Introduction
Sprays consist of discrete liquid droplets dispersed in a carrier gas. In engines, printers, and pharmaceutical inhalers their performance hinges on how droplets atomise, evaporate, and occasionally impinge on walls to create surface films. A film may be beneficial (e.g. protective oil layer) or harmful (fuel wall-wetting, reduced combustion efficiency).
Computational Fluid Dynamics (CFD) lets engineers predict these phenomena before building expensive prototypes.
1 Governing Physics
| Aspect | Spray Domain | Film Domain |
|---|---|---|
| Continuum description | Lagrangian parcels in an Eulerian gas | 2-D Eulerian sheet (mass, momentum, energy) |
| Dominant forces | Drag, turbulence, evaporation | Surface tension, viscosity, wall shear, gravity |
| Phase change | Evaporation / flashing | Evaporation, re-entrainment |
| Film–spray exchange | Deposition, splashing, rebound | Secondary droplet generation |
2 Why These Flows Are Challenging
- Six-way coupling: gas ⇄ droplets ⇄ film ⇄ wall ⇄ heat transfer ⇄ chemistry.
- Multi-scale: micrometre droplets vs. centimetre chambers.
- Highly transient: millisecond injector events demand small time-steps.
- Moving boundaries: pistons, valves, and wipers introduce additional mesh motion.
3 CFD Workflow in OpenFOAM
3.1 Mesh Strategy
- 3-D domain for gas & droplets.
- 2-D surface mesh (extruded one cell) for the film.
- Optional AMR on species or temperature to sharpen vapour plumes.
3.2 Solver Selection & Extension
| Base Solver | Key Features | When to Use |
|---|---|---|
reactingParcelFoam | One-way coupling, chemistry, parcel evaporation | Fast prototyping, laminar jets |
sprayFoam | Two-way coupling, turbulence models, TAB breakup | Engine-like sprays |
| Hybrid (this work) | sprayFoam + Bai–Gosman film module | Wall impingement & film evolution |
reactingParcelFilmFoam (v10+) | Native film; reference for future work | Up-to-date OpenFOAM versions |
3.3 Boundary & Initial Conditions
- Wall:
noSlip, constant‐or temperature‐dependent contact angle, roughness. - Inlet:
coneInjectionorpatchInjectionsub-models. - Outlet: static pressure, wave-transmissive velocity.
3.4 Physical Models Activated
- Break-up: KH-RT, TAB.
- Collision/Splash: O’Rourke, ISS.
- Film: Bai–Gosman, mass & momentum source back to gas.
- Evaporation: Frossling correlations with Antoine vapour pressure.
- AMR: heptane vapour fraction band (Case 5).
4 Case Studies
| # | Scenario | Coupling | Turbulence | Special Feature | Video |
|---|---|---|---|---|---|
| 1 | Vanilla tutorial (reactingParcelFoam) | 1-way | laminar | shows solver limitation @0.12 s | https://youtube.com/shorts/rNratYg63c8 |
| 2 | Cone spray + film | 1-way | laminar | secondary droplets on wall | https://youtube.com/shorts/hFLnjWVP8u8 |
| 3 | AachenBomb w/o film | 2-way | k-ε | dense evaporating plume | https://youtube.com/shorts/dTGGw7Kph3M |
| 4 | AachenBomb + film | 2-way | k-ε | hybrid solver demonstration | https://youtube.com/watch?v=_uihOTGtpS0 |
| 5 | AachenBomb + film + AMR | 2-way | k-ε | AMR on n-heptane vapour | https://youtube.com/shorts/wcdrsiKs1Xw · https://youtube.com/shorts/mUVElgjEyfo |
5 Discussion
5.1 Key Observations
- Case 2 highlights splashing & satellite droplet formation even in laminar gas.
- Two-way coupling (Cases 3-5) lowers gas temperature due to droplet evaporation, altering spray penetration.
- AMR (Case 5) resolves vapour-fuel interface without prohibitive cell counts.
5.2 Strengths & Current Limitations
| Strength | Limitation / Future Work |
|---|---|
| OpenFOAM’s open source allows solver customisation (hybrid film model) | Film AMR and dynamic contact-angle models still absent |
| Eulerian–Lagrangian approach scales well to millions of parcels | Break-up/splash correlations empirical—may need calibration |
| Two-way coupling captures feedback on turbulence & temperature | Combustion chemistry not yet wired into film source terms |
6 Looking Forward
- Dynamic contact angle dependent on local shear & temperature.
- Film AMR to keep cell thickness proportional to film height.
- Hybrid VoF–Lagrangian schemes for ultra-dense sprays.
7 Supplementary Materials
Everything you need—cases, custom solver, and post-processing scripts—lives here:
GitHub – SimuXAI/Surface-Film-Sprays
https://github.com/simuxai/OpenFOAM-7/tree/main/SimuXAI/blog1
Stay Connected
Questions or feedback? Reach us at support@simuxai.com or join the discussion on our community forum.
Spray smart, film smooth! 🚀