2P8 Aerothermal Flashcards
(28 cards)
What does propulsive efficiency measure?
The ratio of the power supplied to the aircraft to the increase in KE of the air through the engine.
What is the thermal efficiency?
The increase in KE of the air / the heat added by the fuel
What is consequence of having finite compressor/turbine efficiencies?
The cycle efficiency has a maximum value when its plotted as a function against pressure ratio.
How can the traditional model of gas turbine be improved?
By varying the gas properties,
Turbine should have a higher cp and a lower γ
Has a significant impact on work out.
Work out gas properties from the tables and changing composition due to exhuast gases.
How has turbine entry temperature increased?
Active cooling, take air from compressor and feed it to turbine blades, this air is still at 900K though
What is cooling effectiveness for turbine blades?
ε = Tg-Tm / (Tg - Tc)
where T g is the temperature of the hot gas stream
Tm is the temperature of the metal
Tc is te temperature of the cooling air
~0.6-0.7 is typical for cutting edge cooling
Equation for stagnation temperature derivation.
enthalpy
h1 + v1^2/2 = h2
where 2 is the stagnation enthalpy
cpT + V^2/2 = cpT0
then substitute cp = γR/(γ-1)
to get T0/T = 1 + (γ-1)/2 M^2
Equation for stagnation pressure derivation.
Use isentropic relationships. This is the point where we assume the decelleration to stagnation is isentropic.
What is a ‘choked’ nozzle?
When M=1 at the throat of a nozzle.
This can be derived by using the isentropic relationship between ρ and T0 and then writing mass flow rate per unit area in terms of T0 and T, then differentiate to find the maximum.
How are the stages inside a turbofan engine labelled?
1 - environmental
2 - just before the fan
BYPASS:
13 - bypass after the fan
19 - after the bypass nozzle
CORE:
23 - before the HPC
3 - entrance to combustor
4 - Turbine entry temp
45 - between HPT and LPT
5 - After LPT
9 - after core nozzle
What effect does increasing bypass ratio have on sfc?
Decreases sfc at a decreasing rate, at a certain point the effect of skin friction drag on the nacelle becomes significant, also the weight increases reducing payload of aircraft
Define bypass fan efficiency.
η = (vj^2 - v^2)/2cp(T013-T02)
Is the assumption of the same jet and core velocity a good one?
No because specific fuel consumption is minimised at around Vjb/Vjc = 0.7-0.8
What is the non dimensional eq for gross thrust and how is it obtained?
(Fg + pa AN) /(D^2 p02)
Apply SFME around engine exhaust FG + pa AN = mdot air V19 + p19 AN
RHS is entirl engine parameters
How can we examine the properties of an engine dimensionlessly?
Consider CV around engine, all properties of engines must depend on ambient coniditons mdot f, pa, Ta, and V
T04/T02 fixes pressure ratios non-dimensional fuel flow and the non-dimensional rotational speeds.
One variable is needed to determine the entire engine condition.
Non-dimensional mass flow rate of air.
mair sqrt(cpT02)/D^2p02
Non dimensional rotational speed of engine
ND/sqrt(γRT02)
speed of tip of engine/speed of sound
Non-dimensional mass flow rate of fuel.
mdotf*LCV/(sqrt(cpT02)D^2p02)
energy/energy
fuel*fuel capacity
/
speed of sound * pressure force on engine.
What order do the rotor and stator come in compressors and turbines respectively?
Compressor
rotor then stator
Turbine
stator then rotor
What happens in a compressor rotor stage?
Area of flow increases, flow decellerates
Flow turns towards axial direction.
What happens during a turbine blade row?
Area decrease => pressure drop
Flow acceleration
Large flow turning (towards tangential direction)
Why do you need more compressor stages than turbine stages?
Compressor, flow is going against an adverse pressure gradient therefore thick boundary layers with risk of seperation, so lots of stages are needed.
Turbine is in a favourable pressure gradient.
What is the flow coefficient.
φ =Vx/U where Vx is the axial velocity and U is the blade speed.
What is the work coefficient.
ψ = Δh0/U^2
stagnation enthalpy change/ blade speed squared
