## Multiphase Flows

#### Where is the Sauter Mean Diameter used?

• FAQ
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In multiphase flows simulations, the knowledge of the Interfacial area between the dispersed and the primary phase is crucial to determine the transfers such as the momentum transfer (through the drag), the mass transfer, the energy transfer and the species transfer. However, if we consider only one dispersed phase of spherical particles, the interfacial area is simply given by N*Pi*d^2 where d is the diameter of each particle and N is the number of particles. In this case, the value of d has to be chosen so that the transfer quantities are well computed. The choice of the Sauter diameter allows the user to define a single diameter representing the accurate interfacial area between the primary and the secondary phase. For example, knowing the local size distribution of spherical particles (based on its evaluation by a Population Balance Model for example), we can determine the total local interfacial area and the total local volume occupied by the particles. Therefore, the ratio between this volume and this interfacial area gives: V/S = (N*Pi * d^3 /6 ) / (N*Pi * d^2) = d/6 where d is the Sauter Diameter Finally using this local sauter diameter in the transport equation of the secondary phase will result in an accurate prediction of the transfer properties.