Sayop Kim

Reduction of a Numerical Grid Dependency in High-pressure Diesel Injection Simulation Using the Lagrangian-Eulerian CFD Method

In the standard CFD code, Lagrangian-Eulerian method is very popular to simulate the liquid spray penetrating into gaseous phase. Though this method can give a simple solution and low computational cost, it have been reported that the Lagrangian spray models have numerical grid dependency, resulting in serious numerical errors. Many researches have shown the grid dependency arise from two sources. The first is due to unaccurate prediction of the droplet-gas relative velocity, and the second is that the probability of binary droplet collision is dependent on the grid resolution. In order to solve the grid dependency problem, the improved spray models are implemented in the KIVA-3V code in this study. For reducing the errors in predicting the relative velocity, the momentum gain from the gaseous phase to liquid particles were resolved according to the gas-jet theory. In addition, the advanced algorithm of the droplet collision modeling which surmounts the grid dependency problem was applied. Then, in order to validate the improved spray model, the computation is compared to the experimental results. By simultaneously regarding the momentum coupling and the droplet collision modeling, successful reduction of the numerical grid dependency could be accomplished in the simulation of the high-pressure injection diesel spray.

Improved Eulerian-Lagrangian Spray Simulation by Using an Enhanced Momentum Coupling Model

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.Copyright