Date of Award
Campus Access Dissertation
Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)
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Department of Mechanical Engineering-Engineering Mechanics
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This PhD dissertation develops a turbulent burning velocity model based on flame stretch concept and couples it with an engine cycle simulation program (GT-Power) to improve its turbulent combustion modeling capability. In a non-turbulent mixture, flame propagation is laminar and the flame has a smooth surface. However, in a turbulent flow field (i.e. internal combustion engines), the flame front is no longer smooth. This was the motivation to experimentally study the burning velocity and flame stretch under engine in-cylinder conditions.
Flame front propagation analysis showed that during the flame propagation period, the flame stretch decreased until the flame front touched the piston surface. This was a common trend for stoichiometric, lean and rich mixtures, which occurred because the flame radius was the dominant factor in flame stretch calculation. In addition, the rich fuel-air mixture (ϕ = 1.18) showed a lower flame stretch compared to stoichiometric (ϕ = 1.0) or lean mixtures (ϕ = 0.84). This was due to the lower Markstein number, the representation of flame sensitivity to flame stretch, for the rich fuel-air mixture compared to the stoichiometric or lean mixtures. The ratio of the thermal to mass diffusivity appeared to be the dominant factor in the Markstein number. Furthermore, comparing the flame stretch at three different engine speeds revealed that increasing the speed increases the flame stretch; especially during the early flame development period.
In addition, dimensional analysis was utilized and a turbulent burning velocity model was developed based on the flame stretch concept. The model showed that the turbulent burning velocity decreased due to flame stretching. Although it was shown that increasing engine speed increases turbulent burning velocity by increasing the turbulent intensity (and hence the turbulent flame surface), a tradeoff between the AT/AL and the flame stretch due to higher engine speed was observed in the model. In cases where the flame distortion was very high, the flame stretch may cancel out any benefits of a large enflamed area.
While the turbulent burning velocity model was developed for an optically-accessible DISI engine at low engine speed and load, it was also tested using data from a four-stroke, liquid-cooled, two-cylinder, carbureted engine at higher speeds and loads.
Comparison of the engine in-cylinder pressure, heat release and performance parameters from simulation and experiments for the engine revealed that the developed turbulent burning velocity model coupled with GT-Power significantly improved the turbulent combustion modeling capability of GT-Power. In addition, simulation results showed that the flame stretch may result in a 35% reduction in turbulent burning velocity at very early (MFB< 1%) stage of combustion compared to the default turbulent burning velocity model in GT-Power which does not account for the effect of flame stretch.
This research also investigated combustion variations using 2D intensity images and compared the results to COV of IMEP computed from in-cylinder pressure data. The results revealed a strong correlation between the variations of the luminosity field during the main phase of combustion and the COV of IMEP. However, during the ignition and early (MFB< 10%) flame kernel formation, utilizing the luminosity field was more powerful than in-cylinder pressure-related parameters to capture combustion variations.
Since the images consist of pixels, uncertainty analysis was conducted to determine the effect of image quality on the flame stretch. Results showed that a maximum relative uncertainty of 4.5% in the flame stretch calculation occurred during the early flame development period and it decreased to less than 1% with increasing flame radius.
Afkhami, Behdad, "DEVELOPMENT OF A TURBULENT BURNING VELOCITY MODEL BASED ON FLAME STRETCH CONCEPT FOR SPARK IGNITION ENGINES", Campus Access Dissertation, Michigan Technological University, 2019.
Available for download on Saturday, August 08, 2020