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1. Improved correlations for the unstretched laminar flame properties of iso-octane/air mixtures

   Li, D. ; Matthews, R. ; Hall, M. ( July 2022 , Proceedings of the International Symposium on Diagnostics and Modeling of Combustion in Internal Combustion Engines)

   Two improved correlations for the laminar flame speed, an improved power law correlation and an improved Arrhenius form correlation, are proposed for iso-octane in this study based on CONVERGE one-dimensional simulation results using the LLNL reaction mechanism. The typical working conditions for a spark-ignition engine, 300-950 K for unburned temperature, 1-120 bar for pressure, and 0.6-1.5 for equivalence ratio, were chosen to generate the results. Each of the two improved correlations has three parameters to be determined and these parameters are all shown as simple functions of equivalence ratio. The predicted unstretched laminar flame speeds using these two correlations were compared with the experimental measurements and with correlations from other researchers. In summary, both improved correlations, using simple and workable expressions, were able to predict the trends and the values of the unstretched laminar flame speed with improved accuracy. The improved Arrhenius form was more accurate and presented good predictions over a large range of operating conditions, and therefore is recommended for practical calculations and predictions. 

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2. Laminar Flame Speed Measurements of Primary Reference Fuels at Extreme Temperatures

   https://doi.org/10.1115/ICEF2022-90501  

   Susa, Adam J. ; Zheng, Lingzhi ; Nygaard, Zach D. ; Ferris, Alison M. ; Hanson, Ronald K. ( October 2022 , ASME 2022 ICE Forward Conference (ICEF2022))

   Experimentally measured values of the laminar flame speed (SL) are reported for the primary reference fuels over a range of unburned-gas temperatures (Tu) spanning from room temperature to above 1,000 K, providing the highest-temperature SL measurements ever reported for gasoline-relevant fuels. Measurements were performed using expanding flames ignited within a shock tube and recorded using side-wall schlieren imaging. The recently introduced area-averaged linear curvature (AA-LC) model is used to extrapolate stretch-free flame speeds from the aspherical flames. High-temperature SL measurements are compared to values simulated using different kinetic mechanisms and are used to assess three functional forms of empirical SL–Tu relationships: the ubiquitous power-law model, an exponential relation, and a non-Arrhenius form. This work demonstrates the significantly enhanced capability of the shock-tube flame speed method to provide engine-relevant SL measurements with the potential to meaningfully improve accuracy and reduce uncertainty of kinetic mechanisms when used to predict global combustion behaviors most relevant to practical engine applications.

    

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3. High Pressure Spherically Expanding Laminar Flame Speed Measurement with Plasma Affected Data

   https://doi.org/10.2514/6.2023-2050  

   Shaffer, James ; Askari, Omid ( January 2023 , AIAA SciTech)

   Measurements of a propagating flame are crucial to the understanding and verification of important flame phenomena. Specifically, laminar, unstretched flame speed is employed to describe complex flame behavior such as turbulence or stability. This data is necessary because advanced and more efficient combustion devices operate at these high pressures where this data is less reliable from the onset of flame instabilities. This research aims to develop a new flame speed measurement technique as an improvement over the traditional method. The analysis utilizes a constant pressure technique where the velocity of a spherical flame is visually measured. Flame propagation of stoichiometric flame from 1-6 atm are examined at radius up to 20mm at 300K. The novel analysis method incorporates flame data which is ignition affected to improve the traditional constant pressure methods. This allows for with the inclusion of ultra-small, highly stretched flame radii (0-10 mm). Electrical measurements of the ignition are utilized with a thermodynamic model to predict the velocity and temperature which result from plasma formation. This predicted temperature is used describe the influence ignition has on the flame velocity and is compared to the traditional adiabatic flame propagation over the radius observed. The extrapolated laminar burning speed measurement is found for both traditional and novel methods where the novel method has the benefit of additional, previously unused, flame propagation at the high stretch regime. Additionally, practical information about the plasma morphology, crucial to the experimental application of this method is also discussed. 

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4. On the ignition kernel formation and propagation: an experimental and modeling approach

   https://doi.org/10.1088/1361-6463/acc411  

   Shaffer, James ; Luna, Steven ; Wang, Weiye ; Egolfopoulos, Fokion N ; Askari, Omid ( April 2023 , Journal of Physics D: Applied Physics)

   Abstract The next generation of advanced combustion devices is being developed to operate under ultra-high-pressure conditions. However, under such extreme conditions, flame tends to become unstable and measurement of fundamental properties such as the laminar flame speed becomes challenging. One potential method to resolve this issue is measuring the ignition-affected region during spherically expanding flame experiments. The flame in this region is more resistant to perturbations and remains smooth due to the high stretch rates (i.e. small radii). Stable flame propagation allows for improved flame measurement, however, the experimentally observed kernel propagation is a function of both inflammation and ignition plasma. Therefore, the goal of the present study is to better understand the plasma formation and propagation during the ignition process, which would allow for reliable laminar flame speed measurements. To accomplish this goal, thermal plasma operating at high pressures is studied with emphasis on the spark energy effects on the formation of the ignition kernel. The thermal effect of the plasma is experimentally observed using a high-speed Schlieren imaging system. The energy dissipated within the plasma is measured with the use of voltage and current probes with a measurement of plasma sheath voltage drop as an input to numerical modeling. The measured kernel propagation rate is used to assess the accuracy of the model. The experiments and modeling are conducted in dry air at 1, 3, and 5 atm as well as in CH 4 -N 2 mixtures at 1 atm, and kernel radius, temperature, and mass are reported. The voltage-drop (as a non-thermal loss) is measured to be approximately 330 ± 5 V (dry air at 1 atm) for glow plasma with a large dependency on pressure, gas composition, electrode surface quality, electrode geometry, electrode shape, and current density. The same loss within the arc plasma is measured to be 15 ± 5 V, however the arc phase loss which agrees with arc propagation is significantly higher (∼45 V) which suggest additional unaccounted for phenomena occurring during the arc phase. With these losses, the modeling results are shown to predict the final kernel radius within 10%–20% of the observed kernel size. The difference found between the modeling and experimental results is determined to be a result of assuming that the primary loss mechanism (voltage drop across sheath formation) remains constant for the duration of glow discharge. The discrepancy for arc discharge is discussed with several potential sources, however, additional studies are required to better understand how the arc formation affects the kernel propagation. 

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5. Development of a fractal engine simulation model in a multidimensional simulation for the cold start process of a gasoline direct injection engine

   https://doi.org/10.1177/14680874231174272  

   Li, Delong ; Hu, Jinghu ; Hall, Matthew ; Matthews, Ron ( September 2023 , International Journal of Engine Research)

   A fractal engine simulation (FES) sub-model was integrated into three-dimensional simulations for modeling turbulent combustion for a gasoline direct injection (GDI) engine. The FES model assumes that the effects of turbulence on flame propagation are to wrinkle and stretch the flame, and fractal geometry is used to predict the surface area increase and thus the turbulent burning velocity. Different formulas for the four sequential stages of combustion in SI engines are proposed to account for the changing effects of turbulence throughout the combustion process. However, most prior studies related to the FES model were quasi-dimensional simulations, with few found in multi-dimensional studies, and none under cold start conditions or stratified charges. This paper describes how the model was implemented into multidimensional simulations in CONVERGE CFD, and what the formulas are in the four sequential stages of combustion in SI engines. The capabilities of the FES model for simulating the cold start cases, under the conditions of the dramatically changing engine speed and mixture stratification in a complex engine geometry, are presented in this study. The FES model was able to not only simulate the steady-state cases with constant engine speed, but also predict the in-cylinder pressure traces in all four cylinders for the very first firing cycle with transient engine speed, and gave good agreement with the experimental measurements under these extremely transient conditions. The uncertain maximum fractal dimension was chosen as 2.37 in this research, and a simple linear correlation with engine speed was used to obtain the coefficient used in calculating the kernel formation time which controls the so-called combustion or ignition delay.

    

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