Exhaust Manifold CHT Analysis

Problem Statements: -

1. Give a brief description of why and where a CHT analysis is used.
2. Perform a CHT analysis on a model of an exhaust manifold. You need to compare the solutions for 2 sizes of meshes. The first one should be the baseline mesh and the second one should have a finer mesh on the solid zone and also with inflation layers. Show the velocity, temperature and heat transfer coefficient plots and explain if refining the mesh produced any changes to the results. -
3. How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

Specification of Geometry: -

Objectives: -

To study conjugate heat transfer phenomenon

To perform steady state CHT analysis on exhaust manifold.

To study effect of body fitted mesh/finer mesh on manifold body.

To study factors affecting the accuracy of HTC in simulations.

Theoretical background: -

To simulate heat transfer between solid and fluid domains by exchanging thermal energy at the interface between them we generally use conjugate heat transfer analysis method.it is basically a process where temperature distribution can be analyzed due to interation between fluid and solid body.

In most of the cases Heat transfer in solid happens due to conduction whereas in fluid it happens due to convection or sometimes radiation. Heat transfer in fluids and solids can also be combined to minimize heat losses in various devices. Heat transfer in solids and heat transfer in fluids are combined in the majority of applications. Such as heat sink, heat exchanger, cooling in electrical circuits, turbocharger, car radiator etc.

Geometry: -

3D model of Exhaust Manifold is designed using solid works application, same model can be imported to Ansys space claim to create fluid domain inside solid domain. Using Edge Extract command, we can create solid domain by simply clicking the edges of inlet and outlet.in order to create conformal mesh we have to create topology to share and also imprint overlapping geometry. This helps to create interaction between solid and fluid domain.

CASE 1

Meshing: -

We have used default meshing technique for baseline meshing with min element size to be 150mm.

No of Elements: -138450

No OF Nodes: -27593

Boundary Conditions: -

As specified in geometry specification hot air inlet from four face with velocity of 5 m/s and temp of hot air is 700K.wall convection is applied with 20 W/m²K on surrounding faces. Simulation is run for 300 interation.

Residual Plot: -

Temperature Plot: -

Velocity Streamline: -

Wall Heat Transfer coefficient: -

Wall Heat Transfer coefficient @ Cut plane: -

CASE 2

Meshing: -

We have used element size of 16 mm with inflation layer provided for body fitted mesh with total thickness 5 mm.

No of Elements: -467426

No OF Nodes: -164078

Boundary Conditions: -

As specified in geometry specification hot air inlet from four face with velocity of 5 m/s and temp of hot air is 700K.wall convection is applied with 20 W/m²K on surrounding faces. Simulation is run for 300 interation.

Residual Plot: -

Temperature Plot: -

Velocity Streamline: -

Wall Heat Transfer coefficient: -

Wall Heat Transfer coefficient @ Cut plane: -

Results and Discussion: -

• From result we can predict that as we reduce the element size, we will be getting more accurate results as well as solution time will be increased. Reducing mesh size means adding inflation will help us to capture accurate data near the fluid solid interation.
• To verify whether the calculated value of Heat transfer coefficient is right or wrong we can go for Nusselt no calculation.

we must first calculate from fluent the heat flux Q (we can obtain the local one, and the average one too). After this, use the following formula: Nu = (QxL)/(kxdelta T)

Q: heat flux

L: characteristic length

k: Thermal conductivity

delta T: temperature difference

• Also, we can calculate Nusselt no using Reynold no and Prandtl’s no comparing the value of Nusselt no from calculation and also by simulation we can predict that results are valid.

• Accuracy of result depends upon how well the domain is meshed, proper application of boundary conditions, creating proper geometry of fluid & Solid Domain.