## Dimensionless key figures

The heat transfer is determined by flow and heat conduction processes. So far, however, only simple cases can be solved mathematically.

As a result, the heat transfer coefficient is determined experimentally $\alpha $necessary. The results obtained on certain models are transferred to physically similar objects and the heat transfer coefficient is determined from them.

In technology it is common $\alpha $ or the dimensionless index Nu (Nusselt index) derived from it as a function of the Reynolds and Prandtl numbers using so-called criteria equations. These criteria equations can also be very complex. For different hydrodynamic conditions and structural features of the reactors, coolers and evaporators, they can be found in the specialist literature (e.g. the VDI heat atlas).