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PREDICTABILITY OF GRANULOMETRY : COALESCENCE

Ziel

The main objective of the project is to prepare a computer code ( CROCODIL) capable of predicting changes in the granulometry of fluid/fluid dispersions. This computer code requires knowledge of collision efficiencies for various pairs of fluids. Thus the second objective of the project is to devise and to use a specific set-up (Couette coalescor equipped with an attenuation probe) allowing an experimental identification of these efficiencies. The current state of knowledge in the research field is characterized by a lack of formulae suitable for industrial use and very little experimental data.
Four main results have obtained :
- A variable axis cylindrical Couette coalescer has been designed
. It has a 40 mm diameter rotating inner cylinder with a 1 mm annular gap to a rotating outer cylinder 200 mm high.
The electrical motor equipped with a variator can provide 9 to 1500 rev min-1 (precision less than 1 %) to the inner or the outer cylinders. A great care has been paid to the optical and geometrical properties of both cylinders : the inner one is reflecting and stainless steel made and the outer one is transparent and glass made.
- An optical probe measuring interfacial area by light attenuation has been designed with a special emphasis en flows with sub-millimetric particles. It permits measurements in liquid-liquid or gas-liquid dispersions without need of introducing empirical correcting factors for the standard exponential decay law of light intensity while keeping an extended application range. This probe was successfully determined basically by hold-up methods. The volume fraction of the dispersed phase was varied between 0. 05 % and 5 % and the particle size between 10 um and 300 um.
- A computer code describing the drop granulometry evolution in time of liquid-liquid dispersions submitted to a laminar Couette (shear) flow, has been established. This code, based on a Smoluchowski model, permits to identify for a given dispersion, the Coalescence Efficiency (C.E. = fraction of collisions between drops that result in coalescences)
- A computer code describing the drop granulometry evolution in space of liquid-liquid dispersions transported along ducts by turbulent flows, has been established. This code, based on a one dimensional population model, has shown that the C.E. tuning was of paramount importance to reach equilibrium conditions. A relationship between the averaged inlet granulometry and the averaged equilibrium has been derived.
Two basic poblems have been encountered: (1) Identification of the coalescence efficiency from Couetteflows was much more difficult than expected. for most
regimes several effects (wall influence, gravity, axial drift...) render those flows not amenable to simple models. Thus only estimations of C.E. have been obtained.
(II) It is not straightforward to transfer C.E. obtained in laminar situations to C.E. used in turbulent situations. A great effort in physical modelling remains to be done.
The first phase has two steps : I/1 Assessment of the attenuation probe : II/1 Creation of a Couette coalescer. The second phase has two steps : I/2 Adaptation of the attenuation probe (assessed in II/1) to the Couette coalescer : II/2 Identification code for the efficiency of coalescence: III/2 Improvement of the computer code ( CROCODIL). The third phase has two steps : II/3 Effective identification of efficiency of coalescence for specific pairs of fluids : III/3 Runs of CROCODIL for the latter pairs.

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Institut National Polytechnique de Grenoble
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Adresse
46 Avenue Felix Viallet
38031 Grenoble
Frankreich

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