MF-e
Different calculation models have been developed to consider the distribution of the involved phases in space and their mixing state.
For practical application, the VDI Heat Atlas recommends model 1 according to Gronewold and Tsotas. This recommendation is based on its extensive comparison with measurement data from literature.
The highest, but then unrealistically high efficiency results from the version of model 3 with Nug. This model version uses the locally valid heat transfer coefficient between fluid and particle, but does not reduce the driving force available for the heat transfer in any way - neither by backmixing of the fluid nor by flow uneven distribution.
Model 2 and Model 3 with Nuapp take the backmixing of the fluid into account. Both result in slightly higher efficiencies than model 1.
The influence of flow uneven distribution only dominates if the heat transfer between the phases is neglected (model 4). Such a model can describe measured data well if the bypass portion ν is used as an adjustment variable, but not necessarily if ν is calculated according to the calculation rule given in chapter M5.
