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Large Eddy Simulation on the Flame Structure for Split Injections of n-dodecane at Different Temperatures and Densities

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Large eddy simulation coupled with linear eddy model is applied for the simulation of n-dodecane spray flames with multiple injections (0.5 ms injection/0.5 ms dwell/0.5 ms injection). The numerical model is validated by comparing the vapour penetration length at inner condition and the predicted ignition delay times at reacting conditions with the experimental results. Good agreement between the predicted and measured data is observed. The effects of initial conditions, including the initial temperature and ambient gas density on the development of double injections are analyzed. By decreasing the ambient gas density or initial temperature, the appearance of high-temperature kernels is delayed. The tip penetration travels faster both for the first and second injections when the initial gas density is reduced from 22.8 to 15.2 kg/m3 at an initial temperature of 900 K. By decreasing the initial gas temperature from 900 to 800 K, the tip penetration travels much slower. This is because that the high-temperature combustion appears very early at 900 K. The combustion of the first injection leads to the increase in local velocity and this effect becomes more obvious as the ignition delay (ID) is shortened. At latter times, the penetration velocity is almost the same for the second injection. Moreover, as the second spray is injected into hotter environment containing different key intermediate species, ID is reduced for the second injection. The reduction in ID is also related to the time when the second spray catches up with the first one. The inhibited mixing between fuel and air leads to richer ignition in the second injection compared with the first injection. For fuel-rich cells with the mixture fraction greater than 0.1, formaldehyde (CH2O) is quickly formed. Moreover, CH2O mass fractions in all of these cells have higher values compared with the first injection.
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Keywords: Multiple injections; autoignition; flame structures; linear eddy model; spray A

Document Type: Research Article

Affiliations: State Key Laboratory of Engines, Tianjin University, Tianjin, China

Publication date: December 2, 2018

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