The superficial velocity is the velocity if the of the phase if it is flowing alone in the cross section. While in a multiphase flow, the area of cross section available to each phase is dependent on its flowrate or its percentage. Therefore, if the flowrate is calculated on the basis of superficial velocity, the velocity of the individual phase will far more than the intended superficial velocity.
Hej,
the superficial velocity at least of one phase in the system is easy to handle and is therefore a good parameter to work with. It is common during description of pneumatic transport systems and fluidization problems. Hence, also a number of correlations for heat and mass transfer rely on the superficial velocity in that area.
Depending on your actual problem you might be interested in the actual velocities - then the superficial velocity is not the right one to look at.
Best regards
Georg
Superficial velocity for multi-phase systems may be used as one of the parameter to correlate the data. To account for fractions of individual phases, holdup is to be used. However, actual velocity for each phase has no direct relationship with superficial velocity with individual phases.
There are situations when individual actual velocity are important then these are also the case when behaviour of individual phase units(let us call particle) are also important. Superficial velocity is not an appropriate parameter in such cases. Actual velocities should be used.
As I understand, the superficial velocity of one phase is the volumetric flow rate of this phase divided by the total cross section area (as it is as if this phase is flowing alone in the cross section). And the actual velocity of this phase is the superficila velocity of this phase divided by the void fraction (in the case of the secondary phase) or divided by (1-void fraction) (in the case of the primary phase).
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