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

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

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 2006-01-0416 (2006). H. Machrafi, P. Guibert, S. Cavadias, Combustion Science and Technology 180 (2008) 1245–1262.

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.

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.

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 [%]

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 = 1.095 [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.

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.

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

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

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!

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 Run through this slide and ask for questions.

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