Exhaust Port Challenge

Conjugate Heat Transfer of Exhaust Port

1. Give a brief description of why and where a CHT analysis is used.

Conjugate heat transfer corresponds with the combination of heat transfer in solids and heat transfer in fluids. In solids conduction dominates whereas in fluids convection dominates. It is the heat transfer between solid and fluid simultaneously. There is a thermal coupling at the interfaces of two regions.

Why-

• For understanding of the complex heat transfer mechanisms and the interactions between those mechanisms to improve the performance and increase the life of the components to be analysed.
• CHT analysis provides the temperature distribution in solids and fluids, and clear insight on velocity distribution & mechanism of heat transfer of fluid like in engines.
• CHT can be used in complex scenarios like in automotive components or in power plant components.
• Multiple number of regions can be simulated. All type of region combination can be simulated like solid-fluid, fluid-fluid etc.

Where is CHT analysis used:

• Analysis of heat transfer mechanism in turbocharger.
• HVAC system.
• Automotive, power plants and various other applications.
• Burner, electronic devices.

work flow of Simulation-

1. Geomtery: Spaceclaim, it is used for import geometry & other CAD related functions like geometry clean-up etc.
2. Mesh: used to generate Ansys mesh and other mesh control option there, also assign name which helps during BCs setup.
3. Setup: used to define physics of problem, like BCs, material & type of solving method, iterations etc.
4. Solution: where runtime results visualization done.
5. Results: CFD-post, where post processing of results will be done.

CAD Cleanup:

Import exhaust port geometry into spaceclaim, As we see below figure is the solid model. We have a solid model of an exhaust port inline 4-cylinder engine, then we are going to modeling both the fluid flow through it & also the heat transfer coefficient.

Prepare->volume extract->select edges->ok, use these for extract the fluid volume. Now we have 2 volumes, fluid volume & solid volume.

Enable share topology to share the entire interface information between solid & fluid. Also to avoid any gaps between these 2 solid and fluid volumes..

workbench->select FFF->share/shareprep

These pink edges show that, geometry as conformal.

Mesh Generation

Before Generate mesh create Name selection.

Inlet- 4 ports through which the hot gases will enter

outlet- 1 port through from which gases come out.

Outer-wall-convection- outer solid surface.

Baseline Mesh:

Now we see below mesh has not common interface between solid and fluid volume, Hence it is not correct. For this we need to enable share topology and shareprep option enabled, so we get conformal mesh.

Elements-138385

Fluent Setup:

Select k-epsilon standard turbulent model

Pressure based and steady state solver.

Material-

Fluid- air

solid- Aluminium

Boundary condition-

At Inlet-

velocity= 5 m/s

Temp= 700 K

At Outlet-

Pressure= 0 Pa (ambient pressure)

Outer-wall-convection-

heat transfer coefficient= 20 w/((m^2)*k)

Assigning different flow quantities:-

• Create temperature contour.
• Hybrid initialize and run the simulation for 150 iterations.

CFD Result:

• Create velocity & pressure contour in entire exhaust port & at throat.
• Check heat transfer coefficient at wall.

Plots:-

Baseline mesh:

Temperature streamline:

Velocity Streamline:

Due to mass conservation velocity at throat increases.

Temperature at Exit port throat:

  Max temp  is 693K

Velocity at the Exit port Throat:

Max velocity is 37.5 m/s

Heat transfer coefficient on inner solid wall:

The internal solid wall heat transfer coefficient is 106 w/m^2*k.

Refined Mesh:

Elements= 477059

Mesh size= 16 mm

Wrong Inflations

Correct Inflations

The inflation layers must be created on both sides of solid and fluid volume.

For this select below fluid volume with excepting inlets and outlet as shown below..

Add Inflation layer of total thickness of 3mm from calculating the Y+= 100 using Y+ online calculator.

Element size of solid component 16 mm.

Plots:

Refind mesh:

Temperature streamline:

Velocity streamline:

Due to mass conservation velocity at throat increases.

Temperature at Exit port throat:

 Max temp  is 687K

Velocity at the Exit Port Throat:

 max. velocity  is 41.88 m/s

Heat transfer coefficient on inner solid wall:

The internal solid wall heat transfer coefficient is 242 w/m^2*k.

Comparsion b/w baseline & refind mesh:

Conclusion:-

1. As the velocity increases, heat transfer rate also increases.
2. In refined mesh, velocity & temperature distribution is more uniform than baseline mesh.
3. Wall HTC of refined mesh is almost accurate but in baseline mesh wall HTC is very less i.e. highly inaccurate. But there no such difference between velocity and temperature value of both meshes.
4. As we refined mesh , computation time also increased.