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© 2016 Convergent Science. All Rights Reserved

Combustion Modeling Development in CONVERGE

Eric Pomraning, PhD

2018 CONVERGE User Conference

Bologna, Italy

Overview

• Combustion model development (3D)- Species based ECFM

- SAGE-RANS Triple-Delta PDF

- LES-SAGE Non-Premixed

- LES-TFM SAGE Premixed

• Chemistry utilizes (0D/1D)- Improved mechanism reduction

- Rapid Compression Machine model

- HCCI engine model

- RON/MON calculator

- Laminar flame speed calculations

Species Based ECFM/ECFM3Z Model• Original ECFM approach uses a passive based

progress variable- Incoherence in transport/diffusion of species

and passives

- Need to use first order upwind

- Progress variable not zero in unburned gases before combustion

• Species based progress variable- Solves the issues above

- Allows the detailed chemistry/emission coupling

- Allows multi-component fuel

5

1 ,F

FT

Yc

Y

2nd order with SB-ECFM3Z

1st order with passive-ECFM3Z

DC3 Engine Sector

SAGE-RANS Triple-Delta PDF• The RANS species transport equation is given by

• If we solve for the chemical reactions directly we can evaluate

• As the chemical source term is non-linear in temperature and species mass fractions, there is an error when assuming that the RANS averaging commutes.

• The RANS commutation error does not reduce to zero with reduced cell size.

• The commutation error is unimportant for small fluctuations

• In a simulation, if a RANS turbulence model does not recover the ensemble average, it is not clear if the commutation error is important or even how it should be evaluated!

• Frequently, a RANS turbulence model does not recover the ensemble average!

Triple-Delta PDF Model

small variance unimodal distribution

large variance bimodal distribution

• To evaluate the commutation error, we need to consider a distribution in species (non-premixed) or temperature (premixed)

• Triple –Delta PDF is developed to consider the fluctuations in turbulence reacting flow.

• Transport mean species mass fraction :

• Probability of three states are based on the mean equivalence ratio and its variance:

Triple-Delta PDF Model: Sandia Flame D

Sandia National Laboratories, TNF Workshop website

• Flame D: – Non-premixed methane-air mixture– Jet flame with burning pilot-stabilizer

• Turbulence model: – Standard k-epsilon model

• Combustion model: – Single-Delta PDF (finite-rate chemistry) and Triple-

Delta PDF• Mechanism:

– 30-species skeletal mechanism based on GRI3.0• The Flame D simulated flow field with k-epsilon is steady

and recovers the experimentally averaged flow field

Triple-Delta PDF Model: Sandia Flame D

x/D=15.0

x/D=30.0

LES SAGE Non-Premixed

• CSI believes LES is better for predictive combustion simulations

• There are outstanding questions for non-premixed combustion with LES

- Grid resolution required

- The importance of the commutation error

• To investigate these issues, an LES combustion study of Flame D was conducted

LES Simulation of Diesel Engine (courtesy of Aramco)

LES SAGE Non-Premixed (Flame D)

LES Turbulence model: Dynamic Structure Model

Combustion model: Single-Delta PDF (finite-rate chemistry)

Inflow: Mean inflow velocity are directly from experimental

measurements, synthetic isotropic turbulence perturbations

are added to the inflow velocity

*Sandia National Laboratories, TNF Workshop website

Finest resolution (Case a) is 0.25 mm. Resolution is

clearly not sufficient for a DNS simulation!

Instantaneous Temperature Distribution (LES)

Cell SizeLarge Small

A (0.25 mm)B (0.375 mm)C (0.5 mm)D (1.0 mm)E (2.0 mm)

Mean Temperature Distribution (LES)

A (0.25 mm)B (0.375 mm)C (0.5 mm)D (1.0 mm)E (2.0 mm)

Cell SizeLarge Small

Case A: 0.25mm

Velocity Temperature Mixture fraction CO mass fraction Near field temperature

Case A: 0.25mm, Centerline profile

LES Engine Combustion Simulation

Aramco Designed Piston With Compression Ratio 20.5

• Compression ignition simulation with combustion

• Simulation with approximately 100 Million cells!

LES Thickened Flame Model (TFM) SAGE Premixed

Turbulent Premixed V-Flame*Yuki Minamoto (Sandia National Laboratories) and Mamoru Tanahashi (Tokyo Institute of Technology)

O. Colin et al., Phys. Fluids A 12 (7) (2000) 1843–1863

• The thickness of a premixed flame is about 0.01-1mm. • Generally, it is computationally too expensive to resolve

flame front (5 to 10 grid points across the flame front).• Thickening the flame is an effective way to resolve the

turbulence premixed flame in LES simulation.

1D Laminar Flame with TFM

Laminar flame speed didn't change too

much with thickening factor from 1 to 20.

2D Flame Growth with Decaying Isotropic Turbulence with TFM

With thickened flame model, simulation of turbulent premixed flame with relative coarse grid can give reasonable results.

DNS results

Well resolved

results

flame front resolved

2D Flame Growth with Decaying Isotropic Turbulence with TFM

Temperature Vorticity

Improved Mechanism Reduction

• Originally chemical mechanism is reduced by looking at the ignition delay.

- Error can be introduced to laminar flame speed.

• Improved method allows user to consider laminar flame speed as targets, beyond the ignition delays.

0D only

0D& 1D

Zero-D: Rapid Compression Machine

Tem

per

atu

re, K

Time, s

Temperature profiles of 0D RCM simulations

● Closed homogeneous reactor.

● Users can specify volume evolution profiles and heat transfer at the boundary.

● Useful tool for reactor simulations of HCCI, RCM etc.

http://ibnox.com/read/7eapnzt/engines-advancements-advantages-and-disadvantages

Zero-D: HCCI Engine Model

• Internal Combustion Homogenous Charge Compression Ignition Model

- Study engine knock, auto-ignition, and HCCI engines

- Engine parameters are user specified (compression ratio, crank radius, RPM, displaced volume, etc…) and used in Heywood (1988)* volume equations.

- Wall Heat Transfer Models (Woschni, Hohenberg, Annand, and user modified) provide further accuracy of engine simulation.

*J. B. Heywood, Internal Combustion Engines Fundamentals, McGraw-Hill Science/Engineering/Math, New York, 1988.

Zero-D: Engine RON/MON

• Based on HCCI Engine Model

• Calculates RON or MON of a fuel composition

- Finds critical compression ratio (CCR) where auto-ignition occurs

- Correlation based on the CCR of PRF fuel composition (i.e., PRF 80 equates to RON/MON of 80) using the LLNL Gasoline Mechanism

- Recommend for gasoline fuels only

- Engine parameters are based on ASTM D2699 (RON) and ASTM D2700 methods (MON)

One-D Premixed Flame Sparse Solver

• A new option to the one-d premixed flame Newton solver

• Up to an order of magnitude faster for larger mechanisms

• Different treatment of species diffusion yields a sparser matrix

- Nitrogen is used to account for inconsistencies introduced by Fick’s law instead of correction velocity

Summary

• ECFM species based has been added to CONVERGE in collaboration with IFPEN

• SAGE RANS Triple PDF model has been added to account for commutation error - Works well for steady solutions that give average result (e.g., Flame D)

- Does not work well for solutions that do not give average result (e.g., engine simulations)

• We are actively researching LES combustion for non-premixed and premixed combustion

- Implementing TFM model in collaboration with IFPEN

• CSI is continuing to improve our 0D and 1D chemistry modeling capabilities

THANK YOU!CONVERGECFD.COM

© 2016 Convergent Science. All Rights Reserved

Thank youpomraning@convergecfd.com

If you are interested in joining C3, please visit fuelmech.org