A.A.B Engines - SCRAMJET Flashcards
Similarities and differences of Ramjet and Scramjet
Similarities:
- Thermodynamically similar
Difference:
- Speed of combustion
- Scramjet reduces static temperature to rise more heat addition
List components of scramjet
- Forebody
- Internal inlet
- Isolator
- Combustor
- Internal nozzle
- Aft body
List 3 applications of Scramjet
- Long-range high-speed weapons
- Long-range high-speed recon/strike
- Prompt TSTO (Two stage to orbit)
Give 3 limits of hypersonic travel
- Vehicle structure limit: dynamic pressure too high
- Combustor blowout limit
- Thermal management limit: aerodynamic overheating
What is the disadvantage of having to use entire aircraft body to compress the air for scramjet aircraft
- Boundary layer at intake 40% of flow
- Interdependent flow fields
- High speeds - high heating - cooling requirements
- Highly sensitive system
List 5 regimes for scramjet
- Perfect Gas
- Two temperature ideal gas
- Dissociated gas
- Ionised gas
- Radiation-dominated
Describe Perfect gas regime
The lower border of this region is around
Mach 5, where ramjets become inefficient, and the upper border around Mach 10-12.
Describe Two temperature ideal gas regime
This is a subset of the perfect gas regime, where the gas can be considered chemically perfect, but the rotational and vibrational temperatures of the gas must be considered separately, leading to two temperature models.
Describe Dissociated gas regime
In this regime, diatomic or polyatomic gases (the gases found in most atmospheres) begin to dissociate as they come into contact with the bow shock generated by the body.
Describe ionised gas regime
The ionised electron population of the stagnated flow becomes significant, and the electrons must be modelled separately. Often the electron temperature is handled separately from the temperature of the remaining gas components. This region occurs for freestream flow velocities around 10–12 km/s.
Describe radiation-dominated regime
Above around 12 km/s, the heat transfer to a vehicle changes from being conductively dominated to radiatively dominated.
What is the challenge with supersonic combustion
Challenge is the short mixing time
What are the 3 fuel choices for scramjets
- Hyrdrogen:
- rapid burning, high mass specific energy content, short ignition delay time
- low density, larger vehicle required
- good diffusivity - Hydrocarbon
- storage/density good
- long ignition time delay - High reactivity fuels
- invention of new fuels
List 5 ways of adding/mixing fuel
- Fuel air mixing (simply add two streams together)
- Fuel injection
- Strut injection
- Ramp injectors
- Cavity injectors
Describe fuel air mixing design
If velocities of two streams are different, shear layer is generated causing lateral transportation of momentum from fast to slow streams
Describe fuel injection
- Jet fuel acts as a cylinder in the flow field
- normal shock set up upstream causes separation and subsonic wake
- this wake adds to flame holding
- reduced efficiencies due to pressure losses however
Describe strut injection
- fuel feed arm
- precompression system
- used in arrays
Describe ramp injectors
- vortex enhances mixing
- injection location important
- stagnation region near loading edge improves ignition
Describe cavity injectors
- generate acoustic oscillations
- characterised by L/D
if L/D < 1 then transverse waves
if L/D > 1 then shear layer attaches to bottom wall, generates longitudinal waves, helps flame holding
What does SABRE stand for
Synergetic Air-Breathing Rocket Engine
Difference between SABRE design and Scramjet
- SABRE combustion much easier to control (uses cryogenic fuel)
- Oxygen is also heavy and using it as fuel is desirable
How does SABRE cool incoming oxygen
Uses the cryogenic fuel on-board to cool incoming air
Then burns air and fuel in a rocket engine combustion chamber
designed to be reusable
What were the problems with HOTOL design
HOTOL - Horizontal take-off and landing
- engine at back and heavy hyrdaulics required for balance
- frost in heat exchanger
- adjustable intake required
List advantages of SYLON design over HOTOL
- cooling inlet air means higher density
- lower temp means lighter alloys can be applied
When do SABRE engines transition from air breathing to rocket
At Mach 5.5
then use rockets to get up to orbital velocity M = 25
Can you draw SABRE engine system
PIC
What is special about expansion deflection nozzle and why
- advanced rocket nozzle that achieves altitude compensation
- has a component at the beginning part of the bell that pushes airflow around it and along nozzle walls
- creates closed wake in a vaccuum
List 5 applications of pulse jets
- Target drone aircraft
- Flying model aircraft
- Fog generators
- Industrial drying
- Home heating equipment
1 big disadvantage of pulse jets
Very noisy
What does PDE stand for
Pulse Detonation Engine
Difference between pulsejet and PDE
PDE - detonation
Pulsejet - deflagration
Advantage of PDE over pulsejet
- Pressure increases significantly during combustion
2. PDE initiates supersonic combustion
Describe detonation
- supersonic wave which propagates through shock compression of the fuel/air mixture
- shock wave heats gas
- ignites chemical reaction / large energy release
- energy pushes shock wave into unreacted gas, which is self-sustaining
Describe deflagration
- subsonic wave propagates by heat conduction
- mass diffusion from the hot burnt products of the chemical reaction and the cold gas mixture ahead
Describe difference between normal shock and detonation
Normal shock:
- downstream velocity always subsonic
Detonation:
- downstream velocity always M = 1
In Rankine-Hugoniot curve describe 5 regions
I - strong detonation M2<1 II - weak detonation M2 > 1 III - weak deflagration M2 < 1 IV - strong deflagration M2 > 1 V - Impossible detonation
On Rankine-Hugoniot curve what are points D and E known as
D - upper CJ point
E - lower CJ point
In most cases how does detonation occur
DDT
Deflagration Detonation Transition
- ignition energy has to be very high to trigger detonation directly
- detonation speed is function of pressure difference across shock wave and initial density
When are PDE’s better than Ramjets
For Mn = 1.35
How much faster is detonation than deflagration
1000x faster
Describe process of detonation
- Combustible material in pipe
- Ignite one end, get laminar flame
- flame front wrinkles increases area and burning rate
- wrinkling generates weak pressure pulses and turbulence ahead of the flame which preheat gas
- as flame speeds up, pressure pulses increase. They coalesce - more preheat until auto-ignition temp reached local explosion
- Rapid expansion causes shock wave
- If turbulence induced, can shorten this process
In theory which is more efficient detonation and deflagration
Detonation is in theory more efficient
PDE is described by which cycle
Humphrey cycle
Advantages of PDE
- No moving parts
- High thermodynamic efficiency
- Operating in high Mach no.s
- Simplicity/flexibility of geometrical config.
- Easy integration to vehicle
- Low cost
Challenges of PDE
- Detonation initiation
- Air inlet design
- Coupling with external flow
- Design optimisation
- Very noisy
Describe idea of RDE - Rotating Detonation Engine
None in production
Engine where one or more detonations continuously travel around an annular channel
(Would be superior to PDE)
Advantage of a RDE
- High intensity of reaction
- Self-pressurisation
- Rapid heat release
