flame stabilisation iin micro combustors

1. By
T SHYAMKUMAR
14TH14F

2. Why micro combustion ?
 Batteries have low specific energy
 Liquid hydrocarbon fuels have a very high specific
energy
 To reduce system weight
 To increase operational lifetimes
 To reduce unit cost
 Easily rechargeable

3. APPLICATIONS
 Micro-satellite thrusters for station keeping require a
small thrust and a high specific impulse for an
operation period of years. An efficient micro-thruster
requires propellants with a large energy density
 Micro-air vehicles for reconnaissance and surveillance
in urban and battle field.

4. SCALING PARAMETERS
1.Based on physical dimension of the combustor
if physical length < 1mm :microcombustion
if physical length is between 1mm -10mm : mesoscale combustion
2.Based on quenching diameter
The combustion is called micro-scale (mesoscale) if the combustor
size is smaller (larger) than the quenching diameter
3. By the relative length scale of the entire device to that of
conventional large scale devices for similar purposes.

5. Thermo physical Issues
 As the characteristic length of the device is reduced, the
Reynolds and Peclet numbers decrease and fluid flow tends to
be less turbulent, so the viscous effects and the diffusive
transport of mass and heat become increasingly important.
 From the point of view of combustion, an important issue is the
magnitude of the residence time, because it decreases as the
length of the combustor decreases, and it has to be larger than
the chemical time, for complete combustion to occur.
 At the length scales of microcombustion devices, heat
transfer by natural convection becomes small because the
induced buoyant flow is small

6.  However, heat transfer by conduction through the gas to
the surrounding surfaces and by forced convection in the
intake and exhaust channels and in the combustion
chamber is significant.
 Heat transfer by radiation also increases as the
characteristic length decreases because of the large view
factor.
 Problematic for combustion chambers due to wall
quenching
 Silica aerogel can be used to insulate the combustor.
 Reduced thermal gradients will reduce thermal expansion
stresses and subsequent misalignments in moving parts.
Biot number for a steel or a silicon device with a length
scale of 10 mm,and subjected to moderate convection and
radiation (combined convection and radiation heat transfer
coefficient of 100 W/m2K), will be of the order of 0.002

7. Combustion
 Physical time available for combustion (residence time) must be larger
than the time required for the chemical reaction to occur (combustion
time).
 Since in general the residence time will be small in micro combustors,
it is important to have small chemical times to ensure completion of
the combustion process within the combustor .
 As engine size or combustion volume decreases, the surface-to-volume
ratio increases, resulting in increased combustor surface heat losses
and increased potential destruction of radical species at the wall.
 Ceramic materials, such as SiO2, SiC, and Si3N4, can withstand
temperatures of around 1700 K, which are high enough to ensure short
chemical times
 Although the increase of the surface-to-volume ratio of the combustor
presents a problem for gas-phase combustion, it favors catalytic
combustion

8. Flame propagation and
stabilization
 When flames (e.g., a torch) are put in narrow
channels, they do not propagate through, when the
gap size is below what is known as the quenching
distance
 Quenching distance is in the range of ~2.5 mm for cold
walls (500 K)
 Burner insulation and thermal management are
important in minimizing heat loss and mitigating
thermal quenching

9. Characteristics of homogeneous
micro burners
Quenching mechanisms
 Thermal - heat loss from the burner walls and gases
 Radical quenching - diffusion of radicals from the
flame to the walls followed by their recombination on
walls to produce stable molecules.
 In the mesoscopic range radical quenching plays only a
secondary role.

10. The excess enthalpy principle
 Qin - enthalpy of the inlet stream .
 Qc - heat generated on combustion.
 Qin + Qc - outlet enthalpy in an adiabatic system.
 Sufficient pre-heating of the incoming gases is
required to stabilize combustion in microburners.
 Qr is recirculted to cold inlet stream.
 Maximum temperature is greater than the adiabatic
temperature rise .

11. The role of burner walls
 Wall conductivity and the wall thickness are key
parameters for flame stability.
 Direction of transverse heat transfer downstream of
the flame is from hot gases to the wall and upstream of
the flame is from the wall to the cold inlet gases
 Micro burner can be split into three distinct zones: the
pre-heating, the combustion and the post-combustion
zones.

12. Micro burner stability
1 The dual role of walls in heat
exchange
 As the wall conductivity decreases, the flame
temperatures become higher and the flame remains
localized but drifts downstream due to insufficient
pre-heating.
 Highly conductive walls, on the other hand, dissipate
heat to the environment and also longitudinally,
causing the entire burner surfaces to be hot
 Two modes of flame quenching occur: a spatially
‘distributed’ type for high wall thermal conductivities,
thick walls, and/or low flow velocities and blowout at
high velocities, low wall conductivity, or thin walls.

13. 2 Effect of flow velocity
 The inlet velocity is easily varied experimentally and
controls the power input to a burner
 At low flow velocities, the power input is low, the
burner temperatures are low and extinction is the
dominant quenching mechanism.
 At high flow velocities the power input is high but the
residence time is low, and as a result blowout is the
dominant quenching mechanism.
 For fast flows, higher wall conductivity facilitates heat
transfer upstream resulting in more stable flames

14. Excess enthalpy burners(heat
recirculation)
 Thermal enthalpy of the
incoming reactants is
increased via preheating
without diminishing the
chemical enthalpy via
chemical reaction.
 A PC with LabView
software is used to control
mass flow controllers for
fuel and air and to record
data from thermocouples
for temperature
measurement. The
exhaust gas composition
was analyzed with a gas
chromatograph.
Electrically heated Kantal
wire is used for ignition

15. Heat recuperation in reverse-flow
micro burners
 Temporal thermal coupling between the hot product
and the cold reactant streams, achieved through
periodic flow reversal in reverse-flow (RF) reactors.
 Requirements - high gas-solid heat transfer rates and
reasonably high heat capacity of the burner solid
structure. Porous beds are required to increase the
gas-solid heat transfer rates
 The RF operation improves the microburner stability
at the blowout limit, but does not affect the stability at
extinction.Micro burner stability increases as the wall
thermal conductivity is reduced.

16. Swiss-roll micro burners
 The central section of a SR
acts as a combustion zone,
where the flame is
stabilized.
 The spiralling inlet and
outlet channels form the
heat exchange section.
 Each inlet channel adjoins
two outlet channels and
vice versa.

17. Continued..
 A catalyst may also be included in the combustion zone to
improve burner stability and to reduce the ignition
temperature
 At low inlet velocities, close to or lower than the laminar
flame speed, the flames are located upstream of the
combustion zone.
 The SR heat exchanger then acts as a net heat sink.
 When the velocity is increased, the flame gets anchored in
the central combustion zone. Further increase in the
velocity pushes the flame downstream and into the
recirculating section. Blowout is reached soon thereafter

18. Micro burner geometry
 The wall thickness, gap width, and reactor length are
the three parameters that can be varied
 Increasing the gap width decreases the net transverse
heat transfer;higher gap sizes are preferable at the
extinction limits, where heat loss from the flame
results in thermal quenching.
 Narrower gap sizes improve the heat transfer that can
stabilize the blowout limit.
 Increasing the reactor length shrinks the region of
self-sustained combustion, due to increased heat
losses through the reactor solid structure.

19. Catalytic micro combustion
 In contrast to gas phase combustion, catalytic combustion
occurs at a surface reaction without a flame.
 Surface reactions are not very sensitive to the reduced size
of micro-combustors, which implies the possibility of
further shrinking of micro-combustors.
 Lower temperature of catalytic combustion makes thermal
stresses, materials limitations and heat losses less
problematic.
 Self-starting fuels and catalysts are highly desirable
because it would eliminate the need for glow plugs,
supplemental battery, electronics, etc

20. Combustion characteristics
 Reactants diffuse from the bulk gas to the washcoat and
then inside the porous support (catalyst layer).
 Reaction takes place on catalyst sites within the support,
and products diffuse outwards through the support and
eventually to gas.

21. Stability of catalytic micro
burners
 Catalytic micro burners have superior stability than
homogeneous ones.
 Blowout stability limit is much higher in catalytic
combustion than in homogeneous combustion.
catalytic burner stability is affected from the fuel
choice.
Catalytic combustion of methane is difficult to sustain
whereas combustion of propane, hydrogen, and
methanol on Pt can be sustained .

22.  .
 Hydrogen dissociation is a non-activated reaction
 Hydrogen self-ignites on Pt at room temperature (i.e.,
it does not require external heating).
 Methane adsorption on noble metals is activated (an
activation energy of~11 kcal/mol on Pt) rendering its
combustion slow. As a result, heat is not generated fast
enough to compensate for heat losses

23. Where is catalyst placed?
 For a microburner length is 2 cm only 1cm is catalytic.
The catalyst is placed in the front, middle, or rear part
of the burner.
 Catalyst placement is more important for low
conductivity walls where axial heat transfer is slow. At
high conductivities, catalyst placement is of secondary
importance.

24. Porous media micro-combustion
 In conventional combustion, the combustion process
occurs in a mostly gaseous environment, convection is the
main mode of pre-heating.
 Whereas in porous media, the combustion takes place in a
three dimensional solid porous matrix having
interconnected pores. Apart from convection, the
conduction and radiation modes of heat transfer are also
activated. This enhances the heat transfer (pre-heating)
from the burned hot gases to unburned mixture.
 These characteristics seem favorable to micro-combustion
because the pre-heating is useful in increasing the flame
temperature, and therefore sustaining flames in a smaller
space.
 A higher and more uniform temperature distribution can
be achieved along the wall of the microcombustor.

25. Conclusions
 Given the nearly hundredfold mass-based greater
energy density of fuels, an improvement over batteries
could be achieved if greater than 1% of the energy
stored in chemical bonds could be converted to
electricity
 Catalytic combustion is superior at these scales over its
homogeneous counterpart and can be sustained for
fairly large heat losses, much lower wall conductivity
materials, and much higher inlet velocities but not
necessarily with complete fuel conversion.

26. References
[1].S.K. Chou, W.M. Yang, K.J. Chua , J. Li K.L. Zhang,” Development of micro
power generators – A review”, Applied Energy 88 (2011) 1–16
[2]. A.Carlos Fernandez-Pello, “micropowergeneration using combustion:Issues
and Approaches”, Proceedings of the Combustion Institute, Volume 29,
2002,pp. 883–899
[3]. Uttam Rana, Suman Chakraborty, S.K. Som,” Thermodynamics of premixed
combustion in a heat recirculating micro combustor”, Energy 68 (2014) 510-518
[4].Niket S. Kaisare a, Dionisios G. Vlachos,” A review on microcombustion:
Fundamentals, devices and applications”, Progress in Energy and Combustion
Science 38 (2012) 321-359
[5]. Nam Kim , Souichiro Kato , Takuya Kataoka , Takeshi Yokomori ,
Shigenao Maruyama, Toshiro Fujimori , Kaoru Maruta ,” Flame stabilization and
emission of small Swiss-roll combustors as heaters”, Combustion and Flame 141
(2005) 229–240..