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ND MEC463 - Advanced Propulsion > Alternative Air-breathing > Flashcards

Alternative Air-breathing Flashcards

1
Q

What is specific impulse

A

Thrust per weight of fuel

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2
Q

What is the basic principle of a ramjet

A

Air is decelerated and compressed in the diffuser
Mixes with fuel and burned in combustion chamber
Exhaust gas expelled through nozzle driven by high pressure in diffuser
Combustion is at constant pressure

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3
Q

What are the advantages of ramjets

A

Simple with no turbomachinery - no lubrication or ignition sources
Low mechanical stresses
Operates at high Mach

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4
Q

What are the disadvantages of ramjets

A

Inability to self start
Performance sensitive to diffuser design so only effective on limited range of flight speeds

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5
Q

What are some applications of ramjets

A

Missiles
Unmanned aircraft with hybrid start systems
Aircraft to launch
Turboramjets
Rocket assisted ramjets

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6
Q

How can pressure recovery of a ramjet be improved

A

Intake design controls shockwaves

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7
Q

Why must stagnation conditions be considered

A

Fluid velocities are high so change in kinetic energy across components has to be considered
Dynamic component of properties must therefore be considered

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8
Q

What is a stagnation condition

A

Properties of flow when it is brought to rest in a prescribed manner

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9
Q

What creates thrust in a supersonic aircraft

A

Intake air is decelerated to low velocity in diffuser
Expansion of hot gas after combustion accelerates air to higher velocity
Difference in velocity being exhaust and inlet creates (momentum) thrust

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10
Q

What implication does isentropic reversible flow have

A

No total pressure loss across modules

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11
Q

Why is the total temperature constant across the intake and the nozzle

A

There is no heat addition or rejection

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12
Q

What effect does full expansion in the nozzle have

A

Static pressure at nozzle edge = static pressure at exit = static atmospheric pressure

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13
Q

Why is the assumption that there is no speed increase wrong if the entry and exit Mach numbers are the same (because the ratios of total to static pressure are the same)

A

There is an increase in temperature which causes an increase in the local speed sound

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14
Q

What are the limiting cases of a Joule cycle

A

Incoming air velocity is too high - high total temperature limits the temperature increase allowed and so limit combustion effectiveness - incomplete combustion

Incoming air velocity is too low - excessive heat lost to environment due to long path to reach maximum T for effective combustion

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15
Q

What losses occur in a real ramjet cycle

A

Pressure losses in intake, combustor and nozzle
Combustion losses as not isentropic
Irreversibilities

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16
Q

What thermodynamic stages occur in a ramjet

A

Ram intake compression
Inlet shock compression
Fuel consumption high at low speed
Combustor similar to reheat system
Combustion chamber subsonic
Fuel injection such that wall temp kept low
Con-Di nozzle accelerates flow

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17
Q

Why can a ramjet not operate well beyond Mach 5 even in ideal conditions

A

Combustor inlet is very hot due to dynamic temperature contribution so it is difficult to burn fuel
Thermal efficiency is low also due to supersonic combustion at high Mach numbers

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18
Q

What is Rayleigh flow

A

About the heating and cooling of flow in a pipe of the same diameter
Heating will accelerate flow if it is subsonic
Heating will decelerate the flow if it is supersonic

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19
Q

What is the primary difference between scram and ramjets

A

Scramjets have supersonic combustion

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20
Q

How does scramjet combustion differ

A

Reduction in static temperature allows more heat addition at high Mach numbers
Large mass of airflow needed

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21
Q

What kind of things occur at the scramjet intake

A

Slightly blunt tip leads to bow shocks
Laminar to turbulent transition
Shock BL interactions with plasma control before fuel injection

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22
Q

What causes an unstart in hypersonic intake

A

Shockwaves form and decelerate airflow to subsonic speeds, if these shockwaves become unstable or shift the airflow cannot be compressed
Causes engine stall, flame out, loss of thrust, oscillations and vibrations

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23
Q

Why is it difficult to test scramjets on the ground

A

Less uniform field and supersonic airflow causes ignition problems

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24
Q

What are some applications of scramjets

A

Long-range high-speed weapons, recon/strike
Prompt two stage to orbit

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25
Q

What limitations on the design are there

A

Vehicle structure limit if dynamic pressure gets too high on body (proportional to density)
Combustion blowout limit if combustion not sustainable
Thermal management limit of aerodynamic overheating
Fuel and air will only burn if mixed at required ratio

26
Q

What fuels are used to compensate for short time to mix

A

Hydrogen is rapid burning, high mass specific energy and short ignition delay time, good diffusivity - not very dense though so large storage

Hydrocarbons are easy storage, handling, high energy density and safe - long ignition delay time though, prone to vaporisation and coking

27
Q

Why are multiple fuel inlets used

A

Shortens length required for mixing - velocities need to be the same to stop shear layer generating

28
Q

What does a shear layer do

A

Causes lateral transportation of momentum and instabilities

29
Q

What fluid effect does a jet of fuel have

A

Acts as a cylinder in flow field causing normal shock upstream, separation and subsonic wake, wake aids flame holding and pressure loss reduces efficiency

30
Q

What methods are used to prevent fuels causing shock

A

Strut injection - fuel feed arm, pre-compression system, used in arrays
Ramp injectors - vortex enhances mixing, pre-compression for ramp face produce favourable points, stagnation region improves ignition
Swept ramp - strengthens vortex for mixing
Cavity injectors - acoustic oscillations, combine with ramp injectors

31
Q

What is the detriment of liquid air cycle engines

A

Need heat exchangers which are big and bulky

32
Q

What is SABRE and its basic makeup

A

Synergetic air-breathing rocket engine
Supersonic intake, pre-cooler inside nacelle, compressor, turbine, rocket engine, nozzle, ramjet

33
Q

What are the advantages of SABRE

A

Cooling inlet air means density significantly higher so more air able to go through engine - higher thrust to weight
Cooling only way to compress air further
Lower temperature means light alloys possible

34
Q

What is the distinctive feature of SABRE

A

Combustion in main core engine occurs at subsonic speed
Easier to control
Can produce thrust at static conditions allowing ground test

35
Q

What fuels are needed in SABRE

A

Cryogenic fuels such as hydrogen or liquefied methane

36
Q

How is altitude compensation achieved

A

Expansion-deflection nozzle at lift-off the out skirt is pushed up to let inner nozzle work properly, at high altitude the outskirt is pushed down to improve performance

37
Q

What is the JAXA

A

Air precooler for turbojet engine
Similar to SABRE but combustion is not in rocket mode so not possible to have SSTO
Easier to accelerate to required velocity for second stage

38
Q

How does a pulse jet work

A

Air sucked into combustion chamber through spring-loaded valve
Spark initiates combustion as approaches constant volume
Gases forced out of tailpipe creating vacuum to open valves
Frequency of cycle a function of size

39
Q

What thermodynamic processes occur

A

Ram effect causes pressure and temperature above ambient at intake
Assuming diffuser is ideal with no losses total temperature and pressure constant across it
Combustion at almost constant volume
Isentropic expansion across tailpipe to ambient pressure

40
Q

How does a valveless pulse jet work

A

Natural oscillations and resonance
Pressure differences control airflow
Momentum comes from exhaust flow
Simple, self-sustaining cycle with everything combining to control airflow naturally

41
Q

Where are pulse jets used

A

Target drone aircraft
Flying model aircraft
Fog generators
Industrial drying and home heating equipment

42
Q

What are drawbacks of pulse jets

A

Noisy
Inefficient

43
Q

What is the major difference between PDE and pulse jets

A

PDE is detonation not deflagration

44
Q

What is a major advantage of PDE

A

Pressure can increase significantly during combustion process in contrast to constant pressure combustion found in gas turbine
PDE can also initiate supersonic detonation easily

45
Q

How does detonation work

A

Supersonic wave propagates through shock compression of fuel/air
Shock heats gas, ignites chemical reactions to large energy releases
Energy pushes shock into unreacted gases so self-sustaining
Interactions between fluid mechanical and thermochemical processes

46
Q

How does deflagration work

A

Combustion and flame
Subsonic wave propagates by heat conduction
Mass diffusion from hot burnt products of chemical reaction and cold gas mixture ahead

47
Q

What is the downstream velocity after a detonation wave

A

It is always the local sonic speed

48
Q

What assumptions are made for foundational analysis of PDE

A

1D and steady flow
Constant area
Ideal gas
Constant and equal specific heats
No body forces
Adiabatic

49
Q

What are Rayleigh lines used for

A

Simultaneous solution of mass and momentum conservation equations

50
Q

What information is found on Rankine-Hugoniot curve

A

For fixed upstream pressure and specific volume
Origin from which Rayleigh lines extend to curve
Regions indicate areas of strong detonation, weak detonation, impossible combustion, weak deflagration and strong deflagration

51
Q

What are features of detonation zones

A

Strong detonations are mathematically possible but difficult to produce in reality
Weak detonations require special conditions to occur e.g. rapid reaction rate

52
Q

What happens in deflagration to detonation transition

A

Ignition energy has to be high to trigger detonation directly
Pressure after shock linked to heat of combustion
Detonation speed is function of pressure difference across shockwave and initial density

53
Q

What is the PDE wave cycle

A

Purging stage with chamber at ambient condition
Filling stage
Ignition/initiation stage
Detonation wave propagation (compression + heat addition)
Detonation wave reaches exit
Exhaust stage (thrust)
Purging starts

54
Q

What is wrinkling in PDE

A

Flame front area and burning rate increase
Turbulence generated and weak pressure pulses ahead of flame pre heats gas and speeds up

55
Q

How can detonation be enhanced

A

Spirals
Grooves
Con-Di in pipe

56
Q

What is thermodynamic cycle of PDE

A

Detonation initiation at closed end of tube
Detonation propagates towards open end
Detonation diffracts outside as decaying shock and reflected expansion wave propagate to closed end starting blowdown
Tube contains burned products at rest
Opening valve sends shock waves purging burned gases
Volume of air injected before reactants for purging
Purging air pushed out by reactants
Reactants fill tube completely and valve closed

57
Q

What is the challenge for deflagration to detonation

A

Minimising the length required for entire process (weight savings)
Minimising time required for entire process (maximum firing frequency)

58
Q

Advantages of PDE

A

No moving parts
High thermodynamic efficiency
Operate in large Mach range
Simple and flexible geometric configuration
Easy integration
Low cost

59
Q

Disadvantages of PDE

A

Issues with detonation initiation
Difficult air inlet design
Fuel/air injection and mixing
Coupling with external flow
Design optimisation
Noisy

60
Q

What is an RDE

A

One or more detonations continuously travel around an annular channel

61
Q

What are limitations of pulse, PDE and RDE for civil application

A

Noisy
Have to consider technical, political and environmental factors in design not just theoretical promise

ND MEC463 - Advanced Propulsion (5 decks)

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