Liquid Steel at Low Pressure: Experimental Investigation of a Water Air Flow in a Convergent Divergent Nozzle

In the continuous casting of steel controlling the flow rate to the mould is a critical issue as a constant mould level is essential for a good quality of the cast product. A stopper rod is a commonly used device to control this flow rate. Agglomeration of solid material near the stopper rod can lead to a reduced cross section and thus to a decreased casting speed or even total blockage (“clogging”). The mechanisms involved in clogging are still not fully understood. 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. This can cause degassing of dissolved gases from the melt, evaporation of alloys and entrainment of air through the refractory material. In the first part of this paper the criteria for degassing of liquid steel at low pressures are analysed and compared to water. It can be shown that the degassing process in liquid steel is mainly taking place directly at the stopper rod and its surrounding. The steel flow past the stopper is highly turbulent. In addition refractory material in general is non wetting to liquid steel. So the first step to understand the flow situation near the stopper is to understand the behaviour of this two phase flow. Accordingly water experiments are conducted using a convergent divergent nozzle. These experiments show the strong impact of the wettability of the wall material on the actual flow structure.