Submerged entry nozzle(SEN) has been instrumental in determining the flow pattern inside the mold cavity. The flow rate, direction, and other flow properties can be optimized to achieve a proper flow pattern. The properly designed nozzle will discourage flow asymmetry, high surface turbulence levels, and high transient level fluctuations. All these occurrences are directly linked with the surface quality of steel and thus nozzle shape is one of the important design variables.
Some of the governing variables affecting the jet conditions are:
• Bore size
• Port angle
• Port opening size
• Nozzle wall thickness
• Port shape (round, oval, square)
• Number of ports (bifurcated or multiport)
• Nozzle bottom design
Refractory makers design nozzles by studying the aspects of fluid flow by applying physical modeling and mathematical simulations to find insights about flow conditions of steel in the mold.
Bore Size
The bore size must be sufficient enough to accommodate the maximum desired flow rate in order to meet productivity demands. Large bore is preferred for higher casting speed. To avoid the conditions of bridging for thin- slab and high speed casting elongated bores are preferred. Although large-sized ports are found effective to compensate for alumina build-up, it restricts the operation of the nozzle, which thereby aggravates flow variations and negative pressure problems.
Nozzle Port Angle and Opening Size
The nozzle ports are designed to reduce internal turbulence and supply appropriate mass flow, velocity and swirl with even distribution on both the sides, resulting in proper and evenly distributed flow pattern on both sides of the port. However, the size of the ports is made larger than the bore to accommodate alumina build up and as a consequence, the flow generally exits through only the bottom of the nozzle port. Owing to the slow speed of the flow and being directed towards the nozzle at the top of the exit port, the angle of the upper edge of the port has considerably very less effect on the angle of the flow exiting out of the nozzle compared to the bottom edge. But the shape of the top edges is important in promoting a smooth curve which helps in attachment of the jet to the nozzle walls, which produces less recirculation and greater flow rate through the port with the curved top edge similar to that caused in clogging.
The nominal angle of the edges of the outlet port has little effect on the jet direction when considering large exit ports. The momentum of the jet through the oversized ports carries the steel jet at a downward angle, irrespective of the angle of the port edges. The flow is angled downward even for port angled upward.
However, when the total port area is less than the bore area, then the angle of the nozzle ports becomes more important, as the flow is forced to conform more closely to the angle of the port edge. It has been observed that the jet leaving a 45-mm-high upward-angled nozzle heads upward, instead of downward for the equivalent 90-mm-high port.
Thus, flow from multiport nozzles tends to be in close conformity to the direction of the nozzle port edges, since each of the exit ports is smaller and is thus more effective at influencing the flow. But, an increase in speed and turbulence of the jets exiting the nozzle is likely with smaller ports.
If port edges are effective, increasing the port angle directs the jet deeper into the liquid cavity, and the flow pattern shifts downward thereby reducing the meniscus surface turbulence and the top surface velocity. Thus, downward port angles are preferred over upward port angles.
Discharging ports with cross-section areas larger than the bore cross-section area of the submerged entry nozzle (SEN) will lead to meniscus turbulence and slag entrainment.
Moreover, large port dimensions lead to recirculating flows just in the upper edge of the port, leading to backflow conditions and aggravating clogging problems.
Nozzle Wall Thickness
The increase in the nozzle wall thickness makes the nozzle port edges more effective at controlling the jet direction. It has been observed that the 12-mm nozzle has a steeper downward jet, while the thick-walled nozzle has a more desirable uniform flow in the same direction as the port angle.
A far as life is concerned, thermal shock resistance and erosion resistance at the slagline are determined by the nozzle wall thickness. However, increasing the nozzle thickness decreases the distance between the nozzle and the wall which can have adverse effects.
Port Shape
Tall and thin ports induce a steeper downward jet angle than short, fat ports when other parameters are being kept constant.
Port shape is less important compared to the port size and angle in controlling the flow. It has been observed that circular ports generate more swirls which have a larger spread angle as compared to the rectangular sections, having the same cross-sectional area.
Number of Ports
Multiport nozzles control the flow direction compared to the bifurcated nozzles. A central hole added to the bottom of a bifurcated nozzle imparts symmetry and stability in the mold flow pattern. The presence of an extra hole hinders the oscillation in the mold flow pattern from side to side for high-speed casting in wide molds or thin slabs. This leads to less surface fluctuation. At the same time, it reduces the flow intensity on the surface by sending the jet momentum deeper into the mold.
The size of the bottom and side ports can be adjusted for square cross-section billet casting for controlling the jet momentum which in turn reduces the surface flow intensity.
Nozzle Bottom Design
Although, the shape of the bottom of the nozzle has little effect on the mass flow and direction of the jets leaving the nozzle but effects can be substantial as far as turbulence leaving the nozzle and the clog deposition is concerned, which has an effect on the flow transients in the mold and subsequently surface turbulence.
A “sump” in the bottom of the nozzle can accommodate more clogging, which can facilitate jet stability exiting the port. Thus slump nozzle is said to produce less transient fluctuations which can improve the surface quality. The shape of the nozzle has a minimal effect even when the ports are angled upwards.
