Process metallurgy Model investigations on the fluid flow in continuous casting moulds of wide dimensions Peter Andrzejewski, Karl-Ulrich Kohler and Wolfgang Pluschkell The control of the liquid metal flow and the damping of surface fluctuations are predominant objects connected with the operation of wide mould casters. Investigations on a full scale water model reveal that the casting speed, the immersion depth of the entry nozzle, and the lift gas injection rate have to be tuned carefully for the benefit of optimal flow conditions in the CC-mould. When casting conditions are changed, it is necessary to adjust the free parameters correspondingly in order to maintain smooth casting operation and high ranking strand quality. Modelluntersuchungen zur Stromung in breiten StranggieBkokillen. Der Betrieb von StranggieBmaschinen mit extra breiter Kokille setzt die Beherrschung der Stromung im Schmelzenvolumen und die Dampfung von Fluktuationen in der Schmelzenoberflache voraus. Versuche an einem Wassermodell im MaBstab 1 :1 zeigen, daB GieBgeschwindigkeit, Eintauchtiefe des GieBrohres und Begasungsrate aufeinander abgestimmt sein mOssen, um eine optimierte Stromungsausbildung zu erreichen. Bei Anderung der GieBbedingungen ist es notwendig, die freien Parameter entsprechend auzupassen, um den optimalen Stromungszustand beizubehalten und damit eine hohe Qualitat des vergossenen Stranges zu gewahrleisten. In recent years, a new generation of continuous casting machines was brought into service. These installations are one-strand slab casters of extra wide mould size. Typically, the ratio of strand width to strand thickness is more than ten, even reaching thirteen. The operation of such casters is difficult, the problems arising from the heavy inlet stream into the mould, which can become even twofold as com- pared with conventional two-strand casters. Choppy flow, wave and vortex formation in the surface of the liquid steel favour the entrapment of mould powder in the strand and also reduce the cleanliness of the solidified steel shell. On the other hand, sufficient heat has to be transferred to the narrow side of the wide mould in order to avoid accre- tions in this sensitive zone. It can be stated, therefore, that properly designed flow in the mould is decisive for the suc- cessful operation of wide mould casters. In 1985, the one-strand caster of Hoesch Stahl AG at Dortmund went on stream. The mould size of this caster is 2765 mm x 220 mm at maximum. To get an insight into the relevant parameters influencing the mould flow, it was decided to install a water model of full scale for investiga- tion purposes. Water models have frequently been operated for mould flow studies 1 )' 5 ). In these investigations smaller moulds were used as compared with the present case. Also, the- oretical computations were performed using principles of hydrodynamics 5 )' 7 ). But fluctuations at free surfaces are in- accessible for computational predictions. Furthermore, the direct visualization of flow characteristics is a strong reason for the installation of a physical model. Full scale water model Figure 1 is a schematic design of the water model. It consists of the mould, the tundish, the storage tank and the tubing with two pumps and various fittings. The mould of 2765 mm x 220 mm size is made of acrylglas so that direct flow observation is possible. The submerged entry nozzle Dr. rer. nat. Peter Andrzejewski; Dr.-lng. Kari-Ulrich Kohler, Hoesch Stahl AG, Dortmund; Professor Dr.-lng. Wolfgang Pluschkell, Institut fiir Eisenhiittenkunde und Giessereiwesen der TU Clausthal, Clausthal- Zellerfeld, Germany. 242 (SEN) and likewise the stopper rod are original compo- nents of the casting plant. Inert gas can be passed through the stopper rod as in reality. The water flow through the model is impressed by the rateable pump no. I acting on the mould reservoir. The tundish is supplied by conveying pump no. 2. A compen- sating storage tank is placed between the two pumps. In order to have constant filling heights in the mould and in the tundish as well, two control systems are installed consisting of pressure gauges and regulators. The first reg- ulator acts on the hydraulic positioner of the stopper rod, the second adjusts the motor valve of the tundish supply tube. The maximum flow rate of water through the model is 1 m 3 min- 1 . The filling heights in the tundish and in the mould remain on the adjusted levels during all the time of the experimental run. Similarity considerations The design and the operation of flow models need due consideration of similarity criteria. The liquid velocity pat- tern measured in the water model corresponds to that in liquid steel, if the criterion of the Reynolds number and that of the Froude number are fulfilled simultaneously. This is the case, when the geometric ratio is in agreement with the equation The indices M and R denoting the model and the real sub- ject respectively; v being the kinematic viscosity of the two liquids under consideration. Using VM = 1.0 · 10-{i m 2 s- 1 for water at 20 °C and VR = 0.95 · 10- 6 m 2 s- 1 for liquid steel at 1550 °C, it follows that LMILR = 1.035. Therefore, the full scale model operated with water is an ideal system for the intended flow studies. With respect to gas injection it can be stated that the rising velocity of gas bubbles is not very different in water and in liquid steel. The gas hold up should be nearly the same provided that the bubble size spectrum is identical. Hence, the lifting effect of the gas on the mould inlet spout is similar in both systems, if the gas flow at the model is sixfold the (STP) gas flow at the caster. The effect of the steel research 63 (1992) No. 6