Exploring STAR-CCM+ Capabilities, Enhancements and Practices for
Aerospace Combustion
Niveditha Krishnamoorthy
CD-adapco
Overview of modeling capability
Applications, Practices and Enhancements
– Gas Turbines
– Rocket Nozzles
– Scramjets
Summary
Outline
Density – and Pressure- based flow
solvers
Incompressible through hypersonic flow
regimes
Full range of turbulence models
– RANS, LES and DES
Multi-physics
– Combustion
– CHT
– Fluid-Structure Interaction
– Aeroacoustics
Overview – (1)
Combustion and Emissions Modeling
– Models for different flame types: Premixed,
non-premixed and partially premixed
– Multi-component liquids, solids and gases
– Homogeneous and Heterogeneous
chemistry
– Global, tabulated chemistry,
reduced/detailed chemistry
– Emission models for Soot, and Nox
• Soot
– Two –equation semi-empirical model
– Method of moments
• Nox
– Fuel
– Prompt
– Thermal
Overview – (2)
Mass Fraction of CO (LES run)
Participating Media Radiation Model
Overview –(3)
• Radiative Transport Equation (RTE) solved using the Discrete Ordinates Method (DOM)
• S2 to S16 Quadratures available
Properties of the medium:
• Gray Thermal Radiation
• User defined
• Weighted Sum of Gray Gases
• Multiband Thermal Radiation
• Properties in wavelength bands
• Particle Radiation
• Scattering: isotropic, gray
• Absorption: gray
Application, Practices and Enhancements
Gas Turbine Combustors
Gas Turbine Combustor
Emissions Fuel Flexibility,Flame Stability
Thermo-acoustic Instability
Mechanical Durability Cost
• UHC• Soot• Nox• CO
• Flame shape• Flame location• Flash-back/ blow-off• Gaseous/liquid Fuels
• Liner temperature• Component temperature
System Level
Combustion Chemistry Heat TransferFluid Dynamics
Unit Level
• Flow and mixing• Swirlers• Bluff bodies
• Fuel formulation• Operating conditions• Chemical kinetics• Thermodynamics
• Conduction• Convection• Radiation
Liquid Droplet Combustion
– Droplet Evaporation• Quasi-steady• User defined
– Droplet Break-up• Primary atomization
– Linear Instability Sheet Atomization (LISA)
• Secondary break-up– Kelvin Helmholtz-Rayleigh Taylor (KHRT)– Taylor Analogy (TAB)– Stochastic Break-up (SSD)
– Droplet Wall-impingement• Bai-Gosman• Satoh
– Collision Detection Model• No Time Counter (NTC)• O’Rourke
– Two-way Coupling
– Turbulence Dispersion• Random Walk Technique
Combustion Chemistry: Models
Global Chemistry– Global multi-step reactions can be calibrated for specific operating conditions
– Calibration carried out using HEEDs and DARS-Basic
[ Freely propagating Flame]
Gas Phase Combustion Modeling
𝜔 = 𝑨𝑒−𝐸𝐴/𝑅𝑇 𝐶 𝒏 𝐷 𝒎
𝐶 + 𝐷 → 𝐸 + 𝐹Sample Reaction:
Reaction Rate:
Variables we can vary
• A : Pre-exponential Factor
• n, m : FORD (forward reaction rate exponents)
Calibration to match flame speed
Blue = Literature
Red = uncalibrated
Green = Calibrated
Purple = Hand Calibrated
5 Step Mechanism
Tabulated Chemistry
– Presumed Probability Distribution Models (PPDF Flamelet)
• Update PPDF Species once per time step without affecting results
• Can delete species not needed for post-processing: affects table size and look-up
times
• Vectorial table retrieval and more efficient table interpolation
– Flamelet Generated Manifold (FGM)
• Flexible definition of progress variable
• Transports progress variable and its variance. Better representation of state-space
than the traditional progress variable model
• Inclusion of heat loss effects in the table always ensures species and enthalpy are
consistent
Gas Phase Combustion enhancements
Performance Improvements: Large Cases (LES)
• Flow Solver improvements in v9.04
Case 1 Case 2
Case 1 Case 2
•Combustion solver improvements in
v9.02 (40-50% speedup)
•Flow and Lagrangian solver
improvements in v9.04
(20-25% speedup from v9.02)
Accurate temperature distributions are required for emission predictions and to assess component life
Heat transfer in solid components requires adequate description of
– Conduction
– Convection
– Radiation
Liners can either be modelled as shells (1-D heat conduction) or with 3-D heat conduction
Heat Transfer
Import CAD
– Single or multiple injectors
– Liners can have all details like dilution holes, effusion holes
– All solid components can be included: splash plate, dome etc.
Repair the CAD to get a closed geometry
Extract fluid domain and all the solid components from the closed CAD
Refine prism layer mesher for CHT components such that Y+ is close to
1
General Meshing Procedure for CHT
Need to ensure there are no
intersecting parts or gaps
between components
Fully conformal mesh possible
Quality of mesh can be checked
by running mesh diagnostic
report
Checks for:
– Face Validity
– Cell quality
– Volume change statistics
– Cell and boundary skewness
angle
Meshing
1:1 matching
Conformal mesh
Lagrangian update can be done once every time-step
Dynamic load balancing for Lagrangian spray helps with speed up
Update species and radiation once per time-step (for LES runs)
Other useful settings
Application, Practices and Enhancements
Rocket Nozzles
Combustion Chamber
– Multicomponent Droplet combustion
– Real Gas Equation of State
– Equilibrium and finite rate chemistry models
Nozzle Flow/Plume Study
– Coupled Solver
– Equilibrium or finite rate chemistry
Heat Transfer (CHT)
– Base heating
– Film Cooling
Application Areas
Combustion with Real Gas Model
Fuel: Methane at 275.8 K
Oxidizer: Oxygen at 105.9 K
Solver:
3D, Steady, k-omega SST
Redlick Kwong EOS,
Coupled Implicit with solution driver
and convergence accelerator (CCA)
Non-premixed, non-adiabatic PPDF
Coupled Implicit, Axisymmetric
Steady, SST K-Omega turbulence
Detailed chemistry: 11 species
DARS-CFD Approximation options:
– In-situ Adaptive Tabulation
• Populates source terms as the
simulation progresses for
subsequent look-up
• Speeds computational time once the
table is populated
– Equilibrium Time-Scale
• Quick approximate solution for
detailed chemistry calculations
• Assumes chemical composition
relaxes to local equilibrium
composition at time-scale
determined by flow and chemistry
Reacting Nozzle Flow
One stream or part of one-stream is
inert, its reactivity neglected
Sole effect of inert stream is to dilute
reacting products
Transport equation solved for
corresponding mixture fractions
Species mass fractions computed as
linear combination from inert stream
and reacting streams
Faster table generation and
interpolation. Smaller table size
Inert Stream model for PPDF Combustion
Yi=Zinert*Yi_inert+(1-Zinert)*Yi_reaction
Application, Practices and Enhancements
Scramjets
Density-based Solver
– Coupled, implicit formulation with
AMG acceleration
– TVD reconstruction
• AUSM+ or Roe inviscid flux schemes
• MUSCL + Venkata limiter
– Advanced initialization and
convergence control
– Real gas models
• Redlich Kwong
• Soave-Redlich Kwong
• Modified Soave-Redlich Kwong
• Peng Robinson
• EBU, PPDF combustion models
High Speed Reacting Flows
Advanced Initialization
– Grid sequencing option
– Fully implicit newton-type solution algorithm
– Controllable number of coarse levels
Continuity Convergence Accelerator (CCA)
– Used for high speed flows where convergence for mass flow is slow
– Solves pressure correction equation using density based Riemann Flux
discretization
– Overall and individual cell mass imbalances are minimized at each iteration
– Option available for Coupled Implicit Solver.
Advanced Initialization and Convergence Control
Supersonic Combustion
• H2 Fueled NASA SCHOLA direct-connect Scramjet engine
• Validate against experiment and NASA VULCAN code
Mesh:
1.4M Hex-dominant
10 Prism Layers
Solver:
Density based solver
Steady,k-w SST, AUSM+FVS
Non-adiabatic PPDF
Supersonic Combustion (2)
Global Chemistry
– Single or multi-step
– Variants of eddy break-up model
• Standard
• Hybrid
• Combined time-scale
• Kinetics only
Tabulated Chemistry
– PPDF Equilibrium
Detailed Chemistry
– DARS-CFD stiff chemistry solver
– Use Equilibrium Time-Scale
approximation for initial guess
– Then switch to finite rate chemistry
• Laminar flame concept
• Eddy dissipation concept
Combustion Modeling in Scramjets
Dual Mode Scramjet
Meshing
– Utilize extruded (directed) mesh as much as possible in long, non-complex,
ductwork (isolator, combustor, etc.)
– Use directional reordering of mesh in streamwise direction
Coupled Flow and Energy
– Coupled inviscid flux scheme: AUSM+
– Coupled Energy: Enable Enthalpy Formulation
Boundary Conditions
– Specify fuel inlets as mass flow. Ramp flow rate over 1000 iterations
– Pressure outlets can have a small area of extrusion w/ free slip wall
General Tips for Scramjets
Solver Settings for Scramjets
Coupled Implicit– CFL = 5.0
– Ramp CFL from 0.1 over first 300 iterations
AMG Linear Solver– Max Cycles = 10
– V-Cycle:
• Pre-Sweeps = 1
• Post-Sweeps = 3
• Max. Levels = 50
Grid Sequencing Initialization– 10 Levels, 150-250 iter./level, Tolerance = 0.005, CFL = 5.0
Expert Driver– CFL Ramp:End Iteration = 250
– Min. Explicit Relaxation = 0.35
– Max. Explicit Relaxation = 0.75
– Target AMG Cycles = 6
Continuity Convergence Accelerator– URF = 0.6 (Ramp from 0.03 over first 100 iterations)
– Enhanced Mass-Imbalance Calculations Enabled
– AMG Solver Convergence Tolerance = 0.05, V-Cycle: 1 Pre-Sweep, 1 Post-Sweep, Max levels = 50
Multi-physics simulations involving high speed flows, reactions and heat
transfer possible using STAR-CCM+
Continued efforts on making the code faster for complex flows
Best practices established for:
– Meshing
– Physics set-up
– Solver settings
– Initial and boundary condition specifications
In each area of application across industry sectors
Summary