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of Non-Premixed Flames in the Counterflow Configuration Ryan - - PowerPoint PPT Presentation

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Structure, Extinction, and Ignition of Non-Premixed Flames in the Counterflow Configuration Ryan Gehmlich STAR Global Conference 2013 Orlanda, Florida March 18-20 1 Outline Background Developing Reaction Mechanisms for Combustion

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SLIDE 1

Structure, Extinction, and Ignition

  • f Non-Premixed Flames in the

Counterflow Configuration

Ryan Gehmlich

1

STAR Global Conference 2013 Orlanda, Florida March 18-20

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SLIDE 2

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

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SLIDE 3

Motivation for chemical kinetic studies in combustion

3

Power generation Gun/Artillery Propellants Rockets/Missiles Ground Transportation Aviation Engines

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SLIDE 4

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

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SLIDE 5

Reaction Mechanisms

5

2 H2 + O2 → 2 H2O(g) + heat Global Reaction of Hydrogen Combustion

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SLIDE 6

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

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SLIDE 7

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

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SLIDE 8

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

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SLIDE 9

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

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SLIDE 10

Numerical Simulation of Flames

 Digital Analysis of Reacting Systems (DARS) Basic

 Includes 0D and 1D reactor models, including a 1D

  • pposed 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

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SLIDE 11

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 𝑈

𝑘, 𝑍 𝑗,𝑘, 𝑞

Write FlameUserSettings.txt 𝑊

𝑘,𝑀, 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

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SLIDE 12

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

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SLIDE 13

Experimental Apparatus 13

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SLIDE 14

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

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SLIDE 15

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.

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Results 16

At a fixed flame temperature (Tst) and location (Zst), replacing O2 by N2O promotes extinction (inhibition) N2O/O2/N2 C2H6/N2

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SLIDE 17

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

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SLIDE 18

Results 18

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

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SLIDE 19

Results 19

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

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SLIDE 20
  • II. Extinction and Ignition of Lightly-Branched

Octane Isomers

20

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SLIDE 21

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

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SLIDE 22

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

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SLIDE 23

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

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SLIDE 24

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

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SLIDE 25

Results: Extinction 25

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

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SLIDE 26

Results: Autoignition 26

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

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SLIDE 27

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