TURBULENT PREMIXED FLAMES AT HIGH KARLOVITZ NUMBERS UNDER OXY-FUEL

CONDITIONS

Yang Chen1, K.H. Luo1,2

1 Center for Combustion Energy, Tsinghua University, Beijing, China2 Department of Mechanical Engineering, University College London, UK

8th Trondheim Conference on CO2 Capture, Transport and Storage16 - 18 June 2015, Trondheim, Norway

Motivations•Turbulent 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:

1) There is a lack of DNS data with detailed chemistry of high Karlovitz number premixed flames to facilitate model development for oxy-methane combustion

2) The understanding and physical insight to combustion characteristics, such as scalar transport in broken flame zone is insufficient.

Objectivesa) Give physical insight to the vortex-flame interactions of turbulent premixed oxy-methane flames in broken flame zones.

b) 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 For oxy-methane flames, the air has been replaced by an O2/CO2 mixture with 67% CO2 by volume (Ref.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).

Governing Equations

The 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.

1 ii

i i

uu

t x x

2( ( ))

3ji i i k

j ijj i j j i k

uu u u upu

t x x x x x x

,k k jk kj k

j j

Y VY Yu

t x x

, ,1 1

( ) ( )N N

p j k k p k k k jk kj j j j

T T T TC u h C Y V

t x x x x

1

Nk

k k

YP RT

W

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.

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

Laminar Flame Files

Fuel consumption rate

2CO H CO OH

2CO H CO OH

2H O O OH

High concentration of CO2 enhances the reaction ,where H is responsible for chain branching reactions by . The competition between CO2 and O2 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 flame

Key ParametersCase OMF1 OMF2 OMF3

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 2.01×10-5 2.01×10-5

Velocity ratio 23.8 73.7 194.2

Ka 160 900 3800

Da 2.1×10-2 6.78×10-3 2.57×10-3

Cell width(m) 10-5 10-5 10-5

2, ,

1

1' ( )

unN

x t x t y yNun

u u uN

0l L

c L

t S lDa

t u

13

22

0

'c L

L

t uKa

t S

Note:RMS velocity is defined as:

Non-dimensional parameters:

Table1. Key parameters in turbulent flame simulations

Global Structures

Fig. 3. Snap shots for CH2O mass fraction (top) and vorticity (bottom, lined by temperature)

YCH2O

Vorticity

OMF1 OMF2 OMF3

Unburned Burned

Global Structures

Fig. 4. Crossing-averaged temperature and fuel consumption rate at t=5τ

OMF1 OMF2 OMF3

5.5 6.9 12.5

1.7 3.5 5.10.05/T L

/T Ls s

Table2. Turbulent flame brush width and speeda

aTime average is performed over t=4 τ-6 τ

PDF Files

Fig.5. PDF files for flame front curvature, density gradient, progress variable and OH mass fraction

Scalar Convection Diffusion and Reaction

Ka

Fig. 6. Convection, diffusion and reaction terms for three flames

Fj

j

YC u

x

1

( )F

j j

YD D

x x

FR

OMF1

OMF2

OMF3

Fig.7. Fuel consumption rate and [CH4]*[OH]

OMF1

OMF2

OMF3

FCR [CH4]*[OH]

Main CH4 consumption routines:

CH4(+M)=>CH3+H(+M) 6.300E+14 0.000 104000

CH4+H=>CH3+H2 2.200E+04 3.000 8751

CH4+OH=>H2O+CH3 1.600E+06 2.100 2460

Reaction A b E

With a lower activation energy, the reactionCH4+OH=>H2O+CH3 is responsible for over 90% of the CH4 consumption.

Fuel Consumption Rate and Radical Fraction

Highlights

Three 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” is gratefully acknowledged