Published August 31, 2017

A Bidirectional Spray Modeling Approach: Eulerian-Lagrangian Spray Atomization in CONVERGE 2.4

Julian Toumey

v2.4 New Feature Series

In CONVERGE, the standard technique for modeling sprays (such as for liquid fuel injection) employs an Eulerian approach for the continuous fluid domain and a Lagrangian approach for the spray parcels.

With the Eulerian approach, CONVERGE treats the fluid as a continuum while with the Lagrangian approach, CONVERGE tracks the discrete spray parcels. Lagrangian particle tracking is computationally efficient because it does not require as fine of a mesh to represent spray physics as required for an Eulerian approach.

One application of spray modeling is to simulate liquid fuel injection such as in an injector at the Engine Combustion Network (ECN) Spray A condition. Engineers are interested in capturing the dynamics of the nozzle fluid flow, which can be quite computationally expensive.

In CONVERGE 2.4, one computational method is VOF-spray one-way coupling. This method consists of two steps: a VOF simulation to model the liquid fuel in the nozzle, and a spray simulation in which the spray is initialized with information at the nozzle exit from the VOF simulation. While this method is appropriate in some situations, it does not capture the effect of ambient flow conditions and droplet motion in the exit chamber on the fluid flow in the nozzle.

Figure 1: Injector and exit chamber
Figure 2: Close-up of injector

Enter ELSA. The Eulerian-Lagrangian Spray Atomization model in CONVERGE 2.4 provides high accuracy by accounting for the downstream effects of ambient conditions and droplet motion via bidirectional coupling.

The method is this: CONVERGE leverages the previously-proven volume of fluid (VOF) model to accurately represent the liquid fuel dynamics in the sac and nozzle. ELSA tracks the liquid in the exit chamber and, when dilute enough, transitions the Eulerian spray to Lagrangian parcels.

As with any Lagrangian spray simulation, you can apply CONVERGE’s physical models for collision, break-up, and evaporation.

Figure 3: Injector mesh

Consider an injector at the ECN Spray A condition. The geometry (shown in Figures 1 and 2) consists of an injector and exit chamber with fuel and ambient conditions as described in the ECN database.

Figure 3 shows the mesh around the nozzle-exit chamber interface. The smallest cell width is 10 microns. Finally, Figures 4 and 5 show the liquid penetration and spray shape for the injection. By capturing the physics of the injection process, the ELSA model produces good agreement with experimental data.

When you need high fidelity resolution of spray physics, employ the ELSA model in CONVERGE 2.4.

Figure 4: Liquid penetration compared between simulation and experiment
Figure 5: Spray shape