Fuel Droplet Evaporation: Diesel-Air Mixture Flow

Diesel-Air Mixture Flow with Fuel Droplet Evaporation, CFD Training

Description

This project investigates the complex dynamics of a diesel-air mixture using ANSYS Fluent, focusing on the interaction between dispersed fuel droplets and the continuous air phase, along with the impact of droplet evaporation on the resulting flow field — analysis directly relevant to engine combustion and fuel spray characterization. The 3D geometry represents a simplified cylindrical combustion chamber, built in SpaceClaim, and meshed in ANSYS Meshing using a grid of over 2 million elements to ensure sufficient resolution for capturing the complex flow phenomena involved.

Methodology

A pressure-based, transient solver was used, with the standard k-epsilon turbulence model and standard wall functions applied to capture the flow's turbulent behavior. The energy equation was enabled to resolve the temperature field. The Species Transport model with Eddy-Dissipation for turbulence-chemistry interaction was used to model the mixing of diesel and air. The Discrete Phase Model (DPM) simulated the movement and evaporation of fuel droplets as discrete entities, with two-way coupling enabled to capture the interaction between the dispersed phase (droplets) and the continuous phase (air). Temperature-dependent latent heat effects were also included to account for the energy consumed during droplet evaporation.

Droplet evaporation was permitted throughout the domain, capturing the associated mass transfer process. Diesel fuel was injected from the center of the inlet, with air entering from the surrounding region, and the simulation ran for 5 seconds to capture the mixture's transient behavior.

Conclusion

The simulation results provide detailed insight into the diesel-air mixture's flow behavior: Density contours show higher mixture density near the injection point, decreasing as the diesel jet spreads and mixes with the surrounding air — reflecting diesel's higher density relative to air. Mass fraction contours for diesel (C10) show the fuel jet extending into the chamber, with concentration progressively decreasing as mixing occurs, while nitrogen mass fraction contours show a corresponding decrease near the injection point as the diesel jet displaces the surrounding air.

Static temperature contours reveal a significant temperature rise where the diesel jet interacts with the air, indicating heat release associated with combustion. Turbulence intensity is elevated near the jet, reflecting strong mixing activity, while velocity magnitude contours show peak velocity at the jet core, gradually decreasing as the jet spreads — illustrating momentum transfer from the injected diesel. Particle trajectories from the DPM model illustrate the dispersion and evaporation of individual fuel droplets within the chamber, with smaller droplets evaporating faster and exhibiting shorter residence times. Particle residence time visualizations further show how time spent within the domain varies depending on droplet size and injection location.