Supersonic combustors
By:
Aditya Lakhotia : 20103026
Siddharth Golccha: 20103002
1
This seminar will include a discussion on:
• The requirements for supersonic combustion to occur
• factors that influence the effectiveness and efficiency of combustion
in supersonic flows
• The various configurations of supersonic combustors and their
working
2
A recap on SCRAMJETS....
• Ram effect and its significance
• Why supersonic combustion?
• Difficulties with combustion in supersonic flows
3
Features characteristic to supersonic
combustion
• Degree of complexity
• Considerable challenge of experimental setup and computational complexity
• Experimental - Costs involved due to
• requirements of advanced materials
• Fuels such as hydrogen can be difficult to work with
• System control for accurate combustion stability and shock location
•
•
• Computational -Non-linear nature of governing equations at hypersonic speeds
Two other factors that affect the above:
Large range of time scales and length scales
4
Time Scales
• Equilibrium
chemistry
vs frozen chemistry
• Residence time
• Heat release time
• Ignition delay time
• Burn time
• Time/ rate
governing factors
5
Fuel-air mixing
• The most critical process for the
purpose of heat release
• Combustion efficiency and
effectivity
• Fuel – air ratio and equivalence
ratios
• Time constraints
• Scales of size in mixing
• Large scale turbulent mixing
• Intermolecular diffusion and
molecular level mixing
Figure : Plot of equilibrium temperature with air- fuel
ratio – source : https://doi.org/10.1016/j.cja.2021.06.002
6
Fuel – air mixing
• Premixed and non-premixed combustion
• Mixing mechanisms:
• Overlap of multiple elementary mixing types – diffusion, turbulent
mixing, recirculation
• Parallel, compressible flows
• Shear layer mixing
• Turbulent mixing
• Mixing enhancers
7
Fuel-air mixing
• Shear Layer Mixing
• Factors affecting
development of shear layer • Velocity
• Temperature
• Compressibility of flows
• Mixing within the shear
layer
Fig: Development of the shear layer between parallel flows
Fig: Transition to turbulent structures in a 2D shear layer
8
Fuel injection
• Reiterate : need for rapid
mixing
• Angles of fuel injection
• Transverse
• Oblique
• Injection configurations
• Mechanism
9
Mixing Enhancers
• Ramps and hypermixers
• Advantages of using a
ramp
• To avoid the strong shocks
associated with transverse
injection
• Ramp configuration:
• Swept vs unswept ramps
• Selecting a ramp angle
10
Mixing Enhancers
Fig : vortical structures that lift the fuel from the wall and enhance mixing
11
Flame holding and flame stability
• Time constraints:
• unless a flame-holding mechanism is in place to extend the residence time,
the exothermic chemical reactions cannot be completed within the
combustion chamber
• achieving a balance between the flame propagation speed and the fluid
velocity
• Flame-holding issue is solved by the generation of some sort of recirculation
region that ensures sufficient residence time
• Role of the recirculation region
• Preventing blowout
• Flame instability and unstart
12
Flame stability – recirculation region
Fig: Recirculation region for a rearward step combustor
13
Flame stability and unstart
Figure – plot of the dependence of
onset of unstart on the
equivalence ratio
Source : doi.org/10.1016/j.energy.2
020.119271
14
Flammability limit and blowout
15
Role of isolator
Figure: Plot of the Ratio of the
backpressure to the max pressure
of the isolator required to avoid
unstart
16
Types of Combustor
17
For Subsonic regime
• Can type
• Annular type
• Canular type
18
19
Can type combustors
20
Annular type combustors
21
22
Cannular type combustors
23
For Supersonic regime
• No moving parts are available in the scramjet engine, which gives
higher thrust to weight ratio compared with any other propulsion
engines.
• Different techniques and approaches are used for getting better
results in the form of improved mixing, momentum, drag
24
Types of Supersonic combustors
• Step Combustors
• Isolator Combustors
• Wall-jet injection combustors
• In-stream injection combustors
• Hypermixer injection combustors
25
Step Combustors
• Used to reduce the BL separation problem
• A3/A2 >1
• Fuel injected in close proximity to the base of the step
• Helps in providing small subsonic recirculation regions for ignition and
flame holding
• Flow characteristics downstream of the of the step are very complex
and generally not amenable to insightful 1-D flow approximations,
only applicable to exit of station 3 that is combustor exit.
26
Step Combustors
27
28
Isolator Combustors
• Inlet isolator + Increased area combustor
• Types of increased area combustor
- Step combustor
- Divergent area wall combustor
29
Isolator Combustors
30
Isolator Combustors
31
Isolator Combustors
32
Combustor energy balance
33
Wall jets, In-stream and Hypermixer
34
• Walljets Give rise of net heat flux around the walls.
35
• Fuel injected axially downstream to the flow
• High combustor velocities leads to increased distance (combustor
length) to achieve adequate mixing
36
37
• Hypermixers have the maximum momentum recovery from the fuel
(The importance of which increases with flight mach number) due to
the axial angled downstream fuel injection
• Challenge with hypermixer is to achieve adequate mixing of nearly
axially injected fuel in reasonable combustor lengths without
offsetting drag increases
38
39