> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ntop.com/llms.txt
> Use this file to discover all available pages before exploring further.

# How to run a Conjugate Heat Transfer (CHT) analysis

## **Objective:**

Learn how to set up and run a Conjugate Heat Transfer (CHT) analysis in nTop. This feature extends the standard Flow Analysis by coupling fluid flow with solid heat conduction. In this guide, we will use a Heat Sink example to demonstrate how to simulate cooling performance by analyzing the interaction between an airflow channel and a solid aluminum heat sink.

## **Applies to:**

* nTop 5.38 or later (requires nTop Fluids capabilities).

## **Procedure:**

**1. Prepare Your Geometry:** Before starting, ensure you have implicit bodies defined for both your fluid and solid regions.

* **Solid Geometry (Heat Sink):** The physical parts that conduct heat, such as the heat sink base and fins.
* **Fluid Geometry (Air Channel):** The negative space or volume surrounding the heat sink where the air flows.

![This images shows an example of Heat Sink for a Solid Geometry and the Domain around it as Fluid Geometry](https://files.learn.ntop.com/help-articles/fluids/50598721924115.png)**2. Set Up the Virtual Model:** We need to add a **Simulation Model** block to act as the container for your simulation setup. You will use its "Virtual Model" overload, which is designed for LBM-based simulations that do not require traditional body-fitted meshing.

* **Add the Fluid Domain (Air):**

  * Add a **Fluid Domain** block to your Virtual Model list.
  * Input your **Fluid Domain** implicit body as the Body input (the air channel).
  * Define the **Fluid Attribute** using the **Air** block (or an **Isotropic Material:** **Isotropic Fluid Property** (Kinematic Viscosity) and **Isotropic Thermal Property** (Conductivity and Specific Heat).
* **Add the Solid Domain (Heat Sink):**

  * Add a **Solid Domain** block to the same list.
  * Input your **Solid Geometry** (the heat sink body).
  * Define the **Solid Attribute** using an **Isotropic Material** (e.g., Aluminum) that includes **Isotropic Thermal Property** values.

![Simulation Model setup for having two Domains Solid and Fluid](https://files.learn.ntop.com/help-articles/fluids/50598721942803.png)*Note: Both materials must have thermal properties defined to enable the CHT solver.*

**3. Define Boundary Conditions:** Input a list of boundary conditions into the **Flow Analysis** block. For CHT, you need to define both flow and thermal conditions on the relevant CAD faces or boundaries.

<Note>
  *Note: You need at least one Temperature Restraint to enable CHT.*
</Note>

* **Flow Conditions:**

  * **Inlet:** Add a **Velocity** boundary (e.g., 1 m/s) to the face where air enters the channel.
  * **Outlet:** Add a **Pressure** boundary (e.g., 0 Pa) to the face where air exits.
* **Thermal Conditions:**

  * **Inlet Temperature:** Add a **Temperature** boundary to the inlet face (e.g., 298 K or 25°C) to define the incoming air temperature.
  * **Heat Source:** Add a **Surface Heat Flux** boundary to the bottom face of the heat sink base. This simulates the heat load from a chip or electronic component (e.g., 2000 W/m²).

![Boundary Condition Setup that has Velocity Inlet, Pressure Outlet, Temperature Inlet and Surface Heat Flux for Heat Source](https://files.learn.ntop.com/help-articles/fluids/50598706699283.png)*Note: nTop Fluids automatically handles the heat transfer interface between the fluid and solid domains defined in your Virtual Model. Any wall or boundary not explicitly assigned a thermal boundary condition is treated as having a zero-temperature gradient. This means the solver assumes these surfaces are perfectly insulated, and no heat is conducted across them.*

**4. Configure the Flow Analysis Block**

* **Model:** Input the **Simulation Model** containing both the Air and Heat Sink domains.
* **Boundary Conditions:** Input the list of flow and thermal boundaries created in Step 3.
* **Cell Size:** Define the resolution of the Cartesian grid (e.g., 5mm).

![Flow Analysis block with Boundary Conditions that include both Flow and Thermal BCs](https://files.learn.ntop.com/help-articles/fluids/50598706743315.png)*Tip: Use at least 3 cells to resolve the smallest flow features (channels/gaps) and at least 2 cells across solid structures for accurate quantitative results.*

**5. Run the Simulation:** Run the **Flow Analysis** block.

The solver will automatically couple the fluid and solid equations, calculating how heat dissipates from the source, conducts through the fins.

![Streamlines from Flow Analysis block in the Display Mode to show Temperature Values](https://files.learn.ntop.com/help-articles/fluids/50598722036627.png)**6. Visualize Results:** Once the analysis is complete, you can inspect the thermal performance.

* Temperature Fields: Switch the HUD variable to Temperature time-averaged to view the heat distribution on the solid fins and in the surrounding air.
* Display Mode to Streamlines: Visualize the airflow path and color the streamlines by Temperature to see where the air picks up heat.
* Surface Plot: Use the temperature time-averaged field to plot colour onto the implicit body to visualise the values and view them in Section Cut (X) mode.

![This image shows an example cut of Surface Plot block with temperature field values](https://files.learn.ntop.com/help-articles/fluids/50598706834067.png)And that's it! You've successfully simulated the cooling performance of a heat sink using CHT.

**Are you still having issues?** Contact the support team, and we'll be happy to help!

## **Download the Example file:**

[Conjugate Heat Transfer Example](https://files.learn.ntop.com/Support%20Article%20Example%20Files/Knowledge%20Base/Fluids/cht.ntop)

## **More on this topic:**

* [How to run a flow analysis](/help-articles/knowledge-base/fluids/how-to-run-a-flow-analysis)
* [How to use a CAD Face in a boundary condition](/help-articles/knowledge-base/structures/how-to-use-a-cad-face-in-a-boundary-condition)

## Keywords:

*Simulation thermal management nTop Fluids Conjugate Heat Transfer Flow Analysis CHT Electronic Cooling*