CFD Analysis of a Double Slope Solar Still in ANSYS Fluent (Free)
CFD analysis of a double slope solar still lets you predict freshwater yield before building any hardware. In this free tutorial you will simulate a double slope solar still in ANSYS Fluent using the VOF multiphase model and the solar load model, across four short video parts covering geometry, meshing, physics setup and post-processing.
2. Part 1 — Geometry and Domain Setup
3. Part 2 — Meshing and Mesh Quality
4. Part 3 — Physics, VOF and Solar Load Model
5. Part 4 — Solution and Post-Processing
6. Validation and Further Work
7. Frequently Asked Questions
Introduction and Governing Physics
A double slope solar still is a shallow basin of saline water enclosed by two inclined transparent glass covers in a symmetric gable arrangement. Solar radiation passes through the glass and is absorbed at the blackened basin, heating the water. Vapour rises from the free surface, condenses on the cooler inner glass, and the condensate runs down both slopes into collection channels as clean distillate.
Reproducing this numerically requires the coupled solution of continuity, momentum, energy and phase change, with buoyancy driven natural convection inside the cavity. The finite volume solver in ANSYS Fluent solves mass, momentum and energy conservation while the VOF model tracks the water and vapour phases and the solar load model supplies the radiative heat input.
Continuity: d(rho)/dt + div(rho u) = 0
Momentum: d(rho u)/dt + div(rho u u) = -grad(p) + div(tau) + rho g + F
Energy: d(rho E)/dt + div(u(rho E + p)) = div(k_eff grad T) + S_h
Part 1 — Geometry and Domain Setup
The first video builds the computational domain: the basin water region, the humid-air cavity and the two glass covers. Defining clear named selections here (basin, glass_east, glass_west, water_surface, walls) makes boundary condition assignment straightforward later.
Part 2 — Meshing and Mesh Quality
Resolving the thermal and velocity boundary layers along the basin floor and glass covers is essential for reliable evaporation rates. Part 2 shows the meshing strategy with inflation layers at the water-air interface and walls, and the mesh metrics (orthogonal quality, skewness, aspect ratio) to verify before solving. A mesh independence check confirms the yield is grid converged.
Part 3 — Physics, VOF and Solar Load Model
This is the analytical core. Part 3 activates the VOF multiphase model with liquid water and water vapour, enables the energy equation and a turbulence model (k-omega SST with buoyancy for this buoyant flow), sets up the Lee phase change model, and configures the solar load model with site latitude and longitude, date, time and sun direction. Glass transmissivity and basin absorptivity are prescribed.
| Setting | Typical choice |
|---|---|
| Multiphase model | VOF (explicit or implicit) |
| Phase change | Lee model (evaporation and condensation) |
| Radiation and solar | Solar Load with DO radiation model |
| Turbulence | k-omega SST (buoyancy terms on) |
| Pressure velocity coupling | PISO or Coupled (transient) |
Part 4 — Solution and Post-Processing
The final video runs the transient solution, monitors convergence through scaled residuals and physically meaningful surface reports (basin temperature, glass temperature and cumulative condensation mass flux), and post-processes the results. Contours of temperature, phase fraction and velocity reveal the buoyant convection cells, while integrating the condensation flux over the glass surfaces gives the cumulative distillate output.
Validation and Further Work
For credible results, compare the numerical distillate yield against experimental data or an established correlation such as Dunkle's model under matched climatic conditions, and report the discrepancy. Useful extensions include parametric studies of water depth, glass cover inclination and basin absorptivity, and adding phase change materials or nanofluids to boost thermal storage and productivity. If you are new to multiphase CFD, first work through the grid independence test tutorial and the first OpenFOAM tutorial to build a solid foundation.
Frequently Asked Questions
Which multiphase model is best for a solar still in ANSYS Fluent?
The Volume of Fluid (VOF) model is the standard choice for a solar still because it tracks the sharp liquid water and water vapour interface at the basin free surface. Evaporation and condensation are added through the Lee phase change model with calibrated mass transfer coefficients.
How is solar radiation applied in the solar still CFD model?
Solar radiation is applied using the Solar Load model with solar ray tracing, combined with the DO radiation model. You specify the site latitude and longitude, date, time and sun direction, and set the glass cover transmissivity and basin absorptivity so the absorbed heat drives evaporation.
How do you calculate the distillate yield from the CFD results?
The distillate yield is obtained by integrating the condensation mass flux over the inner glass cover surfaces over the simulated time. This cumulative condensed mass is the freshwater output and is the main performance metric of the solar still.
Should the solar still simulation be steady or transient?
A solar still is inherently time dependent because solar input and water temperature change through the day, so a transient simulation is used. A pressure based transient solver with PISO or Coupled pressure velocity coupling is typical.
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