FLUENT IC Tut 04 Premixed

Tutorial: Simulate In-Cylinder Premixed Combustion

Introduction

For spark-ignited (SI) engines and some natural gas combustion engines, fuel and air aremixed before entering into the combustion chamber. At the spark event, the mixture canbe assumed to be homogeneous and the combustion process to be premixed.

This tutorial demonstrates how to do the following:

• Set up in-cylinder (IC) premixed combustion.

• Set up the spark model.

• Use the user-defined functions (UDFs) to modify laminar flame speed.

• Use UDF for some IC postprocessing.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1 fromANSYS FLUENT 13.0 Tutorial Guide, and that you are familiar with the ANSYS FLUENTnavigation pane and menu structure. Some steps in the setup and solution procedure willnot be shown explicitly.

It also assumes that you are familiar with ANSYS FLUENT moving deforming mesh (MDM)layering approach. For more information, see Section 11.6, Using Dynamic Meshes in ANSYSFLUENT 13.0 User’s Guide.

Problem Description

This tutorial considers a 2D axi-symmetric geometry of the IC engine cylinder configura-tion. Simulation starts at IVC and ends at EVO, hence there are no valves involved. Theschematic is as shown in Figure 1.

The initial configuration of the system has piston at TDC, hence first mesh motion isperformed to bring the piston to IVC position.

c© Fluent Inc. October 11, 2010 1

http://www.fluentusers.com/fluent/doc/ori/v130/fluent/fluent13.0/help/html/tg/flu_tg_elbow_workbench.htm

http://www.fluentusers.com/fluent/doc/ori/v130/fluent/fluent13.0/help/html/ug/flu_ug_sec_dynamic_using.htm

Simulate In-Cylinder Premixed Combustion

Figure 1: Schematic

Setup and Solution

Preparation

1. Copy the mesh file (IC premixed com.msh.gz) and the UDF source files (initialize.c,laminar flame speed.c, and work.c) to your working folder.

2. Use FLUENT Launcher to start the 2D version of ANSYS FLUENT.

For more information about FLUENT Launcher see Section 1.1.2, StartingANSYS FLUENT Using FLUENT Launcher in the ANSYS FLUENT 13.0 User’s Guide.

3. Enable Double-Precision in the Options list.

4. Click the UDF Compiler tab and ensure that the Setup Compilation Environment forUDF is enabled.

The path to the .bat file which is required to compile the UDF will be displayed assoon as you enable Setup Compilation Environment for UDF.

If the UDF Compiler tab does not appear in the FLUENT Launcher dialog box by default,click the Show More >> button to view the additional settings.

The Display Options are enabled by default. Therefore, after you read in the mesh, itwill be displayed in the embedded graphics window.

2 c© Fluent Inc. October 11, 2010

http://www.fluentusers.com/fluent/doc/ori/v130/fluent/fluent13.0/help/html/ug/flu_ug_sec_mini_launcher.htm

http://www.fluentusers.com/fluent/doc/ori/v130/fluent/fluent13.0/help/html/ug/flu_ug_sec_mini_launcher.htm


Step 1: Mesh

1. Read the mesh file (IC premixed com.msh).

File −→ Read −→Mesh...

As the mesh file is read, ANSYS FLUENT will report the progress in the console.

Step 2: General Settings

1. Define the solver settings.

General

(a) Select Transient in the Time list.

(b) Select Axisymmetric Swirl in the 2D Space list.

2. Check the mesh (see Figure 2).

General −→ Check

ANSYS FLUENT will perform various checks on the mesh and will report the progressin the console. Ensure that the minimum volume reported is a positive number.



Figure 2: Mesh Display

3. Scale the mesh.

General −→ Scale...

(a) Select mm from the Mesh Was Created In drop-down list.

(b) Select mm from the View Length Unit In drop-down list.

(c) Click Scale and close the Scale Mesh dialog box.



Step 3: Dynamic Mesh

1. Set up IC parameters.

Dynamic Mesh

(a) Enable the Dynamic Mesh option.

i. Disable Smoothing in the Mesh Methods group box.

ii. Enable Layering in the Mesh Methods group box.

iii. Click Settings... to open Mesh Method Settings dialog box.

A. Retain the default settings under Layering tab.

B. Click OK to close the Mesh Method Settings dialog box.

iv. Enable In-Cylinder in the Options group box.

v. Click Settings... to open In-Cylinder Settings dialog box.

A. Enter the values as shown in the following table.



Parameter ValueCrank Shaft Speed(rpm) 3000Starting Crank Angle(deg) 360Crank Period(deg) 720Crank Angle Step Size(deg) 0.25Piston Stroke(mm) 80Connecting Rod Length(mm) 140Piston Stroke Cutoff(mm) 0Minimum Valve Lift(mm) 0

B. Click OK to close In-Cylinder Settings dialog box.

2. Set up dynamic zones.

Dynamic Mesh (Dynamic Mesh Zones)−→ Create/Edit...

(a) Define dynamic mesh zone for fluid.

i. Select fluid from the Zone Names drop-down list.

ii. Ensure Rigid Body is selected in the Type group box.

iii. Ensure the selection of **piston-full** from the Motion UDF/Profile drop-down list under Motion Attributes tab.

iv. Ensure that the values of X and Y in Valve Piston Axis group box are 1 and0 respectively.

v. Click Create.

(b) Define dynamic mesh zone for piston.



i. Select piston from the Zone Names drop-down list.

ii. Retain the default settings for Motion Attributes tab.

iii. Retain 0 mm for Cell Height under Meshing Options tab.

iv. Click Create.

(c) Define dynamic mesh zone for top.

i. Select top from the Zone Names drop-down list.

ii. Select Stationary in the Type group box.

iii. Enter 1 mm for Cell Height under Meshing Options tab.

iv. Click Create and close Dynamic Mesh Zones dialog box.

3. Perform a mesh motion preview.

Dynamic Mesh −→ Preview Mesh Motion...



(a) Enter 840 for Number of Time Steps.

(b) Ensure Display Mesh is enabled in the Options group box.

(c) Click Preview.

(d) Close the Mesh Motion dialog box after the preview is done.

At Crank Angle Step Size of 0.25 degree, a mesh motion of 840 steps will bring themesh to CA=570, the starting point of our combustion simulation.

4. Save the case file (IC premixed com CA570.cas.gz).

File −→ Write −→Case...

5. Examine the UDF inputs for initialize.c as shown in Appendix A.

6. Examine the UDF inputs for work.c as shown in Appendix B.

7. Examine the UDF inputs for laminar flame speed.c as shown in Appendix C.

Open the files, initialize.c, work.c, and laminar flame speed.c using a text ed-itor. For this tutorial, the swirl ratio is 3, swirl axis is x coordinate, and the swirlorigin is (0, 0). For this tutorial, there is no need to modify the UDF input.

Step 4: Models

1. Set up the combustion model.

(a) Compile and load the UDF library.

Define −→ User-Defined −→ Functions −→Compiled...

(b) Click Add... and select the files, initialize.c, laminar flame speed.c, andwork.c.

(c) Click OK to close the Select File dialog box.

(d) Click Build to build the library.

Note: Make sure that the UDF source files are in same directory that containthe case and data files.

(e) Click Load to close the Compiled UDFs dialog box.



2. Hook your model to the UDF library.

Define −→ User-Defined −→Function Hooks...

(a) Click Edit... for Initialization to open Initialization Functions dialog box.

i. Select my init function::libudf from the Available Initialization Functions list.

ii. Click Add to add it in the Selected Initialization Functions list.

iii. Click OK.

(b) Click Edit... for Adjust to open Adjust Functions dialog box.

i. Select my T u::libudf from the Available Initialization Functions list.


iii. Click OK.

(c) Click Edit... for Execture At End to open Execture At End Functions dialog box.

i. Select output results::libudf from the Available Initialization Functions list.


iii. Click OK.

(d) Click Edit... for Read Data to open Read Data Functions dialog box.

i. Select read data::libudf from the Available Initialization Functions list.


iii. Click OK.

(e) Click Edit... for Write Data to open Write Data Functions dialog box.

i. Select write data::libudf from the Available Initialization Functions list.


iii. Click OK.



(f) Click OK to close the User-Defined Function Hooks dialog box.

3. Set the user-defined memory location.

Define −→ User-Defined −→Memory...

(a) Increase Number of User-Defined Memory Location from 0 to 1.

(b) Click OK to close the User-Defined Memory dialog box.

4. Enable the standard k-epsilon turbulence model.

Models −→ Viscous −→ Edit...

(a) Select k-epsilon (2 eqn) in the Model group box to open Viscous Model dialog box.

(b) Retain the default settings and click OK to close Viscous Model dialog box.

5. Define the species model.

Models −→ Species −→ Edit...



(a) Select Premixed Combustion in the Model group box.

(b) Select Non-Adiabatic in the Premixed Combustion Model Options group box.

(c) Retain the default selection of C Equation from the Premixed Model group box.

(d) Retain the default selection of zimont from Turbulent Flame Speed Model groupbox.

(e) Click OK.

The information dialog box will appear informing that available material proper-ties or methods have changed, asking you to confirm the property values. ClickOK to close the Information dialog box.



6. Set up the spark ignition model.

Models −→ Spark Ignition −→ Edit...

(a) Set Number of Sparks to 1.

(b) Click Define... to open the Set Spark Ignition dialog box.

i. Select Fixed Spark Size in the Model group box.

ii. Retain selection of Circle in the Shape group box.



iii. Enter 715 (deg) for Start Time.

iv. Enter 0.01 (s) for Diffusion Time.

v. Click OK to close the Set Spark Ignition dialog box.

(c) Close the Spark Ignition dialog box.

Step 5: Materials

Define the material properties.

Materials −→ Fluid −→ Create/Edit...

1. Select ideal-gas from the Density drop-down list.



2. Select piecewise-polynomial from the Cp (Specific Heat) drop-down list. The Piecewise-Polynomial Profile dialog box opens.

(a) Set Range to 2.

(b) Retain the default settings.

(c) Click OK to close Piecewise-Polynomial Profile dialog box.

3. Select user-defined from the Laminar Flame Speed drop-down list.

4. Click Edit... next to Laminar Flame Speed.

(a) Select laminar flame speed from the User-Defined Functions dialog box.

This will hook Metghalchi and Keck model to calculate laminar flame speed. InANSYS FLUENT this model is now available internally. To use it, you have toselect metghalchi-keck-law from the drop-down list and then methane-air from thecombustion mixture dialog box.

(b) Click OK.

5. Enter 5e+07 (j/kg) for Heat of Combustion.

6. Enter 0.0363 for Unburnt Fuel Mass Fraction.

7. Click Change/Create and close the Create/Edit Materials dialog box.



Step 6: Boundary Conditions

Boundary Conditions −→ axis

1. Select axis from the Type drop-down list.

The question dialog box will appear to confirm the change of type of axis from wall toaxis. Click Yes to open the Axis dialog box.

(a) Click OK to close the Axis dialog box.



Step 7: Solution

1. Set the solution parameters.

Solution Methods

(a) Select PISO from the Scheme drop-down list.

(b) Reduce Skewness Correction from 1 to 0.

(c) Select PRESTO! from the Pressure drop-down list.



2. Increase the Under-Relaxation Factors for Pressure from 0.3 to 0.5.

Solution Controls

3. Enable the plotting of residuals during calculation.

Monitors −→ Residuals −→ Edit...

(a) Reduce the Convergence Absolute Criteria for continuity to 0.1.

(b) Click OK to close the Residual Monitors dialog box.



4. Initialize the solution.

Solution Initialization

(a) Enter 0 for Progress Variable.

(b) Retain other default initial values.

(c) Click Initialize.

5. Set auto save option.

Calculation Activities

(a) Enter 90 for Autosave Every (Time Steps).



(b) Click Edit... to open Autosave dialog box.

i. Enter an appropriate file name (IC premixed com CA570.gz).

ii. Click OK to close the Autosave dialog box.

If required, you can set the volume monitors for volume-averaged pressure/temperature.Monitors (Volume Monitors)−→ Create...

Step 9: Animation Setup

1. Mirror the view across the axis.

Display −→Views...

(a) Select axis from the Mirror Planes list.

(b) Click Apply.



2. Display the mesh (see Figure 3).

Graphics and Animations −→ Mesh −→ Set Up...

Figure 3: View Setup

3. Define the plot for animation view (plot-view).

Display −→Views...

(a) Enter plot-view under Save Name and click Save.

(b) Close the Views dialog box.



4. Display the contours of progress variables.

Graphics and Animations −→ Contours −→ Set Up...

(a) Enable Filled in the Options group box.

(b) Select Premixed Combustion... and Progress Variable from the Contours of drop-down list.

(c) Click Display and close the Contours dialog box.

5. Execute commands for the animation setup.

Calculation Activities (Execute Commands)−→ Create/Edit...



(a) Set the Defined Commands to 2.

(b) Enable Active for command-1.

i. Set Every to 4.

ii. Select Time Step from the When drop-down list.

iii. Enter /dis/sw 1 /dis/view/rv plot-view /dis/cont premixc 0 1 forCommand.

(c) Enable Active for command-2.

i. Set Every to 4.

ii. Select Time Step from the When drop-down list.

iii. Enter /dis/save-picture ./flame %t.tif for Command.

(d) Click OK to close the Execute Commands dialog box.

6. Save the hardcopy of display.

File −→Save Picture...

(a) Select TIFF in the Format group box.

(b) Select Color in the Coloring group box.

(c) Click Apply and close the Save Picture dialog box.

7. Save the case and data files (IC premixed com CA570 0000.cas/dat.gz).

File −→ Write −→Case & Data

8. Run the calculation.

Run calculation

(a) Enter 1120 for Number of Time Steps.

(b) Click Calculate.



Step 10: Postprocessing

At the end of the simulation, you have TIFF files for the contours of progress variable atdifferent crank angles. Figures 4–13 show some of the TIFF files.

You can use these tiff files for animation.

Figure 4: Combustion Sequence 1 Figure 5: Combustion Sequence 2









Appendix A: UDF for initialize.c

This shows the UDF for IC initialization with swirl (initialize.c).

/*********************************************************************************************

UDF for IC initialization with swirl

For IC flow, if only combustion and power stroke is of interest. The initial

condition normally contains swirl flow. This udf provides a tool to initialize

the flow field with user specified swirl ratio

How to use the udf:

- Set up your IC case

- Modify the user inputs part of the udf.

- Build the library

- Hook the DEFINE\_INIT udf

- Initialize your flow field

Note:

- UDF works in 2d axisymmetry, and 3d.

- Pure 2d case does not have swirl and thus not supported (a warning will be given).

- UDF works in both serial and parallel.

***********************************************************************************************/

# include "udf.h"

# define RPM RP_Get_Real("dynamesh/in-cyn/crank-rpm")

/********************************* User input starts *****************************************/

/* Initial swirl ratio and swirl axis*/

static real init_swirl_ratio=3.0;

static real swirl_axis[ND_ND]={1, 0};

static real swirl_origin[ND_ND]={0, 0};

/* This variable defines whether the inialization occurs to the whole domain or just some cell zones */

enum

{

whole_domain, defined_cell_zones

}method = whole_domain;

/* If defined_cell_zones is used in the above, then specify cell zone ID list for initialization.

-1 is a flag so please keep it. */

static int Zone_ID[]={2, -1};

/********************************** User input ends ******************************************/

static void initialize_cell_zone(Thread * t, real * omega)

{

cell_t c;

real xc[ND_ND], x[ND_ND];

static int counter=0;

/* loop over all cells */

begin_c_loop(c,t)

{

C_CENTROID(xc,c,t);

NV_VV(x,=,xc,-,swirl_origin);



#if RP_2D

if (rp_axi)

{

C_U(c,t)=NV_CROSS_X(omega, x);

C_V(c,t)=NV_CROSS_Y(omega, x);

C_W(c,t)=NV_CROSS_Z(omega, x);

}

% \end{minipage}

% \pagebreak

else

{

if(counter == 0)

{

Message0("No initialization for pure 2D. Needs to turn on 2d axisymmetric with swirl!");

counter++;

}

}

#else

C_U(c,t)=NV_CROSS_X(omega, x);

C_V(c,t)=NV_CROSS_Y(omega, x);

C_W(c,t)=NV_CROSS_Z(omega, x);

#endif

}

end_c_loop(c,t)

}

DEFINE_INIT(my_init_function, domain)

Thread *t;

int i;

real omega[ND_ND], mag;

/* Normalize swirl axis */

mag=NV_MAG(swirl_axis);

NV_S(swirl_axis, /=, mag);

if (RP_Get_Boolean("dynamesh/models/in-cylinder?")==TRUE)

{

NV_VS(omega, =, swirl_axis, *, RPM/60.*2.*M_PI*init_swirl_ratio);

if(method == whole_domain)

{

/* loop over all cell threads in the domain */

thread_loop_c (t,domain)

{

initialize_cell_zone(t, omega);

}

}

else if (method == defined_cell_zones)

{

i=0;

while(Zone_ID[i]>=0)

{

t=Lookup_Thread(domain, Zone_ID[i]);

initialize_cell_zone(t, omega);

i++;

}

}



{

else

{

Message0("Wrong method for initialization calculation--aborting!!");

exit(0);

}

Init_Face_Flux(domain);

}

else

{

Message0("IC not turned on. No initialization is performed.");

}

}

% \end{minipage}



Appendix B: UDF for work.c.

This shows the UDF for IC indicated work (work.c).

/*********************************************************************************************

UDF for IC indicated work

This UDF does the following

- Calculate indicated work

- Output volume as a function of CA

- Output pressure as a function of CA

- Output burnt fuel mass fraction as a function of CA

How to use the udf:

- Set up your case

- Build the library

- Hook the DEFINE_EXECUTE_AT_END udf

- Hook the two DEFINE_RW_FILE udfs

Note:

- The indicated work will be saved with the data file. So, you CAN restart from

a previously saved cas/dat and continue the run to obtain the indicated work.

- UDF works in 2d, 2d axisymmetry, and 3d. And it works in both serial and parallel

***********************************************************************************************/

# include "udf.h"

# define RPM RP_Get_Real("dynamesh/in-cyn/crank-rpm")


/* Cell zone ID list for pressure output (cell zone ID for combustion chamber). -1 is a flag

so please keep it. */

static int Zone_ID[]={2, -1};

/* Face zone ID list for work output (face zone ID for the piston) */

static int Piston_ID[]={6, -1};

/********************************** User input ends ******************************************/

static real work=0, start_CA=0, end_CA=0;

static int counter=0;

/* Function to calculate cell volume. It takes care of 2d and 2d axisymmetric case as well. */

static real C_MYVOLUME(cell_t c, Thread *tc)

{

real factor=1.0;

#if RP_2D

if (rp_axi) factor=2*M_PI;

#endif

return C_VOLUME(c,tc)*factor;

}

/* Function to calculate face area. It takes care of 2d and 2d axisymmetric case as well. */

static void F_MYAREA(real * x, face_t f, Thread *tf)

{

real factor=1.0;

F_AREA(x, f, tf);



#if RP_2D

if (rp_axi) factor=2*M_PI;

#endif

NV_S(x, *=, factor);

}

DEFINE_ON_DEMAND(Indicated_work)

{

if(counter==0)

{

Message0("\n\n********************** IC Indicated Work Results *****************************\n");

Message0("\nIndicated work calculation has not started. Please do at least one time step.\n");

Message0("\nStart CA :%5.2f (deg) End CA :%5.2f (deg) Work : %9.3e (J)", start_CA, end_CA, work);

Message0("\n\n******************************************************************************\n");

}

else

{

Message0("\n\n********************** IC Indicated Work Results *****************************\n");

Message0("\nIndicated work calculation started at :%7.2f (deg)", start_CA);

Message0("\nIndicated work calculation finished at :%7.2f (deg)", end_CA);

Message0("\nFor the above duration, the indicated work is :%12.4e (J)", work);

Message0("\n\n******************************************************************************\n");

}

}

DEFINE_ON_DEMAND(reset)

{

counter=0;

work=0;

start_CA=0;

end_CA=0;

Message0("\n\n******************************** WARNING *************************************\n");

Message0("\nNote that this resets indicated work and the calculation start CA to zero.");

Message0("\nPlease save the data file to save the new values.");

Message0("\n\n******************************************************************************\n");

}

DEFINE_EXECUTE_AT_END(output_results)

{

#if !RP_HOST

int i;

real pressure, volume, fmf, mass, work_one_dt, x[ND_ND];

Thread *tc, *tf;

cell_t c;

face_t f;

FILE *fp_results;

Domain* domain;

domain=Get_Domain(1);

#if PARALLEL

if(I_AM_NODE_ZERO_P)

#endif

{

if(!(fp_results=fopen("work.txt","a")))

{

Message0("\nCan not open file-aborting!!");

exit(0);

}

}

if(counter==0)

{

start_CA = (CURRENT_TIME-CURRENT_TIMESTEP)*RPM*6.0+RP_Get_Real("dynamesh/in-cyn/crank-start-angle");



#if PARALLEL


#endif

{

fprintf(fp_results, " CA Volume Pressure Yfb Work\n");

}

counter ++;

}

end_CA=CURRENT_TIME*RPM*6.0+RP_Get_Real("dynamesh/in-cyn/crank-start-angle");

#endif

node_to_host_int_1(counter);

node_to_host_real_2(start_CA, end_CA);

#if !RP_HOST

/* Calculate volume weighted pressure and burnt fuel mass fraction */

pressure=0;

volume=0;

fmf=0;

mass=0;

i=0;

while(Zone_ID[i]>=0)

{

tc=Lookup_Thread(domain, Zone_ID[i]);

begin_c_loop_int(c, tc)

{

pressure += C_P(c,tc) * C_MYVOLUME(c,tc);

volume += C_MYVOLUME(c,tc);

if(sg_premixed)

{

fmf += C_PREMIXC(c,tc) * C_R(c,tc) * C_MYVOLUME(c,tc);

}

mass += C_R(c,tc) * C_MYVOLUME(c,tc);

}

end_c_loop_int(c, tc)

i++;

}

pressure = PRF_GRSUM1(pressure);

volume = PRF_GRSUM1(volume);

fmf = PRF_GRSUM1(fmf);

mass = PRF_GRSUM1(mass);

pressure /= volume;

fmf /= mass;

/* Calcualte work by piston. Only pressure force is accounted for and viscous force is neglected. */

work_one_dt=0;

i=0;

while(Piston_ID[i]>=0)

{

tf=Lookup_Thread(domain, Piston_ID[i]);

begin_f_loop(f, tf)

{

F_MYAREA(x, f, tf);

work_one_dt += CURRENT_TIMESTEP * F_P(f,tf) * NVD_DOT(x, WALL_F_GRID_VV(f, tf)[0],

WALL_F_GRID_VV(f, tf)[1], WALL_F_GRID_VV(f, tf)[2]);

}

end_f_loop(f, tf)

i++;

}



work_one_dt = PRF_GRSUM1(work_one_dt);

work += work_one_dt;

#endif

node_to_host_real_1(work);

#if !RP_HOST

/* Output volume, pressure, burnt fuel mass fraction */

#if PARALLEL


#endif

{

fprintf(fp_results, "%8.2f %12.4e %12.4e %12.4e %12.4e\n", end_CA, volume, pressure, fmf, work);

fclose(fp_results);

}

#endif

}

DEFINE_RW_FILE(write_data, fp)

{

Message0("\nWriting user defined data to the data file...\n");

#if PARALLEL

#if RP_HOST

fprintf(fp, "\n%d", counter);

fprintf(fp, "\n%e", work);

fprintf(fp, "\n%e", start_CA);

fprintf(fp, "\n%e", end_CA);

#endif

#else

fprintf(fp, "\n%d", counter);

fprintf(fp, "\n%e", work);

fprintf(fp, "\n%e", start_CA);

fprintf(fp, "\n%e", end_CA);

#endif

}

DEFINE_RW_FILE(read_data, fp)

{

Message0("\nReading user defined data from the data file...\n");

#if PARALLEL

#if RP_HOST

fscanf(fp, "%d", &counter);

fscanf(fp, "%e", &work);

fscanf(fp, "%e", &start_CA);

fscanf(fp, "%e", &end_CA);

#endif

#else

fscanf(fp, "%d", &counter);

fscanf(fp, "%e", &work);

fscanf(fp, "%e", &start_CA);

fscanf(fp, "%e", &end_CA);

#endif

host_to_node_int_1(counter);

host_to_node_real_3(work,start_CA,end_CA);

}



Appendix C: UDF for laminar flame speed.c

This shows the UDF to modify laminar flame speed for premixed combustion (laminar flame speed.c).

/*********************************************************************************************

UDF to modify laminar flame speed for premixed combustion

In Fluent Zimont model, the laminar flame speed by default is a constant. Laminar

flame speed is a strong function of temperature, equivalence ratio. A udf is used

to modify that to be a more realistic value. The correlation is from Metghalchi

and Keck.

How to use the udf:

- Set up your premixed combustion case

- Define one user defined memory

- Modify the user input part of the udf

- Build the library

- Hook up the DEFINE_ADJUST function

- Use the laminar_flame_speed udf in the material panel to replace the default

constant value

Note:

- UDF works in 2d, 2d axisymmetry, and 3d. And it works in both serial and parallel

***********************************************************************************************/

# include "udf.h"


/* Ydil is the mass fraction of diluent, included to account for any EGR product. */

static real Ydil = 0;

/* Fuel type. Supported ones are methane, methanol, isooctane, RMFD_303_indolene. RMFD_303_indolene is a

reserch fuel (also called indolene), which has a controlled composition simulating typical gasolines.*/

enum

{

methane, methanol, propane, isooctane, RMFD_303_indolene, FLAG

}fuel = methane;

/* Fuel mass fraction (required for partially premixed model) */

real Fuel_Mass_Fraction = 0.05;

/********************************** User input ends ******************************************/

/* Fuel molecular weight for each fuel. Assuming RMFD_303_indolene has the same molecuar weight as

isooctance */

static real molecular_weight[]={16.043, 32.040, 44.096, 114.23, 114.23};

/* For one mole of fuel, how many moles of air needed for stoic combustion */

static real stoic_air_moles[]={2.0, 1.5, 5.0, 12.5, 12.5};

/* Unburnt gas temperature */

static real f_T_u(Thread * t)

{

/* determine the unburnt temperature as the weighted average of the unburnt temperature

in each cell ahead of the flame. If c>0.001 everywhere, then the unburnt temperature

is the lowest temperature of the cell zone */



real T_u, vol, T_u_min=1e10;

cell_t c;

T_u=0.;

vol=0.;

begin_c_loop_int (c,t)

{

if( C_PREMIXC(c,t) < 0.001 )

{

T_u += C_T(c,t)*C_VOLUME(c,t);

vol += C_VOLUME(c,t);

}

else

{

if( C_T(c,t) < T_u_min )

{

T_u_min = C_T(c,t);

}

}

}

end_c_loop_int (c,t)

T_u_min=PRF_GRLOW1(T_u_min);

PRF_GRSUM2(T_u, vol);

if(T_u>0. && vol>0.)

{

return T_u/vol;

}

else

{

return T_u_min;

}

}

/* Calculate unburnt gas temperature for laminar flame speed calculation */

DEFINE_ADJUST(my_T_u, domain)

{

#if !RP_HOST

Thread *t;

cell_t c;

real T_u;

thread_loop_c (t,domain)

{

T_u=f_T_u(t);

begin_c_loop_int(c, t)

{

C_UDMI(c, t, 0)= T_u;

}

end_c_loop_int(c, t)

}

#endif

}



/* Laminar flame speed calculation */

DEFINE_PROPERTY(laminar_flame_speed, c, t)

{

real SL, SL_ref, T_u, gama, beta, FIm[FLAG], Bm[FLAG], B2[FLAG], fi, fm;

/* Correlation from Metghalchi and Keck. The author does not have the original paper as of the time of

writing. So, three coefficients for methane is from a plot. More accurate numbers could be from the

original paper.

Coefficients for other fuels listed are from the paper. */

FIm[methane]=1.08;

Bm[methane]=44e-2;

B2[methane]=-152e-2;

FIm[methanol]=1.11;

Bm[methanol]=36.92e-2;

B2[methanol]=-140.51e-2;

FIm[propane]=1.08;

Bm[propane]=34.22e-2;

B2[propane]=-138.65e-2;

FIm[isooctane]=1.13;

Bm[isooctane]=26.32e-2;

B2[isooctane]=-84.72e-2;

FIm[RMFD_303_indolene]=1.13;

Bm[RMFD_303_indolene]=27.58e-2;

B2[RMFD_303_indolene]=-78.34e-2;

if(sg_premixed)

{

fm = THREAD_PROP(t,PROP_premix_unburnt_fuel_mf,0);

}

else

{

fm = Fuel_Mass_Fraction; /* C_FMEAN(c,t) */

}

if(fm<1e-5)

fm=1e-5;

fi = (4.76*stoic_air_moles[fuel]*29/molecular_weight[fuel])/(1.0/fm-1);

SL_ref = Bm[fuel]+B2[fuel]*pow(fi-FIm[fuel],2);

gama = 2.18-0.8*(fi-1);

beta = -0.16+0.22*(fi-1);

T_u = C_UDMI(c,t,0);

SL = SL_ref*pow(T_u/298, gama)*pow((C_P(c, t)+RP_Get_Real("operating-pressure"))/

1.013e5, beta)*(1-2.1*Ydil);

if (SL>0)

return SL;

else

return 0;

}



Further Improvement

UDF will automatically create the file work.txt. This file has combustion chamber pressureand burnt fuel mass fraction as a function of Crank Angle (CA). These can be used togenerate plots as shown (see Figures 14, 15, and 16).

Figure 14: Pressure as a Function of CA

Figure 15: Burnt Fuel Mass Fraction as a Function of CA



Figure 16: P-V Plot

You can use Execute On Demand UDF to calculate the indicated work at any time byexecuting Indicated work::libudf.

Define −→ Use-Defined −→Execute on Demand...

Summary

This tutorial demonstrated the use of premixed combustion model using ANSYS FLUENT.


Date post:	13-Apr-2015
Category:	Documents
Upload:	karthikeyan-srinivasan
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FLUENT IC Tut 04 Premixed

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