Features
of the movement of dispersed impurities in the form of particles in turbulent
gas flows and its reverse influence on the turbulence characteristics of the
carrier phase are key problems in the theory of two – phase flows. The inverse
problem is to study the influence of particles on the characteristics of the
gas flow carrying them. Solving this problem involves determining the
characteristics of a gas in the presence of particles: velocity and temperature
fields, friction and heat transfer coefficients, etc.
[1-7].
Pioneering studies that studied the dissipation of
turbulence energy by relatively low-inertia particles and the generation of
turbulence energy by large particles in vertical and horizontal pipes are the
works [8,9]. There is a fairly large number of experimental studies in which
the authors studied the features of the process of additional dissipation of
turbulence energy due to the presence of particles [10-12]. At the same time,
there are practically no experiments to establish the influence of large
particles on the turbulence energy of the carrier gas. The purpose of this work
is to visualize the moment of turbulence generation behind a large moving
particle.
The
installation for studying two-phase flows with large particles (Fig. 1) is a
vertical channel. A fog generator
1
is installed at the entrance
to the channel, followed by a lemniscate nozzle
2,
to which a
large particle supply unit
3
is attached using special pylons,
through which the dispersed phase is supplied. Next, the air flow passes
through the initial section of circular section
4
(diameter – 100
mm, length – 1000 mm, material – PVC) and then enters working section
5
(square section – side 100 mm, length – 1000 mm, material organic glass). The
outlet section of the working channel is a plug for capturing the dispersed
phase. The airflow leaves the channel using an adjustable fan as part of a
cyclone type filter
7.
Lemniscate nozzle
2,
large
particle supply unit
3
and cyclone filter
7
are
made using additive technologies (FDM) from polylactide (PLA).
Fig. 1 Diagram of the experimental setup:
1 – fog generator; 2 – lemniscate nozzle; 3 – unit for supplying large
particles; 4 – round pipe, initial section; 5 – square channel; 6 – measuring
area and diagnostic tools; 6 (a) – measuring area of square section; 6 (b) –
dual pulse laser; 6 (c) – plane of the laser “sheet”; 6 (d) – cross-correlation
camera; 6 (e) – moving large particle; 7 – cyclone with adjustable fan.
Large
spherical particles (Fig. 2) (material – plastic, physical density of particles
– 1050 kg/m3) with an average diameter of 6 mm are used as a
dispersed phase in the experiment. The surface of the particles is blackened in
order to reduce the effect of light reflection and flare of the receiving
optics.
Fig.
2 Spherical particles
Also,
as a dispersed phase, in order to visualize the air flow, microdroplets
(substance - glycerol, physical density - 1260 kg/m3) were
introduced into the flow using a fog generator (model Safex F2010) with a
diameter of 1 to 5μm(Fig. 3 ).
Fig.3
Micro drops of glycerin
Photography
of turbulent wakes behind large moving particles was carried out using the PIV
(Particle Image Velocimetry) anemometry complex based on particle images. The
measuring complex includes: cross-correlation camera
6 (d),
(model Flow Sense EO 2M) with a resolution of 1600 x 1200 pixels and an
installed lens (model Zeiss 50 mm f/1.4 ZF.2) and dual pulse laser
6 (b)
Dual Power model 145 – 15) with an energy of 145 mJ and a wavelength of 532
nm.
Visualization
of turbulent wakes behind large moving particles was carried out as follows. From
the large particle supply unit, plastic spheres enter the initial section of
the round section, then enter the transparent measuring area of the square
section, where they accelerate to a speed of
=
5,2 m/s. At the same time, micro drops of glycerol in the form of a mist enter
the working channel through the lemniscate nozzle and take on the velocity of
the carrier gas, which on the channel axis is about
=
1,8 m/s. When the plastic sphere reaches the measuring area, it is illuminated
with a flat laser “sheet” and then photographed. Figure 4 shows characteristic
images of moving large particles caught in the frame, surrounded by micro drops
of glycerin. It is worth noting that there are not many successfully
photographed particles; in fig. 4(a) and fig. 4(b) shows particles that did not
fall into the plane of the laser “sheet” and into the focus of the camera. While,
in fig. 4(c) shows the case of a successfully photographed particle; the
shadow of the laser “sheet” is clearly visible in the frame.
Fig.
4
Characteristic images of moving large particles, frame
size 25x40 mm: (a) – particle behind the plane of the laser “sheet”; (b) –
particle in front of the plane of the laser “sheet”; (c) – particle in the plane
of the laser “sheet”
The
use of the optical method of diagnostics of velocity fields “PIV” in the
experiment makes it possible to determine the main characteristics of the
vortex wake behind a large particle. During the experiments, the diagnostic
system was controlled from a computer using the “Dynamic Studio” software
package. Diagnostics were carried out with a frequency of 10 Hz. The thickness
of the laser “sheet” formed by a cylindrical lens was about 1.5 mm in the
measuring area. The measuring area coincided with the central section of the
working channel. The size of the measuring area was about 100 x 100 mm. An
adaptive PIV algorithm was used to calculate the instantaneous velocity field. To
find the characteristics of the flow in the wake of a particle, the maximum
spatial resolution of a vector map with a size of computational areas of 16 x
16 pixels has been determined, which corresponds to a physical size of 0.5 x
0.5 mm2.
The threshold signal/noise value was 6.5. The threshold
value for the height of the correlation peak was 0.45. The limit for detecting
a particle against the background of “noise” was 5. The nominal number of
particles in each of the calculation areas was 10.
Fig.
5 Vector map of the wake vortex behind a large particle
Fig.
6 Velocity map of a vortex wake behind a large particle
Figure
5 shows a vector map of the wake vortex behind a large particle
.
The vector map is of size m x n vectors, approximately 3000 vectors.
Figure
6 shows a velocity map of the wake vortex behind a large particle. Unlike
figure 5, figure 6 shows a map of scalar quantities. From the data presented,
one can see two characteristic regions of the vortex wake behind a large
particle - the region of attached vortex structures and the region of a
detached vortex. The first of the mentioned areas has a length (in the
longitudinal direction) of approximately 5 particle diameters. The second
characteristic area has a diameter approximately equal to the diameter of the
particle.
The
authors demonstrated the possibility of visualizing and measuring the
characteristics of the flow in the wake of large moving particles in a downward
airflow using a fog generator and a PIV velocity field diagnostic system. Velocity
distributions in the region of the rear critical point of the particle are
obtained. Further visual interpretation of the velocity fields showed the
presence of both a region of attached vortices and the presence of detached
large vortices in the flow in the wake of the particle. The proposed
visualization technique can be useful in studying the contribution of large
particles to the characteristics of turbulent and eddy multiphase flows [13-15].
The
work is supported by the Russian Scientific Foundation (Project ¹ 23-19-00734).
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