Computational Fluid Dynamics

During scrap charging, a huge amount of hot gas and dust leave the EAF. Depending on the amount of organic impurities in the scrap, the gas emission can exceed the design throughput of the secondary exhaust gas system. In this case a cloud of dust could enter into the melt shop resulting in too high dust loads in some areas. The video shows that dust particles enter the shop next to the EAF. The hot, dust-free off-gases from the ladle heaters in the left-hand shop area drive the hot, dust-laden off-gases from the left to the right side. This undesired flow pattern can be avoided, e.g. by attaching baffles and/or curtains, and by positioning the ladle heaters in a more symmetrical arrangement. 

The melt flows out of the ladle and into the tundish, a material buffer which links the discontinuous secondary metallurgy with the continuous casting process. An important task of the tundish is to separate non-metallic particles from the melt. Here, a simulation has big advantages as compared to water modeling. Approved numerical two-phase models allow the direct comparison of the separation efficiency of different tundish designs. Moreover, buoyancy effects can be taken into account. Undesired and so-called dead water regions can be identified and avoided by re-design of the interior refractory contour.

In the continuous casting mold the highly sensible solidification process takes place. The mold consists of water-cooled, oscillating copper plates, the melt solidifies and forms a stable strand shell. In the mold domain, the liquid core is covered by a layer of casting flux resulting in a resistance to heat and reducing friction. The supported strand is cooled with spray water. To guide the strand out of the caster horizontally, it is bent and straightened, either partially or fully solidified, depending on the casting technology. However, product defects caused by uncontrolled mold flow phenomena cannot be reversed further downstream in the casting process. CFD simulations allow to calculate a performance map of any SEN for the full casting range (casting speed, width, thickness, immersion depth, etc.). With this performance map the flow behavior in the mold is much more transparent and can be adapted to the casting condition.

Following the casting process, slabs or billets are transported to the hot rolling mill and heated to rolling temperature, e.g. in walking-beam furnaces or pusher-type furnaces. A pusher-type furnace operates semi-continuously and is characterized by high throughput rates and maximum heat transfer capacity. Cold slabs are pushed into the furnace where they initially slide on carrier rails, then on an equalizing surface to the discharge end. Typical issues concerning the heating process are the slab heating rate, the number of burners required and the influence of the carrier rails on the skid marks, i.e. the local temperature distribution, in the slab.

The electrostatic precipitator (ESP) is a dry dedusting system used for BOF primary gas cleaning. For efficient operation of the ESP a balanced off-gas flow over the entire cross section is important. Typical questions that can be answered by CFD concern the transient gas flow distribution inside the ESP. As the gas flow is affected by both the batch-type BOF operation and the routing of the off-gas duct, perforated flow rectifier plates are installed in the inlet region of the ESP to ensure a proper gas exchange. Furthermore, explosive gas mixtures of O2 and CO induced by the BOF batch process have to be avoided; CFD is used to predict the maximum allowable mixture resulting from varying gas properties (density, viscosity). Another approach is to calculate the maximum explosion pressure which may occur. Based on these data, the safety devices (number, size, and location of pressure vent valves) of the overall gas cleaning system are designed.

The NF ladle furnace is used to superheat slags before the slag is treated in a separation plant. The slag is heated up via electricity by electrodes. In order to prevent an inhomogeneous temperature distribution in the slag, inert gas is injected into the water cooled stirring lance. Typical questions concerning the heating process are the flow rate of inert gas and cooling water, the optimum immersion depth of the electrode to produce intensive mixing, and the amperage to obtain the heating rate requested.