Simulating Conjugate Heat Transfer (CHT) analysis in an exhaust port using Ansys Fluent.

Aim: To simulate a conjugate heat transfer analysis in an exhaust port.

Objectives:

1. To set the mesh for exhaust port geometry with the appropriate y+ considerations and it is a conforming mesh between the solid and fluid components.
2. To set up the test case in Ansys Fluent with appropriate boundary condition, run the solution and obtain the contour plots for the temperature distribution.
3. To get the values for the heat transfer coefficient for the exhaust port profile.

Conjugate Heat Transfer:

The conjugate heat transfer refers to the phenomenon where the heat is transfer occurs with the interaction between a solid body and a fluid body that flows over or inside the solid body. The heat transfer within the solid body is through conduction and the heat transfer within the fluid body is of convective type. The physical processes and the governing equations for solving the heat transfer with the two sub-domains are considered separately to each of the sub-domains.

The phenomenon is usually referred to as “Conjugate Convective Heat Transfer” because the heat transfer between the solid and the fluid bodies are convective in nature. Hence, the heat transfer is said to be coupled in nature.

The conjugate heat transfer analysis has a wide range of applications, some which are in the aerospace and nuclear reactor studies. It also finds applications in thermal treatment of materials and different processing techniques used in goods processing. The interactions between the tissues and the internal fluids of the body can also be studied using the conjugate heat transfer phenomenon. Applications can be found in meteorology where the interactions between the atmosphere and the ocean can be studied. A very popular area of study/analysis is in the semi-conductor and electronics department, where the interactions between the solid electronic component and the air flowing over it is studied. The cooling mechanisms in the electronic components are designed and optimized based on the conjugate convective heat transfer within the components. A wider range of applications in the numerical/computational methods can be seen, more so than the experimental studies since the development and advancements in the field of Computational Fluid Dynamics.

Geometry setup:

The geometry under consideration is a typical generic design of an exhaust port of an internal combustion engine. The raw geometry is shown in Fig 1:

Fig 1: Isometric view of the initial 3D geometry.

Before we create a mesh and obtain a computational domain, a certain clean up and activities are to be done to the geometry. These include, deleting the extra edges, creating a fluid domain inside the exhaust port, and enabling the share topology.

Once the extra edges are removed, the fluid domain is created using the volume extract option in SpaceClaim. Fig 2 shows the fluid domain that has been created.

Fig 2: Fluid Domain extracted from the interiors of the exhaust port.

The inner wall of the exhaust por and the outer most fluid layer will have to share topology, it is the only way to achieve a conformal mesh. The share topology option is used in the Workbench section of SpaceClaim, Fig 3 highlights the faces and edges that have shared their facets after the share topology option is used.  

Fig 3: Faces and edges that share topology in SpaceClaim.

Once the Geometry processing is done, the next step is to import it into the meshing module. The following are the named selections that were created:

1. inlets : The 4 ports whose axis is parallel to the y coordinate.
2. Outlet : The port whose axis is parallel to the x coordinate.
3. Inflation_layer : The outer most layer of the fluid domain, which is used as reference for inflation layers in mesh generation.
4. Outer_wall_convection : The outer most layer of the solid domain, upon which the convection boundary condition is applied.

Inflation layers were added to the outermost fluid layer and the inner walls of the solid domain by considering the y+ value to be 100. The following article was referred to decide the y+ value to be considered to get the appropriate first layer thickness for the inflation layers:

https://skill-lync.com/knowledgebase/boundary-layer-y-plus-wall-functions-in-turbulent-flows-2

For the geometry in our consideration, the appropriate values for free stream velocity, fluid density, dynamic viscosity, and the y+ value of 100 were given as inputs to a y+ calculator found on the internet. The following link leads to the online y+ calculator:

https://www.quadco.engineering/en/know-how/cfd-calculate-wall-distance.htm

Fig 4 shows the screen shot of the values obtained from the y+ calculator.

Fig 4: Values obtained from the y+ calculator.

The following sizing options were used for the solid and fluid domains:

• Body sizing for the solid component – element size = 0.018m
• Body sizing for the fluid component – element size = 0.015m

The overall mesh had: 170500 Nodes and 503868 elements.

Fig 5: shows the generated mesh and Fig 6: shows the element quality

Fig 5: Mesh generated for the given final component.

Fig 6: The mesh metrics of the generated mesh.

Solution setup:

1. A pressure based solver is chosen to run the simulation. The solution scheme is chosen to be coupled.
2. The default materials, i.e., air for the fluid domain and aluminum for the solid domain are chosen with their default material properties in Ansys Fluent.
3. The following are the boundary conditions that were applied:
   1. At inlets – The velocity is set at 5 m/sec and the temperature is at 700K.
   2. At outlet – Pressure outlet with 0 gauge pressure.
   3. At outer wall convection – set to Convection with heat transfer coefficient as 20W/(m²K).
   4. All the other named selections are set to be walls.
4. The turbulence model was chosen to be k-epsilon.

The following article was referred to, before deciding on which turbulence model was to be used:

https://skill-lync.com/knowledgebase/choosing-a-right-turbulence-model

The solution was initialized using Hybrid initialization and it converged at 285 iterations. The following are the plots and results:

The above plot shows the variations in scaled over the iterations.

The above image shows the temperature contour after the solution convergence.

The above image shows the pressure distribution at the midplane near the outlet port.

The above image shows the temperature distribution at the mid plane near the outlet port.

The above image shows the temperature streamlines within the fluid domain.

The above image shows the velocity streamlines within the fluid domain.

The above image shows the distribution of the surface heat transfer coefficient at the midplane near the outlet port.

The above image shows the distribution of the surface Nusselt number at the mid plane near the outlet port.

The above image shows the distribution of the wall heat transfer coefficient at the mid plane near the outlet port.

Note on convective heat transfer coefficient:

The following equation describes the convective heat transfer:

Q = hA(T₂ – T₁) where,

Q --> The overall heat transfer rate (W).

h --> Convective heat transfer coefficient (W/m²K).

A --> The surface area where the heat transfer takes place (m²).

T₂ --> Temperature of the surrounding fluid (K).

T₁ --> Temperature of the solid surface (K).

We can deduce the convective heat transfer coefficient to be:

h = q/(T₂ – T₁)

where q --> heat flux (W/m²) = dQ/dA

In our particular case study, the heat transfer takes place by the means of convection within the fluid domain. And the heat transfer within the solid component is by conduction. Hence, we can say that the overall heat transfer depends on the Nusselt number.

Nusselt number:

Nusselt number is a measure of the ratio between the heat transfer conduction and the heat transfer by convection. The equation that describes the Nusselt number is as follows:

Nu = hL/k where,

h --> convective heat transfer coefficient,

L --> Characteristic length,

k --> conductive heat transfer coefficient.

Therefore, if we are able to accurately estimate the Nusselt number in the given case, it can be used to find the accuracy of the estimated heat transfer coefficients.

This concludes the project for Conjugate Heat Transfer analysis in an exhaust port using Ansys Fluent.