Dissolving Objects Model
When a solid dissolves in a liquid, the flow domain changes shape, and the dissolved material redistributes through diffusion and convection. FLOW-3D and FLOW-3D HYDRO include a fully coupled Dissolving Objects Model that captures the dissolution process, the changing geometry of the solid, and the transport of the solute in the fluid.
How It Works
The dissolution is driven by the difference between the local concentration and the saturation concentration of the solute: dissolution proceeds faster where the fluid is less saturated and slows as the surrounding fluid approaches saturation.
The solute is transported through the fluid by convection and diffusion. At the boundary between the solid and the fluid, the dissolution rate is controlled by the rate of diffusion of the solute away from the interface. If diffusion is relatively slow, for example, fluid in the boundary layer becomes saturated stopping further dissolution.
As the solid dissolves, it decreases in size, and its shape changes as well. The area and volume fractions are recomputed at every time step to reflect the gradual loss of the solid material, allowing the flow domain to expand. The mixture densityvaries as a function of the solute concentration, affecting buoyancy.
The model supports both interstitial solutes, where smaller solute molecules fit into the spaces between the molecules of the solvent with little change in the mixture volume, and substitutional solutes, where larger solute molecules displace the solvent ones increasing the mixture volume. A configurable density mixing coefficient covers the range between these two cases, allowing the model to be calibrated to the physical behavior of specific solute-solvent pairs.
Example: Salt Block Dissolution
The images below show a case in which solid medicine dissolves in flowing water. The computational domain contains a bed of cylindrical pins arranged in a regular array sandwiched between two flat plates with the dissolvable tablet placed in the middle. Fluid enters from one end of the domain at a constant rate and leaves at the opposite side. The flow is confined. As dissolution proceeds, the tablet reduces in size and the dissolved substance is carried downstream by the flow.
Applications
The Dissolving Objects Model applies across a wide range of industries and problem types:
- Solution mining for simulating the dissolution of salt or mineral deposits by injected fluid to create underground cavities for resource extraction or energy storage
- Underground gas storage for modeling the creation of salt caverns used for natural gas and compressed air energy storage
- Drug delivery and controlled release for predicting dissolution rates of pharmaceutical coatings and solid drug forms in biological fluids
- Environmental and water quality for simulating dissolution of soluble materials in natural water systems, including contaminant release scenarios
