1. Francesco Contino et al. A Review by John Roberts For:
Experimental and numerical analysis of nitric oxide effect on the ignition of iso-octane in a single cylinder HCCI engine
Francesco Contino et al.
A Review by John Roberts For:
ME 769
Tuesday, March 10, 2015

2. Paper Outline Introduction and Motivation
Overview of the tabulation of dynamic adaptive chemistry (TDAC) technique
Initial 0-D kinetics validation of TDAC
Experimental Results
CFD comparison and exploration
Characterization of speed up time
Conclusion
Run though this slide an outline the parts that you will discuss

3. Introduction and Motivation
Goals of the paper:
Investigate the effect of initial NO concentrations on combustion phasing
Implement the TDAC technique efficiently complete a CFD simulation of this phenomena
Motivation
IC engines need to address emissions and efficiency
HCCI offers a promising solution
Improved methods of controlling ignition timing are required
Ignition timing with Nitric Oxide (NO)
Residual NO may be important in egr - P. Risberg Et al.
Suggested as a means to control the engine - H. Machrafi Et. al
The main goal: investigate the effect of initial NO concentration on ignition delay
Secondary goal: Implement the TDAC technique to perform a CFD comparison to the experimental data
Focus on ignition timing control with nitric oxide.
P. Risberg, J. Andrae, G. Kalghatgi, P. Björnbom, H.-E. Angström, SAE Technical Paper (2006).
H. Machrafi, P. Guibert, S. Cavadias, Combustion Science and Technology 180 (2008) 1245–1262.

4. Modeling Chemistry in CFD - Traditional Methods
Traditional full CFD
Computational domain is subdivided into thousands of fluid cells
Complex chemical mechanism is solved for each fluid cell for each CFD time step
Traditional methods of optimization
Reduce the chemical mechanism down to a skeletal structure
The effect of some reactions may be missed by this technique
Traditional full CFD is already computationally intensive
Adding in the solution to the stiff set of differential equations for a complex mechanism increases computational overhead exponentially!
Traditional methods involve implementing a skeletal mechanism to reduce the intensity of the chemistry component
This technique may miss the effects of some reactions such as initial NO concentration.

5. TDAC Technique
A very efficient and accurate method to reduce the computational overhead of coupled CFD-Kinetics simulations
Two parts:
In-situ adaptive tabulation algorithm (ISAT) – H. Weller Et. al
Dynamic Adaptive Chemistry (DAC) - L. Liang Et. al
ISAT
Tabulates previously solved data for retrieval at a later time
See Weller et al
DAC
A method of chemical mechanism reduction
Determines a skeletal mechanism on the fly
How the method works:
For each time step the algorithm looks at the conditions of each fluid cell
Groups cells with the same physical properties
Compares this to tabular data and retrieves results for cells that meet the criteria
If the cells do not match tabular data, the algorithm moves to the DAC layer
Reduces the mechanism and solves
This is then tabulated for later use
H. Weller, G. Tabor, H. Jasak, C. Fureby, Computers in Physics 12 (1998) 620–631.
L. Liang, J.G. Stevens, J.T. Farrell, Proceedings of the Combustion Institute 32 (2009) 527–534.

6. O-D Kinetics Validation
The TDAC method was validated for a simple 0-D kinetics simulation
Initial investigations for the effect initial NO concentration were performed
Results:
Increasing the initial concentration of NO advances the ignition timing for the mixture
Increasing temperature advances the ignition timing as well
The error associated with the TDAC method are below 1.1% (4 CAD)
Analysis:
Production rate of OH is increased though the following reaction:
Ignition delay vs. Temperature
The authors compared the TDAC method to a full chemical mechanism for various temperatures and initial NO concentrations
Found that the maximum error for ignition delay had a maximum of 1.1%
The species concentration error was higher due to the mechanism reduction
The figure shows the promoting effect of increased NO concentration on ignition delay
Increasing NO concentration reduces ignition delay along with temperature
The increase is due to the activation of a reaction that produces the hydroxyl radical
TDAC method error [%]

7. Experimental Setup
Experiments were conducted to allow comparison between empirical data and CFD modeling
Apparatus:
Four stroke diesel engine (converted to single cylinder operation)
Initial concentrations controlled with coriolis mass flow controllers
Conditions:
Equivalence ratio = 0.3
MAT = 430 [k]
MAP = [bar]
1000 RPM
PFI vaporization for HCCI - Dubreuil Et. al
The port fuel was injected 10 cm upstream and which has been validated by Dubreuil et al
The initial concentration of NO was increased from 0 ppmv to 500 ppmv
500 ppmv was considered to be the HCCI limit of the engine.
A. Dubreuil, F. Foucher, C. Mounaïm-Rousselle, G. Dayma, P. Dagaut, Proceedings of the Combustion Institute 31 (2007) 2879–2886.

8. CFD analysis setup
CFD was completed for the same operating conditions of the engine
The TDAC method was implemented in the analysis
Initial concentration values were increased beyond the HCCI limit of the engine
It had been previousely observed the n-heptance experienced a limit in the reduction of the ignition delay
Beyond this limit the ignition delay was increased and NO actually created an inhibit effect.
The authors attempted to see if this was also experienced by iso-octane when initial concentration s were pushed beyond the HCCI limit of the engine.

9. Experimental and CFD Results
Experimental and CFD results followed the same trends
Data collected below the HCCI limit of the engine
Ignition delay
Additional NO reduces the ignition delay of the mixture
Additional NO increases the HRR of the mixture
The experimental data shows the average of 100 cycles and two standard deviations of the dataset
The experimental data and the CFD data follow the same trends
The hhr increases significantly as the initial NO concentration approaches 500 ppm

10. Experimental and CFD Results - Beyond the HCCI limit
Agreement:
The data collected experimentally follows the same trends as the CFD analysis
Results:
CA50 advances with increasing initial NO concentration
MPRR increases with increased NO concentration
CFD shows a limit in advancing CA50 at around 1000 ppm initial NO concentration
The data for CA50 and max pressure rise rate follow the same trend for both CFD and the experimental data
The CFD simulation shows that at 1000 ppm initial NO concentration, the advance in ignition delay reaches a maximum and then demonstrates an inhibiting effect as observed with n-heptane
This was shown in work completed by Dubreuil et al for n-heptane and the same effect is predicted by the CFD simulation for iso-octane

11. CFD Speed-Up Characterization
Characterization of speed up time:
Full traditional CFD with a complete chemical mechanism would not be possible for this scenario
Five representative points were taken to compare traditional CFD to TDAC
Results:
Traditional CFD – 170,000 CPU hours (19.4 CPU years)
TDAC – 110 CPU hours
Speed up factor around 1500
Traditional CFD method
incorporates an adaptive meshing technique that removes fluid cells during the engine cycle
dark line shows the number of cells which decrease during the compression stroke
dashed line shows the CPU time for 20 averaged cells across the computational domain due to increasing stiffness of the chemical system of ODE’s
TDAC speeds things up about 1500X!

12. Conclusions and Review
Initial NO advances the ignition timing of HCCI combustion
An optimum point of ignition timing advancement is predicted in CFD for iso-octane
This is similar to observations of n-heptane
TDAC is an effect method improving the efficiency of CFD simulations while maintaining desired resolution and accuracy
Final Thoughts:
The authors did a great job of presenting their material and analyzing the subject matter
The paper seemed to be more focused on the methods used than the actual results
It would have been nice to have a more rigorous explanation as to why this phenomena was observed.
It would have been nice to see more experimental correlation at different temperatures and pressures or at least mention this if future work
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13. Questions?