Yang Chen 1 KH Luo 12 1 Center for Combustion Energy Tsinghua University Beijing China 2 Department of Mechanical Engineering University College London UK 8th Trondheim Conference on CO2 Capture Transport and Storage ID: 284876
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TURBULENT PREMIXED FLAMES AT HIGH KARLOVITZ NUMBERS UNDER OXY-FUEL CONDITIONS Yang Chen1, K.H. Luo1,21 Center for Combustion Energy, Tsinghua University, Beijing, China2 Department of Mechanical Engineering, University College London, UK
8th Trondheim Conference on CO2 Capture, Transport and Storage
16 - 18 June 2015, Trondheim, NorwaySlide2
MotivationsTurbulent fluctuation velocity can be 150 times of laminar flame speed in advanced combustion equipments, where the combustion happens in the broken flame zones.There is growing interest in oxy-fuel combustion for power generation due to its potential in capture and sequestration of carbon dioxide .This work is motivated by two observations:There is a lack of DNS data with detailed chemistry of high Karlovitz number premixed flames to facilitate model development for oxy-methane combustionThe understanding and physical insight to combustion characteristics, such as scalar transport in broken flame zone is insufficient. Slide3
ObjectivesGive physical insight to the vortex-flame interactions of turbulent premixed oxy-methane flames in broken flame zones.Highlight the influence of the replacement of N2 by CO2 on the flame characteristics.DNS Test Flames: Oxy-methane flames at Karlovitz numbers 160, 900, 3800 will be presented . For oxy-methane flames, the air has been replaced by an O2/CO2 mixture with 67% CO2
by volume
(Ref.
Sevault
, Dunn, Barlow,
Ditaranto
. 2012, Combust. Flame: 159: 3342-3352).Slide4
Governing EquationsThe compressible continuity equation, Navier-Stokes momentum equations, the energy equation, transport equations of each species together with auxiliary equations such as the state equation for a compressible reacting gas mixture were solved.
Mass:
Momentum:
Energy:
Species:
Equation of state:
Chemical Mechanism
The 16 species, 35 steps chemical mechanism by
Smook
et al (1991) was used in the present work. Slide5
Numerical Approach The spatial discretisation was carried out using a sixth-order compact finite difference scheme and the discretised equations were advanced in time using a third-order fully explicit compact-storage Runge-Kutta scheme.The inlet and the outlet were specified using the Navier-Stokes characteristic boundary conditions (NSCBC). The lateral boundary conditions are treated periodical. The laminar flame file obtained with detailed chemistry was superimposed over the turbulent field. The same chemistry, transport and thermal files were used while resolving the turbulent flames.
Fig. 1. Schematic figure for the
simulation domain Slide6
Laminar Flame FilesFuel consumption rate
High concentration of CO
2
enhances the reaction ,where H is responsible for chain branching reactions by . The competition between CO
2
and O
2
on H decreases the radical mole fractions, therefore decreasing the burning velocity.
Note: Air is composed of 67% N2 and 33 O2 by volume here.
Fig2. Reaction rate and radical fractions. Solid line is for methane-air flame, dashed line is for oxy-methane flameSlide7
Key ParametersCaseOMF1OMF2OMF3
Equivalence
ratio
0.7
0.7
0.7
Flame speed(m/s)
0.31
0.31
0.31
Flame thickness(m)
4.01
×
10
-5
4.01
×
10
-5
4.01
×
10
-5
Domain length(m)
10
-2
10
-2
10
-2
Domain width(m)
10
-2
10
-2
10
-2
RMS velocity
(m/s)
7.38
22.86
60.22
Integral length scale(m)
2.01
×
10
-5
2.01
×
10-52.01×10-5Velocity ratio23.873.7194.2Ka1609003800Da2.1×10-26.78×10-32.57×10-3Cell width(m)10-510-510-5
Note:
RMS velocity is defined as:
Non-dimensional parameters:
Table1. Key parameters in turbulent flame simulationsSlide8
Global StructuresFig. 3. Snap shots for CH2O mass fraction (top) and
vorticity
(bottom, lined by temperature)
Y
CH
2
O
Vorticity
OMF1
OMF2
OMF3
U
nburned
B
urnedSlide9
Global StructuresFig. 4. Crossing-averaged temperature and fuel consumption rate at t=5τ OMF1OMF2
OMF3
5.5
6.9
12.5
1.7
3.5
5.1
Table2. Turbulent flame brush width and speed
a
a
Time average is performed over t=4
τ
-6
τSlide10
PDF FilesFig.5. PDF files for flame front curvature, density gradient, progress variable and OH mass fractionSlide11
Scalar Convection Diffusion and Reaction
Ka
Fig. 6. Convection, diffusion and reaction terms for three flames
OMF1
OMF2
OMF3Slide12
Fig.7. Fuel consumption rate and [CH4]*[OH]OMF1
OMF2
OMF3
FCR
[CH
4
]*[OH]
Main CH
4
consumption routines:
CH
4
(+M)=>CH
3
+H(+M) 6.300E+14 0.000 104000
CH4+H=>CH3+H
2 2.200E+04 3.000 8751CH4+OH=>H2
O+CH3 1.600E+06 2.100 2460
Reaction A b E
With a lower activation energy, the reaction
CH
4+OH=>H2O+CH3
is responsible for over 90% of the CH4 consumption.
Fuel Consumption Rate and Radical FractionSlide13
HighlightsThree DNS cases with Ka from 160 to 3800 was performed with detailed chemistry and transport mechanisms. Fine vortex/flame front interaction process was resolved. Turbulent eddies can survive in flame zone in high Ka flames, indicating that the flame is in broken flame zones.Turbulent flame brush effect broadened the turbulent flame thickness to 12.5 times of the laminar flame thickness at Ka=3800 flames. Fine turbulent eddies give rise to fuel convection and diffusion terms, which enhance fuel consumption rate.The product of CH4 and OH is highly correlated with fuel consumption rate, indicating that it can represent the active flame zones. THANKS
Support
“
Institutional collaboration on CO2 research actions between Norway and China (RANC)
–
Project No. 211755
”
i
s gratefully acknowledged