Introduction - Mr-CFDdl.mr-cfd.com/.../ansys-fluent/04-liquid-fuel-combustion.pdf · Fluent 14.5 Tutorial Guide, and that you are familiar with the ANSYS Fluent navigation pane and

Tutorial: Liquid Fuel Combustion

Introduction

This tutorial demonstrates the evaporation and combustion of liquid fuel using the dispersedphase modeling capability to compute coupled gas flow and liquid spray physics. Themixture-fraction/probability density function (PDF) equilibrium chemistry model is usedto predict the combustion of the vaporized fuel.

This tutorial demonstrates how to do the following:

• Prepare a PDF file for a liquid fuel system.

• Define inputs for PDF chemistry model.

• Define a discrete second phase of evaporating liquid droplets.

• Calculate the flow field using the pressure-based solver, including coupling betweenthe discrete liquid fuel droplets and continuous phase.

The mixture-fraction/PDF modeling approach allows you to model non-premixed turbulentcombustion by solving a transport equation for a single conserved scalar. Multiple chemicalspecies including radicals and intermediate species, may be included in the problem defini-tion and their concentrations may be derived from the predicted mixture fraction using theassumption of equilibrium chemistry. Property data for the species are accessed through achemical database. Turbulence-chemistry interaction is modeled using a β-PDF.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1 from ANSYSFluent 14.5 Tutorial Guide, and that you are familiar with the ANSYS Fluent navigationpane and menu structure. Some steps in the setup and solution procedure will not beshown explicitly. For more information, see chapter 16 Modeling Non-Premixed Combustionin ANSYS Fluent 14.5 User’s Guide.

c© Ansys Inc. December 17, 2012 1

Liquid Fuel Combustion

Problem Description

The liquid fuel combustion system considered here is shown in Figure 1. A liquid sprayof pentane fuel enters a 2D duct in which air is flowing at 650 K and 1.0 m/s. The ductwalls are held at a constant temperature of 1200 K. The model includes a duct length of10H where H is the duct height (1.0 m). The Reynolds number based on inlet conditions isroughly 100,000 and the flow is turbulent. As pentane evaporates, it enters the gas phaseand reacts. The combustion is modeled using the mixture-fraction/PDF approach, withthe equilibrium mixture of chemical species. The spray is assumed to consist of 100 microndiameter liquid droplets injected at 300 K over a filled spray half-angle of 30 degrees atthe duct centerline. The mass flow rate of liquid fuel is 0.004 kg/s, corresponding to veryfuel-lean conditions in the flow.

Figure 1: Problem Schematic

Setup and Solution

Preparation

1. Copy the file (lfuel.msh.gz) to your working folder.

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

3. Enable Double Precision in the Options list.

2 c© ANSYS, Inc. December 17, 2012


Step 1: Mesh

1. Read in the mesh file (lfuel.msh).

File −→ Read −→Mesh...

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

The mesh file is a quadrilateral mesh describing the system geometry. The meshcontains 5200 quadrilateral fluid cells.

Step 2: General Settings

General

1. Retain the default solver settings.

2. Check the mesh (see Figure 2).

General −→ Check

Figure 2: Mesh Display

ANSYS Fluent will perform various checks on the mesh and will report the progress inthe console. Make sure the minimum volume reported is a positive number.

c© ANSYS, Inc. December 17, 2012 3


Step 3: Models

1. Define the standard k-ε turbulence model.

Models −→ Viscous −→ Edit...

(a) Select k-epsilon from the Model list.

(b) Click OK to close the Viscous Model dialog box.

2. Select Non-Premixed Combustion from Model list to open Species Model dialog box.

Models −→ Species −→ Edit...

In this tutorial, the liquid fuel combustor is a non-adiabatic system with heat transfer tothe liquid fuel from the gas, at the combustor wall. Therefore consider a non-adiabaticcombustion system while creating the PDF look-up tables.

(a) Click the Chemistry tab.

i. Select Non-Adiabatic in the Energy Treatment group box.

ii. Enable Inlet Diffusion in the PDF Options group box.

iii. Enter 101000 pascal for the Operating Pressure.

iv. Retain 0.1 for Fuel Stream Rich Flammability limit.



(b) Click the Boundary tab.

i. Enter c5h12 for Boundary Species and click Add.

ii. Enable Mole Fraction in the Specify Species in group box.

iii. Enter 0.79 for n2 and 0.21 for o2 in the Oxid group box.

iv. Enter 1 for c5h12 in the Fuel group box.

v. Enter 303 K for Fuel and 650 K for Oxid in the Temperature group box.

The system pressure and inlet stream temperatures are required for the equi-librium chemistry calculation. The fuel inlet temperature for liquid fuel com-bustion should be the temperature at the onset of vaporization. The oxidizerinlet temperature should correspond to the air inlet temperature.



(c) Click Table tab.

i. Enter 280 K for Minimum Temperature in Table Parameters group box.

ii. Retain default values for other parameters.

iii. Click Apply.

iv. Click Calculate PDF Table.

v. Click OK to close Species Model dialog box.

3. Save the PDF file (lfuel.pdf.gz).

File −→ Write −→PDF...

4. Display the PDF curves.

Display −→PDF Tables/Curves...

(a) Click Display (see Figure 3).



Figure 3: Non-Adiabatic Temperature Look-Up Table on the Adiabatic Enthalpy Slice

ANSYS Fluent displays the maximum value of mean temperature as 2.434304e+03 Koccurring at a mean mixture fraction of 6.751575e-02.

(b) Examine the species/mixture-fraction relationship in the non-adiabatic system.

Display −→PDF Tables/Curves...

i. Select Mole Fraction of c5h12 from the Plot Variable drop-down list.

ii. Enable 2D Curve on 3D Surface in Plot Type group box.

iii. Click Plot (see Figure 4).

Figure 4: Mole Fraction of C5H12



(c) Similarly, plot the instantaneous mole fractions for co (see Figure 5).

Figure 5: Mole Fraction of co

(d) Close the PDF Table dialog box.

5. Define the discrete phase model.

Models −→ Discrete Phase −→ Edit...

In ANSYS Fluent, the discrete phase model is used to model the flow of liquid droplets.This model predicts the trajectories of individual liquid droplets, each representing acontinuous stream (or mass flow) of fuel. Heat, momentum, and mass transfer betweenthe liquid fuel and the air flow are included by alternately computing the discrete phasetrajectories and the gas phase continuum equations.



(a) Enable Interaction with Continuous Phase.

This option enables coupling which allows the discrete phase trajectories (alongwith heat and mass transfer to/from the droplets) to impact the gas phase equa-tions. If this option is not enabled, you can track particles or droplets but theywill not have impact on the continuous phase.

(b) Set Number of Continuous Phase Iterations per DPM Iteration to 5.

The coupling parameter is the number of gas phase iterations performed betweenupdates of the discrete phase trajectory calculations. You might want to increasethis parameter in problems that include a high discrete phase mass loading ora larger mesh size. Less frequent trajectory updates can be beneficial in suchproblems.

(c) Enter 1000 for Max. Number of Steps.

The limit on the number of trajectory time steps is used to stop trajectories ofparticles or droplets that are trapped in the domain (e.g. in a recirculation).

(d) Enable Specify Length Scale and retain the default value.

The length scale controls the time step size used for integration of the discretephase trajectories. Here 0.01 m implies that approximately 1000 time steps willbe used to compute trajectories along the 10 m length of the domain.



(e) Click OK to close the Discrete Phase Model dialog box.

Step 4: Injections

1. Create the discrete phase injections.

Define −→Injections...

The flow of liquid fuel droplets is defined by the initial conditions that describe thedroplets as they enter the air stream. ANSYS Fluent uses these initial conditions asthe starting point for the time integration of the discrete phase equations of motion(the trajectory calculations).

(a) Click Create to open Set Injection Properties dialog box.

You can define the initial conditions of the liquid fuel droplets in Set InjectionProperties dialog box. The fuel stream is defined as a group of 10 distinct initialconditions. All these conditions are identical except for initial velocity, which isvaried to define the filled spray cone of 30 degree half-angle.

i. Select group from the Injection Type drop-down list.

ii. Set the Number of Streams to 10.



ANSYS Fluent represents the range of specified initial conditions by 10 dis-crete droplet streams, each with its own set of discrete initial conditions.Here, the velocity is varied within the injection group.

iii. Select Droplet in the Particle Type group box.

iv. Select n-pentane-liquid from the Material drop-down list.

The liquid fuel droplets are treated as having an uniform diameter of 100microns. The linear option allows you to vary the diameter linearly betweena minimum and maximum value. Set the minimum and maximum diametersto equal values.

v. Select c5h12 in the Evaporating Species list.

vi. Set the parameters as shown in the following table:

Parameter First Point Last PointX-Position (m) 0.001 0.001Y-Position (m) 0.001 0.001X-Velocity (m/s) 100 100Y-Velocity (m/s) 0 57.7Diameter (m) 1.0e-4 1.0e-4Temperature (K) 303 303Flow Rate (kg/s) 2.0e-4 2.0e-4

These initial conditions define the spray of liquid fuel droplets with a uniformdiameter of 100 microns. The filled spray cone of 30 degree half-angle isdefined by the range of Y-Velocity from 0 to 57.7 m/s. The total mass flowrate is 10×2.0e−4 = 0.002 kg/s. This corresponds to the mass flow of liquidfuel in half of the symmetric duct considered here.

vii. Click the Turbulent Dispersion tab.

viii. Enable the Discrete Random Walk Model under Stochastic Tracking and setthe Number of Tries to 10.

Stochastic tracks model the effect of turbulence in the gas phase on the droplettrajectories. Stochastic tracking is important in liquid fuel combustion sim-ulations to simulate realistic droplet dispersion.

ix. Click OK to close the Set Injection Properties dialog box.

The new injection (injection-0) appears in the Injections dialog box.

(b) Close the Injections dialog box.



Step 5: Materials

1. Set the parameters for the discrete phase.

Materials −→ Droplet Particle −→ Create/Edit...

By default, the Material Type is droplet-particle because you have activated dropletsusing the Set Injection Properties dialog box.

(a) Set the following parameters for n-pentane-liquid:

Parameter ValueDensity (kg/m3) 620Cp (j/kg-k) 2300Latent Heat (j/kg) 3.63e5Vaporization Temperature (K) 303Boiling Point (K) 306Volatile Component Fraction (%) 100Binary Diffusivity (m2/s) 6.1e-6Saturation Vapor Pressure (pascal)(constant) 8.2e4Heat of Pyrolysis (j/kg) 0Vaporization Model diffusion-controlled



Some of the default property value settings for n-pentane-liquid from the ANSYSFluent database are similar to the values in the table used to represent the fuel.Here, you will exercise the capability to modify the database properties. You arerequired to select constant from the Saturation Vapor Pressure drop-down list.

(b) Click Change/Create and close the Create/Edit Materials dialog box.

Step 6: Boundary Conditions

1. Set boundary conditions for the inlet (velocity-inlet-7).

Boundary Conditions −→ velocity-inlet-7 −→ Edit...

(a) Select Magnitude and Direction from the Velocity Specification Method drop-downlist.

(b) Enter 1 m/s for the Velocity Magnitude.

(c) Select Intensity and Hydraulic Diameter from the Specification Method drop-downlist.

(d) Enter 10% for the Turbulent Intensity and 2.0 m for the Hydraulic Diameter.

The hydraulic diameter is set equal to twice the duct height. It is used to de-termine the inlet turbulence length scale. For the PDF calculation, you need todefine the inlet mixture fraction and its variance. Here, all fuel enters the systemby evaporation from the discrete phase. Thus, the gas phase inlet has a mixturefraction value of zero.



(e) Click the Thermal tab and enter 650 K for the Temperature.

(f) Click OK to close the Velocity Inlet dialog box.

2. Set boundary condition for the outlet (pressure-outlet-5).

Boundary Conditions −→ pressure-outlet-5 −→ Edit...

(a) Select Intensity and Hydraulic Diameter from the Specification Method drop-downlist.

(b) Enter 10 % for Backflow Turbulent Intensity.

(c) Enter 2 m for the Backflow Hydraulic Diameter.

(d) Click the Thermal tab and enter 1800 K for Backflow Total Temperature.

(e) Click OK to close the Pressure Outlet dialog box.

The Gauge Pressure of zero defines the system pressure at the exit as the operatingpressure. If flow is entrained into the domain through the exit, then the back flowconditions on scalars (temperature, mixture fraction, and turbulence parameters) areused. You should use reasonable values to avoid flow reversal that may occur at theexit at some point during the solution process.

3. Set boundary condition for wall (wall-6).

Boundary Conditions −→ wall-6 −→ Edit...

(a) Click the Thermal tab.

(b) Select Temperature in the Thermal Conditions group box.

(c) Enter 1200 K for Temperature.

(d) Click OK to close the Wall dialog box.

By default, boundary condition for droplets that hit the wall is reflect, as shownunder DPM tab. Here the droplets need not travel more to hit the combustor wall.If needed, select trap (in which the volatile fraction in the droplet is flashed tovapor) from the Boundary Cond. Type drop-down list.

Step 7: Solution

1. Set the DPM parameters.


You first solve for a non-reacting flow solution and then enable the DPM track.

(a) Enter 0 for the Number of Continuous Phase Iterations per DPM Iteration.

(b) Click OK to close the Discrete Phase Model dialog box.

2. Initialize the solution.

Solution Initialization −→ Initialize

Hybrid Initialization is the default Initialization Method in ANSYS Fluent 14.5. Refer tothe section 28.11 Hybrid Initialization, in the ANSYS Fluent 14.5 User’s Guide.



3. Save the case file (lfuel.cas.gz).

File −→ Write −→Case...

4. Start the calculation for 50 iterations.

Run Calculation

Figure 6: Scaled Residuals

The scaled residuals are as shown in Figure 6.

5. Save the data file (lfuel.dat.gz).

File −→ Write −→Data...

6. Set the discrete phase model parameters to solve for the reacting-flow.


(a) Enter 5 for the Number of Continuous Phase Iterations per DPM Iteration.

(b) Click OK to close the Discrete Phase Model dialog box.

7. Save the case file (lfuel-1.cas.gz).

File −→ Write −→Case...

8. Start the calculation for 300 iterations.

Run Calculation

ANSYS Fluent reports the state of the particle (escaped, aborted, and evaporated e.t.c.)during each iteration and also when displaying particle tracks. The solution convergesin approximately 190 additional iterations (see Figure 7).



Figure 7: Scaled Residuals After 240 Iterations

9. Save the data file (lfuel-1.dat.gz).

File −→ Write −→Data...

Step 8: Postprocessing

1. Display the contours of temperature.

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

(a) Enable Filled in the Options group box.

(b) Select Temperature... and Static Temperature from the Contours of drop-downlist.

(c) Click Display (see Figure 8).

Use the Views dialog box to mirror the display about the symmetry plane.To display the contours as shown in Figure 8, open the Display Options dialogbox, disable the Outer Face Culling and Lights On options, and click Apply.



Figure 8: Temperature Contours

2. Display contours of mixture fraction .

(a) Select Pdf... and Mean Mixture Fraction from Contours of drop-down list.

(b) Click Display (see Figure 9).

Figure 9: Contours of Mean Mixture Fraction

The mixture fraction distribution displays where the vaporized fuel exists in the gasphase. It is significant only in the fuel-rich portions of the flame, where oxygen isunavailable to consume the fuel.



3. Display liquid fuel trajectories.

Graphics and Animations −→ Particle Tracks −→ Set Up...

(a) Select injection-0 under Release from Injections.

(b) Select Particle Variables... and Particle Diameter from the Color by drop-down list.

(c) Click Display (see Figure 10).

(d) Close the Particle Tracks dialog box.

Figure 10: Droplet Trajectories



4. Display contours of vaporization rate.

(a) Select Discrete Phase Sources... and DPM Evaporation/Devolatilization from theContours of drop-down list.


Figure 11: Contours of Vaporization Rate

5. Display contours of species concentrations for c5h12.

(a) Select Species... and the Mass fraction of c5h12 from the Contours of drop-downlist.


Figure 12: Mass Fraction of C5H12



6. Similarly, display species concentrations of O2, CO, and CO2 (see Figures 13, 14, and15) respectively.

Figure 13: Mass Fraction of O2

Figure 14: Mass Fraction of CO



Figure 15: Mass Fraction of CO2

Summary

The tutorial has shown that a liquid fuel combustion simulation involves modeling gas phasecombustion chemistry and vaporization using the discrete phase model. In this tutorial, youlearned to use the PDF mixture-fraction equilibrium chemistry model to represent the gasphase combustion chemistry. This equilibrium chemistry model can be applied to otherturbulent, diffusion-reaction systems.

You also learned to set up and solve a problem involving a discrete phase of evaporatingliquid droplets. You created discrete phase injections, activated coupling to the gas phase,and defined the discrete phase material properties.

The procedures used in this tutorial can be used to set up other simulations involvingevaporating liquid droplets (e.g. spray cooling, spray drying, and lime injection).


Documents

Introduction - Mr-CFDdl.mr-cfd.com/.../ansys-fluent/04-liquid-fuel-combustion.pdf · Fluent 14.5 Tutorial Guide, and that you are familiar with the ANSYS Fluent navigation pane and