618
4 DISCUSSIONS AND CONCLUSIONS
According to the evolution of dust dispersion velocity, four stages in the process of dispersion can be classified under the single-forced ventilation condition as shown in Figure 9. The
scattered points denote the velocities of different kinds of dust particles and the black dash
line denotes their average value.
1. Accelerative dispersion stage
High-speed airflow ejected from the ventilation duct and rebounded when reaching the heading face, forming a vortex airflow field between ventilation duct and heading face. Therefore,
at the beginning of dispersion, the dust particles near the heading face accelerated rapidly
because of the strong drag force from the rebounded airflow.
2. Deceleration dispersion stage
The velocity of the rebounded airflow suffered a sharp decrease in a short distance. Accordingly, the dust particles slowed down rapidly. Moreover, in this period, a number of particles
were sucked into the circulation in the vortex region. Consequently, the overall dispersion
velocity experienced a drastic drop.
3. Fluctuant stage
In this stage, a large proportion of the dust particles had migrated into the turbulence region
and the rest of them was still bound in the vortex region. Overall, the dispersion velocity was
fluctuant at a lower level.
4. Stable dispersion stage
As the airflow flew into the stable region, most of the dust particles began to migrate in a
stable dispersion velocity until flowing out from the roadway.
For the dust particles investigated in this study, particles with greater diameters were easier
to disperse forward than those with smaller diameters. This may be attributed to their better
motion stability in the airflow. The smaller dust particles are more susceptible to the airflow
characterized by instability before entering the stable region. As a result, their motion is less
orderly. It has been observed in this study that the smaller dust particles are more likely to be
affected by the entrainment effect of the high-speed ejected airflow and therefore there is a
high possibility of getting incorporated into the dust circulation for them.
Dust particles mainly settled in the multi-direction turbulence region and stable region
where the airflow speed was much less than that in the vortex region. In this study, for larger
dust particles with diameters of 60 um and 80 um, they were more likely to migrate at a lower
height and settle compared with those of diameters of 20 um and 40 um due to the effect
of gravity, which means that smaller dust particles are more responsible for dust pollution.
Figure 9. The average dispersion velocities of all dust particles.
4.0
3.5
3.0
c;;
l
2.5
Z' 2.0
·u
0
Qj 1.5
>
c
0
1.0
.§
Q)
0.5
a. (/)
0 0.0
-0.5
0
I.' .r II
III
IV
,\
. ' \
I 1:
.,:
·I:
.\ ..
.\··
. f ·
l··
1 :Accelerative dispersion stage
11 : Deceleration dispersion stage
111: Fluctuant dispersion slagc
IV: Stable dispersion stage
j• .
} 'i<,.
.\ . : / ... :~;;;,;_ ;,::;:;f.::f.""-''~;;~::,.:;;.;.;;~::::-;;;-!::,..••
. -, ~,
.
:
·r.
,
L
I
·.,
5
10
15
20
25
30
Dispersion time (s)
4 DISCUSSIONS AND CONCLUSIONS
According to the evolution of dust dispersion velocity, four stages in the process of dispersion can be classified under the single-forced ventilation condition as shown in Figure 9. The
scattered points denote the velocities of different kinds of dust particles and the black dash
line denotes their average value.
1. Accelerative dispersion stage
High-speed airflow ejected from the ventilation duct and rebounded when reaching the heading face, forming a vortex airflow field between ventilation duct and heading face. Therefore,
at the beginning of dispersion, the dust particles near the heading face accelerated rapidly
because of the strong drag force from the rebounded airflow.
2. Deceleration dispersion stage
The velocity of the rebounded airflow suffered a sharp decrease in a short distance. Accordingly, the dust particles slowed down rapidly. Moreover, in this period, a number of particles
were sucked into the circulation in the vortex region. Consequently, the overall dispersion
velocity experienced a drastic drop.
3. Fluctuant stage
In this stage, a large proportion of the dust particles had migrated into the turbulence region
and the rest of them was still bound in the vortex region. Overall, the dispersion velocity was
fluctuant at a lower level.
4. Stable dispersion stage
As the airflow flew into the stable region, most of the dust particles began to migrate in a
stable dispersion velocity until flowing out from the roadway.
For the dust particles investigated in this study, particles with greater diameters were easier
to disperse forward than those with smaller diameters. This may be attributed to their better
motion stability in the airflow. The smaller dust particles are more susceptible to the airflow
characterized by instability before entering the stable region. As a result, their motion is less
orderly. It has been observed in this study that the smaller dust particles are more likely to be
affected by the entrainment effect of the high-speed ejected airflow and therefore there is a
high possibility of getting incorporated into the dust circulation for them.
Dust particles mainly settled in the multi-direction turbulence region and stable region
where the airflow speed was much less than that in the vortex region. In this study, for larger
dust particles with diameters of 60 um and 80 um, they were more likely to migrate at a lower
height and settle compared with those of diameters of 20 um and 40 um due to the effect
of gravity, which means that smaller dust particles are more responsible for dust pollution.
Figure 9. The average dispersion velocities of all dust particles.
4.0
3.5
3.0
c;;
l
2.5
Z' 2.0
·u
0
Qj 1.5
>
c
0
1.0
.§
Q)
0.5
a. (/)
0 0.0
-0.5
0
I.' .r II
III
IV
,\
. ' \
I 1:
.,:
·I:
.\ ..
.\··
. f ·
l··
1 :Accelerative dispersion stage
11 : Deceleration dispersion stage
111: Fluctuant dispersion slagc
IV: Stable dispersion stage
j• .
} 'i<,.
.\ . : / ... :~;;;,;_ ;,::;:;f.::f.""-''~;;~::,.:;;.;.;;~::::-;;;-!::,..••
. -, ~,
.
:
·r.
,
L
I
·.,
5
10
15
20
25
30
Dispersion time (s)