Exhaust Port Challenge

Q1. ANS The contemporary conjugate convective heat transfer model was developed after computers came into wide use in order to substitute the actual relation of proportionality of heat flux to temperature difference with heat transfer coefficient which was the only tool in theoretical heat convection since the times of Newton. This model, based on a strictly mathematically stated problem, describes the heat transfer between a body and a fluid flowing over or inside it as a result of the interaction of two objects. The physical processes and solutions of the governing equations are considered separately for each object in two subdomains. Matching conditions for these solutions at the interface provide the distributions of temperature and heat flux along the body–flow interface, eliminating the need for a heat transfer coefficient. Moreover, it may be calculated using these data.

Starting from simple examples in the 1960s, the conjugate heat transfer methods have become a more powerful tool for modeling and investigating nature phenomena and engineering systems in different areas ranging from aerospace and nuclear reactors to thermal goods treatment and food processing, from complex procedures in medicine to atmosphere/ocean thermal interaction in meteorology, and from relatively simple units to multistage, nonlinear processes. The applications in specific areas of conjugate heat transfer at periodic boundary conditions and in exchanger ducts .

Q2 ANS

Analysis is done on two mesh : Coarse mesh (base mesh) and fine mesh.

Base mesh:

In base mesh default values are taken.

The simulation results are as follows:

Streamlines:

Wall Temperature(K)

As we can see the highest temperature is at 574 K.

Velocity Profile(On a plane XY in the outlet pipe):

Both above plots show that maximum velocity reached to 37.82 m/sec.

The velocity will have an impact on the maximum temperature of air and wall heat transfer coefficient.

Velocity contour is not smooth enough at the wall because of coarse mesh and certainly leads to less accurate results.

Temperature Profile(On a plane XY in the outlet pipe):

Effect of course mesh can be seen on the wall of the manifold as we can see the wall is getting heated.

This is called conjugate heat transfer.

Wall Heat Transfer coefficient Profile(On a plane XY in the outlet pipe):-

Wall Heat Transfer coefficient Profile close-up(On a plane XY in the outlet pipe):-

Wall HTC is not good here because of the coarse mesh.

The value of wall heat transfer coefficient is 107.1 W m^-2 K^-1.

Fine mesh:

The mesh is refined and face sizing is done at 15 mm size.

Inflation layers (5) are added at 5 mm thickness on the inlets.

The simulation is done and the results are as follows:

Streamlines:

Wall Temperature:

After doing refinement of mesh we can see the temperature difference as the wall temperature of the manifold is increased with finer mesh.

The wall temperature now is 595 K.

Velocity Profile(On a plane XY in the outlet pipe):

Both above plots show that maximum velocity reached to 41.04 m/sec.

The velocity will have an impact on the maximum temperature of air and wall heat transfer coefficient.

Velocity contour is smooth enough at the wall because of fine mesh and certainly leads to more accurate results.

Temperature Profile(On a plane XY in the outlet pipe):

It shows that the wall is getting heated because of forced convection, which is high at the curvature where velocity is maximum.

Wall Heat Transfer coefficient Profile(On a plane XY in the outlet pipe):-

Wall Heat Transfer coefficient Profile close-up(On a plane XY in the outlet pipe):-

The wall heat transfer coefficient at the wall is looking well and profiles seem smoother than the previous mesh.

The highest wall HTC value is 222.2 W/m2K, which is almost doubled than the previous case value.

So, we can conclude that these mesh results are are good and mesh refinement is necessary.

Q3 ANS

Factors on which accuracy of the predictions are:

1. There should be accurate modeling of the problem,if there is no proper modeling the result will be not good .
2. Geometry of the model should be specific according to the given required dimensions.
3. Fine mesh gives better results then the course mesh so accuracy of the mesh should be higher.
4. Mesh should be studied thoroughly as there are differences in results for different meshes.
5. Overall modeling should be correct to produce correct residual plots.
6. For solution, correct solver should be used to produce good satisfied results.
7. For accurate result , use of actual material and their properties should be used. For example : if the part originally is made up of Aluminium material then in CFD process we should take aluminium as a desired material to produce results.

To confirm the results are accurate CFD process should be done with experimental analysis and numerical analysis and it can be verified and compared to the published data like papers present in various websites.