Conjugate heat transfer simulation of Exhaust Port 

Aim- To calculate the surface heat transfer coefficient on the internal solid surface.

 Conjugate heat transfer (CHT) is used to describe the process of exchange of thermal energy at the interface of the solid and fluid domain.

CHT analysis is mainly used in heat exchangers, engine fins cooling efficiency, cooling tower and air conditioner efficiency.

Geometry:

The four-cylinder exhaust header pipe 

Fluid domain is created inside the pipe 

topology is shared so that there's a matching of nodes at the interface of the solid and fluid region.

Meshing:

Created a baseline mesh to start from and to get an overview of the simulation.

Elements - 136996

Setup 

Using K - omega model which capture the near-wall characteristics efficiently.

All 4 inlet has the same inlet velocity 5m/s with the temperature of 700K.

Outer solid surface has the convection rate of 20w/m^2-k.

Then the solution ran for 150 iterations.

The region near inlet having low temperature but outlet having a higher temperature than the inlet because the inlet 4 and outlet is one therefore fluid velocity increases at conjunction.

which result in the higher heat transfer. 

Due to the coarse mesh its unable to capture the heat transfer properly at the inner solid surface.

Case 2 - Fine mesh

Using the body sizing method on the solid body to capture temperature contours properly

provided the inflation layers at the shared surface on both fluid and solid region 

For the fluid region, Y+ value is calculated to capture the boundary layers properly 

In K-omega Y+ value lies less than 5 

Desired Y+ value 0.95 is selected and wall spacing 0.065mm is calculated.

Setup 

Similar boundary conditions are used.

Result

All the residuals are constant after 30 iteration

As discussed earlier all the points should lie below 5 so it captures boundary layers properly. Only one point is lying above 5 which will not do any significant changes in solution, Therefore we will proceed with this Y+ value.

there is not much difference in the temperature contour but due to finer mesh, it captured the temperature more accurately.

Velocity is high at bends that the where we get higher heat transfer coefficient

The average heat transfer coefficient at the inner solid region is 

906.896 [W m^-2 K^-1]

Conclusion

• Results are more accurate with the finer mesh.
• Wall heat transfer coefficient captured better with a fine mesh having inflation layers.
• Heat transfer increases with an increase in fluid velocity.
• There is no significant change in temperature contours with finer mesh.
• With the proper Y+ value it captures near-wall characteristics properly.

The accuracy of the result can be validated by comparing with the experimental data.

or

By theoretically calculating using Nusselt number to find heat transfer coefficient.

The Nusselt number is the ratio of convective to conductive heat transfer across a boundary. The convection and conduction heat flows are parallel to each other and to the surface normal of the boundary surface and are all perpendicular to the mean fluid flow in the simple case.

The Dittus-Boelter equation gives the heat transfer coefficient (h) for heat transfer from the fluid flowing through a pipe to the pipe walls.

The Dittus-Boelter Equation:

 n=0.4 for the fluid being heated and n=0.3 for the fluid being cooled.

Pr = Prandtl Number

Cp is the constant pressure specific heat, `mu` is the viscosity and k is the thermal conductivity.

By equating both the equation we can predict the heat transfer.

Factors affecting the predictions are

• Element size
• Y+ value and number of layers
• Element quality