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Impact of subgrid modelling and numerical method on autoignition simulation of two-phase flow

Stempka J.

Archives of Thermodynamics

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2018

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Vol. 39, no 2

55--72

EN

The present work focuses on analyses of the autoignition delay time predicted by the large eddy simulation (LES) method by applying different subgrid scales (SGS) models and two different discretization schemes. The analysed flow configuration is a two-phase chemically reacting turbulent flow with monodispersed evaporating fuel droplets. The impact of numerical procedure is investigated in a 3D flow domain with a temporally evolving mixing layer that constituted between the streams of fuel and oxidizer that moved in opposite directions. The upper stream of cold gas carries a dispersed fuel spray (ethanol at 300 K). The lower stream is a hot air at 1000 K. Three commonly used in LES, SGS models are investigated, namely: classical Smagorinsky model, model proposed by Vreman and the σ-model proposed by Nicoud. Additionally, the impact of two discretization schemes, i.e., total variation diminishing (TVD) and weighted essentially nonoscillatory (WENO) is analysed. The analysis shows that SGS model and discretization scheme can play a crucial role in the predictions of the autoignition time. It is observed that for TVD scheme the impact of SGS model is rather small. On the contrary, when the WENO scheme is applied the results are much more dependent on the SGS model.

2

LES study of turbulence intensity impact on spark ignition in a two-phase flow

Stempka J., Kuban L., Tyliszczak A.

Archives of Mechanics

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2018

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Vol. 70, nr 6

551--567

EN

The paper presents large eddy simulation (LES) study aiming at investigations of an influence of flow conditions on a spark ignition process in a two-phase shear dominated flow. Implicit LES approach is applied for the combustion modelling and the spark is modelled using the energy deposition model of Lacaze et al. [20]. We examine an impact of turbulence intensities and randomness of initial distributions of velocity fluctuations on a flame development during the spark duration and shortly after it is switched off. It is found that for a strong spark, as used in IC engines, the turbulence intensity has little effect on the ignition and flame kernel growth and no significant differences are seen even if the turbulence intensities differ four times. It is observed that weak turbulent structures cannot affect fast flame propagation mechanism and its development is conditioned by evaporation and rapid thermal expansion. In such regimes, the turbulence seems to be too weak to significantly alter the flame dynamics. It is found that at the initial stage of the flame development it grows toward the fuel-rich region and spread over the fuel-lean side only after the evaporated fuel diffuses and mixes with the oxidizer stream. The flame size and its shape turn out to be equally dependent on the initial distribution of the turbulence fluctuations and turbulence intensity.

3

Experimental investigation of combustion characteristics for spray combustion by impinging injection in a closed vessel

Morioka K., Kawakami T., Teodorczyk A.

Journal of KONES

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2006

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Vol. 13, No. 2

335-339

EN

Internal combustion engines are the major source of air pollution. Especially, compression ignition engines in today’s automobiles contribute the most to particulate emission and soot, and it is recognized that these emissions have a detrimental effect on human and earth environment. So, an improvement of spray combustion for diesel engines is of urgent necessity. Experiments have been conducted to obtain essential data on spray combustion influenced by impinging injection in a closed vessel. The effect of the impinging injection on maximum burning pressure, total burning time and flame speed is investigated at the condition of 300 K of initial temperature and 0.1 MPa of initial pressure. The travel time of flame front is measured by ionization probes located at two different positions from the center of combustion chamber. The experimental investigations pointed out that the maximum burning pressure for impinging injection is larger than that of the single injection at the same overall equivalence ratio, the total burning time increases with increasing the overall equivalence ratio (after injection) at same equivalence ratio (before injection), the combustion of impinging injection is very effective for increasing the flame speed.