Structure, Extinction, and Ignition of Non-Premixed Flames in the Counterflow Configuration

Ryan Gehmlich

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STAR Global Conference 2013 Orlanda, Florida

March 18-20

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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

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Motivation for chemical kinetic studies in combustion

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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

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Reaction Mechanisms

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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

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Counterflow burner for combustion kinetics

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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

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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

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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!

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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

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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

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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

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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)

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Extinction

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• 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

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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

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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

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Autoignition temperature vs. strain rate for pure ethane-air flame

Results

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Autoignition temperature as a function of N2O mass fraction in the oxidizer stream

II. Extinction and Ignition of Lightly-Branched Octane Isomers

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Motivation

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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

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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

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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

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Results: Extinction

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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

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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

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• 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