Turbo Jet Flashcards by Finlay B C

List stages in a jet engine

Inlet
Compressor
Combustion Chamber
Turbine
Propelling Nozzle

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Giver equation for thrust of a jet engine

T = m(dot)C2 + A2(P2 - Pa)

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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

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Give stages in axial compressor and how flow moves over them

1 - Rotor (accelerates)

2 - Stator (decelerates)

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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)

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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

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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

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What happens in a turbine

Extracts energy from high enthalpy gasses after the combustion chamber and drives the compressor
turbine stages - stator then rotor

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What is turbo fan used for

gets some thrust from the core and most from the fan

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What is the BPR

Bypass ratio = air flows around core / flow enters core

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What does twin-spool mean

two compressors driven by two turbines through concentric drive shafts at different speeds

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What does triple-spool mean

two compressor driven by two turbine. Third turbine drives the fan

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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

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Give equation for work in compressor

-Wc = m(dot)Cp(T2-T1)

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Give equation for work in combustor

Q = m(dot)Cp(T3-T2)

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Give equation for work in turbine

-Wt = m(dot)Cp(T4-T3)

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Give equation for ideal cycle efficiency, η

η = Wt - Wc / Q

η = ( (T3 - T4) - (T2 - T1) ) / (T3-T2)

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Give equation for Carnot efficiency

η = Th - Tc / Th

η = T3 - T1 / T3

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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