Turbulent combustion characteristics of premixed gases in a packed pebble bed in high pressure

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Turbulent combustion characteristics of premixed
gasesin a packed pebble bed in high pressure

• Presenter Wei Liu

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• Background introduction
• Purpose of this research
• Experimental apparatus and methods
• Results and conclusion

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Background introduction

• A one dimensional laminar premixed flame
  propagates relatively to the fresh gases at the
  so-called laminar flame speed SL depending on the
  reactants, the fresh gases temperature and the
  pressure. For usual fuels, the laminar flame
  speed is about 0.1 to 1 m/s.
• When fresh gases are turbulent, the premixed
  flame propagates faster. Its speed ST is called
  the turbulent flame speed and is larger than the
  laminar flame speed (ST gtgt SL).

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Background introduction

• For laminar combustion,its property only depends
  on the thermal and chemical properties
• For turbulent combustion, its property depends on
  the character of the flow, as well as the mixture
  properties

Different flame types of a Bunsen burner
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Background introduction

• Turbulent premixed combustion is of critical
  importance in practical applications
• a) Spark-ignition engines
• b) Gas-turbine engines
• c) Industrial gas burners

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Background introduction

• Premixed combustion in inert porous media
  combustors has been used for a wide range of
  industrial purpose.

• Packed bed burner

• Diesel engine with porous burner

Engine with porous media reactor
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Background introduction

• In past studies, researches on turbulent premixed
  flames had been performed, and the effects of the
  turbulent intensity on the burning velocity and
  flame structures were widely investigated.
• It is reasonable to consider that a certain
  similarity exists in flame speed and turbulent
  flame structures between with and without packed
  pebbles at high pressure.
• Purpose of this paper To obtain experimental
  data on the flame speed and turbulent flow
  characteristics in a packed bed at high pressure
  to clarify the expectations.

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Experimental apparatus and methods

• Alumina pebbles with diameter d5/10/15mm
• Porosity ?0.4/0.42/0.46
• Equivalence ratio ?0.8-1.0 (CH4/air mixture)
• Initial temperature300K
• Pressure measurement Differential-pressure gauge

• 3-D packed bed burner

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Experimental apparatus and methods

• Twenty layers of ceramic rods
• Diameter of rod10mm
• Roughly same value of porosity as 3-D packed bed
• Hot wire anemometer to measure the turbulence at
  the center of the void space
• Sampling frequency300kHz

• 2-D pseudo packed pebble bed

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Experimental apparatus and methods

• Reactor was placed in a high pressure chamber
• Pressure and temperature keep constant in the
  chamber

High pressure combustion test facilities
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?????
Propagating CH4/air premixed flame ?1.0,
P1.0MPa, U50cm/s, d10mm (U- cross section mean
velocity of the premixed gas)
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Sr

Sdisp
U/?2/3

Flame speed in packed bed
Local velocity of the premixed gas in
consideration of the porosity
The displacement speed of the turbulent flame
region
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Results and conclusion

• SL was calculated using PREMIX/CHEMKIN-II/GRI-Mech
  
• The mode change of the flame propagation is
  caused by the increase of both the flow velocity
  and pressure.
• ? has a small effect

Relationship between U and Sr /SL d10mm,?0.8-1.0
,P0.1-1.0MPa
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Results and conclusion

• Pebble Reynolds number
• Rep Ud/?
• _at_Rep150, Sr/SLminimum value
• Rep,cri 120 (By Kobayashi), where the flow start
  to become turbulent
• Similarity between Sr/SL and ST/SL

Relationship between Rep and Sr /SL
d10mm, P0.1-1.0MPa
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Results and conclusion
Relationship between Rep and transition from
laminar to turbulent flow in a 2-D pseudo packed
bed.
Relative turbulence intensities profile along the
central axis of the 2-D pseudo packed bed
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Results and conclusion
 
Sr/SL and ST/SL shows a strong correlation, which
ensured that the flames in the region of Rep
larger than 150 is dominated by the turbulent
combustion, being referred to as turbulent
flame propagation regime (TFPR)
Correlation between ST/SL and Sr/SL
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Results and conclusion
 
 
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Results and conclusion
Tf is the adiabatic flame temperature calculated
by PREMIX
Tu is the temperature of unburnt gas
n is the number of pebbles in the Combustion
region
Hloss exceeds the Hrelease at a certain U,
leading to the flame extinction in a packed
pebble bed
 
?0.9, P 0.5MPa, U 120cm/s, d 10mm
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Results and conclusion

• Sr/SL minimum value_at_Rep150
• Repgt150, transit to turbulent combustion region
• similarity between turbulent premixed flames with
  and without packed pebbles at high pressure
• For propagating turbulent premixed flame,
  heatlossgtheatrelease when the mean velocity
  increase to a certain value, this cause the
  quenching problem.

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• Thank you for your attention.
• Any questiones?