Modeling heat transfer in solids and fluids in COMSOL Multiphysics involves using the Heat Transfer in Solids and Fluids interface, which supports conduction, convection, and radiation. This interface is ideal for conjugate heat transfer problems, where heat exchange occurs between solid and fluid domains. Below is a concise guide to get you started.

Key Steps for Setting Up a Model

1. Select the Physics Interface:
   • Use the Heat Transfer in Solids and Fluids interface, typically included with a Conjugate Heat Transfer multiphysics interface (e.g., with laminar or turbulent flow).
   • The interface defaults to a Solid model for all domains, with a Fluid model available but inactive. Activate the Fluid model for fluid domains to include convection.
2. Define Geometry:
   • Create your geometry, separating solid (e.g., a heat sink) and fluid (e.g., air or water) domains. For example, a heat sink cooling an electronic chip or a pipe with flowing fluid.
   • Use predefined geometries like heat sinks to save time.
3. Assign Material Properties:
   • Define thermal conductivity, density, and heat capacity for solids and fluids. For fluids, include viscosity and temperature-dependent properties if needed.
   • For phase change (e.g., melting), specify latent heat and phase transition properties.
4. Set Boundary Conditions:
   • Solids: Apply heat sources, thermal insulation, or fixed temperatures. Use “Open Boundary” or “Infinite Elements” for continuous material beyond the model.
   • Fluids: Specify inlet/outlet conditions (e.g., temperature, velocity, or pressure). For convection, couple with a fluid flow interface (laminar or turbulent) or define a velocity field manually.
   • Include radiation if needed, using surface-to-surface or participating media models.
5. Mesh the Model:
   • Use a finer mesh at solid-fluid interfaces to capture boundary layer effects. Enable physics-controlled meshing or add boundary layers for fluids.
   • For complex geometries, automated mesh refinement can optimize accuracy.
6. Solve and Postprocess:
   • Solve using a stationary or time-dependent study, depending on your problem (e.g., transient cooling of a thermos).
   • Visualize results like temperature profiles, heat flux, or velocity fields. Use postprocessing tools to compute quantities like cooling power.

Tips

• Start Simple: Begin with a model like a heat sink or thermos cooling example.
• Use One Physics Interface: Avoid separate Heat Transfer in Solids and Fluids interfaces. Define solid and fluid domains within a single Heat Transfer or Conjugate Heat Transfer interface for proper boundary coupling.
• Check Peclet Number: For fluid domains, calculate the Peclet number to assess if convection dominates conduction. A low Peclet number indicates negligible convection.
• Couple with Fluid Flow: For accurate convection, couple the heat transfer interface with a laminar or turbulent flow interface to compute the velocity field.

Common Pitfalls and Fixes

• Unexpected Results: Ensure boundary conditions are correct (e.g., inlet temperature, heat flux). Check for uniform fluid properties or mesh issues.
• No Heat Transfer at Boundaries: Verify solid-fluid interfaces are properly coupled. Use a single physics interface and check boundary layer meshing.
• Non-Smooth Temperature Profiles: Refine the mesh or check material transitions (solid to fluid) for sharp conductivity changes.

For hands-on learning, start with a heat sink model, which covers both solid and fluid domains and guides you through conjugate heat transfer. Extend it to visualize airflow velocity and temperature.