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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
What limitations on the design are there
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
What fuels are used to compensate for short time to mix
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
Why are multiple fuel inlets used
Shortens length required for mixing - velocities need to be the same to stop shear layer generating
28
What does a shear layer do
Causes lateral transportation of momentum and instabilities
29
What fluid effect does a jet of fuel have
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
What methods are used to prevent fuels causing shock
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
What is the detriment of liquid air cycle engines
Need heat exchangers which are big and bulky
32
What is SABRE and its basic makeup
Synergetic air-breathing rocket engine Supersonic intake, pre-cooler inside nacelle, compressor, turbine, rocket engine, nozzle, ramjet
33
What are the advantages of SABRE
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
What is the distinctive feature of SABRE
Combustion in main core engine occurs at subsonic speed Easier to control Can produce thrust at static conditions allowing ground test
35
What fuels are needed in SABRE
Cryogenic fuels such as hydrogen or liquefied methane
36
How is altitude compensation achieved
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
What is the JAXA
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
How does a pulse jet work
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
What thermodynamic processes occur
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
How does a valveless pulse jet work
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
Where are pulse jets used
Target drone aircraft Flying model aircraft Fog generators Industrial drying and home heating equipment
42
What are drawbacks of pulse jets
Noisy Inefficient
43
What is the major difference between PDE and pulse jets
PDE is detonation not deflagration
44
What is a major advantage of PDE
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
How does detonation work
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
How does deflagration work
Combustion and flame Subsonic wave propagates by heat conduction Mass diffusion from hot burnt products of chemical reaction and cold gas mixture ahead
47
What is the downstream velocity after a detonation wave
It is always the local sonic speed
48
What assumptions are made for foundational analysis of PDE
1D and steady flow Constant area Ideal gas Constant and equal specific heats No body forces Adiabatic
49
What are Rayleigh lines used for
Simultaneous solution of mass and momentum conservation equations
50
What information is found on Rankine-Hugoniot curve
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
What are features of detonation zones
Strong detonations are mathematically possible but difficult to produce in reality Weak detonations require special conditions to occur e.g. rapid reaction rate
52
What happens in deflagration to detonation transition
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
What is the PDE wave cycle
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
What is wrinkling in PDE
Flame front area and burning rate increase Turbulence generated and weak pressure pulses ahead of flame pre heats gas and speeds up
55
How can detonation be enhanced
Spirals Grooves Con-Di in pipe
56
What is thermodynamic cycle of PDE
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
What is the challenge for deflagration to detonation
Minimising the length required for entire process (weight savings) Minimising time required for entire process (maximum firing frequency)
58
Advantages of PDE
No moving parts High thermodynamic efficiency Operate in large Mach range Simple and flexible geometric configuration Easy integration Low cost
59
Disadvantages of PDE
Issues with detonation initiation Difficult air inlet design Fuel/air injection and mixing Coupling with external flow Design optimisation Noisy
60
What is an RDE
One or more detonations continuously travel around an annular channel
61
What are limitations of pulse, PDE and RDE for civil application
Noisy Have to consider technical, political and environmental factors in design not just theoretical promise
62
What is the thrust equation for an air-breathing engine
(fuel mass + air mass)*(exit velocity) - (air mass * inlet velocity)
63
What is the exit velocity of a ramjet
Inlet velocity * sqrt(T6/Ta)
64
What is the fuel to air ratio
f = mass of fuel / mass of air
65
What is the energy equation for a ramjet combustor
(m_air * ho2) + (m_fuel * Qr) = (m_air + m_fuel) * ho4
66
What is specific enthalpy equal to
ho1 = Cp * To1
67
How do you find the specific thrust of a ramjet
F = m_air(1 + f)C6 - (m_air * Ca) Divide by m_air and factor out Ca Substitute Ca = M * speed of sound Substitute C6 = Ca * sqrt(To4 / Toa) and use isentropic flow property with gamma and M squared
68
What is the specific fuel consumption of a ramjet
SFC = f / (F/m_air)
69
How do you find the exit velocity of a ramjet
Use the freestream total pressure equation and divide through to get 1 = expression Calculate the nozzle total pressure to exit static pressure ratio and rearrange to get M6 squared Multiply by the first expression found (multiplying by 1 essentially) Use pressure ratios for diffuser, combustor and nozzle Set M6 squared = 2/(gamma_6 - 1) * (m - 1) Find To6/T6 relation (which equals T04/Te) Use this in Mach number equation and insert value of M6 squared
70
How do you find the thrust of a pulsejet
Total pressure ratio across inlet is same but has an efficiency term inside brackets for valve losses Assume constant volume combustion so P/T = constant Combustor energy equation: (m_air * Cpc * To2) + (nb * m_fuel * Qr) = (m_air + m_fuel) * Cph * T03 Rearrange to get fuel air ratio Exhaust velocity: C4 = sqrt(2*Cph*To3(1-Ta/To3)) - replace Ta/To3 with pressure ratio Thrust F = m_air*((1+f)C4 - Ca)
71
What is thrust of a PDE
F = Impulse per unit volume * Volume * frequency
72
What is the efficiency of an ideal ramjet
n = ((F/m_air)*M*a)/(f*Qr)

ND MEC463 - Advanced Propulsion (5 decks)

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