Structure, Extinction, and Ignition of Non-Premixed Flames in the Counterflow Configuration
Ryan Gehmlich
1
STAR Global Conference 2013 Orlanda, Florida
March 18-20
TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AA
Outline Background
Developing Reaction Mechanisms for Combustion Systems
Validating Mechanisms Using Ideal Flames
Case Study I: Extinction and Autoignition of ethane/air/nitrous oxide flames
Case Study II: Extinction and Autoignition of Lightly-Branched Octane Isomers
Summary
2
Motivation for chemical kinetic studies in combustion
3
Power generation
Gun/Artillery Propellants
Rockets/Missiles Ground Transportation
Aviation Engines
Modeling combustion phenomenon Combustion modeling tools are
now able to couple CFD with detailed chemistry
For this to work, we need to develop validated chemical mechanisms!
Validate chemical mechanisms through the use of 1D ideal flames
4
Reaction Mechanisms
5
2 H2 + O2 → 2 H2O(g) + heat
Global Reaction of Hydrogen Combustion
A few combustion mechanisms San Diego Mechanism – C1-C4 hydrocarbons, hydrogen, nitrogen oxides, JP10, heptane
http://web.eng.ucsd.edu/mae/groups/combustion/mechanism.html
GRI-Mech – Natural gas (including NO)
http://www.me.berkeley.edu/gri-mech/version30/text30.html
USC-Mech II – C1-C4 hydrocarbons, hydrogen
http://ignis.usc.edu/Mechanisms/USC-Mech%20II/USC_Mech%20II.htm
Jetsurf 2.0 – Jet fuel surrogates (i.e. n-dodecane, n-butyl-cyclohexane, etc.)
http://melchior.usc.edu/JetSurF/JetSurF2.0/Index.html
Creck Modeling Group – C1-C16 hydrocarbons, alcohols, esters, reference components of surrogates of real fuels
http://creckmodeling.chem.polimi.it/index.php/kinetic-schemes
Lawrence Livermore National Laboratory – C1-C7 hydrocarbons, alcohols, dimethyl ether, etc.
https://www-pls.llnl.gov/?url=science_and_technology-chemistry-combustion-mechanisms
Engine Research Center, UW Madison – n-Heptane, n-butanol, PAH, biodiesel
http://www.erc.wisc.edu/chemicalreaction.php
6
Counterflow burner for combustion kinetics
7
Laminar, opposed-flow diffusion flames can be established experimentally using this simple flow geometry
Counterflow flames can be simulated by applying the equations of continuity, motion, energy, and species concentration
Boundary conditions are well-defined at the duct exits
Properties such as temperature and species concentrations can be modeled in 1-dimensional space
Flow Field Characteristics
8
Oxidizer strain rate,
Flow is momentum balanced such that
Duct separation distance, L = 10 mm (extinction) or 12 mm (ignition)
Three screens of 200 mesh ensure plug flow at the duct exit planes
Flow Visualization
9
Fuel duct
Oxidizer duct
• Illuminated by HeNe laser sheet • Seeded with baby powder (corn
starch), 0.1-0.8 micron diameter • Streamlines demonstrate plug flow
at the oxidizer duct boundary
Numerical Simulation of Flames Digital Analysis of Reacting Systems (DARS) Basic
Includes 0D and 1D reactor models, including a 1D opposed flow diffusion flame model
Visualize mechanisms and species data
Perform sensitivity analyses, flow analyses, and mechanism reduction
Visualize species and temperature profiles, compare predictions with experiments, tune the mechanisms!
10
Using DARS for a 1d opposed flow reactor
Current versions of the DARS GUI do not having looping capabilities for opposed flow reactors
Looping can be achieved using a high level programming or scripting language and the command line tools of the DARS interface (I used MATLAB)
Convergence to solutions tends to be smoother, faster, and more consistent than other commerical codes on the market
11
Select run path
Use previously generated
start solution?
Yes
No
Copy start solution to run path
Write GasComposition.txt
𝑇𝑗, 𝑌𝑖,𝑗 , 𝑝
Write FlameUserSettings.txt
𝑉𝑗,𝐿, grid settings,
solver settings
Copy to run path: InputRedKinMec.txt
InputRedKinTherm.txt
Chemistry set (mechanism, thermo and transport files)
Create folders in the run path for output files
(DARS command line tools cannot do this)
Run Chamble.exe within the run folder
Convergence?
No
Yes
Use better start solution or adjust grid/solver settings
Case Study I: Extinction and Autoignition of Ethane/Air/N2O Flames
12
Improve knowledge of detailed and reduced chemical kinetic mechanism for gas-phase reactions in the ignition of gelled hypergolic propellants
Gas-phase N2O chemistry is a subsystem of nitramine propellant combustion
Data can be used to validate or improve chemical mechanisms for nitrogen chemistry in these systems
Experimental Apparatus
13
Numerical Computations
All computations done using DARS v. 2.06 and 2.08
Used the latest San Diego mechanism including NOx
61 reactive species, 297 reversible reactions
Some cases checked using Creck C1-C3 mechanism with NOx, v. 1201 (111 species, 1,835 reactions, 2,357 including reverse)
14
Extinction
15
• The structure of the reactive flow-field depends on the five independent parameters YF,1, YN2O,2, YO2,2, T1, and T2.
• The experiments were conducted with T1=T2=298 K. This reduces
the number of independent parameters to three.
• To facilitate comparison of predictions of asymptotic analysis with experimental data, the temperature for complete combustion, Tst, and the stoichiometric mixture fraction, Zst, was fixed. This reduced the number of independent parameters by two, leaving only one independent parameter.
• The strain rate at extinction, a2, was recorded as a function of the mass fraction of N2O, YN2O,2.
Results
16
At a fixed flame temperature (Tst) and location (Zst), replacing O2 by N2O promotes extinction (inhibition)
N2O/O2/N2
C2H6/N2
Ignition Mass Fractions and Boundary Temperatures
17
Fuel Stream Balance N2
, measured by a thermocouple below the fuel duct screens
Oxidizer Stream Contains a mixture of N2O, N2, and air
Kept a constant mass fraction of O atoms in the oxidizer stream for varying
T2 is increased slowly until ignition occurs, all flows are constantly recalculated to retain a constant strain rate and a momentum balance
Results
18
Autoignition temperature vs. strain rate for pure ethane-air flame
Results
19
Autoignition temperature as a function of N2O mass fraction in the oxidizer stream
II. Extinction and Ignition of Lightly-Branched Octane Isomers
20
TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AA
Motivation
21
Previous studies on 2-methylalkane and singly methylated alkanes (such as 2-methylheptane) showed significantly different combustion behavior than their linear alkane counterparts
The present study extends this to work with iso-alkanes that have methyl groups on different locations and with more than one methyl substitution
2,5 dimethylhexane (C8H18-25) and 3-methylheptane (C8H18-3) are important components of petroleum-based transportation fuels
Octane 2,5 dimethylhexane 3-methylheptane 2-methylheptane
Experimental Conditions Mass Fractions and Boundary Temperatures - Extinction
22
Fuel Stream
A range of mass fractions of fuel from 0.2-0.5
Balance N2
Oxidizer Stream
Contains undiluted air
Strain rate is increased slowly until extinction occurs
Experimental Conditions Mass Fractions and Boundary Temperatures - Autoignition
23
Fuel Stream
Balance N2
Oxidizer Stream
Contains undiluted air
T2 is increased slowly until ignition occurs, all flows are constantly recalculated to retain a constant strain rate and a momentum balance
Numerical Computations
Mechanism development by Lawrence Livermore National Laboratory in Livermore, CA
Used two mechanisms:
LLNL detailed mechanism – 767 species, 3,961 reversible reactions
LLNL skeletal mechanism – 241 species, 1,587 reversible reactions
24
Results: Extinction
25
Figure 5. Measured and predicted strain rate at
extinction for diluted DME/air counterflow
diffusion flames
Measured and predicted strain rate at extinction
Methyl branch location makes little difference in extinction between 2- and 3-methylheptane
2,5 dimethylhexane extinguishes at lower strain rates
Results: Autoignition
26
Figure 5. Measured and predicted strain rate at
extinction for diluted DME/air counterflow
diffusion flames
Measured and predicted autoignition temperature
Methyl branch location makes little difference in extinction between 2- and 3-methylheptane
2,5 dimethylhexane autoignites at higher temperatures
Summary
27
• DARS 1D solvers are a useful tool in the development, validation, and reduction of reaction mechanisms
• DARS has proven to be an excellent tool in our arsenal– fast, consistent convergence to flame solutions without too much fuss
Thanks: Fabian Mauss, Lars Seidel, Karin Frojd