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LES numerical study on n-hexane injection in a compression ignition engine

100%

Kercheva B. , Kapusta Ł. J. , Teodorczyk A.

tom Vol. 34 nr 2

155--167

The aim of the study was the comparison of different approaches to modeling the injection process in a heavy duty compression ignition engine. The conducted numerical investigation concerned n-hexane direct injection into the engine combustion chamber. Simulations were performed using AVL Fire software, a CFD (Computational Fluid Dynamics) code based on the control volume method. In order to achieve engine conditions, computational model was built basing on piston and cylinder geometry of a real engine and the mesh deformation was defined according to crank mechanism dimensions of the engine. In presented simulations for modeling dispersed phase the Lagrangian approach was used. For capturing the turbulent patterns present in the flow, the Large Eddy Simulation (LES) approach was used. Three different nozzle outflow conditions were compared. In the simplest case, constant flow rate was defined. In the second one, linear stage of increase and decrease of flow rate was defined, and in the third one – the most advanced – data collected during previously done in-injector cavitating flow simulations were used to define the flow parameters on the nozzle outlet. Calculated results for all cases were analyzed and compared. The focus was on the initial stage of the spray. The results show that the way of defining parameters at the outlet influences not only the initial stage of the spray but the whole process.

Numerical simulations of n-heptane spray in high pressure and temperature environments

80%

Smuga W. , Kapusta Ł. J. , Teodorczyk A.

tom Vol. 97, nr 1

1--6

In this study n-heptane spray in supercritical environments was simulated using commercial CFD (Computational Fluid Dynamic) software AVL Fire. The numerical results were analyzed in terms of global spray parameter, and spray penetration. The results obtained were compared with experimental data available at Sandia National Laboratories. N-heptane spray simulations were performed in the same conditions as in the Sandia experiments. The goal of the study was to assess whether the Lagrangian approach performs well in engine relevant conditions in terms of spray global parameters. Not included in this assessment was the influence of supercritical mixing on liquid-gas interphase. The major element was the potential for practical application of the commercial CFD code in terms of properly representing global spray parameters and thus mixture formation in supercritical conditions, which is one of the core aspects in whole engine process simulation. The key part of the study was mesh optimization. Therefore, the influence of mesh density on both the accuracy of calculations and the calculation time was determined, taking into consideration detailed experimental data as initial conditions for the subsequent calculations. This served as a basis to select the optimal mesh with regard to both accuracy of the results obtained and time duration of the calculations. As a determinant of accuracy, the difference within a range of evaporated fuel stream was used. Using selected mesh the set of numerical calculations were performed and the results were compared with experimental ones taken from the literature. Several spray parameters were compared: spray tip penetration, temperature of the gaseous phase and mixture fraction in the gaseous phase. The numerical results were very consistent in respect of spray tip penetration. The other parameters were influenced by specific features of the Lagrangian approach. Nevertheless the results obtained showed that the Lagrangian approach may be used for engine relevant conditions.