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DARS overview, IISc Bangalore

18/03/2014

Outline

• Introduction

• Modeling reactions in CFD

• CFD to DARS

• Introduction to DARS

• DARS capabilities and applications

• Overview of modules

• Homogenous reactors

• Reduction of mechanisms

• Flame modeling in DARS

• SRM models

• DARS 1D models

• Summary

2

Introduction

• CFD was developed to understand the fluid flow

phenomena in various applications

• CFD provides flow, energy, concentration, and

turbulence fields by solving the conservation

equations on a discretized domain

• Although flow characteristics are of major interest in a

CFD solution, if applications involve reactions

species concentrations are also to be resolved

• Following slide shows modeling of reactions in a CFD

framework

3

Modeling reactions in CFD

4

• Simulation of systems in which reactions are also

involved is one of the major interests in combustion

and chemical processing applications

• Currently, CFD can be used to model reactions, by

incorporating detailed kinetic mechanisms or global

reactions into it

• STAR-CCM+ has the capability to solve for reactions

and combustion as well

• On the other hand, it might be time consuming to

solve for species concentrations, which is elaborated

in the next slide

Timescales

5

• In CFD simulations, flow is typically resolved whose

timescales are of the order of milliseconds

• When reactions are involved, they require very low time step

to capture the physics when compared to flow

• As shown in the picture below chemical time steps range from

10-10s to 10s depending upon the chemistry

• Hence, the overall timestep for the simulation has to be very

low(of order of chemical reactions) which would slowdown the

simulations greatly

CFD to DARS

• Apart from the timescales, if the no. of reactions/species

increase in CFD simulations the no. of equations per cell to

solve would increase

• So, if we have to incorporate a detailed mechanism

consisting of many reactions into CFD, it would be

computationally limiting

• Lower timescales and higher no. of reactions/species

suggest us the idea of studying progress of reactions

alone, decoupled from the flow

• This is where DARS comes into picture

6

Introduction to DARS

• DARS- Digital Analysis of Reaction Systems, a

product of CD-adapco, which makes STAR-CCM+

• Used to study progress of reactions without taking flow

into consideration

• Standalone tool for simulating chemical reaction

systems using detailed kinetic mechanisms

• It uses transient, 0D and 1D models to study formation

of various products as described by the mechanism

• Can be used as a precursory code to CFD simulations

to understand the chemistry alone of the system

7

DARS

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• DARS Basic- Standalone tool

• DARS-CFD-coupled with STAR-CCM+

• Emission Libraries-Libraries which can be used in STAR-CCM+

Fast • Based on speedy stochastic reactor models

• Mechanisms optimized to affordable sizes

• Library based combustion and emissions

DARS assets

Accurate

• Detailed chemistry mechanisms

• Methods based on real physics

Easy to Use

Ease of setup and tuning

• One-panel setup for

complex fuel chemistries

• Full GUI support for

setup

• Very few parameters to

tune

• 2D, 3D engine mapping

Coverage

A complete range of models

• IC Engines

• Emissions

• After-treatment

• Fuels

• Flames and burners

• Catalysts and Particulate

Filters

Combustion and

emissions

Industries and Applications

Fuel industry

• Conventional fuels

• Natural gas

• Dual fuel

• Biofuels

• Synthetic fuels

Automotive

• SI

• DICI

• PPC

• HCCI

Environment

• Exhaust manifold

• Catalytic converters

• Diesel particulate filters

Heavy, energy and

chemical industries:

• Power generation

• Gas turbines

• Flames and burners

Map of Chemical Simulation

11

Physical

Model

Chemical

Model

Reactor Tools Detailed Chemistry

Simplified Chemistry CFD

Experiments

DARS Basic Capabilities

DARS Basic

Reactors

Homogeneous

Theoretical Engine Reactors

SI (Two-Zones)

HCCI

SRM

Stochastic Reactor Models

PSR

DARS-SRM-SI

DARS-SRM-HCCI

DARS-SRM-DICI

DARS

1D Models

Piping Coolers After-

treatment

Catalytic Converter

Diesel Particulate Filter (DPF)

Turbo charging

DARS-ESM Flames

Premixed Counter Flow

Flamelet

Single

Library

Chemical Mechanisms

Development

Analysis

Reduction

Transient!

Turbulence and gas

inhomogeneities

Essential for engine modeling with detailed

kinetics

Full powertrain simulation

Adds GT-Power or WAVE capabilites

Enable for ex. dual fuel

applications

Burners

DARS Basic Capabilities (Contd.,)

• On broader classification DARS contains

– Reactors(Homogenous and stochastic) used in the chemical

industries

– Mechanism modules majorly used for analyzing and reducing

the mechanism

– Flame models used for general combustion studies

– Stochastic reactor models(SRM) models to account for

inhomogeneity

– 1D models( stochastic) which are used for catalysis and after

treatment industries

• We would briefly touch upon all the modules to give an

overview of each one of them and their applicability

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Homogeneous Reactors

• Reaction mixture is homogenous throughout, in terms of

physical quantities such as, concentrations, Temperature

etc,. (except plug flow reactor)

• These contain both open and closed reactors

• Conservation of mass, species, energy are solved

• Available modules are:

– Constant pressure and volume reactors

– Perfectly stirred and plug flow reactors

– Rapid compression model: An engine model

• Various modules are explained in detail in the following

slides

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Constant Volume Reactor

• Closed, stationary and homogeneous

• Volume is kept constant. Pressure is allowed to increase

• Used in Calorimetric studies to determine heat of formation

of various fuels

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[ Mass]

[ Species]

[ Specific Internal energy]

Constant Pressure Reactor

• Gas allowed to expand freely in the reactor volume

• Closed, stationary, homogeneous system

• Used for ignition delay times, generating PVM table

16

[ Mass]

[ Species]

[ Specific Enthalpy]

Perfectly Stirred Reactor

• Constant pressure, homogeneous flow system

• Steady-state gas phase combustion

17

[ General Mass]

[ General Species]

[ General Energy]

Plug Flow Reactor

• 1-D model of a tubular reactor

• No axial mixing (diffusive transport = 0)

• Perfect radial mixing (diffusive transport = inf)

• Steady flow

• With or without surface reactions

• Heat transfer options:

– Adiabatic, isothermal, linear

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[ Mass]

[ Species]

[ Momentum]

[ Energy]

Rapid Compression Machine

• Rapid compression machine: Closed system that

represents the time between intake-valve closure and

exhaust valve opening in the engine cycle.

• Equilibrium Model: Can compute adiabatic flame

temperatures for gas-phase systems

– Based on minimization of Gibbs free energy for constant

atomic mass fractions

– Can vary equivalence ratio, temperature, pressure for

multiple runs.

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Rapid compression machine(RCM) is an engine model, included in

the homogenous reactor models of DARS

Reduction of kinetic mechanisms

• Detailed kinetic mechanisms are required to accurately

predict the behavior of reacting systems

• However, the use of these reaction mechanisms for

modeling combustors in Computational Fluid Dynamics

(CFD) is expensive

– The reaction mechanism describing oxidation of n-decane, consists

of 209 species and 1673 reactions, most of them reversible

(Dagaut et al. 2006)

• Reducing a reaction mechanism to a form having less

number of reactions and species

• The reduced mechanisms can be plugged into CFD

simulations

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Degree of Reduction

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• The detailed mechanism can be reduced to any degree of

complexity

• Trade off between accuracy and computational time

• Following diagram shows levels of reduction and their

applications

Analysis of detailed mechanisms

• Identification of less important species/reactions by

analysis of detailed mechanism

• Approximations

– Quasi steady state approximation: If the species is short-

lived, it is assumed that the net rate of production of the

species is zero

– Partial equilibria assumption: Fast reactions are taken to be

in equilibrium

• The most common analysis techniques are:

– Sensitivity analysis

– Reaction flow analysis

– Lifetime analysis

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Sensitivity Analysis

• Sensitivity analysis involves investigation of the change in

a quantity of interest due to small changes in the

controlling parameters

• In analyzing kinetic mechanisms, the quantities of interest

are generally concentrations of species

• The highly influential parameter would be the controlling

parameter, temperature if reactions are temperature

sensitive

• If rate constant is the controlling parameter, then sensitivity

coefficient is defined as

23

Analysis of kinetic mechanisms

Reaction flow analysis Lifetime analysis

• Lifetime analysis is used

for finding species eligible

for the Quasi Steady

State Assumption

(QSSA)

• It gives the time for which

a species is alive

• Species with lower

lifetime and concentration

are identified

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• Reaction flow analysis

determines the pathway

of formation of products

from reactants

• The detailed mechanism

is given, into DARS,

which would solve the

mass fluxes from one

species to another

Illustration: Reaction flow analysis

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• Arrows denote the

reaction pathways

• Thickness of the

lines denotes the

mass flux

• The least

significant (thick)

pathways can be

eliminated from

the mechanism

for reduction

Flames: Introduction

• Flame is a moving combustion zone

• A self-sustaining propagation of a localized combustion

zone

• The two mechanisms for propagation are

– Thermal propagation: the mixture is heated by conduction to

the point where the rate of reaction is sufficiently rapid to

become self-propagating

– Diffusional propagation: diffusion of active species, such as

atoms and radicals, from the reaction zone or the burned gas

into the unreacted mixture causes reaction to occur

• It can vary from laminar to turbulent

• 26

Available flame models

• Characteristics of Flames in DARS

– Flames in DARS are one dimensional with a z-axis

perpendicular to the flame front

– Flames are calculated at constant pressure

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Flames

Premixed

Burner stabilized

Freely propagating

Counterflow

Diffusion

Back to Back

Flamelet

Single

Library

Transient

Premixed Flames

• In a premixed laminar flame the fuel and oxidant

mixture move in the z-direction with the unburned

mixture at z→-∞ and the burnt mixture at z→+∞

• The basic equations solved are:

– Mass conservation

– Species conservation

– Energy Conservation

• Premixed flames can be of two types, burner

stabilized and freely propagating, which are

discussed in the next few slides

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Burner Stabilized Flames

• Burner stabilized flames most often used to study

chemical kinetics

• Modeled as one-dimensional, steady-state flames

• Input: Conditions of the gas at the inlet, burner

configuration (inlet gas velocity)

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Freely Propagating Flames

• Point of reference is a fixed position on the flame

• Flame speed is thus the velocity of unburned gases moving

towards the flame which allows the flame to stay in fixed

• Input: Conditions of the gas at the inlet

– Option to include thermal diffusion and radiation available

– Can calculate temperature profile or read temperature profile

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Counter-flow Flames

• Counterflow flames are produced in the

space between two opposed gas flows

• Can be either premixed or non-

premixed

• Non premixed are complicated than the

premixed flames as, diffusion is the

driving parameter

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Two types of counterflow flames DARS supports are:

• Diffusion: Fuel injected on one side, oxidizer on the other

• Back-to-back: mixture of fuel and oxidizer injected from both sides

Gives two premixed flames

Flamelet

• In turbulent flows, when chemical time scale is

very small compared to the convection/diffusion

timescales combustion occurs in thin zones

• The flame in these thin zones is assumed to be

laminar and are called flamelets

• Features of flamelets

– Turbulent flame considered to be an ensemble of ‘laminar’

flamelets

– Facilitates decoupling of flow and chemistry

– Conservation equations for species and energy expressed in

terms of mixture fraction and scalar dissipation rate

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Flamelet: Models

• The different models supported by DARS are

based on scalar dissipation rates

• They can either be steady or unsteady

• The models are :

– Single Flamelet: Steady state flamelet at user defined scalar

dissipation rate

– Library Steady Flamelet: Runs for a range of scalar

dissipation rates until extinction

– Transient Flamelet model: Solves unsteady equations

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Introduction: Stochastic Reactor Models

• Drawbacks with homogeneous reactors for engines

- Homogeneous composition and temperature

» all gas ignites at once

» overprediction max pressure, temperature, NOx

- Impossibility to account for differences in gas

- Turbulence modeling

• Inhomogeneities exist due to:

- Charge stratification

- Crevices

- Heat transfer to the wall

- Injection (DI engine)

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Stochastic Reactors

What are Stochastic Reactors?

• Stochastic Reactor Model: Quasi 0-D model.

• Homogeneity within the combustion chamber is

replaced by statistical homogeneity, with physical

quantities described by PDFs

• In-cylinder conditions such as species

concentrations, density, pressure, temperature,

cylinder volume, heat release, heat transfer as a

function of time can be determined

• Autoiginition timing and combustion duration also

determined

35

SRM: General features

36

• Gas state (species, enthalpy) is described by PDFs

• Discretization of gas into virtual “particles” (SRM-cells)

• Mixing model (deterministic or stochastic) – to model

turbulence

• Stochastic heat transfer

• Operator splitting technique for solving the system of

differential equations

• An equivalent CFD calculation would take significantly larger resources

• For example, simulation performed on an In-cylinder engine CFD module

consisting around 0.2 million cells takes around 0.5 day (12 hours) of analysis

time on 8 processors using 3D CFD, while it takes less than an hour for

an SRM run in 1D DARS Basic

SRM Modeling

37

• The mixture is described with Probability Density

Functions (PDF): in-cylinder mass is divided into

particles representing the discretized PDF

• Each particle represents a point in the phase space of

species mass fraction, and of enthalpy

• Total heat exchange can also be determined and is

defined by Woschni model

Homogenous vs. Stochastic

38

A comparison between homogeneous and stochastic reactor model for SI

engine shows stochastic reactors capture the phenomena better when

compared to a homogenous reactor model

DARS 1D models

Catalyst Module

• A 1D module in which the entire reactor is split into PSRs

Stochastic Pipe Module

• DARS pipe model is a one-dimensional approach based on

a series of partially stirred reactors

Diesel Particulate Filter

• DARS DPF is a transient 1D model based on a series of

perfectly stirred reactors

Stochastic PaSR Model

• The stochastic PSR is modeled as a single adiabatic,

perfectly stirred reactor at constant pressure and with the

fixed volume of 1 dm3

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Summary

• Necessity of DARS as a standalone tool, with an overview

of its applicability and capabilities are covered

• Homogenous models with a detailed elucidation of all the

ideal reactors are covered

• The need for mechanism reduction, along with the various

analysis and reduction techniques are discussed

• A basic introduction of flame modeling and various

modules of flames are touched upon

• The drawbacks of homogenous models, how SRM

modules are used to rectify them are discussed

• 1D modules, which are predominantly used in catalytic

reactors and particulate filters are briefed

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