Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT...

65
© 2011 ANSYS, Inc. November 4, 2015 1 Modeling reacting flows with detailed chemical kinetics in ANSYS CFD Pravin Nakod Senior Technology Specialist ANSYS Inc.

Transcript of Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT...

Page 1: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 1

Modeling reacting flows with detailed chemical kinetics in ANSYS CFD

Pravin Nakod Senior Technology Specialist

ANSYS Inc.

Page 2: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 2

• Introduction

• Detailed reaction mechanisms

• Detailed chemistry models – Laminar finite rate

– Eddy dissipation concept

– Composition PDF transport

• Chemistry acceleration tools – In-situ adaptive tabulation

– Chemistry agglomeration

– Dimension reduction

– Dynamic mechanism reduction

• Summary

Outline

Page 3: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 3

Detailed chemical kinetics • Tens of species and hundreds of

reactions • Each species participate in a

series of reaction steps – Produced in some steps and

destroyed in some other steps

• Example: H2-O2 reaction mechanism

• Suitable for modeling – Flame ignition and extinction – Pollutants (NOx, CO, UHCs) – Slow (non-equilibrium) chemistry – Intermediate species of interest

(OH…)

Introduction

Page 4: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 4

1000 2000 300010

-15

10-12

10-9

10-6

Sh

ort

est

Sp

eci

es

Tim

e S

cale

, S

ec

Temperature

Ethylene,

p = 1 atmT0 = 1000K

n-Heptane,

p = 50 atmT0 = 800K

Typical flow time

Chemical Stiffness

, K 101

102

103

104

102

103

104

before 2000

2000 to 2005

after 2005

iso-ocatane (LLNL)

iso-ocatane (ENSIC-CNRS)

n-butane (LLNL)

CH4 (Konnov)

neo-pentane (LLNL)

C2H4 (San Diego)

CH4 (Leeds)

Methyl

Decanoate

(LLNL)

C16 (LLNL)

C14 (LLNL)C12 (LLNL)

C10 (LLNL)

USC C1-C4

USC C2H4

PRF

n-heptane (LLNL)

skeletal iso-octane (Lu & Law)

skeletal n-heptane (Lu & Law)

1,3-Butadiene

DME (Curran)C1-C3 (Qin et al)

GRI3.0

Nu

mb

er

of re

actio

ns

Number of species

GRI1.2 pre-2000

2000 – 2005

post-2005

Number of Species

Num

ber

of

Reactions

nC7H16

C11H22O2Large Sizes

Characteristics of detailed reaction mechanisms • Large number of species • Wide range of chemical timescales (Stiffness)

– Small times scales, ~1e-8 (Reactions involving radical species) – Large times scales, ~ 1e-2 (CO oxidation, NOx formation)

Mechanisms for Practical Fuels

Page 5: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 5

• Three types – Detailed, Skeletal and Reduced mechanism

• Detailed mechanism – Large number of species

– Large number of elementary reactions

• Skeletal mechanisms – Shortened detailed mechanisms

– Unimportant species and reactions eliminated

• Reduced mechanisms – Further simplified detailed or skeletal mechanisms

– Quasi-Steady State Assumptions (QSSA)

Types of Detailed Reaction Mechanisms

Co

mp

uta

tio

nal

eco

no

my

Solu

tio

n a

ccu

racy

Low High

Page 6: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 6

Detailed reaction mechanisms available online • http://www.tfd.chalmers.se/~valeri/MECH.html

• http://www.detchem.com/mechanisms.html

• http://web.mit.edu/anish/www/MITcomb.html

• http://www.galcit.caltech.edu/EDL/mechanisms/library/library.html

• http://www.me.berkeley.edu/gri_mech/

• http://www.chem.leeds.ac.uk/Combustion/Combustion.html

• http://melchior.usc.edu/JetSurF/JetSurF2.0/Index.html

• http://www.erc.wisc.edu/chemicalreaction.php

• Reacting Design: Now part of ANSYS

Reaction mechanism is a user input

Acquiring Reaction Mechanisms

Page 7: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 7

CHEMKIN Format

Fluent can import CHEMKIN reaction mechanism files

• Symbolic description of an elementary reaction mechanism

– Information on elements, species and the reaction mechanism

• Appropriate unit conversion is done while importing the CHEMKIN file

• Separate thermodynamic database may be required

Sample file

Pre-exponential Factor Temp. Exponent Activation Energy

Page 8: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 8

Detailed Reactions Modeling in Fluent

• Detailed / skeleton reaction mechanisms – Species limit pre-R14.5: Up to 50 species

– R15: Up to 500 species

• Reduced mechanisms – Quasi Steady State Approximation (QSSA) reduced

mechanisms

– ARM-9 and ARM-19 mechanisms for methane combustion are inbuilt in Fluent

• Reaction rate – Arrhenius with reversible and third body efficiency

– Low and high pressure blend

• Lindemann, Troe, SRI

Page 9: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 9

Detailed Chemistry Models

• Laminar Finite Rate (LFR) model – Laminar flames

– Turbulent flames with weak turbulence-chemistry interaction

• Eddy Dissipation Concept (EDC) model – Assumes reaction occurs in small turbulence scales

– Turbulent flames with turbulence-chemistry interaction

• Composition PDF transport – Turbulent flames with rigorous turbulence-chemistry

interaction

– Computationally most expensive

Page 10: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 10

Laminar Finite Rate Model

• Laminar flows

– 𝝆 𝜕𝒀𝒊

𝜕𝒕+ 𝝆𝑼𝒊

𝜕𝒀𝒊

𝜕𝑿𝒊=𝜕

𝜕𝑿𝒊𝝆𝑫𝜕𝒀𝒊

𝜕𝑿𝒊+ 𝑺

– Diffusivity modeling affects the accuracy

• Turbulent flows

– 𝝆 𝜕𝒀𝒋

𝜕𝒕+ 𝝆 𝑼𝒋

𝜕𝒀𝒋

𝜕𝑿𝒋=𝜕

𝜕𝑿𝒋𝝆𝑫𝜕𝒀𝒋

𝜕𝑿𝒋−𝜕

𝜕𝑿𝒋𝝆 𝑼"𝒋𝒀"𝒋

+𝑺

– 𝑺 evaluated from mean temperature

– Can predict ignition characteristics correctly

– No corrections for temperature or species concentration fluctuations • Works fine for weak turbulence-chemistry interaction

– For larger turbulent fluctuations • Need to use smaller mesh size and/or time step size

Page 11: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 11

• Extension of LFR model to account for turbulence-chemistry interaction

• Suggested by Magnussen in 1981

• Assumptions – Reactants are mixed at molecular level

in the fine turbulent structures

• Of the order of Kolmogorov length scale

– Reactions take place within these structures

– Entire volume of fine scale structures is a fraction of total fluid volume

Eddy Dissipation Concept Model

Bjorn F Magnussen

Fine structures within a computational cell

B. F. Magnussen, 19th AIAA Sc. meeting, St. Louis, USA, 1981

Page 12: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 12

EDC Reaction Rate Evaluation

• Length fraction of the fine scales

• Volume fraction of the fine scales =

• Life-time of small scales

• Reactions proceed within the fine scales over the time, 𝝉∗ – Assuming constant pressure reactor

• Mean reaction rate in mean species equation

25.0

t

4/1

2

*Re

kC

Kolmogorov

2/1

*tC

)YY()1(

S i

*

i3**

2*

i

3*

Fine structures within a computational cell

* Quantities are fine scale quantities; = 2.1377 and = 0.4082 CC

Page 13: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 13

Composition PDF Transport Model

• RANS approach solves Favre averaged species and energy equations – Prone to errors for flows with strong turbulence-chemistry interaction

• A transport equation for joint probability density function (PDF) is obtained from species and energy equations

– Proposed by S.B. Pope (1976)

–𝜕

𝜕𝒕𝝆𝑷 +

𝜕

𝜕𝑿𝒊𝝆𝒖𝒊𝑷 +

𝜕

𝜕𝝋𝒌𝝆𝑺𝒌𝑷 =

𝜕

𝜕𝑿𝒊𝝆 𝒖"𝒊 𝝋 𝑷 +

𝜕

𝜕𝝋𝒌𝝆𝟏

𝝆

𝝏

𝜕𝑿𝒊𝑱𝒊,𝒌 𝝋 𝑷

• P is Favre joint composition PDF • 𝝋 is the composition space = 𝝋 (Y1, Y2, Y3 …… YN , T)

• 𝑨 𝑩 denotes the probability of event A, given the event B occurs

• Ji,k is molecular diffusion flux vector

S.B. Pope, Combustion and Flame, 27, 299-312 (1976)

S B Pope

Page 14: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 14

• Detailed chemistry calculations are computationally expensive

• Chemistry acceleration tools are available to speed – In-situ adaptive tabulation (ISAT)

– Chemistry Agglomeration (CA)

– Dimension Reduction

– Dynamic Mechanism Reduction (DMR)

Chemistry Acceleration Tools

Page 15: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 15

In-Situ Adaptive Tabulation (ISAT)

• Thermodynamic state of fluid characterized by – Mass fractions of species (Yi); Enthalpy (h) and Pressure (P)

• Broad class of flow P ~ P0 (Reference pressure)

• Species and temperature represented by a composition – (Y1, Y2, Y3 …… YN , T)

• Each as a point in D-dimensional composition space – D degree of freedom

• Evolution of

– M is rate of change due to transport

– S is rate of change due to chemical reaction • As t , the trajectory tends to chemical equilibrium

– Linear approximation • To calculate reaction mapping, R( ) for other points close by 0

Evolution of in a composition space

Page 16: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 16

ISAT Functioning

Initially, reacting flow code provides ISAT with

• Time step size, t

• Error Tolerance, tol

• Scaling matrix, B

– It is used to calculate errors

During calculation, reacting flow code gives a query to ISAT

ISAT returns, corresponding mapping within required accuracy

Reacting

Flow Code ISAT

t, B, tol

q

R( q)

Page 17: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 17

ISAT Functioning (cont…)

ISAT passes the composition ( 0) and time step size (t ) to Mapping module

Mapping module does direct integration and returns mapping R( 0) and mapping gradient matrix A( 0) to ISAT

ISAT stores this record in the form of binary tree

• At each leaf , there is a record – Fixed data: Point ( 0); Reaction Mapping R( 0); Mapping gradient A( 0)

– Changing data: EOA information

• At each node , there is a information about cutting plane

ISAT

Mapping

Module

ODE Solver

+ Chemistry

0

R( 0), A( 0)

Page 18: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 18

Direct integration

Algorithm

Initiate binary tree with single leaf

(Exact value of Mapping)

Reacting

Flow Code ISAT

Mapping

Module First Query ( 0)

0

R( 0), A( 0)

Reacting

Flow Code New Query ( q)

Look for

( 0 ) ( q ) & check

( q ) within EOA

ISAT

Traverse binary tree

Linear Approx. Get R( q) Yes No

R(

q)

Return R( q)

Calc. < tol

Yes No

Grow EOA Add new entry

R ( q)

Retrieve

Growth Addition

Page 19: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 19

ISAT: Best Practice Memory • The table size is user input (in Mb) • Set to a large fraction of your maximum available memory

Accuracy • Default ISAT tolerance is 0.001

– Relatively larger; Allows faster convergence • Large performance penalty for error tolerance smaller than it is needed to achieve acceptable

accuracy • For steady state problems:

– Obtain a converged solution with relative large ISAT tolerance (e.g., default value) – Then decrease it gradually and judiciously to obtain accurate thermo-chemical solutions

• For unsteady problems – For small calculations, perform an accuracy study by running the calculation with different

ISAT error tolerances – For large calculations, obtain the necessary ISAT error tolerance by performing an accuracy

study for the considered mechanism in a separate, similar but simple test case

Efficiency • Periodic cleaning of ISAT is more efficient

– For transient problems such as those in IC engines – For initial transition in steady-state problems

• Saving ISAT to reuse it in another calculation is not beneficial • Building the ISAT table from scratch in a calculation is in general preferred

Page 20: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 20

• Lots of cells or particles have similar composition

• Cells close in composition space – Similar in temperatures and mass

fractions of species – Clubbed for calculation of

reaction mapping

• Results in fewer calls to ISAT • Map reaction step back to

original cells

Chemistry Agglomeration

Mixture fraction (f)

T

Page 21: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 21

Chemistry Agglomeration Speed-Up

# of cells Agglomeration setting # of chemistry cal. Per iteration

CPU Time Speed-up

2352 No 2352 2970 1

2352 T + 3 species; 0.01 ~700 1080 2.75

2352 T + 3 species; 0.02 ~520 840 3.54

2352 T + 3 species; 0.03 ~395 610 4.87

2352 T + 3 species; 0.04 ~314 540 5.5

Goldin, Ren, Zahirovic, Combustion Theory and Modeling, 2009

Page 22: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 22

• Reduce the number of transported species – User to select number of the represented species

• Transport equations for the selected species only • Unrepresented species constrained to chemical

equilibrium – Ensure that the unrepresented species are near chemical

equilibrium for better accuracy

• Applications – Dimension reduction is principally to use for mechanisms

with more than 50 species

• Limitations – Equilibrium assumption for unrepresented species

• Not a good assumption for pollutant formation

Dimension Reduction

Page 23: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 23

Dynamic Mechanism Reduction (DMR)

Recall, at each flow iteration* or time-step

• Fluent calls ODE solver to solve chemistry IVP at each cell

• tODEsolver ~ c1Nspecies3 + c2Nspecies

2 + c3Nspecies + c4Nreactions

Dynamic Mechanism Reduction:

• Decrease tODEsolver by reducing Nspecies, Nreactions

– Evaluate kinetic rates before calling ODE solver

• Drop species and reactions with negligible contributions

– Solve smaller ODE set

• Compositions of dropped species are “frozen” throughout ODE solution

– Fewer reaction rates need to be evaluated during ODE solve

• Potentially different mechanism per cell, per flow iteration or time-step

• Mechanism reduction method in R15 : Directed Relation Graph (DRG)

– T.Lu and C.K. Law, 2005

*number of flow iterations per chemistry solve can be adjusted in UI; default = 1

Page 24: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 24

Sandia Flame D, Sydney bluff body flame, Yale flame

• Start from a converged equilibrium solution

• Run a certain number of iterations until convergence with Laminar Finite Rate model

• Compare results between Direct Integration and Dynamic Mechanism Reduction (including speedup factor and scalars accuracy)

Stationary Flames

Page 25: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 25

Sandia Flame D

Co-flow Outlet

Axis

Symmetry

Jet

Pilot

• Mesh 7112 Cells

• Mechanism Methane Flame

– 53 species, 325 Reactions

• DMR tolerance = 0.01 (default)

Page 26: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 26

Sandia Flame D Speedup factor (runtime full mechanism / runtime DMR) = 1.97

Temperature profile (axis)

Page 27: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 28

Sandia Flame D DMR post processing related quantities

Large concentration of chemically active cells in a small region of the domain which gives a high potential for reduction techniques

Speedup factor = 1.97

Page 28: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 29

Initial conditions and set up • Poor initial condition can cause stiff chemistry solver to fail

• Obtain initial solution with any of the fast chemistry model

– Provides a good initial guess for temperature and species

ISAT tolerance • In final solution, decrease the ISAT tolerance to make sure

that the solution is independent of any table interpolation error

With stiff chemistry solver • For slow converging cases, increase the aggressiveness factor

• Aggressiveness factor – Between 0 (most robust but slowest convergence) and 1

– The default aggressiveness factor is 0.5

Solution Strategies

Page 29: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 30

• New chemistry ODE solver CVODE

• Improvements to chemistry solver – More speedup expected for large

mechanism

• Improvements to ISAT efficiency

• Improvements to Dynamic Mechanism Reduction efficiency

Stiff Chemistry Solver R16 New Features

Page 30: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 31

Initial conditions

Pressure (bar) 1.1

Temperature (K) 330

Equivalence ratio 0.4

Turbulent kinetic energy (m2/s2) 0.01

Turbulent dissipation rate (m2/s3) 0.01

Swirl velocity (m/s) 0.01

Boundary conditions

Cylinder temp. (K) 400

Head temp. (K) 450

Piston temp. (K) 550

Test Case

Generic HCCI engine

Setup:

• Compressible flow

• Laminar Finite Rate

• Dynamic mesh : Crank Angle step of 0.5, reduced to 0.1 around ignition

• 20 iterations per time step, 866 time steps

head

piston

cylinder

Mesh : 650 up to 11550 cells

Engine properties : Compression ratio : 13.5 Speed : 1500 RPM Simulation from IVC -143 CA aTDC to EVO 130 CA aTDC

axis

Mechanism : 160 species / 1540 reactions (n-heptane flame)

Page 31: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 32

Test Matrix

Run the case with Direct Integration (no chemistry acceleration tool)

Run the case with ISAT (storage 500 MB, tol = 1e-04, cleaning up the table every 50 time-steps)

Run the case with Dynamic Mechanism Reduction (default tolerance = 0.01)

Run the case using both ISAT and DMR

Compare R15 and R16 in terms of

• Accuracy (averaged temperature time evolution in the cylinder)

• Speedup on the ODE solver runtime

Page 32: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 33

Comparison in Runtimes (R15 and R16)

93%

60%

72%

31%

92%

40%

28%

69%

Percentage represents the ODE solver runtime over the total runtime

In R16 the chemistry ODE solver runtime becomes comparable or sometimes less

than the transient flow solver

Speedup Factors = ODE solver runtime case R15 / ODE solver runtime case R16

Page 33: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 34

Comparison in Accuracy (R15 and R16)

Direct Integration case (no chemistry acceleration tool)

Similar accuracy between R15 and R16

Page 34: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 35

Chemistry Acceleration Tools in R16

Speedup Factors = ODE solver runtime case DI / ODE solver runtime

Page 35: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 36

Chemistry Acceleration Tools in R16

Accuracy Comparison

Page 36: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 37

Test Case: RCM-3

Presented at: 13th International Conference on Numerical Combustion April-2011, Corfu, Greece

Page 37: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 38

MASCOTTE Combustor: RCM-3 Details

Combustor features: • Cylindrical injectors • Square combustion chamber

Thermodynamic conditions

• GH2 Supercritical (Both Pressure and Temperature are supercritical)

• LOx Transcritical (Pressure is supercritical and Temperature is subcritical)

Hydrogen Tc = 32.98 K

Pc = 1.29 MPa

Oxygen Tc = 154.58 K Pc = 5.04 MPa

Page 38: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 39

Geometry

Actual MASCOTTE combustor has a circular injection section and square combustion chamber

However, whole model is considered to be cylindrical for the modeling purpose

• Increased diameter is used to reproduce the volume of the combustor

Injector geometry

Page 39: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 40

2D Axi-symmetric

Inbuilt real gas models

SST k-w turbulence model

Combustion models • Laminar finite rate: 8-species, 21-

reactions – Li, Zhao, Kazakov, and Dryer, Princeton

University, 2003

• Non-premixed model: Equilibrium approach

Transport properties: • NIST data for LOx

• Kinetic theory for other species

Models Set-Up

Page 40: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 41

Cheng & Farmer

Fluent - SRK

Tmax ~3700

Fluent - ARK

Fluent - RK

Fluent - PR

Tmax = 3714

Tmax = 3720

Tmax = 3716

Tmax = 3722

Temp (K)

Tmax ~3700

Tmax = 3626

Tmax = 3622

Tmax = 3623

Tmax = 3626

Temp (K)

Laminar finite rate model Non-premixed equilibrium model

Laminar finite rate model

Temperature Contours

Page 41: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 42

Comparison of Axial Temperature

0

475

950

1425

1900

2375

2850

3325

3800

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Tem

p (

K)

Axial distance from injector exit (m)

Cheng & Farmer

SRKARK

RKPR

0

475

950

1425

1900

2375

2850

3325

3800

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Tem

p (

K)

Axial distance from injector exit (m)

Cheng & FarmerSRKARKRKPR

Laminar finite rate model Non-premixed equilibrium model

Page 42: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 43

Test Case: FIRE-II

Page 43: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 44

Reentry Vehicle Geometry

FIRE-II: Flight Investigation of Reentry

Environment; Flight-II

• A high-velocity flight experiment

• Velocity of 11.35 km/s representative of Apollo

lunar return velocities

Two sections • Fore body

• After body

Only fore body section is considered

2D axi-symmetric model

Two meshes used • Initial solution with coarser mesh

• Final solution with finer mesh

Reentry vehicle geometry

Fore body After body

Page 44: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 45

Mesh

Initial Adapted

Cells 10640 22151

Initial Adapted Initial

Adapted

Dynamic adaption (velocity gradient) Boundary adaption

Page 45: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 46

Test Conditions and Models

FIRE II : Flight Condition

Models

• Solver: DBNS, Implicit

• Flow: Laminar

• Reactions: Laminar finite rate

– 11 species, 20 reactions

– Stiff chemistry solver

Boundary Conditions

• Pressure far field

– Mach No. 37.3

– Pressure 10.5 Pa; Temperature 210 K

– Mass fractions: O2 23.3%; N2 76.7%

• Reentry vehicle wall

– Isothermal: 810 K Axis

Pressure far field

Wall

Pressure outlet

Altitude (km)

Velocity (km/s)

Density (kg/m3)

T

(K) Tw

(K)

71.04 11.31 8.57e-5 210 810

Page 46: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 47

Reaction Model

11 species, 20 reaction Gupta et. al. model

Page 47: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 48

Temperature

DPLR LAURA DPLR-LAURA Present work

Shock stand off distance (m)

0.04598 0.04624 0.04446 0.04573

Temperature (K) contours Axial temperature profile

0

5000

10000

15000

20000

25000

30000

35000

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 0.055 0.06Te

mp

era

ture

(K

) Axial distance from stagnation point (m)

DPLR

LAURA

DPLR-LAURA

Present work

Page 48: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 49

Results

Sharp perpendicular shock near axis

Mach No Velocity (m/s) Static Pressure (Pa)

Page 49: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 50

Electrons Distribution

Electron mole fraction

Y-co

ord

inat

e (m

)

Page 50: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 51

Self-ignition and flame propagation of high-pressure hydrogen jet during sudden discharge from a pipe

Page 51: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 52

Experimental Observations: Ignition is a Possibility

• Not every case results in ignition

• Sometimes, auto-ignition is followed immediately by quenching

• auto-ignition and flame propagation has been observed only for a certain combination of – Pipe diameter – Length of the pipe – Burst Pressure

Page 52: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 53

Test Case

Page 53: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 54

Reaction Mechanism: 8 Species and 21 Reactions

Page 54: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 55

Comparison Along the Axis

Mach Number

Pressure (bar)

Page 55: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 56

Comparison Along the Axis

Axial velocity

Temperature (K)

Page 56: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 57

Temperature Contours at 35s

Xu et. al. Present work

Overall flame capture is reasonable Need to refine mesh and other modeling set up to improve the accuracy

Page 57: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 58

R16 Detailed Chemistry Improvements

Page 58: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 59

New chemistry ODE solver CVODE

Improvements to chemistry solver

• More speedup expected for large mechanism

Improvements to ISAT efficiency

Improvements to Dynamic Mechanism Reduction efficiency

Stiff Chemistry Solver New Features

Page 59: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 60

Initial conditions

Pressure (bar) 1.1

Temperature (K) 330

Equivalence ratio 0.4

Turbulent kinetic energy (m2/s2) 0.01

Turbulent dissipation rate (m2/s3) 0.01

Swirl velocity (m/s) 0.01

Boundary conditions

Cylinder temp. (K) 400

Head temp. (K) 450

Piston temp. (K) 550

Test case

Generic HCCI engine

Setup :

• Compressible flow

• Laminar Finite Rate

• Dynamic mesh : Crank Angle step of 0.5, reduced to 0.1 around ignition

• 20 iterations per time step, 866 time steps

head

piston

cylinder

Mesh : 650 up to 11550 cells

Engine properties : Compression ratio : 13.5 Speed : 1500 RPM Simulation from IVC -143 CA aTDC to EVO 130 CA aTDC

axis

Mechanism : 160 species / 1540 reactions (n-heptane flame)

Page 60: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 61

Test matrix

Run the case with Direct Integration (no chemistry acceleration tool)

Run the case with ISAT (storage 500 MB, tol = 1e-04, cleaning up the table every 50 time-steps)

Run the case with Dynamic Mechanism Reduction (default tolerance = 0.01)

Run the case using both ISAT and DMR

Compare R15 and R16 in terms of

• Accuracy (averaged temperature time evolution in the cylinder)

• Speedup on the ODE solver runtime

Page 61: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 62

Comparison in runtimes between R15 and R16 Speedup Factors = ODE solver runtime case R15 / ODE solver runtime case R16

93%

60%

72%

31%

92%

40%

28%

69%

Percentage represents the ODE solver runtime over the total runtime

In R16 the chemistry ODE solver runtime becomes comparable or sometimes less

than the transient flow solver

Page 62: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 63

Comparison in accuracy between R15 and R16 Direct Integration case (no chemistry acceleration tool)

Similar accuracy between R15 and R16

Page 63: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 64

Chemistry acceleration tools in R16 Speedup Factors = ODE solver runtime case DI / ODE solver runtime

Page 64: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 65

Chemistry acceleration tools in R16 Accuracy comparison

Page 65: Modeling reacting flows with detailed chemical kinetics in ... · ISAT Functioning cont… ISAT passes the composition ( 0) and time step size ( t ) to Mapping module Mapping module

© 2011 ANSYS, Inc. November 4, 2015 66

• Different modes are available in ANSYS CFD to model detailed reactions mechanisms

• A variety of chemistry acceleration tools are also available to speed up the chemistry calculation

– Depending on the size of reaction mechanism they can be combined to achieve calculation speed-up

• Variety of problems are modeled using detailed reactions in FLUENT

– Flames

– Compressible

– Cryogenic conditions

– IC Engine

– Self ignition…….

Summary