GAS TURBINE OPERATION Flashcards
What is the minimum on all Gas Turbine Engines?
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On all gas turbine engines it will minimally include the compressor. combustion chamber. and turbine (Figure 3.1-3). Additionally, on a turbofan, turboprop and turboshaft, it will include their respective fan, propeller or rotor blades. Each of these components plays a vital role in the production of thrust.
BRAYTON CYCLE consist of 4 events that occur simultaneously. 1 2 3 4
1 INTAKE 2. COMPRESSION 3. COMBUSTION 4. ESHAUST THE BRAYTON CYCLE A gas turbine engine follows a cycle of operation known as the Brayton Cycle (Figure 3.1-4). This operating cycle consists of four events which occur simultaneously: intake. compression. combustion and exhaust. It is important to note that this cycle of operation is different than the operating cycle of a reciprocating engine.
- Describe the relationship between pressure and velocity in a closed system.
- Together they make up total pressure.
- How does subsonic airflow react through a convergent opening? A divergent opening?
- Convergent: velocity increases and pressure decreases. Divergent: velocity decreases and pressure increases.
- How does supersonic airflow react through a convergent opening? A divergent opening?
- Divergent: velocity increases and pressure decreases. Convergent: velocity decreases and pressure increases
- What happens to the airflow as it passes through a nozzle? A diffuser?
- Nozzle: velocity increases and pressure decreases. Diffuser: velocity decreases and pressure increases.
- What are the three sections of a gas generator?
- Compressor, burner, turbine.
Q-9B-0020K Sheet 4 of 4
- How does a gas turbine engine produce thrust?
- By rapidly compressing, heating and accelerating a large quantity (mass) of air and fuel.
FOR TRAINING USE ONLY 503
TRAINEE GUIDE Q-9B-0020K Assignment Sheet 5-1-3 Sheet 2 of 4
7. What cycle explains the movement of air through a gas generator?
- Brayton Cycle.
- What happens to the velocity of the gases as they pass through the compressor section? The burner section? The turbines?
- Compressor: Remains fairly constant. Burner and turbine: Increases.
- What is gross thrust?
- Thrust measured at exhaust at standard day conditions 29.92 and 15 °C.
- What is net thrust?
- Thrust measured under normal flight conditions.
- When are gross thrust and net thrust equal?
- When aircraft is in a static position and standard day conditions.
- What affects the density of the air mass?
- Temperature, pressure, and altitude.
- What happens to thrust when air density decreases?
- Thrust decreases.
- What happens to thrust when air temperature increases?
- Thrust decreases.
- What happens to thrust when air pressure decreases?
- Thrust decreases.
- What is the relationship between pressure and temperature as altitude increases?
- Temperature and pressure both decrease with an overall decrease in thrust.
- What is the optimum cruise altitude and why?
- Approximately 36,000=, temperature remains isothermal (A line of equal or constant temperature on a graph or chart, such as a weather map) No more decrease in Temp.
- How does airspeed and the ram effect change the amount of thrust produced at subsonic and supersonic airspeedS?
- Airspeed increase without ram effect causes thrust to decrease.
Airspeed increase with ram effect causes thrust to remain relatively constant at subsonic speeds and increased greatly at supersonic speeds.
Recall that increasing the pressure of airflow will increase its potential energy, while
increasing the velocity of airflow will increase its kinetic energy.
In a gas turbine engine, the inlet, compressor, and diffuser sections build the potential energy of the airflow. In the burner, turbine and exhaust sections, this potential energy, along with the energy from fuel ignition, is converted into kinetic energy. The kinetic energy is then used for thrust production. Therefore, in the first half of a gas turbine engine, high pressures are desired, while high velocities are desired in the second half.
WHY IS THE HI PRESSURE COMPRESSOR AFT OF THE LOW PRESSURE COMPRESSOR?
A basic law of aerodynamics states that the speed of sound increases as the air temperature increases. Since the air temperature is increased through the compression phase, the high-pressure compressor can attain higher speeds without exceeding the speed of sound (Mach). Also, the blades of the high-pressure compressor are shorter than those of the low-pressure compressor, and can turn faster before exceeding their limiting Mach number.
The turbine section is the most highly _________ part of the engine.
The turbine section is the most highly stressed part of the engine. Not only must it operate at temperatures nearing 2,000° F (monitored in the cockpit), but it must do so while undergoing severe centrifugal loads imposed by rotational speeds of over 10,000 rpm. The higher the temperature the turbine section can bear, the higher the thrust that can be produced.
- What is the purpose of the engine inlet?
- Acts as a diffuser and provides a turbulent-free supply of air to the face of the compressor.
- Why should the inlet duct be constructed with a straight section?
- Smooth out turbulent airflow.
- What are the advantages of a single-entrance duct?
- Simplest and most effective at providing smooth airflow.
- What is the shape of a subsonic inlet duct? Why?
- Divergent, decrease velocity I increase pressure.
- What is the shape of a supersonic inlet duct? Why?
- Convergent/Divergent, decrease velocity I increase pressure.
- What is the purpose of a variable geometry inlet duct?
- To allow a supersonic aircraft to fly in all flight regimes (supersonic and subsonic).
- What are the advantages and disadvantages of a centrifugal compressor? An axial flow compressor?
- See the associated sections within Chapter 2.
- What are the parts of a centrifugal compressor and what is the function of each?
- Impeller- increase velocity, pressure and total pressure. Diffuser- decrease velocity, increase pressure.
Manifold - route air mass to the burner.
- What are the components of an axial flow compressor?
- Rotors, stators
- Why do rotor and stator vanes within an axial compressor decrease in length from the front to rear of the compressor?
- The cross sectional area decreases within the compressor from fore to aft to allow the velocity to remain fairly constant as pressure increases. Therefore, the rotor and stator length requirements will decrease.
- What do inlet and exit guide vanes do?
- Redirect the airflow.
- What is the function of the diffuser?
- Final decrease in velocity, and increase in pressure prior to combustion.
- Describe a dual-spool axial flow compressor. Why is it used?
- Two separate compressors, driven by its own turbine, to obtain higher compressor ratios.
- Where is fuel introduced within a gas turbine engine?
- In the burner.
- What are the criteria for a good burner section?
- Minimum pressure loss. 2. High combustion efficiency. 3. Flame must not blow out. 4. Contain the total combustion process.
- What are the three types of combustion chambers? What are the advantages and disadvantages of each?
FOR TRAINING USE ONLY 537
- Can- ease of maintenance, cause cold spots on turbine. Annular- even heat, complicated maintenance. Can-annular- even heat, ease of maintenance, expensive.
- What is primary and secondary air?
- Primary air= 25% mixes fuel for combustion Secondary air= 75% cooling and flame control
FOR TRAINING USE ONLY 540
TRAINEE GUIDE Assignment Sheet 5-2-3
- What is the purpose of the turbine section?
- To turn the compressor and accessories.
- How much of the energy produced is used to turn the compressor and accessories?
- 75% of the total heat energy produced prior to the turbine section.
- What are the main parts of the turbine?
- Stators and rotors.
- What is creep?
- Turbine blade elongation and deformation.
- What is the purpose of the exhaust duct?
- To increase velocity and decrease turbulence.
Q-9B-0020K Sheet 6 of 6
- Describe a subsonic and supersonic exhaust duct and how each operates.
- Subsonic - convergent, supersonic - convergent/divergent. Both will maximize the increase in velocity at the expense of pressure.
- Describe the four parts of an afterburner.
- Spray bars, flame holder, screech liner, variable exhaust nozzle.
- Describe how an afterburner operates.
- Secondary air from burner section along with bypassed air (turbofan) is mixed with fuel and ignited in the afterburner duct to augment thrust.
- What controls the amount of thrust augmentation an afterburner will produce?
- The more fuel introduced, the more thrust produced.
- What is the function of the spray bars? The flame holder?
FOR TRAINING USE ONLY 538
TRAINEE GUIDE Q-9B-0020K Assignment Sheet 5-2-3 Sheet 4 of 6
- Spray bars spray fuel. Flame holders hold the flame and they create local turbulence (eddies) to enable a better fuel/air mixture for combustion.
- What is a screech liner and how does it operate?
- It acts as a shock absorber to guard against pressure fluctuations or cyclic vibrations sometimes associated with AB.
- What type of exhaust nozzle is used on an afterburner equipped aircraft?
- Variableexhaustnozzle.
- How does subsonic airflow react through a convergent opening? A divergent opening?
- Airflow velocity will increase and pressure will decrease. Airflow pressure will increase and velocity will decrease.
The highest total air velocity in the compressor section is at the _______________.
The lowest air velocity in the entire engine is at the _______________
inlet of the last diffuser (Of the compressor section, not total engine)
diffuser outlet & combustor inlet.
The highest static air pressure in the entire engine is at the ___________________.
outlet of the last diffuser &
combustor inlet.
The highest total air velocity is found at the _____________
turbine nozzle prior to the turbine section.
