(237l) Droplet Generation in Two Phase Liquid-Liquid Flow Systems in Millichannels – Effect of Phase Inlet Orientation and Reactant Mass Flux

Droplet
Generation in Two Phase Liquid-Liquid Flow Systems in Millichannels – Effect of
Phase Inlet Orientation and Reactant Mass Flux

Alex Koshy, Gargi Das,
Subhabrata Ray

Department
of Chemical Engineering, IIT Kharagpur

koshyalexkoshy@iitkgp.ac.in, gargi@che.iitkgp.ernet.in, sray@che.iitkgp.ernet.in

The
proposed paper reports droplet generation characteristics for reacting and
non-reacting liquid – liquid flow systems in a 2mm diameter 1m long circular
glass conduit oriented horizontally. The two phases are introduced into the
conduit via a T-entry from diametrically opposite points. Experiments have been
performed with the organic phase introduced from the top limb and aqueous phase
from the bottom limb of the T-entry and vice versa. The non reacting system
chosen is toluene dyed with iodine (organic phase) and water (aqueous phase). For
reacting cases, NaOH is introduced in aqueous phase in excess of stoichiometric
amount. Iodine diffuses from organic phase through the aqueous – organic
interface and reacts instantaneously with NaOH as it reaches the aqueous phase.
The products stay in the aqueous phase and alter its physical properties. Plug
flow is found to be the dominant flow pattern for both reacting and
non-reacting systems over the entire range of phase flow rates selected
(0.1mL/min – 10mL/min). Organic phase is dispersed as plugs in the continuous
aqueous medium which fills the thin gap between plugs and conduit wall.

We
observe that the plugs are generated either by squeezing mechanism or by jetting
mechanism depending on the choice of limbs for phase entry. This is also affected
by the reactant flux across the phase interface at the T-junction inlet where
the plug forms. The diameter of the conduit falls in the mesoscale range [1]
where the effects of body forces cannot be completely ignored. When the lighter
organic phase is introduced from the top limb, there is a tendency for it to stay
at the upper portion of the conduit cross section. In this case, part of the
dispersed phase enters the conduit as a jet, floating over the aqueous phase
and plugs are formed due to shearing at the jet tip by the continuous aqueous
phase [Fig 1(a)]. This phenomenon
which is known as jetting [2], produces small plugs with l/d ratio way lesser
than those formed via squeezing. Squeezing [2] occurs when the lighter phase is
introduced from the bottom limb. The heavier phase coming from top limb tends
to flow below the lighter phase at the T-junction. The lighter phase fills
almost the entire conduit cross section before detaching as plug due to pressure
build up by continuous phase at the junction [Fig 1(c)]. Experiments performed with reacting systems have shown
that the longer plugs produced by squeezing result in higher conversion than
the shorter plugs of jetting. This can be attributed to the presence of longer
continuous phase films where high transfer rates prevail due to high relative
velocities across the interface.

Different
forces act in the region where the plugs are formed by detachment from the
organic stream. The inertia of the dispersed phase tends to shift the plug generation
point away from the T-junction [Fig 2(b)],
whereas the shearing action by continuous phase tends to bring it closer [Fig 2(c)]. Body forces act in such a way
that the lighter (dispersed) phase tends to float on top [Fig 2(a)]. Surface forces also have their role in plug formation.
In reacting cases, an additional force is present which changes the
hydrodynamics of flow of the two phase system. This is the reactant mass flux
acting at the phase interface due to the reactant transport from organic to
aqueous phase [Fig 2(d)]. Reactant
flux opposes the shearing action, thus shifting the plug formation point farther
from the T-junction (longer jets) [Fig
1(b)]. It also induces jetting in cases where squeezing prevailed for non
reacting systems. This is by opposing the tendency of heavier continuous phase
to flow below the lighter dispersed phase at the T-junction [Fig 1(d)]. This pressure force induced
by reactant transfer is quantified and related to measurable parameters. We
thus note that significant influence of body force and reactant mass flux in
millichannels produce a rich physics which is investigated.

Fig 1: Drop generation via jetting
and squeezing mechanisms for reacting and non-reacting cases. (a) Drop
generation via jetting for non reacting case when lighter phase is introduced
from top. (b) When lighter phase is introduced from top for reacting cases,
reactant flux from dispersed phase to continuous phase resists shearing action
by continuous phase, thus shifting the plug generation point away from the inlet.
(c) When lighter dispersed phase is introduced from bottom limb, jetting gives
way to squeezing mechanism for non-reacting cases, leading to longer plugs. (d)
For reacting cases, the reactant flux opposes intrusion of heavier aqueous
phase to the bottom portion, thus preventing shearing at the junction, leading
to jetting. This phenomenon for reacting cases is seen only for those phase
flow rates, when the reactant flux is strong enough to overcome the pressure
exerted by aqueous phase. Else squeezing prevails.

Fig 2: Schematic diagram showing forces
acting at the plug generation point. (a) Body forces keep lighter fluids on top
leading to jetting when the lighter fluid is introduced from top (b) The
inertia of the dispersed phase tends to elongate the jet (c) The shearing
action by continuous phase breaks the jet leading to plug formation (d) The
reactant (iodine) flux from dispersed phase to continuous phase acts opposite
to the shearing action and thus shifts the plug generation point away from the
T-junction

References:

[1]  
Ong, C.L., Thome, J.R., Macro to Micro Transition in Two Phase Flow: Part 1- Two Phase Flow
Patterns and Film Thickness Measurements, Exp. Thermal and Fluid Science,
35 (2011) 37-47

[2]  
Garstecki, P., Fuerstman, M.J., Stone, H.A.,
Whitesides, G.M., Formation of Droplets
and Bubbles in a Microfluidic T-junction – Scaling and Mechanism of Break-up, Lab.
Chip, 6 (2006) 437-446