Conjugate Heat Transfer Analysis on Exhaust Manifold

Aim :

To perform Conjugate Heat Transfer analysis on an exhaust port, and to find out the heat transfer coeffecient on the internal solid surfaces.

Conjugate Heat Transfer Analysis :

Conjugate Heat Transfer, also known as Convective Heat Transfer is used in cases where, there is heat transfer taking place between a solid medium and a liquid medium.In our case, the exhaust pipe is the solid and the exhaust gases, flowing through it, is the liquid medium.When interactions of this kind takes place, CHT analysis is performed.

In case of exhaust pipe, the hot exhaust gases comes in contact with the inner walls of the exhaust manifold casing. During this intercation, heat is convectively transferred from the gases,to the exhaust pipe.

Pre-processing :

-Space Claim:

• At first, the exhaust manifold geometry is repaired by removing any extra edges presents using the extra edges command.
• Once the extra edges are removed, the fluid volume is created, by sekecting the edges of the exhast manifold.
• The solid volume(Exhaust manifold) and the fluid volume are then placed under a single body. The topology is hared inbetween to enable data flow between the two volumes.
• The final geometry is then imported to Meshing module.

-Meshing :

• In the meshing interface, the various sections are named , by using the named selection option or by using the hot key 'N'. The inlet, outlet, Outer wall, Inner wall are named in this way.
• The sizing method is chosed and the entire exhaust port body is selected, including the solid domain and fluid domain and a body sizing of 23mm was given.
• The inflation layer is given in two faces, the first being the Fluid domain surface and the other being, the inner surface of the exhaust port (i.e. the solid portion which is in contact with the fluid.
• After a number of iterations, inorder to get a y+ wall value of less than 1, inflation layer first layer thickness was calculated as 0.052mm.

Inflation Layer

Inflation layer table

Final Mesh

Mesh Statistics

Fluent Solver :

• After the mesh is done, it is updated to the fluent solver.
• The mesh check is done on the imported mesh. For this simulation, a steady state- pressure based solver is used.
• The turbulence model is chosen as K-Epsilon Standard model with Enhanced Wall function ( Enhanced wall function is used instead of standard wall function as it is better in solving the viscous-sublayer. If standard wall function is used then y+ must be > 11 and <30 )

• The material properties used for simulation are that of Aluminium
• The boundary condtions are set the inlet with velocity 5m/s and temperature at 700 K
• The Outer solid wall is given comvection boundary condition , with heat transfer coeffecient of 20 `W/(m^2K)`
• Hybrid initialization is chosen, and the solution is initialized
• The simulation is run for 150 iterations and checked for convergence

Scaled Residuals

Temperatur Contour

Temperature contour

Wall Y+ plus ( Majority cells within y+ value < 1 )

Post-Processing:

• The flow of fluid is illustrated using a stream line funciton

• A Plane is added, and the velocity and temperature contour are studied

Temperature contour

Velocity contour

Heat Transfer Coeffecient

Inference:

From the velocity contour, we can see that, the velocity increases along the curvature of the pipe. Due to increase in velocity, the laminar portion of the boundary layer is reduced, leading to a better mixing in turbulent layer. This inturn increases the Heat Transfer Coeffecient

Verification of Simulation :

From the velocity contour , we can see that the velocity increases along the curvature. As per our inference, velocity is proportional to the Heat transfer coeffecient ( Under same cross sectional dimension). Higher HTC results in better mixing and inturn results in increase in temperature of the inner exhaust manifold wall

By using the pick tool, we can see the temperature

Temperature where velocity is normal

Increase in temperature can be seen where the velocity is higher

Result:

The heat transfer coeffecient on the internal solid surface is found to be 935.42 `W/(m^2K)`