Multiphase Flow and Low Pressure Effects in the SEN
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In the continuous casting of steel a stopper rod is commonly used to control the flow rate from the tundish into the mould. Agglomeration of solid material near the stopper rod and inside the submerged entry nozzle (SEN) can lead to a reduced cross section and thus to a decreased casting speed or even total blockage (?clogging?). It is known that the injection of argon gas at the stopper rod tip reduces the clogging effect. Nevertheless the mechanisms involved in clogging are still not fully understood. A key for a better understanding may be the consideration of the absolute pressure: Single phase considerations of the flow in the region of the stopper rod result in a low or even negative pressure at the smallest cross section due to the high flow velocities. This can cause degassing of dissolved gases from the melt, evaporation of alloys and entrainment of air through the refractory material. According to these considerations, a two phase liquid-gas flow is expected even in the case when no gas is injected in the stopper-rod region. A mathematical model for this two phase flow situation in the SEN is presented that links the throughput and the pressure inside of the SEN with the geometrical parameters, the amount of gas in the SEN and the pressure loss coefficient at the stopper rod. An explanation theory for the diminishing influence of the gas injection on clogging is outlined.
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