Conjugate Heat Transfer analysis of an exhaust port

1. Conjugate Heat Transfer:

Conjugate heat transfer corresponds with the combination of heat transfer in solids and heat transfer in fluids. In solids, conduction often dominates whereas, in fluids, convection usually dominates. Conjugate heat transfer is observed in many situations. For example, heat sinks are optimized to combine heat transfer by conduction in the heat sink with the convection in the surrounding fluid.

Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers, heaters, or heat exchangers.

The fluid usually plays the role of energy carrier on large distances. Forced convection is the most common way to achieve a high heat transfer rate. In some applications, the performances are further improved by combining convection with phase change (for example liquid water to vapor phase change).

Even so, solids are also needed, in particular, to separate fluids in a heat exchanger so that fluids exchange energy without being mixed.

There are many areas of application like heat exchanger, HVAC systems, Exhaust ports etc.

 

2.  Case 1: Basic mesh (default settings)

Mesh: No of elements is 137391.

Temperature outer wall:

From the temperature profile, we can see that inlet pipe temperature is less i.e. 464 K, and as we move towards outlet the temperature increases to 650 K. The low inlet pipe temperature is due to heat transfer coefficient at the surface.

Velocity streamlines:

 

It can be seen that due to the continuity equation the velocity at the outlet increases significantly.

 

Velocity profile at outlet:

 

Heat Transfer Coefficient:

In the above figure, we cannot get clear idea of HTC as it is found only on wall and our mesh is not refined enough to give us the clear picture. Although all the results are good enough but to get boundary values we must ensure body-fitted mesh.

 

Case 2: Refined Mesh (body fitted)

Mesh: No of elements is 371207.

 

An inflation layer was added to capture the boundary phenomenons

 

Temperature at body:

Not much difference can be seen in the temperature distribution as well as velocity variation in both the cases.

Streamlines:

Velocity at outlet:

 

Heat Transfer Coefficient:

In this figure above, we can clearly see the HTC are much better as we have provided an inflation layer which helps in capturing boundary phenomenons clearly.

 

3. Validation of HTC predictions:

From the figures shown above for Case 1 and 2, we can see that the results of HTC in case 1 are very coarse and no proper visualization of HTC can be done. Whereas, in case 2 due to inflation layers we have captured the phenomenon very clearly.

In case 2 we can see that we are having a maximum velocity at the lower bend of the outlet. Higher velocity leads to high Reynold No. and we know that Reynold No. is directly proportional to the heat transfer coefficient. By this fact we can see that our model is behaving exactly as it should be thus, it validates our model.

For further validation, the values of HTC can be compared with either analytical or experimental values.


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