  We investigated the effective influence of grain structures on the heat
transfer between a fluid and solid domain using mathematical homogenization.
The presented model consists of heat equations inside the different domains,
coupled through either perfect or imperfect thermal contact. The size and the
period of the grains are of order $\varepsilon$, therefore forming a thin
layer. The equation parameters inside the grains also depend on $\varepsilon$.
We considered two distinct scenarios: Case (a), where the grains are
disconnected, and Case (b), where the grains form a connected geometry but in a
way such that the fluid and solid are still in contact. In both cases, we
determined the effective differential equations for the limit $\varepsilon \to
0$ via the concept of two-scale convergence for thin layers. We also presented
and studied a numerical algorithm to solve the homogenized problem.
