Fluid flow and mixing characteristics in a gas-stirred molten metal bath.

Abstract

SynopsisFluid flow and mixing characteristics in a molten metal bath are analyzed for inert gas injection through a nozzle at the center of the vessel bottom. It is postulated that the bath consists of two zones; bubble plume zone where gas-liquid mixtures flow upward and annular zone where liquid flows downward. The analysis is made by setting up a steady-state energy balance for the liquid phase. The liquid velocity in the plume zone, the liquid circulating flow rate and the mixing time are calculated for various injecting conditions and correlated as simple functions of gas flow rate, liquid depth and cross-sectional areas of both the plume zone and the vessel. It is found that the cross-sectional area of the plume zone has a significant influence on the circulating flow. Large cross-sectional area of the plume zone is favorable for mixing in the bath. The calculated results of circulating flow rate and mixing time agree with the experimental results obtained previously. I. IntroductionGas injection into molten steel is widely used in steelmaking processes. Fluid-flow phenomena and mixing in the processes have profound influences on the steelmaking reaction rate. Several experimental and theoretical studies have been made on circulating flow and mixing time in the molten steel bath.l-3~ Nakanishi et al.'> obtained a quantitative correlation between mixing time and stirring power of gas. Szekely et a1.2~ interpreted their experimental measurements for tracer dispersion and dissolution patterns of immersed graphite rods in argon-stirred ladles by using a two equation model of turbulent circulating flow. Recently Hsiao et a1.3~ determined the flow velocity of molten steel in argon-stirred ladles by measuring the drag force exerted on a probe immersed in molten steel.In the chemical engineering field, many models of circulating flow in bubble columns are proposed on the basis of momentum balance4-7) and energy balance.8-10> From the models one can calculate the flow velocity and gas holdup.The present study is concerned with analyzing circulating flow and mixing time in a gas-stirred molten metal bath. The analysis is made by setting up a steady-state energy balance for the liquid phase. The liquid velocity in the bubble plume zone and the mixing time are calculated under various injecting conditions. Effects of various factors influencing the fluid-flow phenomena and the mixing time are investigated.

Mixing Time in a Bath Agitated Simultaneously by Bottom Gas Injection and Side Liquid Injection

“…Nowadays it is generally accepted that the power of " is dependent on the mode of energy input and the vessel geometry. 5,12,[19][20][21][22] The same functional relationship as derived by Nakanishi et al 1) was valid for the presently measured values, as shown in Fig. 12 …”

Macroscopic Models for Gas Stirred Ladles

“…The energy balance applicable to gas stirred ladle system as invoked by Sahai and Guthrie 8) as well as Sano and Mori 13) was based on the assumption that the rate at which energy is imparted by the rising bubbles is balanced under steady state conditions by the rate at which turbulence kinetic energy is dissipated within the system. However, because of bubble slippage, waves and droplet formation at the free surface as well as friction at the wall, it is unlikely that turbulence phenomena alone would dissipate all of the supplied input energy to the system.…”

“…This, together with many other characteristics of the gas stirred ladle systems, has accordingly been applied as a basis in many subsequent studies on hydrodynamics, mixing etc. [8][9][10][11][12][13][14] Considerable efforts have been made by various group of researchers to develop expressions to show how plume rise velocity (i.e., average rise velocity of the two phase gas liquid mixture) and average speed of liquid recirculation within the vessel can be predicted ab initio in terms of the key ladle operating variable such as, gas flow rates, and vessel dimension. Thus, Sahai and Gutrhrie 8) were among the first to develop and propose an explicit correlation between plume rise velocity and the operating parameters.…”

A Note on the Determination of Mixing Times in Gas Stirred Ladle Systems.

“…In a series of recent contributions,1~3) Krishnamurthy and coworkers have investigated mixing phenomena in gas stirred liquid baths and have proposed a modelbased on the concept of a "circulation time. "I' 4,5) Their modell) in its updated form2) was used to assess our experimental studies of mixing, which reported on 10cation effects of tracer addition and monitoring poipts on mixing times.6~9) To illustrate this, a sample comparison between Krishnamurthy's calculated results3) and previous experimental data by ourselves,6) reproduced from ref. 3, is shown in Fig.…”