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# Validation Case: Scalar Transport in T-Junction Pipe

The scalar transport in a T-junction pipe validation case belongs to fluid dynamics. This test case aims to validate the following:

• Scalar mixing distribution

In this project, a T-junction pipe is used to simulate passive scalar mixing. The SimScale results are compared to the experimental results reported in  and .

## Geometry

The geometry consists of a pipe with 3 sections: a main pipe, a branch pipe, and a mixing pipe. Figure 1 highlights the pipe dimensions:

Due to the symmetrical nature of the geometry, only half of the pipe is captured. The pipes have a diameter $$D$$ of 51 $$mm$$, which is the same as used in the experimental setup. Details of the pipe dimensions are provided in Table 1:

## Analysis Type and Mesh

Tool Type: OpenFoam®

Analysis Type: Incompressible

Turbulence Model: k-omega SST

Mesh and Element Types: This validation case uses a total of 3 meshes, to perform a mesh independence study. All meshes were created in SimScale with the standard mesher algorithm. In Table 2, an outline of the meshes is presented:

Figure 2 shows the discretization of the mixing pipe obtained with the fine mesh. A total of 10 inflation layers were used to resolve the boundary layer, aiming to achieve a y+ value smaller than 1. Figure 2: Fine standard mesh created in SimScale, showing the end of the mixing pipe (outlet). Ten inflation layers are added to resolve the boundary layer.

## Simulation Setup

Material:

• Water
• Viscosity model: Newtonian
• $$(\nu)$$ Kinematic viscosity: 9.3379e-7 $$m^2/s$$
• $$(\rho)$$ Density: 999 $$kg/m^3$$

Boundary Conditions:

Figure 3 will be used as a reference for the definition of the boundary conditions:

The following boundary conditions are used:

Model:

• $$(Sc_{t})$$ Turb. Schmidt number = 0.1
• Diffusion coefficients = 2.3e-9 $$m^2/s$$

Note

The turbulent Schmidt number is the ratio of momentum diffusivity to mass diffusivity in a turbulent flow$$^3$$.

Following the turbulent Schmidt number sensitivity tests performed by Frank et. al$$^2$$, we will use a value of 0.1 in the simulations.

## Result Comparison

The numerical simulation results for the mixing scalar are compared with experimental data provided by the Laboratory for Nuclear Energy Systems, Institute for Energy Technology (ETHZ), Zürich$$^1$$, and also mentioned by Frank et. al$$^2$$.

A comparison of the mixing scalar distribution obtained with SimScale and experimental results is presented. The scalar distribution is assessed over four lines, placed 51, 91, 191, and 311 mm downstream of the T-junction: Figure 4: The results are assessed over the four red lines, positioned downstream of the T-junction.

Below, a series of figures show the comparison of results from SimScale to the experimental data for the scalar distribution.