introduction to propulsion systems - uliege.be · 2019-09-21 · introduction to propulsion systems...
TRANSCRIPT
Introduction to propulsion systemsIntegrated project: AIAA aircraft design competition1er master Aerospace 2019
Koen [email protected]
Remy [email protected]
Propulsion● Weight → lift
● Lift → drag
● Drag → thrust T
● Propulsive power
Propulsion: action = reaction● Rocket engines: ejection of high speed gas jet
● Acceleration of a stream tube
– Lift based / mechanical acceleration : propeller
– Thermal acceleration : turbojet
– Combination : turbofan
Rocket engine● Nozzle converts heat into speed
● Starting from high pressure oxydant & combustible
Thermodynamics – Brayton cycle● Thermodynamic cycle between Pt1 and Pt2
– Compression 1 → 2 : Wc
– Combustion 2 → 3 : Q
– Expansion 3-4 : Wt
● Energy balance
– Useful work : W= Wt - Wc
– Energy input : Q
– Thermal efficiency = W/Q
Turbojet - M ~ 1 … 3● Gas turbine: hot gas jet
– Recup/RAM inlet velocity : 1’ - 1
– Compressor : 1 - 2
– Combustion : 2 - 3
– Turbine -> compressor : 3 - 4
– Exhaust jet by adjustable nozzle : 4 – 5
● (Thrust vectoring)
Turbojet/fan – airbreathing jet M ~ 1 … 3● Gas turbine: hot gas jet
– Ram inlet velocity : 1’ - 1
– Compressor : 1 - 2
– Combustion : 2 - 3
– Turbine -> compressor : 3 - 4
– Afterburning : 4 – 4’
– Exhaust by adjustable nozzle : 4’ – 5
● (Thrust vectoring)
Turbofan – M = 0 … 1
● Bypass flow mb: acceleration by fan
● Core flow mc : power + hot gas jet
– Fan + compressor : 1 - 2
– Combustion : 2 - 3
– Turbine → fan & compressor 3 - 4
– Exhaust jet : 4 – 5
● Bypass ratio: BPR = mb/(mb+mc)
RAM/Scram: airbreathing M > 3● Cycle without machines
– Convert kinetic energy va in pressure Pt by “RAM” effect : 1 - 2
– Combustion : 2-3
● RAMJET : subsonic combustion
● Supersonic Combustion RAMJET
– Expand in nozzle to form jet : 3 – 4
Combined turbojet & ramjet
Propulsion approach 1 : Propeller● Mechanical acceleration of external flow
Propeller global operation (Rankine-Froude)
● Stream tube through propeller
● Propeller = actuator disk
● No losses up/down - Bernoulli
Propeller global operation (Rankine-Froude)
● Induction factor a
● Thrust computed two ways
– stream tube control volume
– pressure difference
Propeller global operation (Rankine-Froude)
● Thrust
● Power
● Propulsive efficiency
Lift and drag forces on an airfoil
wβ
wβ
L
L
D
D
Propeller: shape and operating parameters● Geometry
– Tip radius Rt
– (Radial distribution of) blade profile
– Radial distribution of chord C(r) / Solidity σ(r) = N C(r)/2 r) = N C(r) = N C(r)/2 r)/2
● Operating parameters
– Stagger or pitch angle ξ
– Rotation speed Ω → local blade speed u = Ω r
– Advance ratio J = va/ut = va/Ω rt
ξ
u
v
Propeller forces (Blade Element Method)● Induction factor a → v(1+a)
● Relative velocity w, flow angle β
● Lift/drag forces L & D
● Compute thrust T and tangential force U
● Recompute induction factor a from T
ξ
u
v(1+a)
w
L
DT
v
β
U
Propeller performance parameters● Thrust coefficient
● Power coefficient
Propeller: force balance
ξ
u
v(1+a)
w
L
DT
β
C
Propeller: variation of advance ratio
ξ
u
v(1+a)
w
L
DT
β
C
Propeller: pitch control
ξ
u
v(1+a)
w
L
DT
β
C
Propeller: use of pitch control
Choosing the propulsion system● From cruise speed
– Propeller : 0.1 < Ma < 0.7
– High BPR turbofan : 0.7 < Ma < 1
– Low BPR turbofan / turbojet : Ma > 1
– RAMJET : Ma > 2
– SCRAMJET : Ma > 5
The type of propulsion system
■How to choose between propeller, turbofan, turbojet or even ramjet?> From your cruise speed !> 0.1 < Mn < 0.7 => propeller> 0.7 < Mn < 1.0 => turbofan
The higher cruise speed, the lower bypass ratio> Mn > 1.0 => turbojet
> Mn > 2 = RAMjet or SCRAMjet> RAMjet uses aircraft speed recovery for compression
P = Ps0 + ½ rho V²
needs to be associated with jet engine for reaching the right speed> SCRAMjet is RAMjet with still supersonic combustion
Classification
■ Propeller> The best efficiency> Big size> Limited cruise speed
■ Open rotor> Very efficient> High cruise speed> Big size> Very noisy> Complexity
■Turbofan> Good efficiency> High cruise speed> Mid size> Low noise> Shroud additionnal weight
■Turbojet / nozzle> Small size & weight> Simplicity> The highest speeds> Poor efficiency