燃烧反应求解策略

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燃烧;数值模拟

Reacting Flow Tips and Tricks

Advanced Combustion Modeling Course

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

General GuidelinesStart in 2DDetermine applicability of model physics Mesh resolution requirements (resolve shear layers) Solution parameters and convergence settings

Boundary conditionsCombustion is often very sensitive to inlet boundary conditionsCorrect 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

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

Initial conditionsWhile 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© 2009 ANSYS, Inc. All rights reserved.

8-2

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Solution strategies for gas phase reactionsNon-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 cp’s to avoid unrealistically high temperatures

8-3

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Solution strategies for gas phase reactionsDefault URF could be too aggressive for complex reacting flow system The effect of underrelaxation is highly non-linearDecrease 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 URF’s as close as possible to 1

Best Practice for the Non-Premixed model are available on the user service center

8-4

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Solution strategies for DPM reacting flow (steady state)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 requiredPatching 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 t

he 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

8-5

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Solution Strategies for DPM - Unsteady FlowsSet the initial condition for discrete phase The DPM source terms are updated only every particle iteration. If the under-relaxation factor is smaller than one, take care that sufficient particle iterations are performed within the time step in order to achieve the full source terms. You must use a factor of 1, when doing only one particle computation per time step. Lower the Number of Continuous Phase Iterations per DPM Iteration if you need to lower the DPM under-relaxation factor. Use smaller time steps if it does not converge. (make sure that the solution is converge within each time step)

8-6

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Troubleshooting DPMStart 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

8-7

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Solution strategies for detailed chemistryInitial conditions and set upA 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 toleranceStart 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 modelThe model is robust and stable, however in some cases tends to converge slowly, to speed up the convergence change the following option from the TUI /define/models/species> set-turb-chem-interaction Acceleration Factor [between 0 (most robust but slowest convergence) and 1] [0] The default for the edc-apmax-fac is 0, change this value with small increment (0.1), in general value above 0.5 will make the solution unstable

8-8

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Solution strategies for Deflagrations&detonations PremixedFuel and oxidizer mixed molecularly, then burn Moving reaction front – usually thin and difficult to model DeflagrationsSubsonic: very difficult to model since the flame speed depends on the chemi

stry as well as the molecular diffusion parameters, and the flame zone must be resolved.

DetonationsSupersonic: ignition due to heat release behind shock. Simpler to model than deflagrations since the shock is not resolved, and detailed molecular transport is not essential.

8-9

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

ConvergenceResiduals 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 balanceThe 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

8-10

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Species Reportsreport/species-mass-flowPrint 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…

8-11

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Mass and Energy Flux Imbalance in DPM ProblemsIn DPM problems, the Report → Fluxes panel will usually give a nonzero mass and energy balance.Occurs in cases involving combustion or heat transfer from the particles/droplets to the gas phase.

In these cases the user should visit the Volume Integrals panel.Request a sum of values for DPM Mass source and DPM Enthalpy source.

At convergence, the Sum of DPM Mass Source and DPM Enthalpy Source should be equal to the Mass and Total Heat Transfer Rate imbalances in the Report Fluxes panel. For large combustion problems, this energy balance can take longer to achieve and is a better indication of convergence than low residuals.

8-12

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

Customizing the reaction rateEddy dissipation/finite rate modelDEFINE_VR_RATE ( name, c, t, r, mw, yi, rr, rr_t)

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

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

Premix model – UDF for the turbulent flame speedDEFINE_TURB_PREMIX_SOURCE ( name, c, t, turb_flame_speed, source)

Detailed chemistry modelDEFINE_NET_REACTION_RATE( name, p, temp, yi, rr, jac)

NOx modelDEFINE_NOX_RATE ( name, c, t, NOx)

8-13

ANSYS, Inc. Proprietary

燃烧;数值模拟

Advanced FLUENT Training Combustion June 2009

SummaryWe have tremendous in-house expertise in combustion modeling.Industrial ex

perience 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 Learning CFD center /login/index.htmAdvanced Reacting Flow tutorial Intermediate tutorial - applications tutorials with a focus on reacting flows

8-14

ANSYS, Inc. Proprietary

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