fluent combustion

2012 ANSYS, Inc. March 12, 2013 1 Release 14.5

14.5 Release

Lecture 7 Tips & Tricks

Advanced Combustion Modeling


General Guidelines

Boundary conditions

Combustion is often very sensitive to inlet boundary conditions Correct velocity and scalar profiles can be critical

Wall heat transfer is challenging to predict; if known, specify wall temperature instead of external convection/radiation BC

Discretization

Start with first order, then converge with second order to improve accuracy Second order discretization especially important for tri/tet meshes

Initial conditions

While steady-state solution is independent of the IC, poor IC may cause divergence due to the number and nonlinearity of the transport equations

Cold flow solution, then gas combustion, then particles, then radiation


Solution strategies for gas phase reactions

Non-premixed model:

in general there is no need to solve first the cold flow, or to patch high temperature

Start with the reacting flow simulation without radiation Enable radiation once the main flow feature and temperature field have been

established

Eddy dissipation/finite rate model:

Start with a cold flow solution Patching of products and/or high temperature is needed to start the reactions Use temperature dependent cps to avoid unrealistically high temperatures


Solution strategies for gas phase reactions

Default URF could be too aggressive for complex reacting flow system

The effect of underrelaxation is highly non-linear

Decrease the diverging residual URF in increments of 0.1

Underrelax density when using the mixture-fraction PDF model (0.7)

Underrelax velocity for high buoyancy flows

Underrelax species to start up the solution (0.9 or lower)

Once solution is stable, attempt to increase species, energy, mixture and radiation URFs as close as possible to 1

Best Practice for the Non-Premixed model are available on the customer portal


Solution strategies for DPM reacting flow (steady)

Converge the non reacting flow using first order discretization

After the flow field has been established start the particle tracking

Depending on the model and conditions patching might be required

Patching high temperature to start the evaporation or devolatilisation If using the Eddy dissipation model patching some products to start the gas phase

reactions

Run the case tracking particle every 20-30 gas phase iterations and lowering the URF for the DPM source term (0.1-0.2)

Enabled radiation only after the main temperature field and flame shape have been established

Solve until a good heat and mass balance have been achieved


Solution strategies for DPM reacting flow (unsteady)

The DPM source terms are updated only every particle iteration. If the DPM under-relaxation factor is smaller than one:

If only one particle iteration is performed within the time step: check the Update DPM source at every iteration option and make sure that enough gas phase iterations are calculated within each time step

Or take care that sufficient particle iterations are performed within the time step in order to achieve the full source terms (not recommended due to CPU penalties)

Use smaller time steps if it does not converge. (make sure that the solution is converge within each time step)


Troubleshooting DPM

Start DPM particle/droplet too early before the flow field has been developed is often the cause of convergence issues in reacting flow DPM problem

Increasing the number of tries and having more iterations between DPM tracks generally helps to make the solution more stable

If there are a large number of incomplete particle you should increase the max number steps in the Discrete Phase model panel


Initial conditions and set up A poor initial condition might cause the stiff chemistry solver to fail A good initial solutions can be calculated using the non-premixed or

eddy dissipation models, that would provide a good initial guess (temperature and species) for the stiff chemistry solver

ISAT tolerance Start with the default 10e-3 and increase the table size from the

default 100 MB

To fully converge the solution decrease the ISAT tolerance and make sure that the solution is independent from any table interpolation error

EDC model, Laminar-Finite Rate (with stiff chemistry solver), Eulerian Composition PDF (with stiff chemistry solver)

For some cases, the models tend to converge slowly, to speed up the convergence change the Aggressiveness Factor

Aggressiveness Factor [between 0 (most robust but slowest convergence) and 1]

Solution strategies for detailed chemistry


Convergence

Residuals should be less than 10-3 except for T and P-1 and species, which should be less than 10-6

The mass and energy flux reports must balance

The fluxes can be checked from the report fluxes menu The flux report will include only heat and mass flux at the boundary (not any

additional source term in the fluid or solid domain)

Monitor variables of interest (e.g. mean temperature at the outlet): solution is stable and not changing if running the case further

Ensure contour plots of field variables are smooth, realistic and steady


report/species-mass-flow

Print list of species mass flow rate at inlets and outlets

Available after performing 1 iteration

These options are more accurate than surface integrals at boundary zones since no interpolation is used.

Report Fluxes

Species Reports


Report Fluxes panel will report the DPM sources (mass and enthalpy) as well

Heat of reaction source is available only after performing 1 iteration

Mass and Energy Flux in DPM Problems


Customizing the reaction rate

Eddy dissipation/finite rate model DEFINE_VR_RATE ( name, c, t, r, mw, yi, rr, rr_t)

Surface chemistry model DEFINE_SR_RATE ( name, f, t, r, my, yi, rr)

Multiple char reaction model DEFINE_PR_RATE ( name, c, t, r, mw, ci, p, sf, dif_index, cat_index, rr)

Premix model UDF for the turbulent flame speed DEFINE_TURB_PREMIX_SOURCE ( name, c, t, turb_flame_speed, source)

Detailed chemistry model DEFINE_NET_REACTION_RATE( name, p, temp, yi, rr, jac)

NOx model DEFINE_NOX_RATE ( name, c, t, NOx)


Summary

We have tremendous in-house expertise in combustion modeling.

Industrial experience World-class developers and consultants

The range of physical models for combustion applications continues to grow.

We are dedicated to providing better service to our customers; we appreciate and encourage your feedback!

Several tutorial are available on the customer portal (to be posted soon)

Advanced Reacting Flow tutorial Intermediate tutorial - applications tutorials with a focus on reacting flows

fluent combustion

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