Partially Premixed Combustion Tutorial

(1)

1 UGM 2002 Confidential

Partially Premixed

Combustion in a Co-axial

Combustor

Graham Goldin

(2)

Problem

u

A swirler at the center of the combustor

introduces the lean methane/air mixture.

u equivalence ratio=0.8

u axial velocity = 30 m/s

u radial velocity = 30 m/s

u axial velocity of air at outer tube = 10 m/s

u major species involved in the combustion process

(3)

3

UGM 2002 Confidential

Setup and Solution

u

Generate PDF look-up table using prePDF

u

Read Grid

u

Define Model

u

Define Material

u

Operating and Boundary Conditions

u

1

st

and 2

nd

Order Solutions

(4)

Generate PDF look-up Table (1)

u

Start prePDF and define

the model type.

Setup:Case…

u Enable Partially Premixed

Model

u Retain the default settings

(5)

5

Generate PDF look-up Table (2)

u

Define the chemical species in the system.

u Setup:Species:Define…

u Under Database Species, select the name

u Set the Species number

u Define the species: CH₄, O₂, CO₂, CO, H₂O,

(6)

Generate PDF look-up Table (3)

u

Define fuel composition.

Setup:Species:Composition…

u Set Species Fraction:

l CH₄=0.0453 l O₂= 0.2264 l CO₂= 0.7283

(7)

7

Generate PDF look-up Table (4)

u

Define oxidizer composition.

(8)

Generate PDF look-up Table (5)

• Define the system operating conditions.

Setup:Operating Conditions…

u Set the Inlet Temperature for Oxidiser to 650

and retain the default values.

u

Retain the default PDF solution parameters

u

Save the input file

ch4-partialpremixed.inp u _{Calculate the PDF table, and save the pdf file,}

ch4-partial-premixed.pdf

(9)

9

Generate PDF look-up Table (6)

u

Examine temperature/mixture fraction, and

species/mixture fraction relationship

(10)

Generate PDF look-up Table (7)

u prePDF automatically fits 3rd-order polynomial

functions (of f ) for unburnt density, temperature, specific heat and thermal diffusivity.

u prePDF automatically fits a piecewise-linear function for

the laminar flame speed for certain fuels and conditions

u H₂, CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₈ u 1atm < pressure < 40atm

u 300K < T_unburnt < 800K

(11)

11

Read Grid

u

Start the 2D version of FLUENT

u

Read the grid file,

par-premixed.msh

u

Scale the grid to inches

u

Display the grid

(12)

Define Model

(13)

13

Define Model

u Define:Models:Species

You will be prompted to read the ch4-partial-premixed.pdf file. When the file is read, the available material properties/methods will

(14)

Material

(15)

15

Operating Conditions

(16)

Boundary Conditions (1)

Set boundary conditions for air inlet.

Set boundary conditions for air-fuel inlet.

(17)

17

Boundary Conditions (2)

(18)

First Order Solutions (1)

(19)

19

First Order Solutions (2)

(20)

First Order Solutions (3)

u Initialize flow field and compute from all zones.

(21)

21

First Order Solutions (4)

u Start the calculation (250 iterations).

u Define a region Adapt:Region…

u Patch a region close to fuel-air

(22)

First Order Solutions (5)

(23)

23

Second Order Solutions (1)

u

Change the discretization for the parameters:

u Pressure: Second Order

u Momentum: Second Order Upwind

u Turbulence Kinetic Energy: Second Order Upwind

u Turbulence Dissipation Rate: Second Order Upwind

u Progress Variable: Second Order Upwind

u Mean Mixture Fraction: Second Order Upwind

(24)

Second Order Solutions (2)

u

Start the calculation (250 iterations).

(25)

25

Postprocessing (1)

uVelocity Vectors.

Set Scale Factor to 10 and Skip Value to 3

uContours of Steam

(26)

Postprocessing (2)

uFilled contours of mean

Progress Variable.

uFilled contours of Static

(27)

27

Postprocessing (3)

(28)

Postprocessing (4)

(29)

29

Results

u

The partially premixed model in FLUENT can

be used to simulate problems with:

u A premixed stream and a non-premixed (or inert

stream such as air)

u Equivalence ratio fluctuations in the premixed inlet

stream

u Can be used in the limit of…

l Perfectly premixed (automatic calculation of props) l Non-premixed (can study mixed and unburnt flows)

(30)

3D Simulation of the IFRF

Industrial Pulverized-Coal

Furnace

Graham Goldin

(31)

31

Overview

u

The International Flame Research Foundation

(IFRF) experimental facility is used to validate

industrial coal combustion models.

u

This tutorial is an extension of the

2-dimensional simulation of this furnace by

Peters and Weber.

u

The mixture fraction/PDF model with the k-e

turbulence model and P-1 radiation model has

been used.

(32)

Problem

u

To simulate a realistic industrial

pulverised-coal furnace and compare with the measured

data.

u 3D analysis of 2.4 MW Swirling,

Pulverized Coal Flame Furnace

u One quarter periodic

model of furnace (shown in fig)

(33)

33

Setup and Solution

u Select a Combustion Model

u Generate PDF look-up table using prePDF

u Read Grid

u Define Model

u Define Materials

u Define Operating Conditions

u Compile UDF

u Define Boundary Conditions

u Define Injections

u Solve for non reacting and reacting flows

(34)

Select a Combustion Model

u Assumptions

u Chemical equilibrium

u Modeling the devolatization and char off-gases as a single mixture

u Combustion Model selected

u Mixture Fraction Model

u Coal Specifications

u Name: Saar Gottelborn hvBb

u High Temperature yield (mole, dry) volatiles 55%, char 36.7%, and ash 8.3%

u Ultimate analysis (mole, dry-ash-free (daf)) C 53%, H 40%, O 6%, and N 1%

(35)

35

Generate PDF look-up Table (1)

u

Start prePDF and define

a case.

Setup:Case…

u Enable Non-Adiabatic

Heat transfer options

u Enable Fuel stream for

Empirically Defined Streams

u Retain the default settings

(36)

Generate PDF look-up Table (2)

u

Define the chemical species in the system.

Setup:Species:Define…

u Under Database Species, select the name

u Set the Species number

u Define the species: C, H, O, N, C(S), O₂, CO₂,

(37)

37

Generate PDF look-up Table (3)

u

Define fuel composition.

Setup:Species:Composition…

l C = 0.53 l H = 0.40 l O = 0.06 l N = 0.01

u Lower Caloric Value = 3.232e+07

(38)

Generate PDF look-up Table (4)

u

Define oxidizer composition.

l O₂= 0. 21 l N₂= 0.79

(39)

39

Generate PDF look-up Table (5)

u

Define the system operating

conditions.

Setup:Operating Conditions…

u Min. Temperature = 370

u Max. Temperature = 2600

u Set the Inlet Temperature

l Fuel = 373

(40)

Generate PDF look-up Table (6)

u

Define the solution

parameters.

u Non-Adiabatic Model:

Enthalpy Points = 20

u Fuel Mixture Fraction

Points = 32

u Mixture Fraction Variance

Points = 16

u Disable Automatic

Distribution

u Distribution Center

u

Calculate the pdf table

and view it with the

graphics routines.

u

Save the pdf file

(41)

41

Grid

u

Start the 3D version of FLUENT

u

Read the grid

file, ifrf

.msh

u

Check and

(42)

Define Models (1)

(43)

43

Define Models (2)

u Define:Models:Species

When prompted read the ifrf.pdf file. When the file is read, the available material properties /methods will change to accomodate the model.

u Define:Models:Radiation

To choose an appropriate radiation model, calculate optical thickness = mean beam length (about 2m) x absorption co-efficient (around 1 /m for hydrocarbon combustion)

Since this optical thickness is greater than unity, the P1 model is appropriate.

(44)

Define Models (3)

u

Define:Models:Discrete

Phase Model

u Set the Max. Number Of

Steps to 25000

u Deactivate Specify

Length Scale

u Set Step Length Factor

(45)

45 UGM 2002 Confidential

Materials

u Define:Materials u Set Absorption Coefficient = wsggm-cell-based u Set Scattering Coefficient = 0.15

(46)

Operating Conditions

(47)

47

Compile Interpreted UDFs

u Create a working directory and save the C

functions.

u Start Fluent from the working directory and read

the case file.

u Compile the UDF using the Interpreted UDFs

panel

u Enter name of the C function (ifrf.c) under Source File Name

u Specify the C preprocessor under CPP Command Name field

u Retain the default Stack Size u Click Compile

(48)

Boundary Conditions (1)

Set boundary conditions for v-1 zone.

Set boundary conditions for v-2 zone.

(49)

49

Boundary Conditions (2)

Set boundary conditions for p-1 zone.

Set boundary conditions for periodic zone.

(50)

Boundary Conditions (3)

Set boundary conditions for wall zones w-1, w-2, w-3, w-4, w-5, w-6, w-7, w-8, and w-9 as per the table

0.5 1073 w-9 0.5 1323 w-8 1 udf-wall7temp w-7 1 udf-wall6temp w-5 1 udf-wall5temp w-5 0.6 1273 w-4 0.6 873 w-3 0.6 573 w-2 0.6 343 w-1 Internal Emissivity Temperature Zone Name

(51)

51

Define Injections (1)

u Create Injections

Define:Injections…

u Click Create in the Injections panel

u Set Injection properties

u Injection Type: Surface

u Release From Surfaces: v1 u Particle Type: Combusting

u Diameter Distribution: rosin-rammler

u Turbulent Dispersion: Stochastic Model u Number Of Tries: 3

(52)

Define Injections (2)

6 Number Of Diameters 1.36 Spread Parameter 4.5e-05 Mean Diameter 0.003 Max. Diameter 1e-06 Min. Diameter 0.01826 Total Flow Rate

343 Temperature 23.11 Z-Velocity Value Parameter

u Under Point Properties, set the

(53)

53

u

Modify the properties for the combusting

particle.

u Name: gottelborn-hy

u Set Properties as per table

Define Injections (3)

Value Parameter kinetics/diffusion-limited Combustion Model 36.7 Combustible Fraction 3e-05 Binary Diffusivity 55.02 Volatile Component Fraction

300 Vaporization Temperature 0 Latent Heat 1100 Cp 1000 Density

Kinetics Limited Rate Pre-exponential Factor = 6.7

Kinetics Limited Rate Activation Energy = 1.1382e+08`

(54)

Solution (1)

u Solve for Non reacting flow

u Disable Energy, P1 and Pdf for equations

u Set pressure discretization to PRESTO!

u Initialize the solution

u Compute from all-zones u Set the initial value for

temperature to 2000

u Plot residuals during calculations

u Request 99 iterations

(55)

55

Solution (2)

u Solve for Reacting flow

u Enable Interaction with Continuous Phase

l Set Number of Continuous Phase

Iterations per DPM Iteration to 20

u Enable Energy, P1 and Pdf equations u Set the under-relaxation factors

u Request another 20 iterations

u Save the data file (ifrf2.dat.gz)

Value Parameter

0.25 Discrete Phase Sources

0.975 P1 0.5 Momentum 0.5 Pressure

(56)

Solution (3)

u Modify the properties of the combusting particle

u Request for an additional 200

iterations

u Save the data file (ifrf3.dat.gz)

Value Parameter Activation Energy = 7.4e+07 Pre-exponential Factor = 2e+05 W single-rate Devolatilization Model 773 Vaporization Temperature

(57)

57

Solution (4)

u Set the discretization to Second Order Upwind for:

u Momentum

u Turbulence Kinetic Energy

u Turbulence Dissipation Rate

u Mean Mixture Fraction

u Mixture Fraction Variance

u Energy

u Request for an additional 500

iterations

(58)

Solution (5)

u Define the NOx Model

Define:Models:Pollutants:NOx...

u Enable the models Thermal NO and Fuel NO

u Under Turbulence Interaction: l PDFMode = Mixture Fraction l Beta PDF Points to 25

u Under Fuel NO Parameters: l Fuel Type = Solid

l Volatile N Mass Fraction = 0.01015 l Char N Mass Fraction = 0.00435 l BET Surface Area = 25000

(59)

59

Solution (6)

u For discrete phase model, set

Number of Continuous Phase Iterations per DPM Iteration = 0

u Set Solution parameters:

u Disable all the equations except NO and HCN

u Under-relaxation factors for NO and HCN to 1

u Discretization scheme as Second Order Upwind

u Convergence Criterion for NO and HCN = 1e-06

u Request for 20 iterations

(60)

Postprocessing (1)

u Check the net in and out fluxes balance.

u Compute gas phase mass fluxes through all boundaries

l Boundaries : Select all zones l Click Compute

u Calculate the net mass transfer to the gas phase from the discrete phase coal particles.

l Options: Sum l Cell Zones: fluid

l Field Variable : Discrete Phase Model...

(61)

61

Postprocessing (2)

u Compute the gas phase energy fluxes through all the boundaries

l Options : Total Heat Transfer Rate l Boundaries : Select all zones

l Click Compute

u Calculate the net mass transfer to the gas phase from the discrete phase coal particles.

l Options: Sum l Cell Zones: fluid

l Field Variable : Discrete Phase

Model... and DPM Enthalpy Source

(62)

Postprocessing (3)

u Static Temperature u Turbulent Viscosity

(63)

63

Postprocessing (4)

(64)

Results

u

The radial profiles and axial plots of time

averaged flow field values at 0.25m and 0.85m

from the quarl end of the combustor were

collected and can be downloaded from the files

listed in the table.

u

Comparison of the experimental data and the

CFD simulation data show an agreement

which can be considered typical.

(65)

65

Center-line (z axis) parts-per-million (dry) radial-NO.xy

Center-line (z axis) carbon-dioxide volume percentage (dry) radial-CO2.xy

Center-line (z axis) parts-per-million (dry) radial-CO.xy

Center-line (z axis) temperature (K) radial-T.xy

Center-line (z axis) oxygen volume percentage (dry) radial-O2.xy Tangential velocity (m/s) at z=0.25m radial-V-1.xy Tangential velocity (m/s) at z=0.85m radial-V-2.xy Axial velocity (m/s) at z=0.25m radial-U-1.xy Axial velocity (m/s) at z=0.85m radial-U-2.xy NO parts-per-million (dry) at z=0.25m radial-NO-1.xy NO parts-per-million (dry) at z=0.85m radial-NO-2.xy

Carbon-monoxide parts-per-million (dry) at z=0.25m radial-CO-1.xy

Carbon-monoxide parts-per-million (dry) at z=0.85m radial-CO-2.xy

Carbon-dioxide volume percentage (dry) at z=0.25m radial-CO2-1.xy

Carbon-dioxide volume percentage (dry) at z=0.25m radial-CO2-2.xy

Oxygen volume percentage (dry) at z=0.85m radial-O2-2.xy

Oxygen volume percentage (dry) at z=0.25m radial-O2-1.xy Temperature (K) at z=0.85m radial-T-2.xy Temperature (K) at z=0.25m radial-T-1.xy Description File Experimental Data : Files of radial profiles and axial plots of time averaged flow field values. Reference :

Peters, A.F. and Weber, R. (1997), Mathematical Modeling of a 2.4 MW Swirling, Pulverized

Coal Flame, Combustion Science and