Heat Exchanger - CFD Simulation SimFlow Tutorial

In this tutorial, we will be simulating the transfer of heat between a hot and cold region in a heat exchanger. We’ll be analyzing two separate flows through the exchanger, each with different parameters. The exchanger wall will be modeled as a single boundary, and we’ll be using thermal resistance to replicate heat conduction through the solid. Heat will be transferred from the hot region to the cold one without fluid mixing. To visualize the results, we’ll be using ParaView.

SimFlow is a general purpose CFD Software

To follow this tutorial, you will need SimFlow free version, you may download it via the following link:
Download SimFlow

Postprocessing for multi-region simulations require a newer version of the ParaView. For purpose of this tutorial we used ParaView 5.9.0. You can download the newest version of ParaView Download ParaView 5.9.0 .
To upgrade ParaView and use it with SimFlow follow these instructions .

Open SimFlow and create a new case named heat_exchanger

Firstly we need to Download GeometriesBafflesExchangerPipes

The STL format does not contain the unit information which are defined during the geometry export. If we do not know the exported unit, we can estimate it based on the total size of imported geometries. The size is displayed next to Geometry size label. In our case, the default unit meter is correct.

After importing geometry, it will appear in the 3D window. We will make the exchanger geometry transparent to see the interior of the heat exchanger.

To reveal edges, we will use Extract Features operation. These edges will indicate additional mesh refinement regions.

Repeat extract operation for the pipes

We need to distinguish surfaces that will be used as the mesh boundaries (both external and internal). We will do it in the 3D graphical panel.

We will repeat the previous operations for the remaining inlets and outlets. The final face groups list for exchanger should look like below:

Since we are going to perform CHT (Conjugate Heat Transfer) simulation, we need to create the mesh per each fluid and solid sub-domain.

In the typical heat exchanger, we would have three regions: hot fluid region, cold fluid region, and solid region representing pipes and baffles.

In this tutorial, we will use the alternative approach, in which we will skip the solid region and represent it by infinitely thin walls. The influence of walls on heat transfer between fluids will be modeled using Thermal Resistance . Therefore, we will need to mesh only two fluid regions: hot and cold fluid regions.

Hot fluid region

We will start meshing from the hot fluid region.

We need to define the base mesh. The box geometry determines the background mesh domain. It encloses both fluid regions – hot and cold.

In order to create the mesh in the hot fluid region, we will place the material point inside the hot fluid sub-domain. The resulting mesh will remain only in this region.

You can specify the point location from the 3D view. Hold the Ctrl key and drag the arrows to the destination.

Now, it’s time to create the mesh of the hot fluid region.

When the meshing process is finished, the hot fluid region mesh appears on the screen.

For the CHT simulations, we need to mark each of the mesh regions as sub-domains. The sub-domains represent a partial mesh that will not be overwritten by meshing operations that use the default region as a target.

Cold fluid region

Now we can mesh the second region – the cold fluid region. We will use already defined geometry parameters for exchanger and pipes. We just need to add baffles to be included in the mesh.

We need to move the material point to be positioned inside the cold fluid region.

Now, it’s time to create the mesh of the cold fluid region. Please note, that the resulting mesh will be automatically assigned to the default region and will exist next to the previously created hot fluid mesh.

The complete mesh should look like in the picture below.

Move the default region into the cold fluid region. The CHT simulation can only use the sub-region meshes.

In the previous steps we have created the mesh for hot and cold fluid regions. At the moment they are treated separately, so the information on the flow cannot be exchanged between them. It’s time to create the interface (coupling) between both fluid regions. Later we will define boundary conditions which will describe the way we want to exchange information between hot and cold fluid region.

For the calculations, we will use a steady-state conjugate heat transfer solver. The family of CHT solvers will activate only if sub-region meshes exist.

We will disable radiation for our simulation.

Now we need to define fluid properties. We will assume that working fluid for hot and cold regions is water.

We will turn off the gravity acceleration and set the reference pressure.

We will leave the default conditions for the baffles and heat exchanger external surfaces – adiabatic (isolated) walls. Now, we will define the inlets and outlets parameters for hot and cold regions.

Now we need to define how hot and cold fluid exchange information between each other. At the beginning of this tutorial, we mentioned we will not model solid walls between two fluid regions. Instead, we will use thermal resistance to replicate heat conduction through the solid.

In the Boundaries list pipes , pipes_inlet_baffle , pipes_outlet_baffle occur twice, as boundaries with the same names exists in both hot and cold regions. Thermal resistance parameters will be displayed only on one from the pair.

image:[image]

We will apply the same settings for other interfaces under hot region: pipe , pipes_inlet_baffle and pipes_outlet_baffle boundaries.

Before we start simulation we need to define the initial conditions. We will adjust the initial velocity and temperature to the inlet conditions of each region.

Finally, we can start our computation.

After computations are finished we can do a complex visualization of our results with ParaView.

If you do not plug in the new ParaView to SimFlow, you can just run the ParaView and open the case file:
` …​/heat_exchanger/heat_exchanger/heat_exchanger.foam `

Load the results of the simulation from SimFlow

We will create the cross-section through the computational domain to display the temperature distribution.