Heat Exchange Material

Ag has the highest thermal conductivity.

Some people prefer liquid sodium. It doesn't have the same high conductivity as silver but the lower mass and relatively low melting temperature permits a fluid to be pumped around for heat transfer.

But it burns in contact with air or water !

As the question was written it deserves MANY and DIFFERENT answers.

If the material is used for a separation wall between 2 fluids then many times the wall has such a low effect on the overall transfer coefficient that the predominant criterion is the cost. All depends on the considered case.

Yes it does burn with air and water. It will also do horrible things to flesh on contact. But by adding the heat exchange mechanism of convection, this will provide more heat to be exchanged.

I'm also getting very tired of a "guest" providing an incredibly tiny amount of information while asking for our help and never returning. As the grammatically poor question is stated the only valid answer I feel is my first answer. To answer any other way requires assuming a different question was intended.

So I'm taking the tack of a theoretical heat exchange medium maximum with out a regard to safety or environment that will work in the temperature ranges found in an automobile. For all I know, this might be a desire to improve the thermal distribution of the rod part of the exhaust valve so that wedging doesn't happen from uneven expansion. In this scenario I could see sodium as a viable possibility inside these valves.

Agreed, Redfred!

Also, what's it got to do with Automotive? There are heat exchangers in vehicles, but most of us don't need to worry about best materials.

Cheers..........Codey

The only thing this has to do with Automotive is that this is the Automotive Section of CR4. When grasping for information to make an assumption to fill in the gaps, anything can be used.

For what? Engineering economics apply.

Best heet exchange just fine. How are you?

In what way?