Alternative Air-breathing Flashcards
What is specific impulse
Thrust per weight of fuel
What is the basic principle of a ramjet
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
What are the advantages of ramjets
Simple with no turbomachinery - no lubrication or ignition sources
Low mechanical stresses
Operates at high Mach
What are the disadvantages of ramjets
Inability to self start
Performance sensitive to diffuser design so only effective on limited range of flight speeds
What are some applications of ramjets
Missiles
Unmanned aircraft with hybrid start systems
Aircraft to launch
Turboramjets
Rocket assisted ramjets
How can pressure recovery of a ramjet be improved
Intake design controls shockwaves
Why must stagnation conditions be considered
Fluid velocities are high so change in kinetic energy across components has to be considered
Dynamic component of properties must therefore be considered
What is a stagnation condition
Properties of flow when it is brought to rest in a prescribed manner
What creates thrust in a supersonic aircraft
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
What implication does isentropic reversible flow have
No total pressure loss across modules
Why is the total temperature constant across the intake and the nozzle
There is no heat addition or rejection
What effect does full expansion in the nozzle have
Static pressure at nozzle edge = static pressure at exit = static atmospheric pressure
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)
There is an increase in temperature which causes an increase in the local speed sound
What are the limiting cases of a Joule cycle
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
What losses occur in a real ramjet cycle
Pressure losses in intake, combustor and nozzle
Combustion losses as not isentropic
Irreversibilities
What thermodynamic stages occur in a ramjet
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
Why can a ramjet not operate well beyond Mach 5 even in ideal conditions
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
What is Rayleigh flow
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
What is the primary difference between scram and ramjets
Scramjets have supersonic combustion
How does scramjet combustion differ
Reduction in static temperature allows more heat addition at high Mach numbers
Large mass of airflow needed
What kind of things occur at the scramjet intake
Slightly blunt tip leads to bow shocks
Laminar to turbulent transition
Shock BL interactions with plasma control before fuel injection
What causes an unstart in hypersonic intake
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
Why is it difficult to test scramjets on the ground
Less uniform field and supersonic airflow causes ignition problems
What are some applications of scramjets
Long-range high-speed weapons, recon/strike
Prompt two stage to orbit
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
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
Why are multiple fuel inlets used
Shortens length required for mixing - velocities need to be the same to stop shear layer generating
What does a shear layer do
Causes lateral transportation of momentum and instabilities
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
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
What is the detriment of liquid air cycle engines
Need heat exchangers which are big and bulky
What is SABRE and its basic makeup
Synergetic air-breathing rocket engine
Supersonic intake, pre-cooler inside nacelle, compressor, turbine, rocket engine, nozzle, ramjet
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
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
What fuels are needed in SABRE
Cryogenic fuels such as hydrogen or liquefied methane
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
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
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
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
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
Where are pulse jets used
Target drone aircraft
Flying model aircraft
Fog generators
Industrial drying and home heating equipment
What are drawbacks of pulse jets
Noisy
Inefficient
What is the major difference between PDE and pulse jets
PDE is detonation not deflagration
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
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
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
What is the downstream velocity after a detonation wave
It is always the local sonic speed
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
What are Rayleigh lines used for
Simultaneous solution of mass and momentum conservation equations
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
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
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
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
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
How can detonation be enhanced
Spirals
Grooves
Con-Di in pipe
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
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)
Advantages of PDE
No moving parts
High thermodynamic efficiency
Operate in large Mach range
Simple and flexible geometric configuration
Easy integration
Low cost
Disadvantages of PDE
Issues with detonation initiation
Difficult air inlet design
Fuel/air injection and mixing
Coupling with external flow
Design optimisation
Noisy
What is an RDE
One or more detonations continuously travel around an annular channel
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
What is the thrust equation for an air-breathing engine
(fuel mass + air mass)*(exit velocity) - (air mass * inlet velocity)
What is the exit velocity of a ramjet
Inlet velocity * sqrt(T6/Ta)
What is the fuel to air ratio
f = mass of fuel / mass of air
What is the energy equation for a ramjet combustor
(m_air * ho2) + (m_fuel * Qr) = (m_air + m_fuel) * ho4
What is specific enthalpy equal to
ho1 = Cp * To1
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
What is the specific fuel consumption of a ramjet
SFC = f / (F/m_air)
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
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(2CphTo3(1-Ta/To3)) - replace Ta/To3 with pressure ratio
Thrust F = m_air*((1+f)C4 - Ca)
What is thrust of a PDE
F = Impulse per unit volume * Volume * frequency
What is the efficiency of an ideal ramjet
n = ((F/m_air)Ma)/(f*Qr)
