(738c) Acoustic Analysis of Particle-Wall Interactions during the Transition of Flow Regimes in Vertical Pneumatic Conveying

Abstract:
Pneumatic conveying is widely used in industry, different flow regimes occur
when conditions change. In order to minimize the energy losses and reduce pipe
wall erosion, a suitable flow regime should be chosen. Thus, it is very
important to determine the transition velocity between the flow regimes. Many
researchers have investigated the influence of operation conditions and
material properties on the transition velocity and minimum pressure drop, such
as pipe diameter, gas velocity, gas density, feed pressure, particle density
and diameter, and so on. Based on the experimental results, a lot of empirical
or analytical approaches have been proposed. These correlations are very useful
when a new conveying system is designed. However, due to the complicated
interactions between gas phase and solid phase at different scales of time and
space, the underlying mechanism of the transition is still not clear and thus
limits the application ranges of these correlations. Acoustic emission signals
are very sensitive to the particle motions and have been widely used in the
detection of in gas-solid two phase flow. In the current work, a comparison
study of acoustic emission signals and pressure signals was proposed, followed
by the investigation of flow behavior inside a vertical pneumatic conveying pipe
during the transition of flow regimes.

Experiments were taken in a vertical
section of the pneumatic conveying system. The inner diameter of the pipe is 25
mm and the length of the vertical section is 3.0 m. Polypropylene (PP) particles
with an average diameter of 2100 µm was used in the experiment, and the particle density is 900
kg/m³. Compressed air was used as the conveying gas. The mass flow
rate of PP particles
was determined by a weighing cell. In the experiments, the superficial gas velocity ranged from 5.0 to 13.0 m/s and the PP particle mass flow rate ranged
from 0.006 to 0.026 kg/s. The online AE
system for collection and analysis was developed by UNILAB Research Center of
Chemical Engineering in Zhejiang University. The system consists of an AE
sensor, a preamplifier, a main amplifier, A/D conversion module and a computer.
The gain of preamplifier is 40 dB. The AE sensor used in this work is a
piezoelectric accelerometer (AE 144S, Fuji ceramics corporation). The AE sensor
was mounted on the outer surface of the vertical pipe, and the sampling
frequency used was 900 kHz, sampling time was 5 s. Pressure drop of the pipe
was detected by a couple of pressure probes (CTG121P, China) with a distance of
1.2 m between them. The pressure signal was recorded by the computer, and the sampling
frequency used was 400 Hz, sampling time was 30 s. All the experiments were repeated 3 times and the
average results were used.

The results
showed that for all the mass flow rates conducted, as the gas velocity
decreased, both the pressure drop and energy of acoustic signal decreased to a
minimum and then increased. Moreover, the transition velocity detected by these
two kinds of method agreed with each other very well. The similarity of these
two kinds of signals can be interpreted from the sight of energy consumption.
To be specific, the pressure drop stands for the energy needed to transport the
particles while the energy of acoustic signals reveals the energy loss due to
particle-wall collisions and frictions. Additionally, for the gas velocity
beyond and below the minimum transport velocity, a nearly linear relationship
can be found for the acoustic energy and pressure drop. However, the slopes of
these two lines were not the same which indicate that different mechanisms
dominated. Wavelet analysis was further applied and the acoustic signals were decomposed
into 1-10 of scales detailed signals (d₁-d₁₀) and the 10^th
scale approximated signal (a₁₀) using Daubechies 2^nd order wavelet
transform.
According to our previous work (He et al., Ind. Eng. Chem. Res., 2014), the detailed
signals d₁-d₃, and d₄-d₅ were recomposed
as the particle-wall collision signals and particle-wall friction signals, which
represent particle-wall interaction in the normal and tangential direction respectively.
The results showed that as the gas velocity decreased, energy fraction (energy
of detailed signal divided by the energy of original signal) of particle-wall
collision signals decreased to a minimum and then increased while that of
particle-wall friction signals varied the contrary. When the gas velocity was
larger than the transition velocity, energy fraction of particle-wall collision
signals decreased while that of particle-wall friction signals increased as the
gas velocity decreased. This was due to the increase of gas phase turbulence
which led to the increase of particle velocity fluctuation and thus larger
normal velocity was gained. On the other hand, when the gas velocity was lower
than the transition velocity, particle-wall collision energy fraction increased
while particle-wall friction energy fraction decreased as the gas velocity decreased.
Further analysis showed that the increased solid concentration led to the
increase of collisions for the reversed particles with those moving upwards.
Based on these analysis, it can be concluded that particle-wall interactions is
one of the main cause for the change of pressure drop, and the transition of
pressure drop was due to the change of the ways particles collide with each
other.

Keywords: transition velocity; acoustic emission; flow regimes.