# Bug in new implicit Conjugate heat transfer Solver

### Summary

I am highly appreciate the implicit coupled patch which has been released in v2112.

I also understand it is more than a boundary condition since in conjugate heat transfer solver, if it detects implicit patch, it will solve all regions in one matrix. This leads a very fast convergence. This is a desirable solver which we have been chasing many years in OF. However, I think I found a suspcious bug in this new solver.

In the coupled patch, it will calculate a source term to balance the enthalpy jump between different region in coupled patch in following code.

However, if you compare the result of new implicit coupled CHT solver with original segregated solver. In steady state CHT, When solution reach convergence , the solid temperature would be less than segregated solver. In fact, the segregated solver has good temperature correlation with other commercial software. I highly think this source term affects the results. I know if I remove this source to keep it as zero, the temperature will get a wierd results. However, if I do a trick in steady state solver, like setup all the Cp in solid region same as fluid, then I can get a perfect result with similar leverl temperature as segregated CHT solver and other commercial software. Also, if you check the final results wall heat flux, in implicit solver, some solid region not reach the wall heat flux balance. Example, in tutorial CpuCabinet case, Fin. The problem is theoratically, in steady state, the Cp should not affect the final results. Therefore, I think use this formula source to balance enthalpy jump is a problem.

Could you please check it?

Cheers,

### Steps to reproduce

Just use implicit CHT solver

### Example case

even for tutorial CpuCabinet.

###
*bug* behaviour?

What is the current gives lower temperature and imbalance wall heat flux.

###
*correct* behavior?

What is the expected similary level temperatuer as before, and balanced heat flux

### Relevant logs and/or images

### Environment information

- OpenFOAM version : v2112
- Operating system :
- Hardware info :
- Compiler :