## Solver: chtMultiRegionFoam Description

chtMultiRegionFoam is a solver designed for transient simulations involving buoyant, turbulent fluid flow and solid heat conduction. It is dedicated to handling conjugate heat transfer (CHT) between solid and fluid regions, making it suitable for a broad range of multiphysics problems where interaction between fluid and solid phases is a key aspect.

The solver uses the **PIMPLE** (merged **PISO-SIMPLE**) algorithm for pressure-momentum coupling. This algorithm leverages the strengths of both PISO and SIMPLE methods for pressure-velocity coupling, ensuring robustness in handling transient flows with large time steps. This approach is supplemented by under-relaxation techniques to secure convergence stability. It supports Multiple Reference Frame (MRF), porosity modeling and allows easy integration of passive scalar transport equations and source terms.

The solver’s transient nature, combined with its ability to simulate conjugate heat transfer (CHT) and handle secondary circuits, makes it highly versatile. It can be applicable in the HVAC (Heating, Ventilation, and Air Conditioning) industry for heating and cooling system calculations, and in the automotive industry for engine cooling studies. Solar loads or heat exchanges are typical applications for the solver.

## Solver: chtMultiRegionFoam Features

**Transient****Compressible****Multi-Region Mesh (Solid/Fluid)**

- Arbitrary Fluid and Solid Regions
- Conjugate Heat Transfer (CHT)
- Heat Source
- Radiation

- Laminar and Turbulent (RANS, LES, DES)
- Equation of State Models
- Pressure-Based Solver
- Passive Scalar
- Porosity Modeling
- Buoyancy
- Source Term (explicit/implicit)
- PIMPLE Algorithm
- Solution Limiters:
- Velocity Damping
- Pressure Limit
- Temperature Limit

## Solver: chtMultiRegionFoam Application

**Electronics**

- CPU and/or GPU Cooling (e.g. Electronics cooling)
- Mother Board Heating Component Placement
- Heat Sink Design
- Fan Sizing and Placement

**Lightning design**

- LED Fixtures Cooling
- LED Spacing, PCB Choice
- Active vs. Passive Cooling

**Energy**

- Heat Exchangers for Heavy Industry (e.g. Heat exchanger)

**HVAC**

- Heating and Cooling Systems
- Heat Exchangers
- Solar Load

**Automotive**

- Engine Cooling
- Windshield Condensation
- Windshield Defrost

## Solver: chtMultiRegionFoam Heat Transfer Solvers Comparison

Heat Transfer Solvers In this group, we have included solvers that are designed to model: **Heat Transfer**, **Radiation**, **Natural** and **Forced Convection**, **Conjugate Heat Transfer (CHT)**.

**Heat Transfer, Single Fluid**

- buoyantSimpleFoam steady-state, compressible, buoyancy-driven flow
- buoyantPimpleFoam transient, compressible, buoyancy-driven flow

**Heat Transfer, Single Fluid - Boussinesq**

- buoyantBoussinesqSimpleFoam steady-state, incompressible, buoyancy using Boussinesq approximation
- buoyantBoussinesqPimpleFoam transient, incompressible, buoyancy using Boussinesq approximation

**Heat Transfer, Single Solid**

- laplacianFoam steady-state and transient, thermal conduction in solid
- overLaplacianDyMFoam extension of laplacianFoam with Overset

**CHT, Multiple Fluids / Solids**

- chtMultiRegionSimpleFoam steady-state, compressible, arbitrary fluid and solid regions
- chtMultiRegionFoam transient, compressible, arbitrary fluid and solid regions

- CHT - Conjugate Heat Transfer
- MRF - Multiple Reference Frame
- Overset - also known as Chimera Grid (Method)

## Solver: chtMultiRegionFoam Tutorial

- Transient conjugate heat transfer (CHT) process of cooling a steel cylinder with a water stream, using a simplified 2D mesh.

## Solver: chtMultiRegionFoam Validation Cases

- External cooling of a water pipe with cool air analysis, considering both free and forced convection scenarios.

## Solver: chtMultiRegionFoam Results Fields

This solver provides the following results fields:

**Primary Results Fields**- quantities produced by the solver as default outputs**Derivative Results**- quantities that can be computed based on primary results and supplementary models. They are not initially produced by the solver as default outputs.

**Primary Results Fields**

Velocity | \(U\) [\(\frac{m}{s}\)] |

Temperature | \(T\) [\(K\)] |

Pressure | \(p\) [\(Pa\)] |

Hydrostatic Perturbation Pressure | \(p - \rho gh\) [\(Pa\)] |

**Hydrostatic Perturbation Pressure** This value represents the pressure without the hydrostatic component (minus gravitational potential). Read More: Hydrostatic Pressure Effects

**Derivative Results**

Density | \(\rho\) [\(\frac{kg}{m^{3}}\)] |

Vorticity | \(\omega\) [\(\frac{1}{s}\)] |

Mach Number | \(Ma\) [\(-\)] |

Courant Number | \(Co\) [\(-\)] |

Peclet Number | \(Pe\) [\(-\)] |

Stream Function | \(\psi\) [\(\frac{m^2}{s}\)] |

Q Criterion | \(Q\) [\(-\)] |

Wall Functions (for RANS/LES turbulence) | \(y^+\) [\(-\)] |

Wall Shear Stress | \(WSS\) [\(Pa\)] |

Wall Heat Flux | \(\phi_q\) [\(W/m^2\)] |

Turbulent Fields (for RANS/LES turbulence) | \(k\) \(\epsilon\) \(\omega\) \(R\) \(L\) \(I\) \(\nu_t\) \(\alpha_t\) |

Volumetric Stream | \(\phi\) [\(\frac{m^{3}}{s}\)] |

Passive Scalar | \(scalar_i\) [\(-\)] |

Forces and Torque acting on the Boundary | \(F\) [\(N\)] \(M\) [\(-\)] |

Force Coefficients | \(C_l\) [\(-\)] \(C_d\) [\(-\)] \(C_m\) [\(-\)] |

Average, Minimum or Maximum in Volume from any Result Field | \(Avg\) \(Min\) \(Max\) |