1. High Fidelity Numerical Simulations of Turbulent Combustion
Ramanan Sankaran
Evatt R. Hawkes
Chunsang Yoo
David O. Lignell
Jacqueline H. Chen (PI)
Combustion Research Facility
Sandia National Laboratories, Livermore, CA

2. Direct numerical simulation (DNS) of turbulent combustion
Turbulent Combustion is a Grand Challenge
DNS Approach and Role
Fully resolve all continuum scales without using sub-grid models
Only a limited range of scales is computationally feasible
Terascale computing = DNS with O(103) scales for cold flow.
DNS is limited to small domains. Device-scale simulations are out of reach
Turbulent Combustion involves coupled phenomena at a wide range of scales
O(104) continuum scales
Combustor size~1m
Molecular reactions ~ 1nm

3. S3D – Sandia’s DNS solver
Solves compressible reacting Navier-Stokes equations
High fidelity numerical methods
8th order finite-difference
4th order explicit RK integrator
Detailed chemistry and transport models
Multi-physics (sprays, radiation and soot) from SciDAC-TSTC
Ported to all major platforms
DNS provides unique fundamental insight into the chemistry-turbulence interaction

4. S3D - Parallel Performance
90% parallel efficiency on 5000 XT3 processors
Ramanan: “This figure will be replaced by a new scaling figure. With dual core xt3 scaling on it.”

5. DNS of lean premixed combustion
Goals
Better understanding of lean premixed combustion in natural-gas-based stationary gas turbines
Model validation and development
Simulation details
Detailed CH4/air chemistry; 18 degrees of freedom
Slot burner configuration with mean shear
Laboratory configuration and statistically stationary -Better suited for model development
A unique and first-of-its-kind DNS
Three different simulations at increasing turbulence intensities to clarify the effect of turbulent stirring on flame structure and burning speed
Better images from more recent simulations can go here. When they are ready. May be an animation too.

6. Effect on flame structure
Progress variable defined based on O2 mass fraction
Instantaneous slices shown on the left for Case 1
Progress variable from 0 (blue) to 1 (red) in color
Heat release rate as line contours
Considerable influence on preheat zone
Reaction zone relatively intact
Fresh
1/4
Burned
u’/SL=3
u’/SL=6
1/2
3/4

7. DNS of extinction/reignition in a jet flame
Hawkes, Sankaran, Sutherland, Chen
Scalar dissipation rate in DNS of a jet flame
Understanding extinction/reignition in non-premixed combustion is key to flame stability and emission control in aircraft and power producing gas-turbines
The largest ever simulations of combustion have been performed to advance this goal:
500 million grid points
11 species and 21 reactions
16 DOF per grid point
512 Cray X1E processors
30 TB raw data
2.5M hours on IBM SP NERSC (INCITE); 400K hours on Cray X1E (ORNL)
Burning
Extinguished

8. DNS of turbulent flame-wall interaction
A. Gruber, R. Sankaran, E. R. Hawkes, and J. H. Chen
DNS of a premixed flame interacting with turbulence in a wall-bounded channel flow
Detailed H2/Air chemistry (14 D.O.F.)
Inflow turbulence obtained from a separate simulation of inert channel flow
Cost: 100k hours on X1E
Goals
Material failure from fluctuating thermal stress in micro-gas turbines
Study spatial and temporal patterns of wall heat flux
Wall heat fluxes in turbulent flame wall interaction at low Reynolds number

9. DNS of non-premixed sooting ethylene flames (planned)
D. O. Lignell, P. J. Smith, and J. H. Chen
Motivation
Soot is a pollutant and lowers combustion efficiencies in diesel engines
Soot radiation is the major heat transfer mode
Goals
Quantify soot formation and transport mechanisms in turbulent flames
Approach
DNS of turbulent sooting flames with detailed chemistry, transport, radiation
2–3 moment soot particle model with semi-empirical soot chemistry
3M cpu-hours on Cray XT3 at ORNL to observe slow soot processes
fuel
air
Fuel
Air
Picture shows DNS as a small region of a larger, more physically relevant flow. Smoke breakthrough/shielding near the top.
Ethylene flame with extinction.
Colored by temperature with soot contours
Plot is mean soot mass fraction versus mixture fraction and time for the blue Kelvin Helmolz plot.
Soot breakthrough is important in fires since smoke can shield the environment from radiation.
How soot gets from fuel rich regions to a queched state outside the fire is a mystery.

10. DNS of stabilization in lifted auto-igniting flames (planned)
Goal: Determine stabilization mechanisms in lifted, auto-igniting flames relevant to efficient, clean burning low-temperature compression ignition engines and flashback in aircraft gas turbine engines
Hydrogen lifted flame
300K hydrogen turbulent jet; 1100K air coflow jet
Important submechanism for n-heptane
Comparison with experiment (Chung, Cabra)
Auto-ignition may occur below the base of the lifted flame due to high co-flow temperature
Approximately 1.2 million CPU hours per simulation on Jaguar at NCCS
9 species; 21 elementary reaction steps (Li et al. 2006)
24x20x3.6 mm3 domain size; 1600x1000 x240 grid resolution= ~400M grids
4–6 flow-though times; Umean = 350 m/s
Total 3~4 simulations (~3.0 million cpu-hours on CrayXT4 at ORNL 2006–2007)
Heat release (gold) and vorticity in a lifted autoigniting H2/air jet flame

11. Contacts Jacqueline H. Chen Ramanan Sankaran
Combustion Research Facility
Sandia National Laboratories
Livermore, CA
Ramanan Sankaran
National Center for Computational Sciences
Oak Ridge National Laboratory
11 Sankaran_Combustion_0611