Systems Final BGT stuff Flashcards
Bernoulli’s principle is stated as follows:
when fluid or gas is supplied at constant flow rate thru a duct, the sum of potential and kinetic energy is constant.
velocity pressure (ram air aka dynamic pressure) is inversely proportional to static pressure
how does Bernoulli’s act in diverging and converging ducts
if the air exits a converging duct, the pressure will be lower so velocity will be higher (increased pressure during compression)
if the air exits a diverging duct, pressure will be higher and velocity will be lower (increased velocity into turbine)
what is the gas turbine engine cycle called and it’s features
the constant pressure cycle aka The Brayton Cycle.
continuous thermodynamic cycle of gas turbine engine.
Called constant pressure because pressure is relatively constant across the compressor section as volume attempts to increase - increasing gas velocity.
DRAW/REFER DIAGRAM
What are the advantages of centrifugal compressors
- picks up air and accelerates it outwards
- more robust
- easier to manufacture and develop
- favoured for smaller engines
- used in combination with axial flow
- simplicity and ruggedness
what are the advantages of axial flow compressors
- each rotor stage accelerates air rearwards avoiding energy loss
- consumes more air
- higher pressure ratios
- more thrust for same frontal area
- increase pressure ratio by addition of stages
- improved efficiency
- improved SFC per unit thrust
- easier to maintain
What are the characteristics of guide vanes
- stationary vanes ahead of first stage
- directs airflow into 1st rotor stage at most desirable angle (not considered a stage (part of inlet)
what are the characteristics of rotor blades
- purpose is to move air rearward thru each stage (is a rotating unit within the compressor)
- are designed aerodynamically and have an angle of incidence (twist) similar to that of a propeller.
What is the purpose of stator blades
- to receive high velocity air and diffuse it, changing kinetic energy to potential as pressure (stationary unit within compressor)
- aerofoil in shape
What are the advantages of Multi-spool compressors
- operational flexibility
- high compression ratios
- control of stall
- improved efficiency
- As ALT increases, decreasing air density allows N1 to speed up which recovers pressure loss from N2.
What is a compressor stall?
- abrupt loss of axial flow efficiency in a turbine engine when compressor blade AoA becomes excessive.
surge can occur if there’s aerodynamically induced vibration aswell as fluctuating temp, fuel flow and thrust indications.
What is a positive and negative incidence compressor stall?
Positive incidence - low pressure stage at high speed
Negative incidence - high pressure stage at high speed
explain how the compressor stall occurs
- inlet air velocity and compressor rpm effect on airflow vectors form an angle of attack approaching aerofoil (comp blade). A compressor stall is an imbalance between these 2 vector quantities
- this causes the air to slow down through the compressor and can stagnate or reverse direction depending on the stall intensity.
what are the symptoms of compressor stall
- sound of air pulsating
- a mild flutter
- a louder pulsating sound
- sound of a violent backfire or explosion.
What are Transient and Hung compressor stalls
- A transient stall is a mild stall condition that’s usually not harmful to the engine and can correct itself after 1 or 2 pulsations. Cockpit gauges often won’t show a mild stall condition.
- A hung stall can significantly decay engine performance, cause loss of power, damage the engine or cause it to fail.
what is the list of common causes of compressor stalls
1) turbulent or disrupted airflow to the engine inlet
- reduces velocity vector
2) excessive fuel flow caused by abrupt engine accelerations
- increases velocity vector by increasing combustor Pb.
3) excessively lean fuel mixture caused by engine decelerations
- increases velocity vector by reducing combustor Pb.
4) contaminated or damaged compressor(s)
- velocity vector increased as compression goes down
5) damaged turbine components, causing loss of power to the compressor and low compression
- velocity vector increased as compression goes down
6) engine operation above or below designed RPM
what’s an example of how disrupted airflow causes a compressor stall
eg. a twin business jet with rear mounted engines
- if it goes into a sharp turn, fuselage will partially block airflow to engine causing reduced inlet air velocity and increased effective AoA leading to the stall.
how will the pilot know the engine is experiencing compressor stall, what is the appropriate action
- audible noise
- RPM fluctuations
- increase in EGT
OR a combo of the 3
action is to reduce power to allow inlet air velocity and RPM to return to normal
how does FOD or fuel system malfunction affect the compressor
- reversal of airflow occurs at such a force that bending stresses on the rear of compressor blades can cause them to contact stator vanes. at this stage a series of material failures can occur and disintegrate the rotor system leading to complete engine failure.
what is the function of a turbine? how does it produce shaft power?
- the function of the turbine is to drive the compressor but can also drive accessories and shaft or turbo prop or turbo shaft engines.
- converts kinetic and heat energy into mechanical work to drive components
what is the principle of shaft power creation in a turbine
** when gas reaches turbine entry it’s at it’s hottest
- extracts energy from gases by lowering pressure
- lowering pressure in converted to velocity by convergent nozzles at TE of stator and rotor blades.
what are the characteristics of the impulse blade design
- A stator and rotor blade arrangement whereby the vanes from convergent ducts and the blades form straight ducts. The rotor is then turned by impulse as gases impinge on blades
what are the characteristics of the impulse reaction design
- consists of rotor blades with compound curvature
- twist in the blade is designed to distribute the workload evenly along the blade length by keeping the exit pressure and velocity uniform from base to tip
- accomplished by extracting different amounts of Ek at various blade sections.
- 50% impulse 50% reaction is most efficient (reaction greatest at tip and impulse at root)
describe how pressure thrust is formed
- exit velocity reaches speed of sound in relation to EGT and nozzle is choked (no further speed increase available)
- upstream total pressure rises above prop nozzle choking pressure and the static pressure at exit rises above atmospheric pressure.
- pressure difference across nozzle is the pressure thrust which is additional thrust due to momentum change of gas flow.
what is the role of convergent and divergent passages
convergent - the shape causes gas to accelerate
divergent - the shape causes gas velocity to decrease and pressure to increase.
