Energy Balance Around Gas Injection into Oxygen Steelmaking
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IN oxygen steelmaking process, supersonic high-purity oxygen jets impinge the molten bath and remove carbon, silicon, manganese, and phosphorus through oxidation. There have been various studies[1–5] on the optimization of the steelmaking process; however, no study has been reported that establishes an energy balance of the injection part of the process. The present work can provide an outline of determining the amount of energy consumed by different processes around gas injection. In modern day oxygen steelmaking, Laval nozzles (i.e., converging-diverging nozzles) are used to produce supersonic oxygen jet into the bath.[6] Supersonic gas jets are used because it creates higher dynamic pressure than subsonic jets and thus provides deep penetration and better stirring.[7] As the distance from nozzle exit increases, the diameter of the core gradually decreases. Usually, the span of the potential core is three to eight times of the nozzle diameter. After the decay of the potential core, the jet begins to spread and entrainment effects set in. Air or gases from the ambient atmosphere enters into the jet flow and jet diameter increases with decrease in the jet velocity.[6] This layer is called turbulent mixing layer (as shown in Figure 1) where mixing of jet with the surrounding air takes place.[8] A SHABNAM SABAH, Faculty of Science, Engineering and Technology, and GEOFFREY BROOKS, Pro-Vice Chancellor, are with Swinburne University of Technology, Hawthorn, VIC 3122, Australia. Contact e-mail: [email protected] Manuscript submitted July 6, 2015. Article published online October 13, 2015. 458—VOLUME 47B, FEBRUARY 2016
supersonic jet issuing from a Laval nozzle can be divided into three different regions:[8] (a) Potential core region: In this region, the axial velocity of the gas is equal to the nozzle exit velocity. The length of the potential core region is affected by upstream pressure and ambient temperature. (b) Transition region: It is the region between potential core region and fully developed flow region. (c) Fully developed flow region: In this layer, the flow becomes fully turbulent and jet spreading begins. As the jet strikes the bath surface, it creates a cavity on the bath surface. The cavity is unstable in nature and oscillates vertically and horizontally.[9] Formation of waves was found to form inside the cavity.[10,11] The general frequency of this oscillation was found to be 8 to 16 Hz,[11] 7 to 8 Hz,[12] and 10 to 12.[13] Molloy[14] termed three types of cavity shapes as dimpling mode, splashing mode, and penetrating mode (as shown in Figure 2). In dimpling mode, there is a slight depression with no droplet formation. In splashing mode, there is a shallow cavity with large outwardly directed splash. In case of penetrating mode, there is decrease in outwardly splash with deep cavity shape. Incorporating Kelvin–Helmholtz instability criteria into the droplet generation, Subagyo et al.[3] introduced a new dimensionless number termed as ‘Blowing number’ to predict droplet generation. In the current ana
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