3.2.1. Drying curves and effects of acoustic field
Figs. 6 and 7 showed the variations of moisture ratios with
drying time at a sound frequency varying from 1 kHz to 4 kHz and
sound pressure level in the range of 90–140 dB, respectively.
These Figures demonstrated that acoustic energy has the ability
to remove moisture, and consequently mass transfer processes
can be accelerated. In addition, high-intensity acoustic waves produce
cavitations of water molecules inside the lignite, which may
be beneficial for the removal of strongly attached moisture [16].
As seen fromFig. 6, the influence of acoustic waveswas negligible at
a lowsound frequency. Thatmeans that acoustic energy inside particles
and the drag force and collision force by sound waves were weakened
remarkably during the drying process at lower sound frequency. In
the present case, the threshold above which acoustic wave effect was
found significant is in the range of 1.25 – 2 kHz. These results agreed
with those obtained at the minimum fluidization velocity of lignite