Improved Eulerian-Lagrangian spray simulation by using an enhanced momentum coupling model

초록

This study describes a strategy for reducing grid-size dependency that mainly comes from inaccurate calculation of the droplet-gas interactions and droplet collision modeling. The present paper suggests an enhanced momentum coupling (EMC) model and introduces the improved collision models to obtain the goal of reducing grid dependency. For conventional CFD codes, due to the low computational cost and effort, the Eulerian-Lagrangian method is preferred in simulating the multiphase flow, for instance, liquid spray penetrating into gaseous phase. However, it is well known that the spray computations are highly dependent on the grid resolution because momentum gain from liquid droplet less or more transferred to unit gaseous mass according to the grid cell volume, resulting in inaccurate prediction of droplet-gas relative velocity. For this reason, the grid-size dependency leads to inaccurate prediction of spray tip penetration and mean droplet size. To overcome the problem, in the present study, enhanced sub-models for reducing the grid dependency are introduced and implemented in the three dimensional engine simulation code, KIVA-3V. In the EMC model, keeping the standard Eulerian-Lagrangian method, the momentum coupling term in the momentum conservation equation of the Eulerian phase is revised and lack of momentum transfer due to inadequate cell resolution is compensated regarding the gaseous volume receiving the effective spray momentum. Computations were conducted using the EMC model, the gas-velocity interpolation scheme, and grid-size independent collision model under the high-pressure diesel injection conditions. From the results, the improved model composed of the EMC model and the grid independent collision model give a dramatic decrease in grid dependency of spray tip penetration and overall droplet size.

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