Name General Intake Requirements
• Supply engine with sufficient massflow
• Convert velocity to static pressure at convenient Mach number at the fan face
• Allow low inner and outer total pressure losses
• Supply uniform inlet flow profile to the fan
• Provide noise reduction
• Prevent ice accretion
• Achieve low weight at high structural stiffness
• Integration with fuselage/wing
• Low initial and operating cost
Name two types of Intakes and their resprective characteristics
Subsonic Flight Intake
rounded edges, relatively short arrangement
Low static pressure rise
Low total pressure loss
Causes distorsion to fan and compressor at high incidence or cross-wind
Supersonic Flight Intake
sharp edges, long arrangement, potentially variable
High static pressure rise
High total pressure loss
causes distorsion to fan and compressor at any flight condition
Can develop intake buzz
Helicopter Intake Arrangements
Pressure loss from required particle filter
Loss and distortion from ducting to engine
Unsteady impact of rotor blades on filter inflow
Asymmetric „one- sided“ entry of flow to compressor inlet face
Figures of Merit of an intake system
Total pressure recovery (<1 due to losses) varies with Ma and AoA
Static pressure rise coefficient
Static pressure rise efficiency
Plot the static pressure rise efficiency vs the hade angle of a diffuser
Plot the streamline behavior in front of the intake at Standstill, Slow Flight and Cruise conditions
Intake Aerodynamics - Some Questions
True/False
Aero-engine intake operating conditions do not vary much due to an immutable and undisturbed upstream flow field an immutable and undisturbed upstream flow field
The flow field in a subsonic intake always remains subsonic once the approaching flow reaches the intake lip
As the main duty of an intake is to achieve a static pressure rise and flow decelleration it is very sensitive to design parameter choices
Because of friction and loss generation within the intake an axial force component occurs, which contributes to engine thrust
False
True
False (There is a suction on the lip however)
Name two effects that condensation can have on the cycle
1. Temperature increase in the intake due to condensation:
-> Degradation of the thermodynamic cycle
2. Temperature drop in the compressor due to evaporation:
→ Improvement of the thermodynamic cycle
Combination of both effects give a small change in engine overall thermal efficiency, positive or negative
Name three types of Thrust Loss due to Ice Accretion
Compressor surge:
Ice debris released from surfaces causes compressor surge, hence reduction of rotational speed and massflow
Combustor extinction:
Pieces of ice cause the flame to quench
Engine damage:
Pieces of ice damageindividual or full rows of aerofoils
Plot smith diagrams of a multi-stage compressor when
The compressor is throttled/de-throttled
The compressor speed is in-/decreased
What is described by the work line?
The sum of all operating points
Why are the rear stages the first to fail at high speeds?
Why does the HPC behave differently than the IPC during acceleration/deceleration?
What defines the maximum speed of the compressor?
Choking of the compressor
Why are the front stages the first to fail at low speeds?
At lower speeds the compressor delivers less pressure ratio, therefore also a lower density ratio.
The gaspath in the engine converges and is too tight for the less dense gas in the back -> Acceleration of the flow in the back
In the front the gas path is too big for the dense gas, therefore the velocity goes down and these stages are prone to stall.
How does the compressor pump or stop?
When the compressor reaches the surge line it, the flow detaches from the airfoils. The compressor cant deliver the required pressure and the higher pressure in the back of the engine accelerates to the front. After the gas is expelled out the front, the flow attaches to the airfoils and the compressor can deliver pressure again…
Plot a compressor map, label the axes correctly. Also plot airfoils near chocke, stall and under normal conditions
Why do the operating lines of the compressors IPC and HPC (Op. Line) have different slope?
How is acceleration and deceleration of the engine controlled?
How is it possible to influence the stability of the compressor during transient manoeuvers?
How is the engine controlled and monitored?
Which secondary systems are necessary for operation?
Name the euler equation and which assumptions were taken to derive it
Incompressible working fluid
Adiabatic and inviscid flow
Fully pressurized and rotationally symmetrical impeller (Blade nr. = ∞)
Gravitational influence is negligible
Name two Loading limits relevant for airfoil design
De-Haller Number:
Max. feasible diffusion → Condition to avoid separation at the hub & shroud:
DHRotor = w2/w1 ≥ 0.7
DHStator = c3/c2 ≥ 0.7
Diffusion Factor:
Max possible deflection → Conditions to avoid separation on the profile:
DFRotor = Δwu/u ≤ 0.5
DFStator = Δcu/u ≤ 0.5
What are the main reasons for the choice of the fan speed when scaling a fan
Performance cycle (Low PR should be achieved with low speed)
Aerodynamics (Mach no., loss, stability; all these get non-linearly worse when tip speed goes up)
Structural strength (High tip speeds cause high weight of drum and casing)
Noise (Noise emission regulations impose different needs for different fan sizes)
Plot a Fan Bypass Map inlucding working lines at Ma=0 & Ma=0.85 and mark the flight conditions MTO,CR,MCL
Fan Operation - Some Questions
Aero engine fans at different size can be derived from each other by photographically scaling the geometry
Due to subsonic flow conditions present in the bypass nozzle throat the fan working line remains unaffected by altitude and flight Ma number
Most fan maps are further constrained by aero-elastic blade vibration occuring in the design speed region
Bypass ratio is a fixed design parameter, which does not vary across different thrust settings during a flight mission
Name two basic approaches to containment of the fan
Softwall fan casing:
Made from aluminum alloy, over-wound with dry aramid fibers
Energy-absorbing capability, blade get stuck in the fibers
Hardwall fan casing:
Made from aluminum alloy only and designed to reflect the blade back into the engine
Enables building in a smaller radial envelope
Can cause secondary damage well in excess of softwall design
Innovation:
Use of ductile carbon-fiber reinforced polymer
Fan Design for Safety - Some Questions
Potential fan blade damage can be dynamically simulated with regard to bird particle trajectories and bird fragmentation
Certification authorities prescribe identical combinations of bird size and impact location for integrity tests on all fan blades
A fan blade-off event has little impact on the full engine structure and prediction of blade containment dynamics is obsolete
Modern simulations of blade containment events allow to predict time-resoved part stresses and displacements across the full engine
What is the difference in the Static Pressure Rise Mechanism of subsonic and supersonic sections?
Subsonic Sections (Ma1rel < 1) :
• Static pressure rise is achieved through continuous diffusion in the blade passage
• Strong degree of turning required
Supersonic Sections (Ma1rel > 1) :
• Shocks increase pressure, either a strong normal shock appears or multiple weaker oblique shocks
• Continuous diffusion through the remaining rear part of the blade passage
• Little turning required, shock achieves most of the overall pressure rise
Unique Incidence in Fan Blade Passage
Lowest losses
Relevance of Reynolds Number on Fan Blades
Operation at high altitude (low Re number) changes boundary layer state
Dominating source of loss and potential flow instability is the interaction between the passage shock and the suction side boundary layer
Boundary layer instabilities may occur, resulting in poor fan performance or vibrations
How are the losses of a fan in relation to the blade height
Fan Aerodynamic Design Aspects - Some Questions
„Unique incidence“ is a term relevant in the design of blade sections with supersonic inlet flow conditions in the relative frame
When de-throttling a blade section inlet flow angle and Mach number stop changing and a choke condition (=const. red. Massflow) is reached
Due to the well-defined choke regime of supersonic blade sections the aerodynamic design of these is straight-forward with low effort
What are the two main causes for fan noise?
Buzz saw noise:
Supersonic flow allows pressure surges on the fantip
Rotor Stator interaction:
Interaction of the wakes of the Rotor with the front edges of the stators
Name the main three types of nozzels and their applicability, pros&cons
Nozzle+Mixing (Civil, Low BPR):
+Reduction in noise
+Thrust improvement
-Long nacelle, bypass duct
Separate Jet/Nozzle (Civil, High BPR):
+Short nacelle, bypass duct
+Lower impact of nozzle on fan and core throttling
-No Thrust improvement
-No noise suppression
Variable Nozzle (Military):
+Thrust vectoring
+Used for afterburner
- Long arrangement
-High impact of nozzle on core throttling
Nozzle Configurations - Some Questions
Using lobed mixers in the engine exhaust system requires separate nozzles for core and bypass flow
Due to the strong vortex intensity and turbulence in lobed mixers these devices cause increased jet noise level and loss of net thrust
The quality of thrust generation in an engine nozzle strongly depends on the intensity of radial velocity vector components and endwall geometry
When Ma=1 occurs in the exit area of a divergent nozzle (p_ex=p_crit) the flow further accellerates/expands into supersonic flow conditions
Name three different nozzle types
Name the three main operating conditions of a transonic fan, sketch and explain them
Analyze the following picture
What ist the SAS and what is meant by “Build and Forget“?
Secondary Air = Air mass flow that do not primarily contribute to the thrust generation
„Build and Forget“- System, i.e. the system is installed and must function without any control or external intervention. However it can and must be controlled via measuring points
Name & explain the three different air extraction methods for the SAS
Air extraction at the housing:
Direct air extraction at the casing -> aerodynamic effects on the compressor
Air extraction at the outlet:
Combustion chamber and first turbine stage can only be cooled by this air (pressure has to greater than at the needed location)
Air extraction at the hub:
Vortex reducer necessary
Name the different fuel injection concepts for supersonic combustion and their respective advantages/disadvantages
Wall Injection:
+Simple manufacturing
+Good penetration in the nearfield
-Mixing in main stream insufficient
-High pressure loss
Step Injector:
+Minimal pressure rise(Area higher)
+Recirculation zone(flame holder)
-No wakes in logitudinal direction
-H2 stay near channel bottom
Cavity Injector:
+cavities can anchor and stabilize flames
-high thermal stress
-in total small exchange with the surrounding area
Swept Ramp Injector:
+better wake prudction -> better mixing
-relatively high total pressure loss
What nozzle types are used for ram and scramjets and why?
Ramjets:
Convergent-Divergent Nozzle
Subsonic->Supersonic
Scramjet:
Purely Divergent Nozzle (Mostly SERN, due to high expansion ratios)
Supersonic->Supersonic
Advantages/Disadvantages of SERN nozzles
Advantages:
Expansion self adapting to ambient pressure
Weight saving
Large scale integration
Disadvantages:
Pitching moment
How is the engine thrust measured nowadays and why?
No direct measurement of thrust, due to complex, heavy & costly designs
N1C:
+N1-Signal is in any case available for the control system
+The Fan-inlet temperature as well
-N1C vs. Thrust is a nonlinear function
-The relation between thrust and fan speed changes by fan degradation
→ New Trim Procedure !
EPR:
+Due to the direct correlation of nozzle pressure and thrust there is no need for additional trim procedures
+N1C is a redundant signal
-An additional reliable and precise pressure measurement is needed
-For EPR instead of IEPR only the bypass flow is considered
Whats a FADEC?
Full authority digital engine control
How is a helicopter engine controlled?
The engine load depends on:
Rotor Speed
Pitch Angle
Air Density
Measurement Value:
Pitch Angle Signal
Advantages and Disadvantages of Brush Seals
Leakage up to 70% less than labyrinth seal
Compensation of moving sealing gaps, e.g. rotor movement
Insensitive to shock and pollutant
Damping effect
Simple and fast replacement
Application Limits:
Max. pressure difference: 1500 [kPa]
Max. temperature: 700 [° C]
Max. sliding speed: 400 [m/s]
Only one rotating direction
Draw a M-n diagramm for a starting sequence
Pro & Contra Arguments for Heat Exchangers in Aero Engines
Pro:
Enhanced cycle efficiency
Potentially lower IR-signature
In some cases crucial component for new concepts
Con:
Additional pressure losses
Additional mass
Rather big dimensions
Lifetime considerations
Additional cost
Plot the working line of the different cmponents in the compressor/turbine maps
Name advantages and disadvantages of Two-Phase Heat Exchangers
Increase effective temperature difference (since (nearly) constant temperature on phase-changing side)
Reduce the required size of the heat exchanger (due to very high heat transfer on phase-changing side)
Reduce the fluid content on the phase-changing side (reduce mass)
Disadvantage:
Complex modeling and operation of two-phase flow
Potential instabilities
Pressure loss on phase-changing side tends to be rather high
Accuracy of available correlations quite low
Transient behaviour of HPC (Acceleration)
Transient behaviour of HPC (Decceleration)
Danger for the combustion chamber : „flame-out“
Transient behaviour of MPC (Acceleration)
Transient behaviour of MPC (Decceleration)
Bleed of Valves in the MPC are opened during deccelerataion to avoid surging
How does a HPC bleed influence the working line of the HPC front and rear stages?
How does a VSV influence the compressor map of a HPC?
How and why is Maximum Takeoff limited?
Limited by TET (Tt4)
High Thermal Stresses and gradients in temperature lead to limited cycles
How and why is Maximum Climb limited?
Highest aerodynamic loads on the engine
Engine runs at maximum rpm
High axial and circumferential velocities within the engine place this flight state at the edge of the components’ operability at the choke boundary
How is maximum cruise characterized?
Usually beginning of cruise (loss of weight results in lower thrust requirement)
Most time is spent in cruise (engine is optimized for these conditions)
How and why is Idle characterized?
There are two idle states: Ground Idle & Flight Idle
Lowest states, in which a safe operation is assured
Limiting is the aerodynamic stability in the components (safe distance from the surge line) and flame stability in the combustor
At ground idle, the rpm is usually at 25%-50% of N1 and around 60% N2
At flight idle, the rpm is even higher than at ground idle due to lower density
Fan Flutter
Aero-elastic phenomenon resulting from coupled interaction of aerodynamics and a flap or torsional mode of the fan blade
-> self-excited high-amplitude vibration
-> Can lead to HCF
-> increased risk of flutter on blisks, since the mechanical damping is lower
Pros and Cons of N1 and EPR measurements
N1:
Last changed4 months ago
