Turbo Jet Flashcards
List stages in a jet engine
Inlet Compressor Combustion Chamber Turbine Propelling Nozzle
Giver equation for thrust of a jet engine
T = m(dot)C2 + A2(P2 - Pa)
Give reason for inlet
Critical for good performance - it guides the flow into the engine and aims to provide the fan with a uniform flow
Give stages in axial compressor and how flow moves over them
1 - Rotor (accelerates)
2 - Stator (decelerates)
When is centrifugal compressor used
Used where only single stage is required to produce a large pressure rise
- has larger frontal areas than axial (higher casing drag)
How does a centrifugal compressor work
Flow enters the rotor ‘eye’ and is accelerated as it flows radially outward to the tip
Flow then decelerates and increases pressure
What happens in a combustor
raises enthalpy of the working fluid by burning fuel in the compressed air delivered from the compressor
arranged in an annulus (doughnut) around the drive shaft
What happens in a turbine
Extracts energy from high enthalpy gasses after the combustion chamber and drives the compressor
turbine stages - stator then rotor
What is turbo fan used for
gets some thrust from the core and most from the fan
What is the BPR
Bypass ratio = air flows around core / flow enters core
What does twin-spool mean
two compressors driven by two turbines through concentric drive shafts at different speeds
What does triple-spool mean
two compressor driven by two turbine. Third turbine drives the fan
Explain using a T-S diagram where energy comes from to provide thrust
Distance between 2-1 is smaller than distance 4-3. Therefore due to combustion stage, enthalpy of the gasses is higher and so has more energy. Part is used to power the compressor, the rest is exited out a nozzle to accelerate air and convert enthalpy to kinetic energy
Give equation for work in compressor
-Wc = m(dot)Cp(T2-T1)
Give equation for work in combustor
Q = m(dot)Cp(T3-T2)
Give equation for work in turbine
-Wt = m(dot)Cp(T4-T3)
Give equation for ideal cycle efficiency, η
η = Wt - Wc / Q
η = ( (T3 - T4) - (T2 - T1) ) / (T3-T2)
Give equation for Carnot efficiency
η = Th - Tc / Th
η = T3 - T1 / T3
Give equation for turbine work ratio
ηt = Wt - Wc / Wt
ηt = useful turbine work / total turbine work
Which is less efficient - Compressor or Turbine - Why
Compressor less efficient than turbine. Flow decelerates across stator …
boundary layer
separation
more losses
List efficiency losses in jet engine components
Fluid friction - intakes -combustion chambers - exhaust ducts - connecting ducts Friction losses in shaft - transfer energy from turbine to compressor Auxiliary power requirement - oil and fuel pumps Air cooling blade roots and turbine disks
Give equation for compressor efficiency
ηc = T02’ - T01 / T02 - T01
T02’ is isentropic stagnation temp
T02 is stagnation temp
Give equation for turbine efficiency
ηt = T03 - T04 / T03 - T04’
T04’ is isentropic stagnation temp
T04 is stagnation temp
Define polytropic efficiency
the isentropic efficiency for an elemental stage in a process such that it is constant through the whole process
reason: the increase in temp due to friction in one stage, results in more work being required in the next stage
Describe how isentropic efficiency varies with pressure ratio for a compressor and for a turbine
Compressor:
medium, converges lower
Turbine:
medium, converges higher
Both start at same point at polytropic efficiency
Equation for propulsive efficiency
ηp = Thrust power / Thrust power + Jet kinetic power
ηp = 2Ca / (Ca + Cj)
if Cj is almost Ca this means almost no thrust … where exit speed is almost equal to inlet speed
Describe graph that shows relationship between ηp and Cj/Ca
1/x graph
high ηp to start, decays rapidly and the converges almost at 0 for increasing Cj/Ca
What is specific thrust
Specific thrust -
the thrust per unit mass of air flow
Fs = F / m(dot)
What is SFC
Specific Fuel Consumption -
SFC = mf(dot) / F
What are the mechanical losses
Turbine power transmitted directly to the compressor, however losses in bearing and windage
assume = 0.99 if not given
Give Cp for air and exhaust gasses
Cp air - 1.4
Cp gasses - 1.333
How do inlets effect air flow for different conditions
Stationary - inlet acts to accelerate airflow into the engine
cruise speed - inlet acts as a diffuser and slows down airflow
What could distortion of inlet air lead to
flame out or mechanical damage due to surge
How to calculate stagnation temperature
T02 = T1 + C1^2/2Cp
What is RAM pressure
pressure from moving through a fluid
is taken into account in inlet efficiency
ηi = P01 - Pa / P0a - Pa
What is used for thrust reversing
Thrust is reversed by using clamshell doors to direct airflow in opposite direction
How does thrust vary with altitude
Thrust decreases initially with increased altitude due to increased momentum drag
then will increase due to beneficial effects of the RAM pressure rise
EPR is
Engine Pressure Ratio = P04 / P01
RPR is
Ram pressure ratio = P01 / Pa
OPR is
Operational pressure ratio
OPR = EPR x RPR
OPR = P04 / Pa
Describes possible methods to increase thrust
- re-design for increase in mass flow rate or increase in T3
- temporary thrust increase:
- liquid injection
- afterburning
- methanol injection:
spray water methanol mixture at compressor inlet during take-off and climb, equivalent drop in compressor temp therefore less work required to run, so turbine can use more energy on thrust
this also increase mass flow rate too
How do afterburners work
- burn additional fuel in the jet pipe
- no rotating parts so max temperature increases (2000K)
- accept penalty in SFC
- efficiency drops a lot!
Difference in noise between turbojet and turbofan
noise reduction in turbofan is massive
What is the problem with having the fan aft of the engine
- turbine blades for hot stream and fan for cold stream
- sealing
If mhot and mcold mix in the nozzle of a turbofan, how should this be treated when calculating
Assume:
- full mixing of the streams
- inlet static pressure equal
Conduct:
- energy balance to get exit temperature
- momentum balance to get exit pressure
Give equation for thrust in a turboprop
T(tp) = (SP)ηpr + FCa
Sp - shaft power
Designs usually have an unchoked nozzle with Mcruise = 0.6
