(181n) Visualization of Gas-Liquid Mass Transfer and Reaction Inside a Single Droplet Using PIV and PLIF Technique

In
present work, planar laser-induced fluorescence (PLIF) technique was employed
to visualize the gas-liquid mass transfer accompanied by an irreversible
chemical reaction inside a single droplet hanging out of a capillary. The
dynamic change of fluorescence signal intensity in the droplet emitted by the
fluorescence dye, Rhodamine B, was quantitatively captured by a 12-bit
high-speed CCD camera (IMPERX, IPX-VGA210-L), where the concentration of the
dye varies in time and space due to the liquid micro-mixing coupled with an
oxidation reaction by ozone environment in the gas phase. The reaction process
inside the droplet can then be recorded quantitatively, integrated the process
of gas-liquid mass transfer and liquid-phase reaction.

The
experiments were conducted under different conditions, i.e., a nearly stationary
gas flow around the droplet with little disturbance, an uneven gas flow across
the droplet with certain disturbance on the droplet, and the droplet growing
process. The results implied that the mass transfer was almost entirely
achieved by molecular diffusion under nearly stationary gas flow condition. The
fluorescence dye in the liquid phase was oxidized by ozone gradually from the
surface to the interior region of the droplet. However, as an uneven gas flow was
directly exerted on the droplet, the reaction no longer proceeded from outside
to inside. In order to investigate the effect of environment gas
flow conditions on the reaction and mass transfer process between the gas and
liquid droplet, particle image velocimetry (PIV) was employed to visualize the
flow patterns inside the droplet under the different conditions mentioned
above. The results showed that the gas flow which induced certain disturbance
on the droplet had a significant impact on the internal flow pattern of the
droplet and thus intensified the mass transfer and reaction rate. Besides, the
reaction and mass transfer as well as the velocity field was measured during
the droplet growing process with different growing rates. The straightforward visualization
measurement illustrated a rich variety of micro-mixing behaviors with the
improved understanding on the mechanisms of gas-liquid mass transfer and
micro-mixing inside a droplet.