are available, it can be beneficial to be able to burn several different types. Additionally, a gas turbine typically has a higher power to weight ratio than a piston engine.

7. In which type of gas turbine does a certain portion of the air entering the engine bypass the combustor and the turbine?

Answer: Turbofan

In a turbofan the bypassing air contributes to the thrust of the engine. In typical jet fighter turbofans only a fraction of the air entering the engine is bypassed. In the turbofans of modern airliners the majority of the incoming air is bypassed. Land based gas turbines, in which thrust is not a desired attribute, do not bypass any of the incoming air.

8. The gas turbines developed for the HTRE project in the late 1950s and early 1960s and intended for use on extremely long-range bombers were unique in that:

Answer: A nuclear reactor took the place of a combustor

Modified jet engines were used for the HTRE-1, HTRE-2, and HTRE-3 experiments in which a relatively small nuclear reactor took the place of the traditional fuel-burning combustor. The engines were never used to power an aircraft although they achieved nearly full thrust during ground tests.

More recently interest has been shown in a land-based gas turbine which utilizes nuclear power for the heat addition phase of the gas turbine cycle. Preheating of air prior to entering the compressor would not increase the efficiency of a gas turbine.

However, gas turbines with regeneration will heat air exiting the compressor using otherwise wasted exhaust heat. This does result in an increase of efficiency.

9. Some of the earliest gas turbine work was performed by Hans von Ohain in Germany during the 1930s. What was the fuel von Ohain originally used in the first prototype built in conjunction with the Heinkel company?

Answer: Hydrogen

Von Ohain's first gas turbines had difficulty with combustion stability, resulting in combustion occurring after the fuel had already passed through the turbine. The higher flame speed of hydrogen gas solved the problem until improved combustors utilizing gasoline could be installed.

10. The Rolls-Royce RB211 series of turbofans is unconventional among typical airliner turbofans in that:

Answer: It has a triple spool design

Standard turbofans used on airliners have a double spool design. This means that the engine has a low pressure compressor followed by a high pressure compressor and a high pressure turbine followed by a low pressure turbine. Concentric shafts connect the low pressure compressor to the low pressure turbine and the high pressure compressor to the high pressure turbine respectively.

A triple spool design includes an intermediate pressure compressor and an intermediate pressure turbine.

Source: Author wilhelm33

This quiz was reviewed by FunTrivia editor crisw before going online.
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Gas Turbines Trivia Quiz

This quiz is about gas turbines, both on land and in the air.

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