two parts with two bodies vs one part with two bodies for regenerative cooling
 

I am trying to simulate regenerative cooling. I am totally confused. In DesignModeler whether we make two parts with two bodies for solid part and fluid part or we should chose one part with two bodies for liquid and solid knowing that fluid will pass through the solid and heat will be exchanged between the two domains.
I performed both in an attempt to check, but there was a huge difference between the two cases though the models were same.

what is the correct choice in this case please ??

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