SWIRLING JET MIXING OF A BATH COVERED WITH A TOP SLAG LAYER
M. IGUCHI
Graduate School of Engineering, Hokkaido University, North 13, West 8, Kita-ku, Sapporo,
060-8628 Japan
Y. SASAKI
Graduate School of Engineering, Tohoku University, 02 Aza-aoba, Aramaki, Aoba-ku, Sendai,
980-8579, Japan.
D. IGUCHI
Graduate School of Engineering, Hokkaido University, North 13, West 8, Kita-ku, Sapporo,
060-8628 Japan
T. OHMI
Graduate School of Engineering, Hokkaido University, North 13, West 8, Kita-ku, Sapporo,
060-8628 Japan
ABSTRACT
Mixing of a bath by gas injection is widely used in many engineering fields. In particular, molten iron and steel baths are
commonly agitated by Argon gas injection for homogenizing temperature and components and for removing non-metallic
inclusions and impurities from molten steel. In recent years it also finds its applications to parlor wastewater and sludge
treatments. Injected gas induces a swirl motion of a bath under specific conditions. The bath is much more strongly mixed in
the presence of the swirl motion than in the absence of it. A similar swirl motion appears when a liquid or a mixture of gas and
liquid is injected into a bath contained in a cylindrical vessel. In this study the effects of a top layer on these swirl motions are
investigated.
Introduction
When gas is injected into a bath through a single-hole centered bottom nozzle under a subsonic condition, the gas
disintegrates successively into many bubbles at the nozzle exit and then they rise in the bath while entraining the surrounding
liquid behind their wakes [1-2]. As a result, a bubbling jet is formed above the nozzle. The bubbling jet did not always rise
straight upwards, but rotated around the centerline of the bath under specific conditions. Two types of swirl motions appeared
depending on the bath depth: the shallow-water wave type and deep-water wave type [3]
The shallow-water wave type swirl motion appeared for an aspect ratio, HL/D, smaller than approximately 0.3 and the deepwater wave type swirl motion appeared for an aspect ratio, HL/D, between approximately 0.3 and unity. Here, HL and D denote
the bath depth and the diameter of the reactor, respectively. These swirl motions were caused by bath surface oscillations
associated with the impingement of bubbling jet onto the bath surface. The direction of the swirl motion was not uniquely
decided because the vessel was not large enough to be affected by the Coriolis force.
Splashing of molten steel became frequent with an increase in the gas flow rate, Qg. As the splashing of molten steel is very
dangerous, practical application of the swirl motion of a bubbling jet is not acceptable to the steelmaking industry. Accordingly,
the commonly used reactors in the steelmaking industry are designed to operate for HL/D ≤0.3 (converters) and HL/D ≥1.0
(ladles).
The swirl motion of the deep-water wave type however has high mixing ability compared to a bubbling jet rising straight
upwards. The swirl motion therefore was applied to wastewater and sludge treatments because the splashing of water is not
dangerous at all [4, 5]. Particular attention was paid in this study on the deep-water wave type swirl motion of a liquid jet [6].
Experimental apparatus and procedure
Figure 1 shows a schematic of the experimental apparatus. Water was used as the working fluid and injected into a water bath
contained in a cylindrical vessel through a centered single-hole bottom nozzle. The water flow rate, QL, was controlled with a
valve and a flow meter. The water in the bath was drained through four bottom nozzles at the same flow rate as the injected
one for keeping the bath depth constant. The occurrence region of the liquid jet was determined by eye inspection. As the
basic characteristics, the period, Ts, amplitude, A, starting time, Ts,s, and the damping time, Ts,d, of a swirl motion of a liquid jet
were obtained.
Vessel diameter
D
=0.100∼0.309m
Vessel
Inner diameter of
nozzle dnen =0.013m
Water
Liquid flow rate
QL=0-750cm3/s
HL
0<
D
Pump
≤1
5
Flow meter
Fig.1 Experimental apparatus and conditions
The period of the swirl motion, Ts, hardly depended on the water flow rate and it was correlated by the following theoretical
equation derived for the rotary sloshing caused by oscillating a cylindrical bath in the vertical or horizontal direction, as shown
in Fig.2.
Ts = 2π/ω1
1/2
ω1 = [(2gλ1/D)･tanh(2λ1HL /D)]
λ1 = 1.84
(1)
(2)
(3)
where
ω1 is the angular frequency, and
g is the acceleration due to gravity.
This is because the swirl motion and the rotary sloshing are the most stable motion of liquid contained in the cylindrical vessel.
The amplitude, A, of the swirl motion increased with an increase in the water flow rate, Qg. Accordingly, the bath agitation
became strong with an increase in the water flow rate. The measured values of the amplitude of a swirl motion were correlated
as a function of a modified Rossby number, Rom, as shown in Fig.3.
0.5
A/D = 3.5Rom
(4)
In practical applications the starting time, TS,S, and damping time, TS,d, of a swirl motion are of essential importance. The
period from the start of liquid injection to the moment at which a swirl motion becomes steady is named the starting time. The
damping time is defined as the period from the stoppage of gas injection to the moment at which the amplitude becomes less
than 0.5mm.
11)
Figure 4 shows that the measured values of the starting time, TS,S, can be approximated by the following empirical equation .
0.5
0.5 1.75
TS,S (g/D ) =0.253[TC (g/D ) ]
(5)
The damping time of a swirl motion was mainly influenced by the vessel diameter, as shown in Fig.5.
TS,d (g/D )
1/2
= 500
(6)
Solid line:
4
Eq.(4)
Fig. 3 Amplitude of swirl motion of liquid jet
Fig.2 Period of swirl motion of liquid jet
1000
1000
+70%
D =0.130m
D =0.200m
D =0.309m
-70%
TS,d(g/D)0.5
100
S,S
T (g/D)
0.5
+25%
-25%
D =0.130m
D =0.200m
D =0.309m
10
10
100
T (g/D)
0.5
C
Fig.4 Starting time of swirl motion of liquid jet
100
0.001
0.01
0.1
Rom
Fig.5 Damping time of swirl motion of liquid jet
Further experiments were carried out to understand the effect of a top oil layer on the swirl motion of a liquid jet. The top oil
layer is a model for a molten slag layer placed on the surface of the molten metal contained in a reactor in the steelmaking
industries.
Two types of swirl motions were observed in the presence of a top oil layer, as shown in Fig.6. One appeared when the top oil
layer was thin. The top oil layer swirled together with the molten metal and the oil was strongly mixed with the molten metal.
The other appeared when the top oil layer was thick. The swirl motion appeared at the interface between the two layers, as if
the gravitational acceleration became small. This type of swirl motion is not useful for agitating the molten metal bath. The
boundary between the two types of swirl motions is described by a ratio of the top layer thickness to the molten metal layer
thickness of about 0.3.
The occurrence regions of the two types of swirl motions depended on the thickness and physical properties of the top layer.
The amplitude, starting time, and damping time of the swirl motion also were influenced by the top oil layer. The period
however was not affected by the top oil layer.
Empirical equations were proposed for the amplitude, starting time and damping time. In addition, the occurrence condition of
the swirl motion was clarified.
(Type A)
(Type B)
Fig.6 Two types of swirl motions in the presence of top layer
Nomenclature
A : amplitude of swirl motion
D: vessel diameter
dnen: inner diameter of nozzle for liquid injection
g: acceleration due to gravity
HL: bath depth
QL: liquid flow rate
Qg: gas flow rate
Rom : modified Rossby number
Ts,: period of swirl motion
TS,d : damping time of swirl motion
Ts,s: starting time of swirl motion
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