Gas Turbine Flashcards
Describe the operation of a gas turbine engine
The gas turbine engine has four stages of work: suck, squeeze, bang and blow.
- When the compressor turns, it generates a low air pressure which sucks ambient air into the inlet. The airflow is then squeezed by rotating blades in the compressor to increase the air density, pressure and temperature before entering the combustion chamber.
- Once it reached the combustion chamber, fuel is added to form a fuel air mixture, then burned at constant pressure to generate a high velocity flow by converting the chemical energy of fuel into kinetic energy of the airflow.
- The high energy exhaust is then blasted through the turbines, which is connected to the compressors by the same shaft. The turbines convert some of the kinetic energy in the flow into mechanical energy that drives the shaft, therefore drives the compressor and produces power.
- The flow is then passing through the exhaust nozzle to convert the remaining kinetic energy into thrust.
For a typical gas turbine engine, plot a diagram illustrating the change in gas velocity, temperature and pressure for each stage of the engine, explaining with the changes are occurring.
Check Page 33 of the slides
https://app.box.com/s/zrhyh546jdezkzpn8ydytnyht5tunn6t/file/476130785521
Q4 b
List three major components of the gas turbine engine, stating current materials solutions for these parts.
- compressor blades - titanium or nickel based superalloys
- combustion chamber - ceramic matrix composites (CMCs) and nickel-based superalloys.
- turbine blades - nickel-based superalloys or advanced CMCs that offer superior thermal and mechanical properties.
Describe the key differences between a two shaft and a three shaft engine in terms of turbine and compressor components.
In three shaft engines, the big fan, high-pressure and the low-pressure compressors are driven by three different shafts so that they operate at different rpm; in two shaft engines, the big fan and low-pressure compressors are driven by the same shaft.
Describe the four forces on a fluid element which are considered when deriving thrust equations, and show that thrust is essentially conveyed through area change.
End Face Force
Sidewall Force
Friction Force
Body Force
For Mathematical proof, go on Page 43
Describe the operation of an axial compressor in a gas turbine engine. How does a centrifugal compressor differ from this and why do modern engine designs show a preference for axial compressors?
- Compressor in a gas turbine converts the kinetic energy of the drive shaft into the potential energy of the airflow.
- Axial compressor has multiple steps of rotors and stators, their blades form divergent ducts, which slow down the incoming air flow and increase the air pressure and temperature.
- The annulus height is decreasing throughout the length of the axial compressor to increase air density and maintain constant axial flow speed. In an axial compressor the stage pressure increase is smaller than what is seen in a centrifugal compressor. Centrifugal compressor has a bigger pressure change due to bigger frontal area and has some weight issues.
Draw and describe the Brayton cycle upon which gas turbine operation is based.
Description:
- Brayton cycle is the basis for gas turbine operation
- Fresh air is drawn into the compressor at ambient conditions
- Compressor raises temperature and pressure of air
- High pressure air enters the combustion chamber where fuel is burned at constant pressure
- High temperature gases from combustion enter the turbine and expand to atmospheric pressure, producing power
- Exhaust gases are not recirculated, making the cycle open
- The open cycle can be modeled as a closed cycle using air standard assumptions
- In the closed cycle, combustion and exhaust processes are replaced by constant pressure heat addition and rejection from external sources
- Ideal Brayton cycle has 4 internally reversible processes: isentropic compression, constant pressure heat addition, isentropic expansion, and constant pressure heat rejection.
Drawing : Slide 201 of Mega PDF
Describe the criteria for a fan blade material in the gas turbine engine
- Light weight – due to large size of these components any weight saving can be critical.
- High strength/stiffness – CF Loads on fan blades are typically 100 tonnes.
- Good fatigue performance – LCF and HCF issues associated with rotation/vibration
- Good impact resistance – Bird strike/FOD etc
- Adequate fracture toughness
- Temperatures usually -50 °C to 50 °C. Not really a performance driver
Gas turbine engines can be classified into the following; turbojets, turbofans, turboprops, turboshafts, ramjets. Describe what is meant by each of these and how they may be used.
Gas turbine engines classification:
- Turbojets: Engines that produce thrust solely by jet propulsion from exhaust gases. Used in high-speed aircraft where speed is a priority.
- Turbofans: Engines that produce thrust from both jet propulsion and bypass air. Used in commercial airliners for efficiency and reduced noise.
- Turboprops: Engines that drive a propeller directly for propulsion. Used in smaller aircraft for general aviation and regional air transportation.
- Turboshafts: Engines that transfer power through a shaft to drive external machinery, such as helicopters and industrial applications.
- Ramjets: Engines that rely on high-speed forward motion to compress and combust air. Used in high-speed missiles and specialized applications.
Draw a plot of altitude vs. Mach number for propulsion systems, indicating limitations of different engine types.
Q1 part C
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What is meant by the bypass ratio of a gas turbine, and how is bypass air beneficial to overall efficiency?
Bypass ratio is the mass flow rate of air through fans, propellers or helicopter blades over the mass flow rate of air through the gas generator.
High bypass ratio indicates a larger mass of air is accelerated to a lower velocity for a higher propulsive efficiency, which increase the overall efficiency.
Describe the operation of the compressor in a gas turbine engine. Include in your answer comments about temperature and pressure changes across an axial compressor.
- Compressor in a gas turbine converts the kinetic energy of the drive shaft into the potential energy of the airflow.
- Axial compressor has multiple steps of rotors and stators, their blades form divergent ducts, which slow down the incoming air flow and increase the air pressure and temperature.
- The annulus height is decreasing throughout the length of the axial compressor to increase air density and maintain constant axial flow speed.
Explain the difference between installed and uninstalled thrust specific fuel consumption and why the differences arise.
The relationship between installed thrust specific fuel consumption (S) and uninstalled thrust specific fuel consumption (TSFC) is given by:
𝑆 = 𝑇𝑆𝐹𝐶(1 − 𝛷𝑖𝑛𝑙𝑒𝑡 − 𝛷𝑛𝑜𝑧𝑧𝑙𝑒)
- Where 𝛷𝑖𝑛𝑙𝑒𝑡 and 𝛷𝑛𝑜𝑧𝑧𝑙𝑒 represents inlet loss coefficient and nozzle loss coefficient respectively.
- These two coefficients are directly related to the friction drag from the inlet and nozzle, which decrease the installed engine thrust value and thus decrease the value of S, making S always smaller than TSFC.
Explain why afterburning may be used in the gas turbine engine.
- The afterburner uses unburned oxygen in the jet exhaust to burn additional fuel.
- An extended exhaust pipe is placed between the turbine and the final nozzle. With flame-on, the extra heat energy is added to the gas stream, thus raises the gas temperature and local sonic speed.
- With afterburner, the gas can be accelerated to a higher sonic choking speed in the nozzle and thereby generate higher thrust.
Describe processes by which the efficiency of the gas turbine engine has been increased. Your answer should focus mainly on developments in the compressor and turbine sections of the engine.
For compressors:
- Reducing tip losses due to leakage between moving rotors and stationary walls (tighter clearance)
- Minimizing surface/volume ratios
- Developing low-drag laminar-flow blade cascades
- Careful study of 3-D flow fields to minimize secondary flow loss
- Using centrifugal compressors for small engines
For turbines:
- Using single-crystal structure blades, especially nozzle guide vanes
- Implementing multi-pass cooling in turbine blades
- Applying thermal barrier coating to the blades
- Casting bleed air holes into the blades’ trailing edge
- Manufacturing the blades and discs as a single structure
